3494
NEUROLOGIC SYSTEM
Paula Vernon-Levett
CENTRAL NERVOUS SYSTEM
Developmental Anatomy
A. Embryogenesis
The nervous system is one of the first organ systems to develop in the embryo (neurula).
1. All body tissues are derived from three different germ cell layers:
a. The mesoderm forms future muscle, skeleton, connective tissue, and the cardiovascular and urogenital systems; it assists in the early development of neural tissue; and forms the notochord, which is incorporated into the future spinal column.
b. The endoderm forms the future gut and associated organs.
c. The surface ectoderm forms future skin, nails, epidermis, hair, and mammary glands, whereas the neuroectoderm forms future neural tissue.
2. Neurulation is the process of neural tube formation and cerebrum development (Figure 4.1).
a. The neural plate is composed of specialized neuroectoderm cells arising from the embryo that thicken on either side of the neural groove (on the dorsal surface), forming a flat plate with distinct lateral edges present at approximately 20 days gestation.
b. The neural groove is the anteroposterior groove in the ectoderm that appears at 2½ weeks gestation. Development proceeds cranially.
c. The neural crest contains the specialized cells that originate from the neural plate but separate from it to form a parallel band that extends the length of the neural plate. The neural crest gives rise to the future peripheral nervous system (PNS), spinal and autonomic ganglia, and some nonneural tissue (including the meninges). It is present at 3½ weeks gestation.
d. The neural tube is formed by the lateral edges (folds) of the neural plate, which fold and grow medially until they meet and form a tube. 350The cavity of the neural tube becomes the future ventricular system of the brain and central canal of the spinal cord. It is closed at 4 weeks gestation.
FIGURE 4.1 Cross-sections showing early development from neural groove to cerebrum.
Source: From Waxman, S. G. (1996). Correlative neuroanatomy (23rd ed.). Stamford, CT: Appleton & Lange.
e. Epidermal (sensory) placodes are composed of nine or 10 pairs that arise from separate ectoderm thickening in the head region. Together with the neural crest, they give rise to cranial nerves and cranial sensory organs.
3. Brain Development. Further specialization of the neural tube forms three distinct swellings (vesicles) at the rostral end of the tube.
a. The neural tube forms three bulges (primary brain vesicles) at its cephalic end that develop future parts of the brain (represented at 4 weeks gestation): prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain; Figure 4.2).
b. Early in the second fetal month, two of the three primary brain vesicles further subdivide to form secondary vesicles. The prosencephalon forms the telencephalon (future preoptic region and paired cerebral hemispheres of the mature brain) and the diencephalon (future hypothalamus and thalamus of the mature brain). The mesencephalon remains the midbrain (future superior and inferior colliculi; red, reticular, and black nuclei; and cerebral peduncles of the mature brain). The rhombencephalon forms the metencephalon (future pons and cerebellum of the mature brain) and the myelencephalon (future medulla of the mature brain).
FIGURE 4.2 The brain vesicles indicate the adult derivatives of their walls and cavities. The rostral part of the third ventricle form from the cavity of the telencephalon. Most of the ventricles are derived from the cavity of the diencephalon.
Source: Reprinted with permission from Moore, K. L., & Persaud, T. V. N. (1993). The developing human: Clinically oriented embryology (5th ed.). Philadelphia, PA: Elsevier.
c. Fissure formation begins in the fourth fetal month with development of the lateral sulcus (of the cerebrum) and the posterolateral sulcus (of the cerebellum). The central sulcus, calcarine sulcus, and parietooccipital sulcus are visible in the fifth fetal month. All main gyri and sulci are present by the seventh fetal month.
d. Myelination of the brain begins at approximately 24 weeks gestation and progresses rapidly during the first 2 years of life with slower maturation into adulthood. Oligodendrocytes and Schwann cells form myelin (primarily composed of lipids and proteins), which wraps around axons to form a sheath. The myelin sheath acts as an insulator of axonal segments and is responsible for high-velocity nerve conduction.
4. Spinal cord development begins from the caudal portion of the neural tube. The earliest nerve fiber tracts appear around the fifth gestational week. Long association tracts appear at the third gestational month. Pyramidal tracts appear in the fifth gestational month.
3515. Neural Tissue Specialization
a. Neural tissue further differentiates into four concentric zones around the central canal that develop into specific areas of the mature brain. The ventricular zone is located adjacent to the central canal and is a precursor to neurons and macroglia. The subventricular zone generates certain classes of neurons and macroglia and some deep structures of the cerebrum. The intermediate (mantle) zone evolves into gray matter. The marginal zone has no primary cells of its own but evolves into most of the white matter.
b. Sensory components of the central nervous system (CNS) develop from further divisions of the neural tube. The basal plate is the ventral portion of the neural tube, which contributes to the efferent (motor) system. The alar plate is the dorsal portion of the neural tube that contributes to the afferent (sensory) system.
B. Neuron and Associated Cells
1. The neuron is the functional and anatomic unit of the nervous system (Figure 4.3). The cell body (soma) contains the nucleus and comprises most of the gray matter. The perikaryon (neuroplasm) is cytoplasm surrounding the nucleus. The perikaryon contains granular, filamentous, and membranous organelles. Two types of neuronal processes include the dendrites and axons.
FIGURE 4.3 The principal morphologic features of a multipolar neuron.
a. Each neuron usually contains several dendrites, which conduct impulses toward the cell body (afferent).
b. Each neuron only has one axon. A myelin sheath encases some axons (white matter) and increases transmission of impulses. Nodes of Ranvier are normal interruptions in the myelin sheath that have ion channels required for regenerating an action potential. Axons conduct impulses away from the cell body (efferent).
c. The synapse is the site of contact of one neuron with another neuron. Transmission of electrical or chemical signals occurs between these neurons. The neuron sending the message is the presynaptic cell and the neuron receiving the message is the postsynaptic cell. The synaptic cleft is the space between the axonal terminal swelling (bouton) of the presynaptic neuron and the postsynaptic location on another neuron, usually a dendrite. The neuromuscular junction is the termination of a nerve fiber in a muscle cell, and the neuroglandular junction is the termination of a nerve fiber in a glandular cell.
i. Chemical transmission. The presynaptic axon has a vesicle in the cytoplasm of the bouton that contains an active neurotransmitter agent. Once the neurotransmitter is released into the synaptic cleft, it attaches to a receptor on the postsynaptic membrane; 352the resulting response may be excitatory or inhibitory.
ii. Electrical transmission. The membrane of one cell is closely connected to the membrane of another cell via small pores known as gap junctions. The connection is also known as an electrical synapse. As the membrane potential of the presynaptic neuron changes, ions travel to the postsynaptic neuron and depolarize the neuron.
2. Neuroglias are the supporting and nourishing structures of the nervous system. There are four main types: oligodendrocytes (produce myelin), astrocytes (support, bind, and nourish neurons), microglia (phagocytic properties), and ependyma (line the ventricular system and choroid plexus and produce cerebrospinal fluid [CSF]).
C. Extracerebral Structures
1. The scalp is composed of skin, subcutaneous tissue, galea aponeurotica, and pericranium.
2. The skull of the infant consists of eight bones: two frontal (one bone after metopic suture fuses), two parietal, two temporal, one occipital, one ethmoid, and one sphenoid.
3. The sutures (Figure 4.4) are dense, white, fibrous, connective-tissue membranes that separate the bones. The sagittal suture separates the two parietal bones on top of the skull. The coronal suture (frontoparietal) connects the frontal and parietal bones transversely. The basilar suture is created by the junction of the basilar surface of the occipital bone with the posterior surface of the sphenoid bone. The lambdoid suture connects the parietal and occipital bones transversely.
FIGURE 4.4 Cranial sutures view from top of head.
4. The fontanelles are areas where several sutures join together. The posterior fontanelle is formed by the intersection of the sagittal and lambdoid sutures. Two anterolateral fontanelles are formed by the intersection of the frontal, parietal, temporal, and sphenoid bones. The anterior fontanelle is formed by the intersection of the coronal and sagittal sutures. Two posterolateral fontanelles are formed by the intersection of the parietal, occipital, and temporal bones. The posterior and anterolateral fontanelles close at 2 months after birth. The anterior fontanelle closes between 12 and 18 months. The posterolateral closes at 24 months.
5. Meninges are three membranous connective-tissue layers that cover the brain. The dura mater is the outermost layer and consists of two layers. The outer periosteum adheres to the inner surface of the skull and vertebrae. The inner layer folds inward as dural reflections, dividing the cerebral hemispheres (falx cerebri), separating the cerebrum from the cerebellum and brainstem (tentorium cerebelli), and dividing the two cerebellar hemispheres (falx cerebelli). The arachnoid is the middle transparent avascular covering with many fine collagen strands (trabeculae). The pia mater is the inner, delicate, clear membrane that adheres directly to the surface of the brain and spinal cord.
6. Ventricular System and CSF Circulation
a. The ventricles are four interconnecting chambers lined by ependyma. The paired lateral ventricles are contained within the cerebral hemispheres, subdivided into four parts: the anterior horn located in the frontal lobe, the body located in the parietal lobe, the inferior horn located in the temporal lobe, and the occipital horn located in the occipital lobe. The third ventricle is connected to the lateral ventricles via the foramen of Monro and connected to the fourth ventricle via the aqueduct of Sylvius. The fourth ventricle communicates with the third ventricle and subarachnoid space around the brain and spinal cord via three exit points. The foramen of Magendie exits to the cisterna magna (central canal of spinal cord) and the spinal subarachnoid space. The foramina of Luschka exit to the cisterna magna and the subarachnoid space around the brain.
b. The choroid plexuses are branched and highly vascular structures and consist of numerous villi. The choroid plexus is a three-layer membrane consisting of the choroid capillary endothelium, pial cells, and the choroid epithelium. It is located in all four ventricles and parenchyma and is responsible for the majority of CSF production.
353c. CSF is produced from arterial blood by the choroid plexuses located in the ventricles and, to a lesser degree, by ependymal cells lining the ventricles and spinal cord. The hourly rate of CSF production fluctuates and output increases logarithmically with age and body weight. Gender also plays a role with males producing more than females. The rate of CSF production is highest in infants. On average, the rate of production is approximately 0.3 to 0.4 mL/min and the total volume is replaced every 6.5 to 9.0 hours. The total static CSF volume in the ventricular system also varies by age and is approximately 150 mL in adults (Cartwright & Wallace, 2007; Sakka, Coll, & Chazal, 2011) and is proportionately less in infants and children.
d. Historically, the purpose and function of CSF was considered to be for protection of the CNS via mechanical support. Recent data suggest that it also plays an important role in homeostasis of the interstitial fluid, removal of waste products, and regulation of neuronal function (Sakka et al., 2011).
e. The classical hypothesis for circulation of CSF is detailed in Figure 4.5. The four ventricles of the brain are a series of communicating cavities that contain and circulate CSF. From the two lateral ventricles, CSF passes to the third ventricle via the foramina of Monro. It travels from the third ventricle to the fourth ventricle via the aqueduct of Sylvius. The combined CSF volume passes through two lateral foramina of Luschka and the medial foramen of Magendie into the cisterna magna. CSF travels upward around the cerebrum via the subarachnoid space and downward around the spinal cord via the spinal subarachnoid space.
FIGURE 4.5 Location scheme of the choroid plexuses and the distribution of CSF in the human central nervous system. CSF is shown as the gray area and the arrows point the direction of CSF circulation and the sites of CSF absorption.
CSF, cerebrospinal fluid.
f. Absorption occurs primarily through the arachnoid villi. The fingerlike projections from the arachnoid layer extend into the superior sagittal sinus function as one-way valves allowing CSF to exit the sagittal sinus, but the projections prevent blood from entering the subarachnoid space. The rate of absorption depends on CSF pressure (higher pressures result in more absorption to a certain point) and venous pressure (higher venous pressures can impede absorption). With normal physiological conditions, CSF secretion in the ventricles and absorption of CSF in the venous sinuses are balanced (Orešković & Klarica, 2010).
354g. Characteristics of normal CSF include the following: CSF is primarily composed of water. It is clear and odorless; glucose concentration is one half to one third of the serum glucose concentration; protein concentration is 15 to 45 mg/dL (higher in term infants, median 74 mg/dL), white blood cells (WBCs) are usually absent (however, a few may be present, especially in neonates, median 3 cells/μL), and red blood cells (RBCs) are absent except during traumatic lumbar tap (Srinivasan et al., 2012). The opening CSF pressure obtained during a lumbar puncture is dynamic and is related to the patient’s body position, age, depth of sedation, activities, and physiologic condition. A CSF opening pressure of <28 cm H2O (equivalent to 20.5 mmHg) is considered normal for most children. However, it should not be interpreted in isolation, but combined with other clinical assessments (Avery, 2014).
D. The Brain is Divided Into the Cerebrum, Diencephalon, Brainstem, Reticular Formation, and the Cerebellum (Figure 4.6)
1. Cerebrum (Telencephalon)
a. The cerebral hemispheres consist of four lobes: The frontal lobes hold the primary motor cortex, Broca’s motor speech area (written and spoken language), and personality. The temporal lobes are responsible for reception and interpretation of auditory information, emotional and visceral responses, and retention of recent memory. The parietal lobe is responsible for comprehension of language, orientation of spatial relationships, and initial processing of tactile and proprioceptive information. The occipital lobe is responsible for the reception and interpretation of visual stimuli.
FIGURE 4.6 Lateral view of the cerebral hemispheres, brainstem, and cerebellum with basic functions.
b. Basal ganglia are located deep in the cerebral hemispheres and are composed of four nuclei providing unconscious control of lower motor neurons. The basal ganglia are processing stations, linking the cerebral cortex to specific thalamic nuclei.
c. The corpus callosum is the largest commissural tract and is composed of a bundle of transverse nerve fibers connecting the two cerebral hemispheres. It transfers information between cerebral hemispheres and makes up the roof of the lateral ventricles and the third ventricle.
d. The limbic system refers to several structures, including the limbic lobe, hippocampus and connections, amygdala, septal nuclei, hypothalamus, anterior thalamic nuclei, and portions of the basal ganglia. This system is primarily responsible for affective behavior and autonomic control.
3552. The diencephalon is the rostral end of the brainstem and is located deep within the cerebrum. Sometimes it is classified as part of the brainstem. Anatomically, it is divided into the following:
a. The epithalamus is a narrow band forming the roof of the diencephalon. It includes the habenula and pineal gland. The epithalamus’s exact function is not well understood, but it is associated with the limbic system, optic reflexes, and reproductive activity.
b. The thalami are the largest subdivision of the diencephalon. Two egg-shaped masses are located deep in each cerebral hemisphere, and their primary function is to be a relay station for sensory input.
c. The hypothalamus forms the base of the diencephalon and the floor and inferior lateral walls of the lateral ventricles. It is a very small structure that contains several nuclei that connects the CNS to the endocrine system. The primary function of the hypothalamus is to maintain physiologic homeostasis by regulating a number of visceral responses, as well as more complex behavioral and emotional responses.
d. The subthalamus is located lateral to the hypothalamus and is functionally integrated with the pyramidal pathways.
FIGURE 4.7 Gross anatomic features of the brainstem, lateral view.
3. The brainstem consists of three continuous structures located beneath the thalamus (Figure 4.7).
a. The mesencephalon (midbrain) is located rostral on the brainstem between the diencephalon and metencephalon and is the origin of cranial nerves III and IV. The primary function of the mesencephalon is to be a relay center for visual and auditory reflexes. It is also the center for postural reflexes and the righting reflex (maintains the head in an upright position).
b. The metencephalon (pons) is located above the medulla and ventral to the cerebellum and serves as the origin of cranial nerves V, VI, VII, and VIII. It contains nerve fibers that form the reticular formation and are continuous with other parts of the brain. The metencephalon also contains the medial longitudinal fasciculus (MLF), which is composed of efferent fibers. The pons helps to regulate respiration.
c. The myelencephalon (medulla oblongata) is continuous with the pons rostrally and the spinal cord caudally, and is the origin of cranial nerves IX through XII. Predominant functions of the myelencephalon include the primary respiratory and cardiac centers and the vasomotor centers.
4. The reticular formation is a diffuse network of neurons located in the brainstem. It begins at the 356upper end of the spinal cord and extends upward to the hypothalamus and adjacent areas. This formation contains both sensory and motor neurons, nuclei that interact with the extrapyramidal motor control system. The reticular formation is the site of the reticular activating system (RAS), which assists in regulating awareness (sleep–wake cycles).
5. The cerebellum is located superior to the fourth ventricle and contains two lobes. It is connected to the brainstem via three pairs of fiber bundles (cerebellar peduncles). Primary functions of the cerebellum include coordination of voluntary movements, control of muscle tone, and maintenance of equilibrium.
E. Cerebral Circulation
1. Arterial blood is supplied by two paired vessels: the common carotid arteries and the vertebral arteries.
a. The common carotid arteries are located anteriorly; each bifurcates into two vessels. The internal carotid artery enters the cranial cavity and extends to the circle of Willis, where several major vessels meet. The anterior cerebral arteries supply the medial aspect of the cerebral hemispheres and the frontoparietal regions. The middle cerebral artery supplies much of the lateral aspect of the cerebral hemispheres and basal ganglia. The posterior cerebral arteries supply the lateral, medial, and inferior occipital cortex. The posterior communicating arteries connect anterior and posterior circulation.
b. The external carotid arteries supply arterial circulation to the extracerebral structures (skin and muscle of the face and scalp).
c. The vertebral arteries are located posteriorly; they originate from the subclavian arteries and join to form the basilar artery. Numerous vessels arise from the vertebral and basilar arteries and include the superior cerebellar artery, anterior inferior cerebellar artery, posterior inferior cerebellar artery, meningeal artery, anterior and posterior spinal arteries, and posterior cerebral arteries. Collectively, all the vessels supply the cerebellum, brainstem, occipital lobe, and inferior and medial surfaces of the temporal lobes.
2. Venous blood is supplied by a network composed of valveless, thin-walled cerebral veins. The superficial veins drain the external surfaces of the brain and include the superior cerebral vein, middle cerebral vein, and inferior cerebral vein, which empty into the dural venous sinuses. The deep veins drain internal areas of the brain and include the basal veins, vein of Rosenthal, and the great vein of Galen. All venous drainage empties at the base of the skull via the internal jugular veins.
3. The blood–brain barrier (BBB) is a dynamic component of the neurovascular unit. It is composed of the anatomic structures and physiologic processes that separate the brain and blood compartments. Brain capillaries are characterized by tight junctions between endothelial cells, astrocytes with foot processes that encase capillaries and neurons, and endothelial cells with large numbers of mitochondria (responsible for energy-dependent transport). The BBB is believed to be incompletely developed in the preterm neonate. Physiologic properties of the morphologic barrier prevent rapid transport of blood to the brain and maintain a delicate homeostatic balance within the internal brain environment. Chemical barriers and tight junctions restrict some substances such as large serum protein molecules and some chemotherapeutic agents, but it also responds to nutrient requirements and local chemical signals in both healthy and pathologic conditions (Benarroch, 2012). Substances easily transported across the membrane include water, oxygen, carbon dioxide, glucose, and some lipid-soluble substances such as alcohol and anesthetics.
4. The blood–CSF barrier is composed of the anatomic structures and physiologic processes that separate the brain and CSF compartments (functionally similar to BBB). The morphologic barrier is created by high impermeability of choroid epithelial cells to most substances.
F. Spinal Cord and Column
1. The spinal column consists of 33 vertebrae: seven cervical, 12 thoracic, five lumbar, five fused sacral, and four fused coccygeal segments (Figure 4.8).
2. Vertebrae. The cylinder body is located anteriorly and increases in size as it progresses downward. The posterior arch has two pedicles and two laminae. The pedicles project posterolaterally from the bodies and form part of the transverse foramen. The two laminae are located posteriorly and are thin and relatively long. The spinous processes are formed by fusion of the two laminae and vary in shape, size, and direction depending on location. The transverse process is located on each side of the arch, providing a lever for muscle attachment. The articular processes (two superior and two inferior) form synovial joints with corresponding processes on adjacent vertebrae. The intervertebral foramina are formed by notches on the superior and inferior borders of the pedicles of the adjacent vertebrae, providing a channel for spinal vessels and nerves. The intervertebral discs are fibrocartilage tissue interposed between adjacent vertebrae consisting of an outer concentric layer of fibrous tissue (annulus fibrosus) and a central spongy pulp (nucleus pulposus). The discs provide an elastic buffer to absorb mechanical shocks.
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FIGURE 4.8 The vertebral column.
Source: Gray, H. (1918). Anatomy of the human body (20th ed.). Phliadelphia, PA: Lea and Febiger.
3. The spinal cord is an extension of the medulla oblongata. It extends downward, tapering (conus medullaris), and terminates at the lower border of the first lumbar vertebra in the adult and at the third lumbar vertebra in the neonate. The filum terminale is a slender, median, fibrous thread that extends from the conus medullaris to the coccyx.
a. Outer coverings are continuous with the corresponding cerebral meninges. The dura mater consists of only one layer, does not adhere to vertebrae, and merges with the filum terminale. The spinal cord is suspended from the dura mater via a series of 22 pairs of denticulate ligaments. The epidural space is located between the dura layer and periosteum of the vertebrae. It contains venous plexuses and fat and is the location for injection of anesthetics. The arachnoid mater is nonvascular and extends caudally to the second sacral level, where it merges with the filum terminale. The subarachnoid space contains CSF and blood vessels and surrounds the spinal cord (spinal or lumbar cistern). The pia mater is directly attached to the spinal cord, its roots, and the filum terminale and is vascular.
b. The inner core of the spinal cord is composed of gray and white matter. Butterfly- or H-shaped gray matter consists of cell bodies and unmyelinated fibers. They are anatomically and functionally divided into regions (Figure 4.9). The anterior (ventral) horns contain the neuronal cell bodies of motor neurons supplying the skeletal muscles. The posterior (dorsal) horns contain the neuronal cell bodies involved in sensory input to the spinal cord. The lateral horns contain preganglionic fibers of the autonomic nervous system (ANS).
c. White matter surrounds the gray matter and consists of myelinated (predominate) and unmyelinated fibers. They are arranged into three pairs of funiculi (columns): posterior, lateral, and anterior. Funiculi are subdivided into bundles of nerve fibers (tracts or fasciculi) that are functionally distinct. Ascending (sensory) pathways transmit sensory information from peripheral receptors to the cerebral and cerebellar cortex and transmit pain, touch, temperature, spatial relationships, vibration, passive movement, and position sense. Descending (motor) pathways contain upper motor neurons, originate from the cerebrum, and descend to the spinal cord (and brainstem). They play a major role in voluntary motor movement. The central canal is lined with ependymal cells, contains CSF, and is continuous with the fourth ventricle in the medulla oblongata. Tracts are of clinical significance and are named based on the column in which the tract travels, origination of cells, and termination of fibers. Table 4.1 lists the names of ascending and descending spinal tracts and their primary function.
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FIGURE 4.9 Spinal cord with gray and white matter numbered by anatomical region.
Source: Polarlys.
TABLE 4.1 Common Ascending and Descending Spinal Tracts
Tract Name | Function |
Ascending (sensory) | |
Dorsal (posterior) spinocerebellar | Proprioception |
Ventral (anterior) spinocerebellar | Proprioception |
Lateral spinothalamic | Pain, temperature |
Ventral (anterior) spinothalamic | Touch, pressure |
Descending (motor) | |
Corticospinal (pyramidal tracts) | |
Ventral (anterior) corticospinal | Skilled voluntary movements |
Lateral corticospinal | Skilled voluntary movements |
Rubrospinal | Fine movements, muscle tone |
Vestibulospinal | Aid equilibrium, extensor muscle tone |
Reticulospinal | Posture, muscle tone |
Tectospinal | Mediates optic and auditory reflex movement |
Source: From Curley, M. A., & Patricia, M. A. (2001). Critical care nursing of infants and children (2nd ed.). Philadelphia, PA: Mosby Elsevier. Copyright Elsevier (2001).
3594. The reflex arc is an intrinsic neural circuit that, once activated, follows a specific response without conscious control. The monosynaptic reflex arc consists of a sensory end organ (receptor), afferent nerve fibers, one synapse, efferent nerve fibers, and muscle fiber or glandular cell (effector). A classic example is a deep-tendon reflex (DTR; Figure 4.10). The polysynaptic reflex arc consists of a sensory end organ, afferent nerve fibers, multiple interneurons and synapses, efferent nerve fibers, and an effector. A classic example is withdrawal of an extremity from pain stimuli.
G. Spinal Column Circulation
Arterial blood is supplied from branches of vertebral arteries and the radicular arteries derived from segmental vessels (i.e., deep cervical, intercostal, lumbar, and sacral arteries). The arteries pass through the intervertebral foramina and divide into two branches: the smaller anterior spinal artery and the larger posterior spinal artery. Venous drainage is via the venous plexus and the veins that parallel arteries.
FIGURE 4.10 Simple reflex arc (knee-jerk reflex). (1) the receptor, the sensory nerve fiber that first picks up the impulse as the hammer strikes the tendon; (2) the sensory transmitter, the afferent neuron that passes the impulse to the spinal cord; (3) the motor transmitter, the efferent neuron that passes the impulse to the effector (muscle); (4) the neuroeffector junction, a specialized endplate of motor nerves; (5) the effector, a muscle that carries out the actual response (jerking of knee).
DEVELOPMENTAL PHYSIOLOGY
A. Impulse Conduction
1. During the resting membrane potential (RMP; i.e., the inability to conduct a nerve impulse), the intracellular fluid (neuroplasm) of a neuron has a more negative electrical charge compared to the extracellular fluid, resulting in an imbalance in electrical charge across the cell membrane. Sodium (Na+) and chloride (Cl−) are in higher concentration in extracellular fluid. Potassium (K+) is in higher concentration in intracellular fluid. This electrical imbalance across the cell membrane results from three main processes:
a. There are selective ion channels that allow K+ ions to pass freely across the cell membrane into the extracellular compartment.
b. A negatively charged protein molecule inside the cell cannot cross the membrane.
c. There is an ionic pump that moves three Na+ out of the cell for every two K+ it moves into the cell.
3602. As depolarization occurs in response to an electrochemical stimulus, the cell membrane becomes more permeable to Na+. Na+ enters the cell, and the membrane becomes less negative internally. Initial depolarization must be greater than a certain threshold value for depolarization to continue.
3. The action potential is the response of the neuron to depolarization. Impulsive flow of ionic current is produced briefly. After a brief delay, the membrane potential shifts back to negative. Na+ flow is inactivated, and K+ permeability increases.
4. Repolarization is the reestablishment of negative polarity of the RMP. The cell membrane becomes impermeable to Na+ and more permeable to K+. RMP returns to normal via the sodium–potassium pump.
5. The action potential is self-propagating and is an all-or-none phenomenon. The impulse travels as a full-blown force or not at all. The action potential in a myelinated nerve fiber is propagated by saltatory conduction, jumping from one node of Ranvier to the next node of Ranvier. Myelin improves conduction of action potentials.
6. The presynaptic membrane action potential activates the release of neurotransmitters contained in vesicles. Neurotransmitters diffuse across the synapse, producing a synaptic delay.
7. The postsynaptic membrane contains receptors that combine with neurotransmitters to alter the membrane permeability to specific ions. Excitatory neurotransmitters include glutamate, aspartate, and acetylcholine. The receptor responds with increased permeability for Na+and K+, net influx of Na+, and cell membrane changes in a depolarizing direction (excitatory postsynaptic potential [EPSP]) and initiates an action potential. Inhibitory neurotransmitters include glycine and γ-aminobutyric acid (GABA). The receptor responds with an increase in permeability for K+ and Cl− but not Na+, an outward flow of K+, and cell membrane potential shifts in a hypopolarizing direction (inhibitory postsynaptic potential [IPSP]), which decreases excitability and inhibits an action potential.
B. Intracranial Pressure Dynamics
1. Modified Monro-Kellie Doctrine. The rigid skull contains three volume compartments: brain tissue (80%–90%), CSF (5%–10%), and blood (5%–10%). The brain tissue, CSF, and blood exist in a state of volume equilibrium. If there is an increase in any one or more of the volume compartments, there must be a reciprocal change in one or more of the other volume compartments to maintain pressure equilibrium:
Intracranial volume = Volbrain + VolCSF + Volblood
2. Pressure–Volume Relationships
a. Intracranial pressure (ICP) is generally measured in mmHg to allow for comparison with mean arterial pressure (MAP) and allows for quick calculation of cerebral perfusion pressure (CPP). Normal ICP varies in different age groups and is lowest during infancy (Welch, 1980).
i. Newborn. 0.7 to 1.5 mmHg
ii. Infants. 1.5 to 6 mmHg
iii. Children. 3 to 7.5 mmHg
iv. Adult. Less than 10 mmHg
b. The volume–pressure curve represents the relationship between changes in intracranial volume and the resulting ICP (Figure 4.11). Elastance is the change in pressure that occurs with a change in volume (∆P/∆V). Compliance is the inverse relationship of elastance (∆V/∆P). The ICP curve is not linear but is a three-phase hyperbolic curve. Phase 1, the compensatory phase, is the flat portion of the curve, reflecting good compliance and normal ICP. Temporary increases in ICP are “buffered” by several mechanisms: CSF translocation to the spinal subarachnoid space, venous blood displaced to the extracranial compartment through valveless 361veins, and decreased production or increased reabsorption of CSF. Phase 2 is the exponential portion of the curve, representing early decompensation with normal ICP but poor compliance (i.e., slight increases in volume are not tolerated). The critical point when compliance is lost varies and depends on several factors: rate of volumetric change (rapid increases in ICP are not tolerated well), age (younger child has less buffering capacity with acute increases in ICP), and medical interventions. Phase 3 is the steep portion of the curve, representing the failure of compensation with increased ICP and poor compliance.
FIGURE 4.11 The relationship between ICP and intracranial volume. The relationship between pressure and volume is not linear. The flat portion of the curve “1” represents normal ICP and good compliance. The exponential portion of the curve “2” represents normal ICP with poor compliance. The steep portion of the curve “3” represents increased ICP with poor compliance, compensatory mechanisms have been exhausted.
ICP, intracranial pressure.
3. Brain Metabolism
a. Oxygen. Twenty percent of cardiac output is delivered to the brain, although the brain comprises only 2% of the total body weight. Brain cells require a constant and consistent delivery of oxygen and are dependent on aerobic metabolism. The cerebral metabolic rate of oxygen (CMRO2) is approximately 3 to 3.5 mL/100 g/min in the adult but is not constant throughout the brain; gray matter consumes more than white matter (Tameem & Krovvidi, 2013). The exact CMRO2 in the neonate and infant is unknown.
b. Glucose. Glucose stores in the brain are minimal; therefore, cells also require a constant and consistent delivery of glucose. Glucose is associated with significant brain cellular processes, including protein synthesis, amino acid metabolism, neurotransmitter release, membrane function, and pH homeostasis. The cerebral metabolic rate of glucose (CMRglucose) is 4.0 to 5.0 mg/100 g/min in the adult (Tameem & Krovvidi, 2013). The exact CMRglucose in the neonate and infant is unknown. Hypoglycemia and hyperglycemia can cause neurologic damage.
4. Cerebral Blood Flow
a. Normal cerebral blood flow (CBF) in the brain of the neonate is widely variable with an unknown lower limit. The actual delivery of oxygen to the tissues is affected by the percentage of circulating fetal hemoglobin. Normal CBF varies throughout the brain; it is highest in gray matter and lowest in white matter. Average (whole brain) CBF in the child is 105 mL/100 g/min (Jensen, 1980). Adolescents and adults have an average CBF of 45 to 55 mL/100 g/min.
b. Determinants of CBF are best described by the following equation for laminar flow:
where ∆P = cerebral perfusion pressure
r = radius
η = viscosity of blood
l = length of blood vessel
π = constant, 3.14
CBF is directly related to CPP and vessel caliber and inversely related to vessel length and blood viscosity.
i. The smaller the arteriolar radius, the greater the resistance to CBF. Mechanisms that change the caliber of the vessel are cerebral autoregulation and chemical regulation. Autoregulation is a compensatory mechanism that matches CBF to CMR by altering the radius of the cerebral vessels (i.e., vasoconstriction or vasodilation). A constant CBF is maintained when the MAP is between 50 and 150 mmHg. Outside this range, autoregulation may break down with inadequate or excessive cerebral perfusion. Autoregulation may be impaired in patients with neurologic injury and rapid and severe brain swelling may occur. PacO2 (pH) affects the cerebral arteriolar radius: low PacO2 (elevated pH) causes vasoconstriction and high PacO2 (decreased pH) causes vasodilation. PaO2, to a lesser extent, affects the cerebral arteriolar radius: PaO2 less than 50 mmHg causes vasodilation and PaO2 greater than 50 mmHg causes vasoconstriction, but remains constant. Figure 4.12 illustrates autoregulation by showing the effects of CPP, oxygen, and carbon dioxide on CBF. Increased blood viscosity (polycythemia) decreases CBF. The length of the vascular bed is constant at any point in time. Increased length increases resistance to CBF.
ii. CPP represents the pressure difference between inflow (arterial) pressure and outflow (venous) pressure across the cerebral vascular bed. Clinically, it is most often calculated by the following equation: CPP = MAP − ICP. CPP measurement varies depending on where the arterial and ICP transducers are leveled. There is consensus that external ICP transducers are leveled at a location that approximates the lateral ventricles; for example, the tragus of the ear. There is considerable variation in the literature and clinical practice with respect to leveling the arterial transducer. Some centers level the arterial transducer at the right atrium, whereas other centers level it as they do the ICP transducer. The level of the transducer is not an issue when patients are maintained in a flat position, but there can be significant differences in CPP calculations when the upper body and head are elevated. The hydrostatic difference between both transducers increases as the patient’s head is elevated. Until a consensus is reached on how to measure CPP, caution is required when interpreting CPP-targeted therapies and consistency in leveling technique should be maintained (Rao, Klepstad, Losvik, & Solheim, 2013).
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FIGURE 4.12 Autoregulation.
CPP, cerebral perfusion pressure; PaCO2, partial pressure of carbon dioxide; PaO2, partial pressure of oxygen.
Source: Reprinted with permission from Shardlow, E., & Jackson, A. (2008). Cerebral blood flow and intracranial pressure. Anaesthesia & Intensive Care Medicine, 9(5), 222–225. doi:10.1016/j.mpaic.2008.03.009 Copyright 2008 with permission from Elsevier.
iii. The optimal CPP in the pediatric patient is unknown. In normal adolescents and adults, CPP is variable and ranges between 70 and 90 mmHg with a constant CBF (Tameem & Krovvidi, 2013). The lower threshold of CPP when targeting therapy is unknown. Current guidelines in children with traumatic brain injury (TBI) recommend a lower threshold of 40 to 50 mmHg, infants at the lower range and adolescents at the upper range (Kochanek et al., 2012).
PERIPHERAL NERVOUS SYSTEM
Developmental Anatomy
A. Sensory and Motor Components
1. Spinal nerves are mixed nerves, which carry sensory, motor, and autonomic signals between the spinal cord and the body. They are connected to the spinal cord via two roots.
a. The dorsal (posterior) root carries afferent (sensory) fibers that transmit impulses from sensory receptors in the body to the spinal cord. The fibers supply the innervation for a particular segment of the body called a dermatome (Figure 4.13). Afferent fibers are subdivided according to function. General somatic afferent (GSA) fibers transmit impulses from sensors in the extremities and body wall. General visceral afferent (GVA) fibers transmit impulses from sensors in the viscera.
b. The ventral (anterior) root carries efferent (motor) fibers that transmit impulses from the spinal cord. Efferent fibers are subdivided according to function. General somatic efferent (GSE) fibers innervate voluntary striated muscle. General visceral efferent (GVE) fibers innervate involuntary smooth muscles, cardiac muscle, and glands.
c. Fusion of the roots forms 31 spinal nerves: eight pairs of cervical, 12 pairs of thoracic, five pairs of lumbar, five pairs of sacral, and one coccygeal. Cauda equina (horse’s tail) is the long root of lumbar and sacral nerves contained within the spinal cistern (the spinal cord is shorter than the vertebral column). Thoracic, lumbar, and sacral nerves are numbered according to the vertebra just rostral to the foramen through which they pass. Cervical nerves are numbered for the vertebra just caudal to the foramen through which they pass. Fibers are also classified functionally according to conduction velocity.
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FIGURE 4.13 Dermatomes.
2. Cranial nerves are the peripheral nerves of the brain. Cranial nerve I fibers originate from the olfactory mucosa in the nasal cavity, cranial nerve II fibers originate in the retina, and cranial nerves III through XII originate from different locations of the brainstem. Classification by type and function is described in Table 4.2.
3. Autonomic Nervous System
a. Components of the ANS are located in the CNS and the PNS. The primary (preganglionic) neuron originates in the CNS. The axon of the primary neuron travels outside the CNS to synapse on a secondary (postganglionic) neuron found in one of the autonomic ganglia. The postganglionic fiber terminates in an organ or structure.
b. In the sympathetic (thoracolumbar) division (Figure 4.14), preganglionic fibers originate in the intermediolateral cell column of segments T-1 through T-12. Fibers emerge from the spinal cord through the ventral roots and branch into the white rami communicants. White rami communicants send fibers to the paired trunk ganglia (located laterally on each side of the thoracic and lumbar vertebrae), where they synapse with postganglionic fibers. The postganglionic fibers exit the trunk ganglia and innervate different organs and structures. T-1 to T-5 innervates the head and neck. T-1 and T-2 innervates the eye. T-2 to T-6 innervates the heart and lungs. T-6 to L-2 innervates the abdominal viscera. L-1 and L-2 are to the urinary, genital, and lower digestive systems.
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c. In the parasympathetic (craniosacral) division (Figure 4.15), preganglionic fibers originate from two areas: brainstem preganglionic fibers (often travel in the cranial nerves, specifically, cranial nerves III, VII, IX, and X) and the middle segments of the sacral region. Nerve fibers are distributed exclusively to visceral organs. Most preganglionic fibers have long axons that synapse with a few postganglionic fibers with short axons. The synapse usually occurs in the end organ. The cranial fibers innervate visceral structures including the head, thoracic cavity, and abdominal cavity. The sacral fibers give rise to the pelvic nerve, which innervates most of the large intestine, pelvic viscera, and genitalia.
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FIGURE 4.14 Sympathetic division of the autonomic nervous system (left half ).
CG, celiac ganglion; IMG, inferior mesenteric ganglion; SMG, superior mesenteric ganglion.
ESSENTIAL PHYSIOLOGY
A. Neural Transmission
Neural transmission in the ANS occurs via neurotransmitters. Sympathetic division preganglionic nerve terminals secrete acetylcholine (cholinergic), postganglionic nerve terminals secrete norepinephrine (adrenergic), and the postganglionic nerve terminals to sweat glands secrete acetylcholine. Parasympathetic division preganglionic nerve terminals secrete acetylcholine, and postganglionic nerve terminals secrete acetylcholine. Acetylcholine is deactivated by cholinesterase. Norepinephrine is deactivated by monoamine oxidase (MAO) and catechol O-methyltransferase (COMT).
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FIGURE 4.15 Parasympathetic division of the autonomic nervous system (only left half shown).
B. Systemic Effects of ANS Innervation
Systemic effects of ANS innervation (Table 4.3): The sympathetic division is organized to exert influences over widespread body regions. Stimulation prepares the body for the intense muscular activity needed for the “fight-or-flight” response. The parasympathetic division is organized to exert influences in localized discrete areas of the body. Stimulation prepares the body primarily for “resting” bodily functions.
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CLINICAL ASSESSMENT OF NEUROLOGIC FUNCTION
The nervous system is incompletely developed at birth and takes several years to mature. Consequently, neurologic assessment of the infant and young child must be individualized to reflect neurodevelopment and temperament of the child.
A. History
1. Chief Complaint. Use the parents’ own words and description and solicit information from school-aged and older children when their condition permits.
2. Present Illness. Describe onset and development, associated symptoms, and factors that relieve or exacerbate symptoms.
3. Past History. Infant and toddler history should summarize antenatal, perinatal, and postnatal courses, including maternal infections, medications taken during pregnancy, Apgar scores, gestational age, and birth complications such as meconium aspiration, seizure activity, or respiratory status (oxygen requirements). History should include a chronologic list of developmental milestones, childhood illnesses, immunization status, significant or chronic illnesses (e.g., seizures, diabetes, and head injury), and medications.
4. Family History. Some neurologic disorders manifest themselves as disturbances in other body systems; therefore, it is important to review the patient’s past history.
a. Neurologic disorders may be static or progressive and may be traumatic (acquired) or congenital.
b. Endocrine disorders with neurologic implications include diabetic coma and thyroid disease and hormonal imbalances (growth disorders).
c. Cardiovascular disorders with neurologic implications include cyanotic heart disease (risk for brain infarcts and abscesses) and aneurysms (may have a higher incidence in families with a known history).
d. Congenital disorders with neurologic implications include neural tube defects (NTDs) and metabolic disorders (e.g., phenylketonuria [PKU], cretinism).
e. Genetic disorders may have a neurologic origin or may affect the neurologic system. Most neurodegenerative disorders are transmitted as a recessive gene. Epilepsies and migraine headaches tend to be transmitted as a dominant trait.
f. Renal disorders may produce metabolic imbalances that affect neurologic functioning (e.g., acute renal failure and the increased risk of cerebral edema).
5. Social history should include school performance, types of play activities and recreation, substance abuse, and smoking.
369B. Physical Examination
1. General appearance evaluation includes behavior, dress, speech and conversation, gait, emotional state, and symmetry of body structures.
2. Skull Examination
a. Inspection of the skull includes occipitofrontal head circumference, shape and symmetry of head, transillumination (increased with serous fluid [caput succedaneum] and decreased with blood fluid [cephalhematoma]). Extreme downward rotation of the eyes and paralysis of upward gaze (setting sun sign) is often seen with hydrocephalus. Craniofacial malformation may be present with craniosynostosis from premature closure of one or more cranial sutures.
b. Palpation of the fontanelles should occur while the infant is upright and quiet. Fontanelles that remain open beyond the usual period of closure may be related to disorders that abnormally increase the intracranial contents (e.g., tumors, hydrocephalus). The anterior fontanelle is usually 4 to 6 cm at its largest diameter at birth, and the posterior fontanelle is usually 1 to 2 cm at its largest diameter at birth. The anterior fontanelle may be full and tense with increased ICP, crying, vomiting, or coughing in the infant. Pulsations of the anterior fontanelle reflect the peripheral pulse and are normally barely palpable. Palpation of sutures reveals overriding sutures (common with vaginal deliveries) or premature closure of the sutures. Widely separated sutures may suggest hydrocephalus.
c. Auscultation of the skull using a bell stethoscope with the child in an erect position is performed over six areas: the temporal fossae, both globes, and the reticulo auricular or mastoid regions. In all cases, a transmitted cardiac murmur should be excluded. Bruit (spontaneous) in the young child may be normal, but an abnormal bruit is often loud, harsh, and asymmetric or accompanied by a thrill, or both.
d. Percussion of the skull is normally dull. A “cracked-pot” sound (Macewen’s sign) is heard with separated sutures and increased ICP.
3. Level of Consciousness
a. Consciousness is a state of awareness of self and environment. There are two major components of consciousness and disease states affect them differently. First, there is the content of consciousness, which represents mental function, or higher intellectual activity and is primarily at the level of the cerebral hemispheres. The second component of consciousness is arousal, or wakefulness, and its function is primarily located throughout the brainstem, an extensive network of nuclei and interconnecting fibers known as the RAS. Maintaining alertness requires intact function of the cerebral hemispheres and arousal mechanisms located in the RAS. Impaired consciousness is from impairment of both cerebral hemispheres or dysfunction of the RAS.
b. Altered states of consciousness are on a continuum and represent varying degrees of neurologic dysfunction. Many terms are used to describe acute altered states of consciousness and several of these terms are used incorrectly or mean different things to different observers. Clouding of consciousness is characterized by reduced wakefulness, confusion, and alternating drowsiness and hyperexcitability. In its mildest form, the patient is distracted, has inattention, and judgment is reduced. In more severe states, the patient is more confused and may misinterpret stimuli. Delirium is a neurocognitive disorder due to a somatic illness or treatment. Although a child may experience delirium anywhere, it is more likely to occur in the hospital. Many conditions can cause delirium such as fever, infections, medications, or metabolic disorders. It is usually a temporary and reversible state when the underlying condition is identified and treated. It is a disturbance of consciousness and is characterized by disorientation, fear, irritability, visual hallucinations, and agitation. The patient has reduced ability to focus and sustain attention. Memory may be altered with language disturbances. This disturbance usually develops over a short period and may fluctuate throughout the day. There is evidence to suggest that there is a relationship between severity of illness and pediatric delirium (Silver et al., 2015). Emergence/agitation delirium is a condition that may occur in children in the immediate postoperative or postprocedure period. It usually resolves in 30 to 45 minutes without intervention. Several bedside tools have been introduced to clinically screen and guide treatment for delirium in pediatric patients, such as the Pediatric Anesthesia Emergence Delirium Scale (PAED; Sikich & Lerman, 2004), the pediatric Confusion Assessment Method for ICU (pCAM-ICU; Smith et al., 2011), the Cornell Assessment Pediatric Delirium tool (CAP-D; Traube et al., 2014), and the Sophia Observation Withdrawal Symptoms-Pediatric Delirium scale (SOS-PD; Ista, van Dijk, de Hoog, Tibboel, & Duivenvoorden, 2009). 370Several variables (e.g., age, gender, severity of illness) may affect the validity of a tool and should be taken into consideration when using them (Luetz et al. 2016). Obtundation is characterized by mild to moderate reduction in alertness, reduced interest in the environment, and increased periods of sleep. Stimuli of mild to moderate intensity fail to arouse the patient. If arousal does occur, the patient is slow to respond. Stupor is characterized by unresponsiveness except to vigorous and repeated stimuli. Once the stimulus is removed, the patient drifts back to a deep sleep-like state. Coma is indicated by severely reduced or absent verbal or motor response to environmental stimuli. The patient may respond to noxious stimuli with abnormal motor movements, but they lack localization or defensive movements (Posner, Saper, Schiff, & Plum, 2007).
c. Mental status assessment in infants and children is more complex than in older patients because of their immature neurologic development. There are also significant normal developmental ranges among infants and toddlers. Establishing the child’s preillness baseline is imperative and requires caregiver cooperation and accurate recall. In infants, assess the quality of the cry, alertness and level of activity, feeding patterns, language development, presence or absence of primitive reflexes, patterns of sleep and wakefulness, and responses to caregivers. In children, assess attention, alertness, orientation, cognition, memory, affect, and perception. The mental status of older children and adolescents can be assessed using more traditional methods (e.g., orientation to person, place and time; short-term and long-term memory; and speech and language).
d. The cause of coma may be structural or metabolic. If the cause is structural, brain imaging studies are useful in identifying the extent and location of the brain lesion. If the cause of coma is metabolic, laboratory studies (e.g., metabolic panels or toxic screen) and perfusion scans are useful. Coma scales are used to grade the degree of unresponsiveness by standardized assessments. The Glasgow Coma Scale (GCS) is the most widely used scoring system and assesses arousability in relation to three responses: eye opening (arousal state), verbal response (content of consciousness), and motor response (arousal state and content of consciousness). Each response is given the best number for a given response. The sum of the numbers ranges between 3 (least responsive) and 15 (normal). The GCS cannot be applied directly to children younger than 5 years, particularly the verbal scale. For example, the best response under Verbal is “oriented” and the best response under Motor is “obeys commands”; these are not realistic responses in young children. A number of coma scales have been developed to accommodate preverbal children and infants, and typically, are scored in a similar fashion to the GCS. When assessing the unconscious state in a child, it is important to note the level of intensity of stimulation necessary to arouse the child and their specific response. Stimuli used to elicit a response should start with voice or touch. If the patient does not respond, the nurse should escalate to a noxious stimulus (e.g., sternal rub or nail bed compression). Careful attention is given to preventing injury, especially in patients with a coagulopathy. Table 4.4 compares the GCS to a modified version that can be used in children less than 5 years of age.
4. Pain and Sedation Monitoring
a. Infants and children admitted to the pediatric intensive care unit (PICU) frequently experience discomfort from their preexisting injury or condition. Pain and agitation may continue as they undergo numerous procedures and as a consequence of multidisciplinary management.
b. Assessing pain and agitation in patients with altered states of consciousness is challenging and requires close scrutiny of the patient’s physiologic status and clinical response to analgesia and sedation. The goal is maximal comfort without diminishing patient responsiveness. Sedation and analgesia need to be balanced so that meaningful serial neurological assessments can be performed while minimizing pain, agitation, and stress.
c. The use of pain assessment tools and scales is recommended to provide consistency among observers, to recognize and trend pain levels, and to determine the effectiveness of comfort alleviating interventions. Commonly used pain assessment scales in the PICU include the COMFORT Behavior Scale (Ambuel, Hamlett, Marx, & Blumer, 1992) COMFORT-B Scale (Bai, Hsu, Tang, & van Dijk, 2012), and the FLACC Scale (Merkel, Voepel-Lewis, & Malviya, 2002).
d. Optimal sedation level is important to improve patient outcomes (e.g., length of stay and ventilator days) and to minimize patient distress. Commonly used sedation assessment scales in the PICU include the COMFORT Behavior Scale (Ambuel et al., 1992), COMFORT-B scale (Bai et al., 2012), and the State Behavior Scale (Curley, Harris, & Fraser, 2006).
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5. Motor Function. Assessment of normal motor development proceeds cephalocaudally and proximodistally.
a. Assess primitive reflexes in infants and toddlers and determine their presence or absence, time of disappearance, and the symmetry of the reflex. The most commonly evaluated reflexes include the following:
i. Moro: Elicited by a sudden movement of the body that causes a change in equilibrium. The response is extension and abduction of the upper extremities (fingers fan), followed by flexion and adduction. The reflex appears between 28 to 32 weeks gestation and disappears by 3 to 5 months after birth.
ii. Palmar grasp. Elicited by the examiner’s placing his or her index finger into the ulnar side of the infant’s hand and pressing against the palmar surface. The response is immediate flexion of the infant’s fingers around the examiner’s finger. The reflex appears at 28 weeks gestation and disappears between 4 and 6 months after birth.
iii. Parachute. Elicited by holding the infant in a ventral position. A sudden plunge downward produces extension and abduction of the infant’s arms and fingers. The reflex appears at 4 to 9 months after birth and persists throughout life (the response is usually covered up with voluntary movement in older individuals).
iv. Rooting. Elicited by stroking the perioral skin at the corner of the mouth; moving 372laterally toward the cheek, upper lip, and lower lip. The infant turns his or her head toward the stimulated side with sucking movements. The reflex appears at 28 weeks gestation and disappears between 3 and 4 months after birth.
v. Placing. Elicited with the infant supported in a vertical position with the dorsum of one foot pressed against a hard surface. The infant’s foot will flex and extend, simulating walking. The reflex appears at 35 to 37 weeks gestation and disappears at 1 to 2 months after birth.
vi. Asymmetric tonic neck response. Elicited by rotating the infant’s head to the side while the infant’s chest is maintained in a flat position. The arm and leg extend on the side to which the infant’s face is turned, and the opposite arm and leg flex. The reflex appears at birth to 2 months and disappears between 4 and 6 months.
b. Assess for developmental milestones (e.g., sitting, crawling, and walking).
c. If the patient can follow commands, assess muscle strength and tone, symmetry of movement, and DTRs. Not all DTRs are present in the infant because of the immaturity of the corticospinal tracts. DTRs are tested based on the segmental level they innervate. The usual reflexes include biceps (segmental levels C-5 and C-6), brachioradialis (segmental levels C-5 and C-6), triceps (segmental levels C-7 and C-8), knee (segmental levels L-2, L-3, and L-4), and ankle (segmental levels S-1 and S-2). The technique for eliciting DTRs is similar to that used for adults; however, the hammer may be replaced with the examiner’s semiflexed index finger. The technique includes positioning the limb so that the muscle is slightly stretched, striking the tendon briskly to create an additional sudden tendon stretch, and testing both muscle groups on each side of the body. Reflex responses are usually graded on a scale from 0 (no response) to 4+ (very brisk, hyperactive; may be indicative of disease). Abnormal findings include very brisk or asymmetric responses or deviations from a previous assessment.
d. Superficial reflexes include the Babinski, abdominal, and cremasteric reflexes. The technique for eliciting Babinski’s reflex includes using a sharp object (thumbnail) to stimulate the plantar surface of the foot. Stimulation begins at the heel and travels along the lateral border of the sole, crossing over the base of the metatarsals to the great toe. A normal response in children younger than 1 to 2 years is immediate dorsiflexion of the great toe and subsequent separation (fanning) of the other toes; this response is abnormal in older children and adults. A normal response beyond the second year is plantar flexion of the toes. The abdominal reflex is elicited by lightly, but briskly stroking each side of the abdomen, above and below the umbilicus. A normal response is contraction of the abdominal muscles and deviation of the umbilicus toward the stimulus. The reflex may not be present at birth but is consistently present at 6 months of age. An asymmetric response is abnormal. The cremasteric reflex (only found in males) is elicited by lightly but briskly stroking the inner aspect of each of the upper thighs. A normal response is elevation of the testicle on the side stimulated. It may not be present at birth but occurs consistently at approximately 6 months of age. An asymmetric response is abnormal.
e. Abnormal motor responses in the comatose patient include decorticate posturing (consisting of flexion and adduction of the upper extremities and extension of the lower extremities with plantar flexion representing dysfunction of the cerebral hemispheres or upper part of the brainstem), decerebrate posturing (extensor posturing consisting of extension, adduction, and hyperpronation of the upper and lower extremities and plantar flexion representing dysfunction at the pontomesencephalic level), and flaccidity (no motor response to external stimuli representing severe dysfunction of the lower brainstem and vital centers for which spinal cord injury [SCI] and stroke must be ruled out).
f. All extremities should be assessed independently. Specific stimuli to elicit a motor response should be documented.
6. Sensory Function
a. In infants, sensory testing results are variable and less reliable than in the older child. Light touch is assessed by stroking an extremity (the normal response is to withdraw the limb). Vibration sense is assessed with a tuning fork over bony areas (the normal response is cessation of movement and often a look of surprise). Proprioception cannot be tested in infants or comatose patients because it requires participation of the patient. Pain sensation is assessed with nail bed pressure (at the end of examination).
373b. Light touch and superficial pain are assessed in older children in all four extremities. If abnormalities are noted, a more detailed segmental assessment is done. Proprioception is tested by asking the child to close his or her eyes and move a finger or toe up or down and then asking the child to identify whether the movement is up or down. Pain sensation is tested at the end of the examination by using a pin to test the various dermatomes (see Figure 4.13).
7. Cerebellar Function
a. In infants and toddlers, cerebellar function should be assessed by observing the child during play or usual activities. Abnormal findings include tremors, which are rhythmic alterations in movement and, unlike spontaneous seizures, are usually precipitated by a variety of stimuli (e.g., sudden changes in movement) with no alteration in the level of consciousness. Dysmetria (inability to control the range of movement in muscle action) or gait abnormalities (e.g., wide-based or waddling type of gait) may also be noticed.
b. Maneuvers to assess older children include the finger-to-nose test, the heel–shin test, observation of gait, and toe-to-heel walking. The finger-to-nose test is performed while the child stands erect with arms extended at the sides; the child is then asked to touch his or her nose with alternating index fingers. An abnormal finding is seen if the child completely misses his or her nose. The heel–shin test is done while the child is in a supine position. The child is asked to place one heel rapidly down the shin from the knee to the ankle and repeat on the other side. Movements should be coordinated and accurate. The child can also be instructed to touch each finger to the thumb of the same hand in rapid succession. Each hand is tested, and the response should be symmetric. Observation of gait can be made while the child walks toward and away from the examiner. The child should have good posture and balance. During toe-to-heel walking, the child places the heel of one foot to the toe of the other foot and continues this maneuver for a distance of several feet. The child should have good balance.
8. Cranial Nerve Function. The order and specific nerves to be tested depend on the age and condition of the child. Age-specific techniques used to assess cranial nerves in infants and children are included in Table 4.5.
9. Funduscopic Examination
a. Normal findings include a red reflex that is orange-red and fairly uniform in color, a creamy pink optic disc with an indented center (physiologic depression) and smooth margins, and veins that are slightly wider than arteries.
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b. Abnormal findings include papilledema, which is characterized by blurring of the nasal and upper margins of the optic disc. Papilledema is seen with increased ICP in the older child or in the infant with acute, rapid increased ICP. Retinal hemorrhage may occur with subarachnoid hemorrhage (SAH), severe diabetes, and shaken baby syndrome.
10. Vital Signs
a. Respiratory patterns are the first of the vital signs to change with neurologic dysfunction. 380Respiratory patterns are more informative than is the respiratory rate. Patterns may overlap or change depending on progression of neurologic dysfunction and are difficult to assess in the intubated and ventilated patient.
i. Cheyne–Stokes respirations are described as periodic breathing with phases of hyperpnea alternating with apnea. The location of the lesion is bilateral, hemispheric, or diencephalic.
ii. Central neurogenic hyperventilation is sustained, rapid, and fairly deep hyperpnea. The exact mechanism in the brain (if any) that causes this is unknown.
iii. An apneustic respiratory pattern is characterized by a prolonged inspiration with a pause at full inspiration lasting 2 to 3 seconds. This represents damage to the brainstem near the level of the fifth cranial nerve nucleus.
iv. Ataxic respirations consist of a completely irregular breathing pattern with deep and shallow breaths. They are seen in patients with damage to the respiratory centers in the medulla.
b. Temperature. Temperature changes are nonspecific in most patients with neurologic dysfunction. Hyperthermia may result from abnormalities of the brain itself or from toxic substances that affect the temperature-regulating centers. Patients at risk for hyperthermia include neonates (due to immature development of thermoregulation centers), children with CNS infections (due to the effects of pyrogens on the hypothalamus), and children with status epilepticus (SE; may be due to hypothalamic dysfunction as well as increased total body oxygen consumption). Hypothermia may be seen in neonates (same as mentioned earlier) and in brain death because of loss of hypothalamic function.
c. Pulse and blood pressure. Changes in pulse and blood pressure are very late and ominous signs of neurologic dysfunction. Cushing’s reflex is an increase in systolic pressure greater than diastolic pressure (i.e., widened pulse pressure) and bradycardia. Cardiac dysrhythmias are seen with some TBIs. Vasodilation with systemic hypotension may be seen with spinal trauma or sympathetic insufficiency.
C. Brain-Death Determination
1. Most states have adopted guidelines to define death the same way for infants, children, and adults. One of two conditions must exist:
a. Irreversible cessation of breathing and circulation or
b. Irreversible cessation of whole-brain function (i.e., cortical and brainstem)
2. The difference in brain-death determination between children and adults is not the legal definition of death but the process of confirming brain death. Several sets of guidelines have been published for the determination of brain death in children. The most widely accepted guidelines are those originally published by the Ad Hoc Task Force for the Determination of Brain Death in Children in 1987 and updated in 2011 (Nakagawa, Ashwal, Mathur, Mysore, & The Committee for Determination of Brain Death in Infants Children, 2012).
a. History. The cause of coma must be known to establish irreversibility. Prerequisites must be met prior to initiating the clinical evaluation. There must be an absence of complicating factors, such as hemodynamic instability, supra-therapeutic levels of sedatives and analgesics, paralyzing agents, severe hypothermia, or metabolic disturbances. Cardiopulmonary resuscitation may interfere with the reliability of the neurologic exam and it should be delayed for ≥24 to 48 hours postresuscitation.
b. To determine brain death, the physical examination should demonstrate that coma and apnea coexist. The clinical examination should demonstrate a lack of function in the entire brain. Clinical signs include flaccidity, absence of movement (except for spinal cord reflexes), and absence of brainstem function. Loss of brainstem function is determined by nonreactive, midposition or fully dilated pupils; absence of oculocephalic (doll’s eyes) and oculovestibular (cold calorics) reflexes (refer to Table 4.5); and absence of corneal, gag, cough, sucking, and rooting reflexes.
c. Apnea testing must be performed with the clinical examination and the patient must have complete absence of respiratory effort with standardized apnea testing. Following disconnection from the ventilator several conditions must occur: adequate time (5–10 minutes) to allow PaCO2 to increase to levels sufficient to stimulate respiration, adequate oxygenation, and absence of cardiovascular instability. The PaCO2 must be 20 mmHg above the baseline PaCO2 and ≥60 mmHg.
d. Two examinations with apnea testing separated by an observation period are required. Consistent examination techniques are required 381throughout the observation period and must be performed by two separate attending physicians; apnea testing may be done by the same physician. The recommended observation periods are
i. Infants 37 weeks gestational age to 30 days: 24-hour observation
ii. Children older than 30 days to 18 years: 12-hour observation
e. Ancillary studies (EEG and CBF) are not required to diagnose brain death and should not be used as a substitute for the clinical examination. Ancillary studies may be used when the clinical examination or apnea testing cannot be completed, if there is uncertainty about the results, to reduce the observation period, or if there is a medication that may affect the exam (Nakagawa et al., 2012).
3. There is insufficient evidence in the literature to support recommendations for the preterm newborn. The exact cause of coma is often unknown in the preterm newborn and the accuracy of ancillary tests in this age group is uncertain.
INVASIVE AND NONINVASIVE DIAGNOSTIC STUDIES
A. Assessing Anatomic Integrity
1. Radiograph
a. Description. Roentgenographic films of skull and spine demonstrate structural deficits only. The radiation penetrates all body tissues and is absorbed to varying degrees, resulting in different shadow intensities.
b. Clinical use. Skull films are used to determine fractures, widened sutures, tumors, calcification, and bone erosion. Spinal films are used to evaluate the integrity of the vertebral structures, including the vertebral body, disc interspace, lamina, and pedicles. Spinal films are also used to evaluate fractures, dislocations, and degeneration of bone.
c. Nursing implications. Immobilize fractures with splints, cervical collars, traction devices, or age-appropriate immobilizers. Provide routine safe transport care, including use of appropriate monitoring devices, elevation of side rails, good alignment of affected body structure(s), securing standby emergency equipment, and serial monitoring of neurovascular status. Explain the procedure to older children or parents, including the length of the procedure, purpose of the procedure, sensations and appearance of the environment, and expectations of the child during the procedure (e.g., to remain quiet without body movement).
2. CT Scan
a. Description. CT scans use multiple x-ray beams that pass through the brain at different angles. The beams are picked up by receptors that digitally send the information to a computer. The computer formats the information and displays an image for every section of the brain that is studied. Different anatomical structures of the brain absorb various levels of radiation energy depending on tissue density of the structure. CT scan differentiates tissue density relative to water via a computer. Highly dense structures (e.g., bone, fresh blood) appear white, and low-density areas (e.g., air, CSF, fat) appear dark. Contrast may be used with a CT and provides good visualization of vascular structures and leaks in blood vessels. Advances continue to be made in CT technology and include increased patient comfort, faster scanning times, and higher resolution. Disadvantages include radiation exposure and use of contrast.
b. Interpretation. CT scans are interpreted sequentially and systematically. The left side of the brain is displayed on the right of the scan as the viewer faces it. Each CT scan section is examined for characteristic anatomical landmarks, some requiring measurement. The first image usually begins with a cut through the posterior fossa followed by sections that advance superiorly. Anatomical structures are viewed in terms of size, location, and symmetry. Sections are then examined for abnormal densities (e.g., blood clots, tumors) and enlarged structures. The basilar cistern is the area of subarachnoid space that surrounds the midbrain. Diffuse brain swelling is recognized on CT scan as a decrease in ventricular size, absence or compression of the basilar cistern, and loss of differentiation between gray and white matter. Visual loss of the third ventricle and loss of sulci indicate an increase in brain bulk (swelling). Asymmetry is almost always an abnormal sign that indicates volume changes between compartments. Figure 4.16 demonstrates four CT scans that show abnormalities.
c. Clinical use. Because of cost, speed, and availability, CT scan is used for examination of acute neurologic dysfunction. CT scan is superior to MRI in detecting blood (especially SAH) and in evaluating cortical bone structures of skull and spine. Contrast-enhanced CT (CECT) is used to detect lesions that cause BBB breakdown, to visualize blood vessels and well-vascularized lesions, and to rule out cerebral metastases. It is difficult to visualize the posterior fossa because of bone obstruction.
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