Principles of Intracranial Pressure
Abstract
The principles of intracranial pressure, including pathology, variations, and treatment, form the foundation to understanding neurologic function. Increased intracranial pressure may be a symptom or a side effect of a variety of neurologic diseases and injuries. Understanding these principles is essential for nurses caring for any patient with an actual or potential neurologic disorder.
Keywords: cerebral blood flow, cerebral perfusion pressure, cerebrospinal fluid, external ventricular drain, Glasgow Coma Scale, Monro–Kellie doctrine
3.1 Intracranial Pressure
Knowledge of the principles of intracranial pressure (ICP) and its variations is the foundation for understanding neurologic function. Increased pressure in the head has been known for centuries as a predictor of poor outcome—nearly 2,500 years ago, Hippocrates wrote that alleviating pressure in the head helped relieve “dropsy of the brain.” This belief may have been the basis for the ancient practice of trepanation, that is, drilling holes in the skull to relieve pressure. Increased ICP can occur in many neurologic disorders and portends a poor neurologic outcome (Box 3.1 Conditions Associated with Increased Intracranial Pressure).
Intracranial pressure refers to the pressure exerted on the brain and its contents. Normal ICP is between 0 mm Hg and 15 mm Hg. Increased ICP, also referred to as intracranial hypertension, occurs when the pressure within the brain is increased to more than 15 mm Hg, as first described in the Monro–Kellie doctrine in 1783.
Box 3.1 Conditions Associated with Increased Intracranial Pressure
Conditions that increase brain volume
A space-occupying lesion, such as intracranial tumor, hematoma, hemorrhagic stroke, abscess, aneurysm, vascular malformation, and cerebral edema
Conditions that increase blood volume
Hyperemia
Increased intrathoracic or intra-abdominal pressure (Valsalva maneuver)
Venous outflow obstruction
Conditions that increase cerebrospinal fluid (CSF) volume
Hydrocephalus due to obstruction, poor CSF resorption, tumor, increased production of CSF
3.1.1 Basic Principles of the Monro–Kellie Doctrine
Intracranial contents include brain parenchyma, cerebrospinal fluid (CSF), and blood
Intracranial contents are contained in a fixed vault (the skull); therefore, their total volume must remain constant
A change in volume of any one component mandates a change in the other components
3.1.2 Compensatory Mechanisms
Compensatory mechanisms are a necessary part of the brain’s ability to alter the volume of one of its components to regulate the volume of the others, thereby maintaining normal ICP. These mechanisms may include
Alterations in blood volume
Alterations in CSF volume
Compression of brain tissue
Deterioration of these compensatory mechanisms results in neurologic decline.
Compliance
Compliance is described as a
Measure of brain “stiffness”
Measure of the brain’s ability to maintain equilibrium in the presence of internal and external challenges
Mechanism that allows the brain to accommodate slow-growing, space-occupying lesions (e.g., tumors)
Cerebral Blood Flow
Cerebral blood flow (CBF) refers to the amount of blood that passes through 100 g of brain tissue in 1 minute.
Normal value is approximately 50 mL/min
The brain uses approximately 20% of the body’s oxygen
Cerebral Perfusion Pressure
Cerebral perfusion pressure (CPP) is the net pressure gradient driving the CBF that delivers oxygen and metabolites. It is defined as
The difference between mean arterial pressure (MAP) and ICP
Normal CPP is 70 to 100 mm Hg (Box 3.2 Calculation of Cerebral Perfusion Pressure)
Cerebral perfusion pressure serves as the stimulus for autoregulation to provide adequate CBF.
Box 3.2 Calculation of Cerebral Perfusion Pressure
Cerebral perfusion pressure is the difference between mean arterial pressure (MAP) and intracranial pressure (ICP)
CPP = MAP – ICP
MAP can be calculated using systolic blood pressure (SBP) and diastolic blood pressure (DBP), using the formula
MAP = DBP + 1/3 (SBP – DBP)
Normal CPP = 70–100 mm Hg; Normal ICP = 0–15 mm Hg
Autoregulation
Process in which the brain alters its own vasculature to accommodate changes in ICP
Failure of the brain to autoregulate results in neurologic decline
Cerebral Edema
Refers to an increase of water in the brain parenchyma
Cytotoxic edema
Accumulation of intracellular water
Results from a hypoxic event
May or may not be reversible
Vasogenic edema (▶ Fig. 3.1)
Alteration in vascular permeability, with disruption of the blood-brain barrier
Increased extracellular space
Frequently seen in neoplastic diseases
Treated with osmotic agents
May coexist with cytotoxic edema
May be irreversibly damaging
Fig. 3.1 Vasogenic edema.
3.1.3 Herniation Syndromes
Herniation of the brain can occur when the intracranial contents are shifted away from their usual location in the skull (▶ Fig. 3.2). This shifting can be caused by numerous factors, such as abscess, hemorrhage, hydrocephalus, stroke, and radiotherapy.
Fig. 3.2 Herniation syndromes.
Cingulate Herniation
Also referred to as subfalcine herniation
Caused by an expanding lesion in one hemisphere that causes it to shift toward the opposite hemisphere
Causes compression, which restricts local blood flow
May cause cerebral edema, local tissue ischemia, and increased ICP
Resulting deficits depend on the specific location and extent of herniation
Transtentorial Herniation
Also referred to as central herniation
Caused by a lesion that displaces the brain downward through the tentorial notch
May cause cerebral edema, local tissue ischemia, and hemorrhage
May result in decreased level of consciousness (LOC), respiratory depression, and contralateral motor weakness
Resulting deficits depend on the specific location and the extent of herniation
Uncal Herniation
Most common type of brain herniation
Caused by an expanding lesion, often in the temporal lobe (Box 3.3 Conditions at Risk for Uncal Herniation)
Pressure is exerted across the midline, pushing the uncus into the edge of the tentorium
Sluggish ipsilateral pupil is a common early sign
Contralateral hemiparesis, decreased LOC, and altered respiratory patterns are common late signs
Brainstem compression may result
A dilated nonreactive pupil is a very late sign of uncal herniation (Box 3.4 Herniation Syndromes: Nursing Implications)
Box 3.3 Conditions at Risk for Uncal Herniation
Any supratentorial space-occupying lesion (especially in the temporal lobe), including
Tumor
Cerebral contusion
Intracranial hemorrhage
Subdural or epidural hemorrhage
Gunshot wound
Abscess
Box 3.4 Herniation Syndromes: Nursing Implications
Herniation syndromes are a neurologic emergency and must be reported immediately
Nursing intervention is aimed at early recognition and reporting of neurologic decline
3.1.4 Clinical Manifestations of Increased Intracranial Pressure
Clinical manifestations of increased ICP depend on the location within the brain where pressure is exerted and the rate and extent of increased ICP. Some areas of the brain called eloquent areas, which are directly involved in speech, motor function, sensory reception, and cranial nerve function, are less tolerant of increased ICP than others. The brainstem is particularly sensitive to ICP. It is the origin of many cranial nerves, and it is a small area with little room for edema. It is also the location of the brain’s respiratory center. Clinical manifestations of ICP affecting the brainstem may include
Decreased LOC
Cranial nerve deficits
Motor and/or sensory changes
Decorticate or decerebrate posturing
Headache
Nausea and vomiting
Changes in vital signs (usually seen with increasing progression of brainstem compression); see also Chapter 2, Assessment
3.1.5 Diagnosis of Increased Intracranial Pressure
Clinical assessment (Box 3.5 Clinical Alert: Early Diagnosis of Increased Intracranial Pressure)
ICP monitoring
Imaging studies, such as magnetic resonance imaging (MRI) and computed tomography (CT) (▶ Table 3.1)
Box 3.5 Clinical Alert: Early Diagnosis of Increased Intracranial Pressure
The importance of early and accurate nursing assessment cannot be overstated
Identification and reporting of early signs of increasing ICP can be the difference between recovery and neurologic devastation
Diagnostic tests | Expected findings | Nursing implications |
CT and MRI | Can confirm presence of cerebral edema, hydrocephalus, and space-occupying lesion | Ask if the patient has allergy to contrast agents; notify physician if an allergy is present If the patient is undergoing an MRI, perform metal screening. In addition, determine whether the patient is claustrophobic; if so, notify physician for sedation orders NPO status for MRI with sedation |
Lumbar puncture | Measures opening and closing pressure indications of ICP | Contraindicated in the presence of a space-occupying lesion (or suspicion of lesion) Informed consent required |
TCD ultrasonography | Measures velocity of arterial and venous flows, indicative of ICP | Important to note TCD trends Elevated TCD may indicate vasospasm, which could lead to increased ICP |
Cerebral angiography (performed if cerebrovascular abnormality is suspected) | Indicates vasospasm | Patient must be NPO before angiography Patient must lie flat for predetermined period (usually 4–6 hours) Close observation of puncture site (usually right or left femoral artery) is required to detect groin hematomas |
Abbreviations: CT, computed tomography; ICP, intracranial pressure; MRI, magnetic resonance imaging; NPO, nil per os (nothing by mouth); TCD, transcranial Doppler. | ||
3.1.6 Treatment of Increased Intracranial Pressure
Medical Treatment
Hyperosmolar therapy
Nursing interventions to decrease ICP (Box 3.6 Hospital Activities that May Exacerbate Increased Intracranial Pressure)
Respiratory support
Sedation (▶ Table 3.2)
Barbiturate coma (Box 3.7 Barbiturate Coma)
Box 3.6 Hospital Activities that May Exacerbate Increased Intracranial Pressure
Improper positioning of head and neck
Straining (e.g., during a bowel movement)
Coughing
Suctioning
Valsalva maneuver
Emotional stress
Excessive environmental stimuli (television, radio, and conversation)
Nursing activities (assessment, positioning, administration of medications, and bathing)
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
