INTRODUCTION

Epilepsy is one of the most common neurological disorders seen in pregnant women. It is also responsible for increased maternal and fetal mortality and morbidity rates in the UK. In the most recent Confidential Enquiries into Maternal and Child Health 2000–2002 (Lewis & Drife 2004), there were 13 deaths from epilepsy during childbirth. Epilepsy is a disorder of the central nervous system in which neurons are inappropriately stimulated causing abnormal sensations and seizures. Other disorders of the nervous system that may be seen by the midwife are multiple sclerosis and myasthenia gravis, which will be briefly discussed at the end of the chapter.

Epilepsy is a condition in which nerve cells – neurons – are stimulated in a disorganized way, resulting in abnormal movements, such as fits, strange sensations, flashing lights, or smells that do not actually exist. Epilepsy occurs commonly in young people and therefore a midwife must understand the condition and its effects on pregnancy and childbirth.

The goal in caring for the pregnant woman with epilepsy is to keep her relatively free of seizures and to minimize the effects of epilepsy on both the pregnant woman and her fetus. Antiepileptic drugs can have a teratogenic effect on the fetus. During an epileptic fit, the woman may fall and injure herself or her baby, and may suffer from hypoxia, which could also affect the fetus. Knowledge of the condition, its signs and symptoms and how to prevent epileptic seizures by supporting women to comply with treatment, is essential in midwifery practice.

RELEVANT ANATOMY AND PHYSIOLOGY

The central nervous system is made up of the brain containing the cerebral cortex, which contains centres of motor and sensory control, and the spinal cord, through which nerve pathways carry neurons to all parts of the body (Fig. 8.1). Neurons detect changes in both the internal and external environment and respond to them to maintain homeostasis with body organs and systems. Epilepsy is an abnormal functioning of these neurons.

Figure 8.1 Gross structure of the central nervous system.

The cerebrum is composed of two cerebral hemispheres which are separated from each other, except at a point deep in the substance of the brain where a broad band of nerve fibres, the corpus callosum, enables communication between the two hemispheres. Each cerebral hemisphere is a mirror twin of the other and each has a complete set of centres responsible for sending, receiving and interpreting information. The cerebral hemispheres are responsible for the functions of the opposite side of the body but are capable of taking over some of the functions of the other hemisphere if the corresponding area has been damaged.

The central nervous system is composed of two principal types of tissue: grey matter which contains the cell bodies of neurons, and white matter through which neurons travel. The white appearance is due to the colour of the myelin sheath that surrounds neurons (see below). Cell bodies appear grey as they are not surrounded by myelin.

In the cerebral cortex, the outer 2–4 mm of tissue contains cell bodies, appears grey and is known as the cerebral cortex. It is in this region of the brain where the majority of functional centres are situated and these interpret and act upon information received by the nervous system. The lighter ‘white’ matter of the brain fills the substance of the cerebrum. Interspersed throughout the white matter are other visible areas of grey matter known as nuclei, in which specialist interpretation of the environment is carried out.

In the spinal cord, the order of the white and grey matter is reversed, with white matter enclosing an H-shaped area of grey matter. This arrangement provides the cell bodies of neurons passing through the cord with maximum protection from injury. Once damaged, cell bodies cannot be repaired.

Other regions of the central nervous system include:

The nervous system is made up of millions of small functional units called neurons supported by a network of specialist connective tissue called neuroglia. Neurons are cells that have been modified to transmit information in the form of an action impulse or electrical pulse between different parts of the body.

A neuron consists of (Fig. 8.2):

Figure 8.2 Anatomy of a typical neuron.

There are two principal types of neuron: those with a myelin sheath surrounding the axon and those without. Myelin is a fatty substance which insulates the axon from the surrounding environment preventing the loss of substances including ions which are responsible for the production of an action impulse. Neurons surrounded by myelin transmit an impulse much more quickly than those without: myelinated at on average of 120 m/s, unmyelinated at around 5–10 m/s.

The function of a neuron is to transmit information in the form of an action potential from one area to another. An action potential is a change in the electrical potential across the cell membrane of the neuron. This alters from −70 mV to +35 mV. This change in electrical potential is brought about by the movement of ions across the cell membrane.

Most cell membranes in the body maintain an electrical imbalance between the intracellular fluids within the cell and the extracellular fluids surrounding the cell. This is brought about by an imbalance in the concentration of potassium cations (K⁺), sodium cations (Na⁺) and associated anions. The fluids inside the cell contain an increased concentration of K⁺ and a decreased concentration of Na⁺. Those outside the cell have a decreased concentration of K⁺ and an increased concentration of Na⁺. This imbalance is maintained by a sodium potassium pump and by the greater permeability of the cell membrane to K⁺ than to Na⁺ and is called the resting potential of the cell membrane.

In the neuron, this imbalance is also present and is used to create a rapid change in electrical charge along the length of the axon (Fig. 8.3). As a result of stimulation of a dendrite (by a change in temperature, stretch in an organ, damage to cells for example), sodium channels in the cell membrane near to the dendrite open and allow Na⁺ to flood into the cell. This process is called depolarization and the result is that the inside of the cell at this point on the axon becomes positively charged in comparison with the outside of the cell.

Figure 8.3 Conduction of an impulse in a neuron.(a) Unmyelinated neuron. (b) Myelinated neuron.

The next region of the axon will still be maintaining its resting potential and because unlike charges attract, the positive ions within the cell will be attracted towards the area of negativity; thus the impulse will spread down the length of the axon.

Once a section of the cell membrane has be-come depolarized, K⁺ ions leak out of the cells and the sodium channels close again, so restoring the resting potential. This is called repolarization. Until resting potential has been restored, the neuron cannot respond to another stimulus.

The whole cycle from resting potential, through depolarization and repolarization and back to resting potential, takes only a millisecond and when stimulated, up to 200 action potentials/s can be passed along the neuron.

Transmission of information

An action potential occurs as the result of stimulation of a dendrite by a stimulus. This stimulus may be the result of a sensation that needs to be sent to the cerebral cortex or may be in response to the interpretation of sensory input requiring an action or a motor output such as contraction of a muscle. Senses that stimulate an action impulse can take many forms. These may be touch, light, heat or damage to organs or tissues, changes in ion concentration, chemical concentration, or pressure. As a result of dendritic stimulation, an action potential is triggered in the neuron detecting the sensation. However, this neuron does not stretch all the way from the source of the stimulus to the brain. Several neurons are found along the pathway. Thus, the stimulus must be passed from one neuron to the next. At the terminal end of the neuron, branches of the axon extend in several potential directions ending with a synaptic end bulb. Synapses are present between synaptic end bulbs and dendrites of the adjoining neurons. Dendrites and cell bodies are often covered in synaptic end bulbs from many different neurons (Fig. 8.4).

Figure 8.4 Relationship between synaptic end bulbsand cell bodies.

A synapse is a space across which an action potential must be transmitted. This process is carried out by the production of a chemical by the terminal synaptic end bulb. This substance diffuses across the space and results in an increase or decrease in the permeability of the cell wall of the dendrite of the adjoining neuron (Fig. 8.5). Thus, an action potential will be initiated or the impulse inhibited from spreading along the axon of the receiving neuron. In this capacity, these chemicals are known as neurotransmitters.

Figure 8.5 A synapse.

Neurotransmitters are synthesized in the synaptic end bulb or in the cell body of the neuron; in the latter case the chemical will need to be transported along the axon to the synapse. Many chemicals perform this function including acetylcholine and dopamine, oxytocin and the endorphins.

EPILEPSY

Epilepsy is a condition in which abnormal electrical activity in the brain generates inappropriate action potentials resulting in strange sensations, behaviour, muscle spasm or seizures. Most people will be familiar with the sight of an individual having a typical spasmodic fit. However, there are several types of epilepsy and these are described below.

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