Therapy Analysis - Epilepsy
What is epilepsy?
During normal brain activity, electrical impulses known as action potentials are transmitted along the length of a neurone. Action potentials consist of brief changes in voltage across the cell membrane, caused by an influx of sodium and efflux of potassium ions. These impulses are passed between neighbouring neurones by neurotransmitters, which are released into the gap between cells following stimulation by the electrical activity. The amplitude of the action potential does not vary greatly; instead messages are encoded in the frequency of action potential transmissions along the cells. This means that a balance between the initiation and termination of electrical activity is essential to accurately encode messages sent throughout the nervous system; in addition, the spread of electrical activity is also strictly limited. In the onset of a seizure, this balance is lost and there is a 'storm' of uncontrolled and excessive electrical activity in groups of neurones, leading to sudden and unprovoked changes in behaviour and altered states of consciousness. Epilepsy is diagnosed when more than one of these seizures occurs. The underlying cause can be attributed to injury, illness or abnormal development that affects the brain's normal pattern of behaviour. However, in most cases the cause is unknown and patients are therefore diagnosed with idiopathic epilepsy. One potential cause of epilepsy is an imbalance in excitatory and inhibitory neurotransmission. These two mechanisms allow for 'on' and 'off' signals to be sent, to further control electrical signalling. Gamma-aminobutyric acid (GABA) is a key inhibitory neurotransmitter that temporarily prevents the firing of excitatory action potentials. Both epilepsy and an increased seizure risk have been linked to lower levels of GABA. Indeed, many of the drugs launched to treat epilepsy work by augmenting GABA neurotransmission in the central nervous system (CNS).
The underlying electrical disruption causing epileptic seizures can occur in any location in the CNS, therefore the presentation varies dramatically between patients. As such, the current system used for classifying the types of epilepsy presentation was developed by the International League Against Epilepsy (ILAE) in order to standardise how seizures are classed by physicians. The first system, proposed in 1981, concerns the classification of individual seizures through the use of electroencephalogram (EEG) recordings and clinical observations. Seizures are initially grouped according to the localisation of the source of the seizure. Partial or focal seizures affect only a part of the brain, whereas generalised seizures affect both sides of the brain. Partial seizures are further classified by the extent to which they affect consciousness, into complex or simple partial seizures. Complex partial seizures involve some interruption to consciousness whereas simple partial seizures do not. Additionally there is a subset of partial seizures that develop into generalised seizures.
Generalised seizures can also be subdivided according to accompanying behavioural effects, into categories including absence seizures, myoclonic seizures, clonic seizures and tonic-clonic seizures. Absence seizures involve a disruption to consciousness, with the sufferer appearing vacant for a short time. Myoclonic seizures are characterised by involuntary muscle twitches without alterations in consciousness, occurring singly or in groups. Clonic seizures also involve the brief, rapid muscle contractions of myoclonus, however, these repeat in a regular, rhythmic pattern roughly every few seconds. Tonic-clonic seizures (often referred to as Grand Mal) are perhaps the type of seizures most people understand as an 'epileptic fit'. They primarily entail a sudden, abrupt loss of consciousness then a 'tonic' contraction of the muscles, which may involve tongue biting, and an interruption of breathing. This is followed by a clonic phase of rhythmic motor activity, producing limb flexing and extension. This classification system is relatively simple, and an apparent problem arose regarding patients that suffer from more than one type of seizure over the course of their illness. Therefore in 1989 the ILAE proposed an addition to the classification system where epileptic syndromes (defined as a group of symptoms occurring together) are placed into four broad groups: localisation-related, generalised, undetermined and special syndromes.
