21st June 2023

Classification of epilepsy

Aetiology of epilepsy

Definitions

The National Institute of Health and Care Excellence (NICE) defines a seizure as a transient acute change in neurological function produced by abnormal excessive or synchronous activity in the brain. Therefore, epilepsy is a predisposition to having recurrent seizures and can only be definitively diagnosed after a person has had two or more seizures.

Causes of Epilepsy

The causes of epilepsy are heterogenous. The table below summarises the main causes of epilepsy:

Infectious

This is the commonest aetiology causing epilepsy worldwide. CNS infections can cause secondary epilepsy.

Examples of CNS infections that can cause epilepsy are bacterial meningitis, encephalitis, human immunodeficiency virus (HIV), malaria, toxoplasmosis, cytomegalovirus (CMV), and tuberculosis (TB).

Classification of Seizures

Seizures are classified by the origin of the seizure within the brain.

Focal seizures

Focal seizures originate from a single region within the brain.

There are two types of focal seizures:

• Focal seizure with retained consciousness – a seizure originating from a single region, which does not spread and does not cause impairment of consciousness
• Focal seizure with impairment of consciousness – a seizure which originates from a localised region of the cortex but then spreads generally; associated with impairment of consciousness.

Generalised seizures

Generalised seizures affect both hemispheres of the brain and cause impaired consciousness at its onset. Generalised seizures include:

• Absence seizure – a seizure lasting seconds where the individual suddenly becomes unaware and usually involves a blank appearance and clonic (rhythmic jerking) movements, occurring at 3Hz, of the face – commonly the eyelids, the perioral region or the chin. EEG classically shows regular 3Hz generalised spike-and-wave patterns. Post-ictally, there is more likely to be confusion. If it lasts more than 45 seconds, it is more likely to be a focal seizure. Focal seizures are more common in children.
• Atonic seizure – sudden loss of muscle tone lasting seconds, typically involving the head, trunk or limbs. EEG shows slow waves during atonia.
• Tonic-clonic seizures – a seizure that starts with a tonic phase, presenting as bilateral increased tone, followed by a clonic phase, presenting as bilateral repetitive rhythmic jerking of the whole body. EEG in the tonic phase may show generalised fast rhythmic spikes, and in the clonic phase, bursts of spikes followed by synchronous slow waves may be seen. The post-ictal phase commonly presents as drowsiness, headache, confusion, and incontinence.

Features of seizures

The features that result from the seizure depend on the region affected; this is usually sub-classified as motor features and non-motor features. These features can be seen in focal and generalised seizures.

Seizures with motor features:

• Clonic seizure – rhythmic jerking of one body part.
• Myoclonic seizure – a cluster of muscle contractions or jerks lasting milliseconds. On EEG, a myoclonic jerk correlates with a generalised spike-and-wave or polyspike-and-wave.
• Tonic seizure – increased muscle tone in one body part. This usually recovers quickly. EEG shows generalised and accelerating low amplitude paroxysmal activity.
• Hyperkinetic seizure – irregular large movements of a group of muscles, such as thrusting, rocking or jumping.
• Automatism seizure – repetitive activity, which mimics voluntary movement, such as lip smacking, head nodding, walking or grunting.
• Commonly after focal motor seizures, patients develop Todd’s paresis, which is characterised by post-ictal weakness or paralysis of the contralateral limb or contralateral side of the body.

Seizures with non-motor features:

• Sensory seizures – involve a sensory experience, such as sensation, visual, olfactory, auditory, vestibular, and gustatory.
• Cognitive seizures – involve change to a cognitive function, such as expressive dysphasia, dyslexia, déjà vu or jamais vu (feeling of unfamiliarity).
• Autonomic seizures – involve autonomic symptoms, such as palpitations, nausea, pallor, altered respiration, etc.
• Emotional seizures – involve changes in mood or emotion, such as anxiety, crying, anger, etc.

Status epilepticus

NICE defines status epilepticus as a seizure that lasts for more than 5 minutes or recurrent seizures, one after the other, without recovery in between. Status epilepticus can be convulsive or non-convulsive. Non-convulsive status epilepticus causes a change in mental status or behaviour. Status epilepticus can cause disability or death.