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Epileptic seizures are classified both by their patterns of activity in the
brain and their effects on behaviour.
In terms of their pattern of activity, seizures may be described as either
partial (focal) or generalised. Partial seizures only involve a localised part
of the brain, whereas generalised seizures involve the entire cortex. The term
'secondary generalisation' may be used to describe a partial seizure that later
spreads to the whole of the cortex and becomes generalised.
Partial seizures may be further subdivided into both simple and complex
seizures. This refers to the effect of such a seizure on consciousness; simple
seizures cause no interruption to consciousness (although they may cause sensory
distortions or other sensations), whereas complex seizures interrupt
consciousness to varying degrees. This does not necessarily mean that the person
experiencing this sort of seizure will fall unconscious (like fainting). For
example, a complex partial seizure may involve the unconscious repetition of
simple actions, gestures or verbal utterances, or simply a blank stare and
apparent unawareness of the occurrence of the seizure, followed by no memory of
the seizure. Other patients may report a feeling of tunnel vision or
dissociation, which represents a diminishment of awareness without full loss of
consciousness. Still other patients can perform complicated actions, such as
travel or shopping, while in the midst of a complex partial seizure.
The effects of partial seizures can be quite dependent on the area of the brain
in which they are active. For example, a partial seizure in areas involved in
perception may cause a particular sensory experience (for example, the
perception of a scent, music or flashes of light) whereas, when centred in the
motor cortex, a partial seizure might cause movement in particular groups of
muscles. This type of seizure may also produce particular thoughts or internal
visual images or even experiences which may be distinct but not easily
described. Seizures centred on the temporal lobes are known to produce mystical
or ecstatic experiences in some people. These may result in a misdiagnosis of
psychosis or even schizophrenia, if other symptoms of seizure are disregarded
and other tests are not performed. Unfortunately for those with epilepsy,
anti-psychotic medications prescribed without anti-convulsants in this case can
actually lower the seizure threshold further and worsen the symptoms.
When the effects of a partial seizure appear as a 'warning sign' before a more
serious seizure, they are known as an aura: it is frequently the case that a
partial seizure will spread to other parts of the brain and eventually become
generalized, resulting in a tonic-clonic convulsion. The subjective experience
of an aura, like other partial seizures, will tend to reflect the function of
the affected part of the brain.
Generalised seizures can be sub-classified into a number of categories,
depending on their behavioural effects:
* Absence seizures (sometimes referred to as petit mal seizures) involve an
interruption to consciousness where the person experiencing the seizure seems to
become vacant and unresponsive for a short period of time (usually up to 30
seconds). Slight muscle twitching may occur.
* Tonic-clonic seizures (sometimes referred to as grand mal seizures),
involve an initial contraction of the muscles (tonic phase) which may involve
tongue biting, urinary incontinence and the absence of breathing. This is
followed by rhythmic muscle contractions (clonic phase). This type of seizure is
usually what is referred to when the term 'epileptic fit' is used colloquially.
* Myoclonic seizures involve sporadic muscle contraction and can result in
jerky movements of muscles or muscle groups.
* Atonic seizures involve the loss of muscle tone, causing the person to fall
to the ground. These are sometimes called 'drop attacks' but should be
distinguished from similar looking attacks that may occur in narcolepsy or
cataplexy.
* Status epilepticus refers to continuous seizure activity with no recovery
between successive tonic-clonic seizures. This is a life-threatening condition
and emergency medical assistance should be called immediately if this is
suspected. A tonic-clonic seizure lasting longer than 5 minutes (or two minutes
longer than a given person's usual seizures) is usually considered grounds for
calling the emergency services.
* Epilepsia partialis continua is a rare type of focal motor seizure (hands
and face) which recurs every few seconds or minutes for extended periods (days
or years). It is usually due to strokes in adults and focal cortical
inflammatory processes in children (Rasmussen's encephalitis), possibly caused
by chronic viral infections or autoimmune processes.
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