In terms of their origin within the brain, seizures may be described as either partial (focal) or generalized. Partial seizures only involve a localized part of the brain, whereas generalized seizures involve the whole of both hemispheres. The term 'secondary generalisation' may be used to describe a partial seizure that later spreads to the whole of the cortex and becomes generalized.
Whilst most seizures can be neatly split into partial and generalized, there exists some that don't fit. For example: the seizure may be generalized only within one hemisphere. Alternatively there may be many focal points (multifocal seizures) that are distributed in a symmetrical or asymmetrical pattern.
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 centered 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 anticonvulsants 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 larger 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.
Primarily generalized seizures can be sub-classified into a number of categories, depending on their behavioural effects:
Status epilepticus refers to continuous seizure activity with no recovery between successive seizures. When the seizures are convulsive, it 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.
In 1997, the ILAE began work on revising the classification of seizures, epilepsies and epileptic syndromes. This revision remains in gestation and has not superseded the 1981 classification.
Proposed changes to terminology include:
The hierarchy presented has the structure:
The 1981 classification was a revision of the one devised by Henri Gastaut for the ILAE and published in 1970. A significant difference was the distinction between simple and complex partial seizures. In the 1970 classification, the distinction was whether the symptoms involved elementary sensory or motor functions (simple) or whether "higher functions" were involved (complex). This was changed to consider whether consciousness was fully retained or not. As a result, studies that group patients according to these classifications are not directly comparable from one generation to another. The 1970 classification was important for standardising the modern terms for many seizure types. Prior to this, terms such as petit mal, grand mal, Jacksonian, psychomotor and temporal-lobe seizures were used.
The earliest classification of seizures can be attributed to Babylonian scholars who inscribed their medical knowledge into stone tablets know as the Sakikku (meaning All Diseases). This dates from the reign of the Babylonian king Adad-apla-iddina of the Second Dynasty of Isin - reckoned to be between 1067 and 1046 BC. Many types of seizures are described, each attributed to a certain demon or departed spirit and given a prognosis.