Epilepsy can be defined in a number of ways. Firstly, an epileptic seizure can be described as either a generalised seizure, that encompasses all of the brain or a partial seizure that involves only part of the brain that is known as a focus. Generalised seizures fall within two categories that are primary generalised seizures and secondary generalised seizures. Primary generalised seizures are sudden onset seizures that involve both hemispheres of the brain. Secondary generalised seizures start with a focus as in a partial seizure and the seizure activity spreads across and to both hemispheres of the brain, (Rogan 1992). An example of a generalised seizure is that of a tonic clonic seizure whereby the sufferer falls to the ground and exhibits tonus which is stiffening of the muscles and clonus which is twitching of the muscles. Tonus and clonus are usually then followed by a state of confusion before the sufferer proceeds into a deep sleep. Other examples of generalised seizures are absence seizures, myoclonic seizures and atonic seizures, (Rogan 1992).

Partial seizures are confined to one part of the brain. Partial seizures have also been divided into two categories that are simple partial seizures and complex partial seizures. Simple partial seizures can be further subdivided into two categories which are focal motor and focal sensory. A typical focal motor seizure would produce twitching of the muscles or bodily movements that are relative to the area of the brain that is experiencing abnormal electrical discharges. Focal sensory seizures are similar to focal motor seizures except the disturbances occur in perception and sensation such as visual or auditory distortions. Focal sensory seizures can occur in the temporal lobe which can produce feelings of deja’vu, (Rogan 1992).

Complex partial seizures involve an impairment of consciousness and can display behaviours that are inconsistent with the sufferer’s normal behaviour. The person may smack his/her lips, move objects and undertake activities without no obvious reasons for doing so. Complex Partial Seizures include seizure behaviour that used to be classified under Temporal Lobe Epilepsy and Psychomotor Epilepsy. Both simple and complex partial seizures can become secondary generalised seizures, (Rogan 1992). 

There has been some controversy regarding the classifications of epilepsy. The findings of The Commission on Classification on Terminology of the International League Against Epilepsy (1981) differed markedly to the previous findings of the First International Classification of Seizures (Gastaut 1970; Gastaut 1973) which was used widely between 1969 and 1981. The revised system argues that the evolution of seizures should be described. The revised system argues that a seizure such as an absence which then proceeds to a tonic clonic seizure, would be described as a convulsive generalised seizure under the earlier classification system with no acknowledgement of the evolution of the seizure. Under the revised classification system, the aforementioned seizure would be designated as a non convulsive to convulsive seizure. Furthermore, the revised system argued that partial seizures should not be divided into categories dependent on clinical manifestations. Firstly a partial seizure that occurs without any conscious impairment is designated as a simple partial seizure. A partial seizure that involves conscious impairment is designated as a complex partial seizure. Under the original classification system, a complex partial seizure was defined as any partial seizure leading to complicated clinical manifestations with or without amnesia. This has led to considerable confusion with regard to what terms such as generalised seizure and complex partial seizure mean when being used without reference to the classification system being used.