Generalised tonic-clonic seizures (GTCSs) occur in many different types of epilepsy, and making an accurate diagnosis based solely on a described or even observed GTCS is difficult. In general, the first distinction is to determine if the GTCS is truly an unprovoked seizure event; a variety of provoking insults can cause GTCSs in a patient who does not have epilepsy.[27] external link opens in a new window

If no provoking stimulus can be identified, the second step is to determine if there is an underlying generalised-onset epilepsy versus a focal-onset epilepsy; this is necessary to direct the appropriate treatment steps. This can be accomplished through careful history or the use of electroencephalogram (EEG) and/or magnetic resonance imaging (MRI) studies; occasionally long-term video-EEG monitoring (telemetry) is necessary to characterise the epilepsy syndrome fully.

If it is determined that the GTCS reflects generalised-onset epilepsy, further categorisation is necessary, based upon a careful assessment of seizure types, age of onset, family history, response to therapy, and supportive data. A history of other generalised seizure types may be of use; these include absence, myoclonic, tonic-clonic, atonic, tonic, and clonic seizures. Any given patient may experience one seizure type or any combination of multiple seizure types, depending on the underlying epilepsy syndrome. As a result, presentations can often be quite variable, and a complete synthesis of the patient's history, examination, and supportive data is necessary to form a diagnosis.

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