A seizure is a transient alteration of behavior due to the disordered, synchronous, and rhythmic firing of populations of brain neurons. Epilepsy refers to a disorder of brain function characterized by the periodic and unpredictable occurrence of seizures. Epileptic seizures have been classified into partial seizures, which begin focally in a cortical site, and generalized seizures, which involve both hemispheres widely from the outset. The behavioral manifestations of a seizure are determined by the functions normally served by the cortical site at which the seizure arises. For example, a seizure involving motor cortex is associated with clonic jerking of the body part controlled by this region of cortex. A simple partial seizure is associated with preservation of consciousness. A complex partial seizure is associated with impairment of consciousness. The majority of complex partial seizures originate from the temporal lobe. Examples of generalized seizures include absence, myoclonic, and tonic-clonic.
Classification of epileptic syndromes guides clinical assessment and management, and in some cases, selection of antiseizure drugs (Table 19-1). More than 40 distinct epileptic syndromes have been categorized into partial versus generalized epilepsies. The partial epilepsies may consist of any of the partial seizure types and account for ~60%o of all epilepsies; the etiology commonly consists of a lesion in some part of the cortex (e.g., tumor, developmental malformation, damage due to trauma or stroke), but may also be genetic. The generalized epilepsies account for ~40%c of all epilepsies and usually are genetic. The most common generalized epilepsy is juvenile myoclonic epilepsy, accounting for ~10% of all epileptic syndromes. This disorder presents in the early teens and is characterized by myoclonic, tonic-clonic, and often absence seizures. Like most generalized-onset epilepsies, juvenile myoclonic epilepsy is probably due to inheritance of multiple susceptibility genes.
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