Atonic phenomena and partial seizures. A reappraisal.

At least three types of ictal atomic phenomena can be seen in partial seizures. Focal atomic seizures are partial seizures with ictal paresis or paralysis of one or more body parts preceding or independent of any convulsive activity. A diagnosis may be difficult without simultaneous EEG recording supplemented by video monitoring or polygraphy. Epileptic negative myoclonus manifests as brief lapses in tone in one or more muscle groups that interferes with motor coordination and postural control. Drop attacks can also occur in partial seizures, as have been well documented in patients with temporal or frontal lobe epilepsy. Different mechanisms may underlie these three phenomena. Focal atonic seizures probably result from relatively circumscribed seizure activity in one or more cortical areas that contribute to the elaboration and execution of motor functions. Whether this represents inactivation or interference in motor integration, or activation of inhibitory system, remains unresolved. The pathophysiology of epileptic negative myoclonus, like that of its positive counterpart, is probably to be found at a more local level, possibly from transiently enhanced local inhibition in the motor cortex. Drop attacks in partial seizures may be close in mechanism to that also seen in generalized seizures, possibly related to ictal activation of descending inhibitory pathways in the brainstem.