Most electroencephalographic seizures are unilateral and have a restricted electrical field. Seizures with bilateral electroencephalographic onset are rare (Figure 3.1).

Figure 3.1.— An electroencephalographic seizure. Generalized high voltage sharp waves followed by a period of depression and a sequence of rhythmic discharges with a well-defined onset, body, offset, and electrical field that do not have the appearance of an artifact or a physiologic activity.

The term electroclinical seizure refers to a clinical paroxysmal event that is associated with an electroencephalographic seizure. The term convulsion refers to an electroclinical seizure characterized by increased motor activity. The term clinically silent electroencephalographic seizure refers to a scalp-recorded electroencephalographic seizure that occurs during the course of normal neonatal activity. In neonates, clinically silent electroencephalographic seizures probably occur more frequently than electroclinical seizures. Clinically silent electroencephalographic seizures and electroclinical seizures have similar single photon emission computed tomography findings in neonates with hemimegalencephaly, which suggests that they may have similar consequences and should be treated the same way (Figure 3.2).