Seizure disorders in Down syndrome. The prevalence, onset, and type of seizure disorders, as well as seizure control, were studied in a large cohort of 405 individuals with Down syndrome (age range, 6 months to 45 years). The evaluation of a questionnaire completed by the subjects' parents and of the patients' medical records indicated that 33 (8.1%) of 405 persons with Down syndrome had seizure disorder. With regard to the onset of seizures, a bimodal distribution was noted: 40% of patients began having seizures before the age of 1 year, and another 40% started with seizure activity in the third decade of life. In the younger age group, primarily infantile spasms and tonic-clonic seizures with myoclonus were observed, and the older patients often had partial simplex or partial complex seizures as well as tonic-clonic seizures. 