Objective: To define the clinical and EEG features of the epileptic syndromes occurring in adult and infantile mitochondrial encephalopathies (ME).
Methods: Thirty-one patients with recurrent and apparently unprovoked seizures associated with primary ME were included in the study. Diagnosis of ME was based on the recognition of a morphologic, biochemical, or molecular defect.
Results: Epileptic seizures were the first recognized symptom in 53% of the patients. Many adults (43%) and most infants (70%) had nontypical ME phenotypes. Partial seizures, mainly with elementary motor symptoms, and focal or multifocal EEG epileptiform activities characterized the epileptic presentation in 71% of the patients. Generalized myoclonic seizures were an early and consistent symptom only in the five patients with an A8344G mitochondrial DNA point mutation with classic myoclonus epilepsy with ragged red fibers (MERRF) syndrome or "overlapping" characteristics. Photoparoxysmal EEG responses were observed not only in patients with typical MERRF, but also in adult patients with ME with lactic acidosis and strokelike episodes (MELAS), or overlapping phenotypes, and in one child with Leigh syndrome.
Conclusions: Epilepsy is an important sign in the early presentation of ME and may be the most apparent neurologic sign of nontypical ME, often leading to the diagnostic workup. Except for those with an A8344G mitochondrial DNA point mutation, most of our patients had partial seizures or EEG signs indicating a focal origin.