Seizure semiology and EEG findings in mitochondrial diseases

Seizures constitute a frequent yet under-described manifestation of mitochondrial disorders (MDs). The aim of this study was to describe electroencephalography (EEG) findings and clinical seizure types in a population of children and adults with mitochondrial disease.

Retrospective chart review of 165 records of children and adults with mitochondrial disease seen in the University of Texas Houston Mitochondrial Center between 2007 and 2012 was performed; all subjects were diagnosed with confirmed mitochondrial disease. EEG findings and clinical data, including seizure semiology and response to antiepileptic drugs (AEDs), were analyzed and categorized.

Sixty-six percent (109/165) of subjects had a routine EEG performed. Sixty-one percent (67/109) of EEG studies were abnormal and 85% (56/67) had epileptiform discharges. The most common EEG finding was generalized slowing (40/67, 60%). The most frequent category of epileptiform activity seen was multifocal discharges (41%), followed by focal (39%) and generalized (39%) discharges. Clinical seizures were seen in 55% of subjects and the most common types of seizures observed were complex partial (37%) and generalized tonic-clonic (GTC; 37%). The most common seizure type in patients with mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) was GTC (33%), with generalized or focal discharges seen on EEG. In Leigh syndrome GTC (11%) and complex partial (11%) seizures were the most frequent types. Of 60 subjects with clinical seizures, 28% were intractable to medical treatment.

Mitochondrial disorder should be included in the list of differential diagnosis in any child that presents with encephalopathy, seizures, and a fluctuating clinical course. Given the relatively high prevalence of EEG abnormalities in patients with MD, EEG should be performed during initial evaluation in all patients with MD, not only upon clinical suspicion of epilepsy.