Is it ADEM, POLG, or both?

OBJECTIVE To describe a child with apparent brain biopsy-confirmed acute disseminated encephalomyelitis (ADEM) but genetic confirmation of compound heterozygosity for DNA mutations of the polymerase gamma (POLG) gene. DESIGN Case report. SETTING Tertiary referral center. PATIENT A 4-year-old boy presented with ataxia and encephalopathy. RESULTS Magnetic resonance imaging demonstrated multiple focal areas of T2 prolongation. The patient's family refused steroid treatment. His symptoms improved then progressed. Magnetic resonance imaging findings also progressed. A cerebrospinal fluid specimen revealed myelin basic protein and oligoclonal bands. A brain biopsy specimen demonstrated demyelination, suggesting progression of ADEM. However, polymerase chain reaction amplification and sequencing revealed 2 heterozygous mutations of the POLG gene, suggesting mitochondrial disease. The patient died 9 months after his initial presentation. CONCLUSIONS This case raises interesting questions about whether ADEM triggered severe neurologic degeneration in a patient with mitochondrial disease, whether mitochondrial disease predisposed to a pathologic immune response, or whether mitochondrial disease can mimic an autoimmune disease. Mitochondrial disease-causing mutations may help explain the poor outcome in some cases of apparent autoimmune central nervous system disease.