A rare inherited metabolic disorder that affects the central nervous system. Caused by mutations in mitochondrial or nuclear DNA, or by pyruvate dehydrogenase deficiency. Symptoms typically begin in infancy (<1 year), and patients present with nausea, vomiting, diarrhea, poor suck, dysphagia, and irritability, leading to failure to thrive. Patients deteriorate quickly and develop hypotonia, weakness, dystonia, ataxia, neuropathy, vision loss, nystagmus, encephalopathy, or seizures. There may be episodes of rapid decline and worsening encephalopathy following infections or illnesses. Prognosis is poor and affected individuals typically die within a few years, most commonly of respiratory failure.