Mitochondrial function in 4 patients with alternating hemiplegia (AH) was evaluated by magnetic resonance spectroscopy of resting muscle at the Montreal Neurological Institute, McGill University, Montreal, Canada. All patients had abnormally high resonance intensities from inorganic phosphate and an abnormally low calculated cytosolic phosphorylation potential. These changes were consistent with mitochondrial dysfunction. 
COMMENT. Alternating hemiplegia of childhood may represent a phenotype of mitochondrial disease. Mitochondrial diseases share with AH the occurrence of alternating hemiplegia, seizures, and neurodevelopmental deterioration. The clinical characteristics and differential diagnosis of AH are reviewed in a report of 22 cases from the Hopital des Enfants Malades, Paris, France . SPECT images of two patients showed focal areas of decreased uptake of the radiotracer, representing impaired regional blood flow during and between hemiplegic episodes and a possible mechanism of the neurologic deficits.