Ultrastructural diagnosis of mitochondrial encephalomyopathies revisited.

Mitochondrial encephalomyopathies (MEs) are a heterogeneous group of multisystem disorders with extreme variability in clinical phenotype. Due to their complex nature, accurate diagnosis requires a coordinated approach, based on clinical and various laboratory data. Despite the introduction of biochemical assay of mitochondrial enzymes and the availability of mtDNA mutation analysis, the diagnosis of MEs still relies heavily on morphological methods. The latter include histology, histochemistry, and electron microscopy. A comparative study was undertaken to define the contemporary role of electron microscopy in the morphological diagnosis of MEs. Muscle biopsies from 20 patients with MEs, 9 children and 11 adults, were evaluated by histology, enzyme histochemistry, and electron microscopy. The results clearly demonstrate that electron microscopy is of importance in providing essential diagnostic information in pediatric patients, but is of lesser value in the diagnosis of adult cases, where it provides only supplementary information.