Histopathological features of peripheral nerve and muscle in mitochondrial disease

Despite enormous strides in the molecular diagnosis of mitochondrial disease, this approach is currently applicable to only a minority of patients who are affected with these disorders. The phenotypic spectrum in this category of disease is large and, in the absence of genotypic confirmation, a pattern recognition paradigm is probably the most sensitive means to reinforce the suspicion of mitochondrial disease. Along with clinical, biochemical, radiographic, and electrophysiological markers, histopathological features from nerve and muscle biopsy are useful indices to factor into a complex equation permitting a presumptive diagnosis or to justify more elaborate diagnostic undertakings. The combination of electrophysiological evidence of demyelinating neuropathy on nerve conduction studies and mild myopathic features on electromyography is one such constellation that should instigate a high index of suspicion for mitochondrial disease. The histopathological hallmarks of mitochondrial cytopathies on muscle biopsy are the "ragged-red fiber" on light level evaluation and paracrystalline inclusions at the electron microscopic level. Neither of these is exclusive to mitochondrial disease and both may be identified among other nonspecific changes seen in biopsy specimens. Histopathological evaluation of muscle and nerve can provide information to reinforce the likelihood of mitochondrial disease or to indicate an alternative diagnosis as the more probable cause of a patient's symptoms.