Mitochondrial encephalomyopathies: a review of routine morphological diagnostic methods with emphasis on the role of electron microscopy.

Mitochondrial encephalomyopathies (MEs) are a group of clinically and genetically heterogeneous diseases. They can be caused by defects in both mitochondrial or nuclear coded genes. Their phenotypic expression is governed by unique biological phenomena such as the dual genetic control, mitotic segregation, heteroplasmy and threshold effects. Currently, the correct diagnosis of ME relies on a multidisciplinary approach which includes clinical information as well as laboratory data from muscle morphology, biochemistry and molecular genetics. Among the morphological methods, histology, histochemistry and electron microscopy were historically instrumental in the diagnosis of MEs. However, with the development of molecular genetics, the diagnostic value of morphology and of electron microscopy in particular have been questioned. The aim of the present review is to present a comparative assessment of the diagnostic contribution of histology, histochemistry and electron microscopy in a group of 48 patients with a diagnosis of ME.