In muscle histopathological hallmarks for OXPHOS disorders are the so-called ragged-red fiber, the COX-negative or COX-deficient fiber, and the paracrystalline inclusions in mitochondria. Ragged-red fibers may be found in cases with mitochondrial DNA mutations. Up to now no morphological hallmarks were found for nuclear DNA mutations in genes coding for OXPHOS proteins. However, mutations in (nuclear) assembly genes for Complex IV may give rise to severe COX deficiency.
In the central nervous system, the peripheral nervous system, and in other organs, histopathological changes may be severe and specific for a particular OXPHOS disorder. However, direct indications to OXPHOS disorders are generally not present.