Fig. 1.

Mitochondrial disease symptoms and skeletal muscle biopsies. Mitochondrial diseases are multisystemic disorders that present with a wide variety of neurological, muscular, hepatic, and gastrointestinal symptoms, among others (left). Myopathy is associated with mitochondrial disease and often leads to exercise intolerance, cramps, and fatigue. Skeletal muscle biopsies are commonly taken for diagnostic purposes. When reacted for COX/SDH histochemistry, a mosaic pattern of affected (COX-deficient) and unaffected (COX-positive) muscle fibers can be seen in transverse sections (light blue). When muscle biopsies are examined longitudinally, COX-deficiency can be seen restricted to small segments along the length of the fiber surrounded by COX-positive regions (orange)