[Mitochondrial diseases; thinking beyond organ specialism necessary].

Mitochondrial disorders are caused by a defect in intracellular energy production. In general, these are multi-system disorders, predominantly affecting organs with high energy requirements. Due to the fact that mitochondrial disorders are not as rare as is generally assumed, and due to the diversity of symptoms, many different medical specialists will at some time be confronted with these patients. Early recognition ofa mitochondrial disorder reduces patient anxiety and avoids unnecessary ancillary investigations and potentially hazardous treatments. A mitochondrial disease should be considered in the event of dysfunction of more than 2 organ systems or processes with high energy requirements, certainly if there is a positive maternal family history. If fatigue includes exercise-induced muscle pain or muscle weakness, and if muscle pain predominantly occurs during exertion, a mitochondrial disease should be considered. The combination ofdiabetes mellitus and deafness is also a strong indicator of mitochondrial disease. An extensive family history should always be taken. In adults, the most frequently occurring mitochondrial syndromes are chronic progressive external ophthalmoplegia (CPEO), maternally inherited diabetes and deafness syndrome (MIDDS) and Leber's hereditary optic neuropathy. Since much research effort is currently being invested in the development of causal medical treatments, the importance of an early diagnosis is likely to become of increasing importance in the future.