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Symmetrical polyneuropathy is a common feature of mitochondrial disease. Both axonal and demyelinating types are described, with Schwann cell abnormalities demonstrated on nerve biopsy. Some authors have also suggested an increased incidence of entrapment neuropathy. We identified 738 adult patients with proven mitochondrial disease seen in our centre in the past 25 years. One-hundred sixty seven of these patients had undergone nerve conduction studies as part of their routine clinical care, and the results of these studies were reviewed. We found an incidence rate of carpal tunnel syndrome ( CTS) of 50.7 per 100,000 person-years; 32.5 per 100,000 person-years for men and 65.3 per 100,000 person-years for women. One other patient had evidence of ulnar neuropathy at the elbow. The incidence of CTS in mitochondrial disease is similar to published rates for the UK general population. We found no evidence that mitochondrial disease per se increases the risk of entrapment neuropathy. We suggest that the pathophysiological mechanisms for the development of polyneuropathy in mitochondrial disease are quite distinct from the pathophysiology of CTS. Furthermore, it is essential that patients with mitochondrial disease who present with upper limb paraesthesia be referred for neurophysiological testing, so that treatable CTS is not missed. [ABSTRACT FROM AUTHOR]