If clinical suspicion is strong and the blood results are negative, then mtDNA from a muscle biopsy should be tested. Since the disorders are highly clinically heterogeneous and there is considerable phenotypic overlap, the classic MERRF and MELAS point mutations and the KSS deletions frequently are ordered together and tested as a mitochon-drial test panel. The A8344G MERRF mutation is associated with other phenotypes, including Leigh syndrome, myoclonus or myopathy with truncal lipomas, and proximal myopathy. The A3243G MELAS mutation can result in milder phenotypes, including sensorineural deafness with diabetes. Furthermore, these mutations sometimes are found in asymptomatic relatives of the index case.
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