Abstract
A 38-year-old man complained of lifelong intolerance to exercise. He described premature fatigue and myalgia even after moderate exercise, such as walking on level ground for 20min. Walking uphill or upstairs, he could only tolerate for a few minutes. If he forced himself to exercise, he felt exhausted on the following day, and the muscles he used felt sore. He never noted pigmenturia. He believed his legs were more affected than his arms. A cardiovascular consultation revealed no cardiac dysfunction. The neurological examination was normal, including muscle bulk and strength. Resting serum CK was normal. Lactic acid was 5.2mEq/L (normal values, 0.5–2.2mEq/L) and rose excessively after a standardized aerobic exercise (9.2mEq/L after 1min). Electromyography findings were normal. His family history was negative for neuromuscular disorders; specifically, his mother and two sisters did not complain of exercise intolerance. Biochemical analysis showed a distinct complex I deficiency in his frozen muscle biopsy, and histochemistry revealed scattered hyper-SDH stained and COX-positive (ragged blue) fibers; Sanger sequencing identified a pathogenic mutation (m.11832G>A) in MTND4 encoding the ND4 subunit of complex I.
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