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This is a case of a 28‐year‐old man who presented with a 6‐month history of gradually progressive proximal leg weakness and pain that worsened on exercise and was relieved by rest. He had no symptoms in his upper limbs. Apart from intermittent mild ptosis and diplopia, he had no other cranial nerve symptoms. Sensation, bladder, and bowel function were normal. His examination revealed fatigable ptosis and diplopia with a positive ice‐pack test. His motor examination showed mild fatigable hip flexion weakness and hyporeflexia that recovered on brief exercise. Electrophysiologic studies revealed small motor nerve amplitudes that improved with brief exercise, and a significant decrement on slow repetitive nerve stimulation. He had a positive anti‐VGCC antibody. The clinical, electrophysiological, and antibody findings supported a diagnosis of Lambert Eaton Myasthenic syndrome (LEMS). LEMS commonly presents as a paraneoplastic syndrome associated with small cell lung cancer in older male smokers. However, some forms are primarily autoimmune, as was the case in our patient.

Neuromuscular features are common in mitochondrial DNA (mtDNA) disorders. The genetic architecture of mtDNA disorders in diverse populations is poorly understood. We analysed mtDNA variants from whole‐exome sequencing data in neuromuscular patients from South Africa, Brazil, India, Turkey and Zambia. In 998 individuals, there were two definite diagnoses, two possible diagnoses and eight secondary findings. Surprisingly, common pathogenic mtDNA variants found in people of European ancestry were very rare. Whole‐exome or ‐genome sequencing from undiagnosed patients with neuromuscular symptoms should be re‐analysed for mtDNA variants, but the landscape of pathogenic mtDNA variants differs around the world.