Brody disease: when myotonia is not myotonia

A 56-year-old man presented with painless impairment of muscle relaxation on vigorous contraction (eg, eyelid closure, hand grip, running). There were no episodes of paralysis, symptom progression, weakness or extramuscular symptoms. Five of his fifteen siblings had similar complaints. His serum creatine kinase was normal. Electromyography showed electrical silence on muscle relaxation, without myotonic discharges. DMPK, ClCN1 and SCN4A genetic testing was normal, but he had a homozygous pathogenic variant of ATP2A1 (c.1315G>A; pGlu439Lys). Brody disease is a rare autosomal recessive myopathy due to ATP2A1 mutations that reduce sarcoplasmic reticulum calcium-ATPase1 activity, hence delaying muscle relaxation.