Brody myopathy demonstrates a pseudo-increment on repetitive nerve stimulation

Brody myopathy (BM) is a recessive condition caused by mutations in the ATP2A1 gene and usually induces impaired muscle relaxation during and after exercise. Diagnosis relies on needle electromyography showing electrical silence, muscle biopsy with decreased sarcoplasmic reticulum calcium adenosine triphosphatase activity, and genetic analysis. Electrodiagnostic functional analyses are useful in the diagnosis of channelopathies, and thus may be impaired in BM.

We performed exercise tests and repetitive nerve stimulation (RNS; 10 supramaximal stimuli at 3 Hz) in 10 patients with BM.

All participants showed incremental responses on RNS. Compound muscle action potential amplitude was increased and duration was decreased, especially in the ulnar nerve (+30.2 ± 7.1% and - 30.3 ± 2.8%, respectively; both P < .001).

Easily accessible, this sign, referred to as the Arzel sign, could prove to be a very useful tool in BM diagnosis and in broadening its phenotype.