EMG diagnosis of McArdle disease with long exercise test

Standard electromyography (EMG) is routinely performed during the diagnostic process of myopathies. However, its role is limited in metabolic myopathies with exercise intolerance, such as McArdle disease (glycogenosis type V, GSDV). The aim of the study was to evaluate a provocative test (the Long Exercise Test, LET) in the EMG diagnosis of GSDV. Twenty-five patients (17 males, 41 ± 17 years) with confirmed diagnosis of GSDV underwent an EMG study (including nerve conduction studies, repetitive nerve stimulation, needle EMG) completed with LET. Briefly, the compound muscle action potential (CMAP) responses were recorded from right abductor digiti minimi (ADM) muscle before and after 5 min of maximal ADM isometric contraction. Needle EMG showed myopathic pattern only in 5/25 patients. LET disclosed a post-exercise decrease in CMAP amplitude in 22/25 patients (88%). The decrement appeared immediately after exercise (average −20%) and reached its maximum (−30%) at 30 min, after a transient plateau phase (lasting 5 to 15 min). This pattern was not observed in controls (healthy subjects, other metabolic myopathies, periodic paralysis). Interestingly, GSDV patients with normal LET (n = 3) presented milder symptoms and minimal myophosphorylase activity on muscle biopsy. In conclusion, EMG combined with LET detects a peculiar pattern in McArdle disease. This test is sensitive, safe and non-invasive. It may orient molecular analysis toward myophosphorylase gene in patients referred for exercise intolerance with hyperCKemia at rest. The normal response of LET in patients with milder symptoms indicates a correlation with clinical severity, thus suggesting its use as a possible outcome measure in clinical trials.