The short exercise test is normal in proximal myotonic myopathy

Objectives: Proximal myotonic myopathy (PROMM) is a multisystem disorder that may mimic myotonic dystrophy (MD). Previously we demonstrated that the 60 s exercise test was normal in two siblings with PROMM. The test enabled distinction of PROMM from MD, as there is a well documented immediate post-exercise compound muscle action potential (CMAP) amplitude decline in MD. Methods: We now performed exercise testing using several exercise durations in 8 PROMM patients from 6 kinships, and one MD patient, extending our previous observations. Repetitive stimulation and needle electromyography findings were also recorded. Results: The 10 (n=8), 30 (n=5), and 60 (n=5) s, and the 5 min (n=1) exercise tests were normal in all PROMM patients. Specifically, the maximum post-exercise CMAP amplitude decline was 8%. In contrast, the MD patient had CMAP amplitude declines of 48% (10 s exercise test) and 26% (30 s exercise test). The distribution of repetitive stimulation and motor unit duration abnormalities were variable and less diagnostically useful. Conclusions: The 10, 30, and 60 s exercise tests help distinguish PROMM from MD. As the 10 s exercise test is rapid and easily tolerated, we recommend this test for clinical testing.