SM is an autosomal dominant herculean painful myopathy recently described in Canada, which leads to muscle hypertrophy and herculean strength, progresses to cramps and fatigability, followed by incapacitating myalgia. Myalgia is the principal symptom reducing quality of life. A missense variant in the DCST2 gene has been reported in two French-Canadian family. DCST2  might play an important role  in calcium homeostasis. Dantrolene acts directly on skeletal muscle by interfering with release of calcium ions from the sarcoplasmic reticulum and reduces myoplasmic calcium ion concentration.

Three SM patients with myalgia post-activity, muscle cramps, increased muscle bulk, percussion pseudo-myotonia and positive first-degree family history of superior strenght were followed over three months. Patients received 12.5 to 25 mg of dantrolene. They performed the Verbal Numerical Rating Scale (VNRS) prior to treatment initiation and at 3 months follow-up. The Clinical Global Impression Improvement Scale (CGI-I) was also completed at 3 months follow-up to evaluate improvement.

For each patient VNRS decreased from 10, 10 and 8, to 4, 7 and 4, respectively. Overall, this suggested a mean improvement in VNRS of 44% (pre-treatment: 9 +/- 1.5; 3 months follow-up: 5 +/- 1.5). For CGI-I, scores were 3, 2 and 2 and the mean 2.3 (+/- 0.6) represented minimal to much improved. No adverse events were reported. Patients reported improvement of flexibility.