Introduction Myotonia is found in a number of muscle diseases, including myotonic dystrophy and non-dystrophic myotonia. The resulting symptoms of myotonia can interfere with daily activities such as walking or climbing the stairs. Due to the rarity of both these conditions, pharmacological treatment of myotonia is largely anecdotal and is led by specialist clinicians who tend to favour the use of mexiletine, a class 1b antiarrhythmic sodium antagonist.

Objective To identify and review randomised controlled trials in order to assess the efficacy and safety of use of mexiletine in myotonic dystrophy and non-dystrophic myotonia for two different patient cases.

Search methods The literature search was conducted using MEDLINE, EMBASE and The Cochrane Library (from January 1990 to December 2014). Specialist neurology centres were also contacted.

Selection criteria All randomised controlled trials between January 1990 and December 2014 which compared the use of mexiletine for the treatment of myotonia in patients who suffer from myotonic dystrophy and non-dystrophic were included in this review. Primary outcome: reduction of clinical myotonia.

Results Two randomised controlled trials were included for review. Both studies are underpowered; however, there is evidence to support the use of mexiletine for the improvement of clinical myotonia.

Conclusions Larger randomised controlled trials are required, which look at the functional effect of myotonia as a primary outcome (ie, stair test) and the long-term use of mexiletine. This is needed to establish the ongoing efficacy and safety of the long-term use of mexiletine in the management of myotonia.