Mexiletine is a sodium channel blocker indicated in France for the symptomatic treatment of myotonic syndromes (myotonic dystrophies and non-dystrophic myotonias or channelopathies). The Summary of Product Characteristics, however, recalls that mexiletine has anti-arrhythmic properties and “may cause the occurrence of an arrhythmia or worsen a pre-existing arrhythmia diagnosed or not” requiring the establishment of cardiological checkups with electrocardiogram before and during the treatment.
A study of the long-term effects of mexiletine was conducted in Italy between 1999 and 2019. Offered to 112 patients with non-dystrophic myotonia aged 2 to 78 years, the treatment was finally followed by 59 patients (i.e. 52% ) in relation to the severity of the myotonia and the discomfort it caused. During a follow-up ranging from one month to 20 years, the drug was well tolerated. No patient developed an arrhythmia. Half of the participants reported no side effects. The others complained most often of dyspepsia. Treatment was however stopped by a quarter of participants (22%) who preferred flecainide, considered that they did not need mexiletine, following the occurrence of uncontrolled dyspepsia or due to pre-existing gastric cancer.
These results confirm those of another retrospective study carried out on 63 patients suffering from channelopathy for which the authors had concluded that the long-term tolerance of mexiletine was good and stressed the need to gradually increase the doses of mexiletine when starting on the way to treatment and to accompany it with anti-dyspeptic treatment.
