Long term safety and efficacy of lamotrigine in patients with non-dystrophic myotonia, a single-centre prospective study.

The non-dystrophic myotonias are rare genetic conditions characterised by delayed muscle relaxation, muscle stiffness, pain and fatigue. This study examines the experience of the UK National Referral Centre in using lamotrigine to treat non-dystrophic myotonias. A prospective cohort of 37 patients with genetically confirmed non-dystrophic myotonias (23 with CLCN1 variants and 14 with SCN4A variants) who were prescribed lamotrigine as part of routine clinical care were followed for a mean of 26 months. Treatment efficacy was assessed using the Myotonia Behaviour Score. 26 participants were included in the efficacy analysis where lamotrigine was found to reduce the mean Myotonia Behaviour Score from 3.3 to 1.8 (p < .001, Cohen's d = 1.2), demonstrating significant symptom improvement. Side effects, mainly rash and headache, led to discontinuation in nine out of 37 patients, but no severe adverse events occurred. In most cases, lamotrigine was chosen due to prior side effects with mexiletine or cardiac contraindications to mexiletine and proved to be an effective alternative. This study provides real-world evidence supporting the long-term use of lamotrigine and the integration of lamotrigine into personalised treatment strategies for non-dystrophic myotonias.