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Case summary A 2-year-old female intact domestic shorthair cat was referred to the neurology service at the Foster Hospital for Small Animals as a result of lifelong weakness, seizure-like episodes after excitement, muscle spasms, stiffness of the limbs and shortness of breath. A diagnosis of myotonia congenita (MC) was made based on compatible history, clinical signs and electromyography findings, and excluding other causes of muscle weakness using muscle biopsy, baseline bloodwork and echocardiography. Initial treatment with procainamide had overall low efficacy. The cat was then switched to carbamazepine, which provided long-term improvement in clinical signs at a dosage of 7.4 mg/kg PO q12h with no observed clinical side effects. Relevance and novel information To date, there is only a single case report describing the successful treatment of MC in cats, and no report describing the use of carbamazepine to treat this condition in this species. This anti-convulsant medication was shown to be anecdotally effective in the case reported. Further research is needed to categorize the metabolism, pharmacokinetics, tolerance, dose range and efficacy of carbamazepine in cats with and without MC.