Equine muscular dystrophy with myotonia

Objectives: To describe a case of equine muscular dystrophy with myotonia. Methods: A 5-year-old horse presented with hypertrophy and delayed relaxati on of the muscles of the hindlimbs from age 2 months. Testicular atrophy de veloped from 2 years of age. Action and percussion myotonia was associated with weakness in these muscles, and EMG showed diffuse myotonic discharges and myopathic features. Biopsy of the gluteal muscle showed adipose and con nective tissue infiltration, marked variation in muscle fibre size, and mot h-eaten, ring and whorled fibres. Results: Injection of apamin, a peptide blocker of calcium-activated potass ium channels, which inhibits myotonia in human myotonic dystrophy. was inef fective in blocking myotonic discharges. Discharges promptly abated with 2% lidocaine injection. Conclusions: Myotonia in this horse is associated with dystrophic changes s imilar to human myotonic dystrophy, though there are some pharmacological d ifferences. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.