Atypical myotonia congenita in a foal

Myotonia congenita is a rare congenital muscle disorder in horses characterized by myotonia, electromyographic changes, mild pathologic changes on muscle biopsy, and normal serum creatine kinase (CK) activity. In the foal described in this case report, exertional rhabdomyolysis and muscle fasciculations without characteristic electromyographic changes were initially found at 3 weeks of age. When the foal was evaluated at 6 months of age, typical electromyographic changes were present that were consistent with myotonia. However, the foal continued to have elevations in serum CK activity and had osseous deformation of the face. Myotonia, persistent elevations in CK activity, and facial dysmorphism are characteristic of Schwartz-Jampel syndrome of humans. A definitive diagnosis of Schwartz-Jampel syndrome, however, could not be made.