Paradoxical pseudomyotonia in English Springer and Cocker Spaniels

BACKGROUND: Paramyotonia congenita and Brody disease are well-described conditions in humans, characterized by exercise-induced myotonic-like muscle stiffness. A syndrome similar to Brody disease has been reported in cattle. Reports of a similar syndrome in dogs are scarce. OBJECTIVES: To define and describe the clinical, diagnostic, and genetic features and disease course of paradoxical pseudomyotonia in Spaniel dogs. ANIMALS: Seven client-owned dogs (4 English Springer Spaniels and 3 English Cocker Spaniels) with clinically confirmed episodes of exercise-induced generalized myotonic-like muscle stiffness. METHODS: Sequential case study. RESULTS: All dogs were <24 months of age at onset. The episodes of myotonic-like generalized muscle stiffness always occurred with exercise, and spontaneously resolved with rest in <45 seconds in all but 1 dog. Extreme outside temperatures seemed to considerably worsen episode frequency and severity in most dogs. Complete blood count, serum biochemistry including electrolytes, urinalysis, brain magnetic resonance imaging, cerebrospinal fluid analysis, electromyography, motor nerve conduction velocity, ECG, and echocardiography were unremarkable. Muscle biopsy samples showed moderate but nonspecific muscle atrophy. The episodes seemed to remain stable or decrease in severity and frequency in 6/7 dogs, and often could be decreased or prevented by avoiding the episode triggers. The underlying genetic cause is not identified yet, because no disease-causing variants could be found in the coding sequence or splice sites of the 2 major candidate genes, SCN4A and ATP2A1. CONCLUSIONS AND CLINICAL IMPORTANCE: Paradoxical pseudomyotonia is a disease affecting Spaniels. It is of variable severity but benign in most cases.