|
NEUROMUSCULAR CASE OF THE MONTH - JUNE 2001
A syndrome of exercise-induced muscle hypertonicity was described several years ago in young Cavalier King Charles Spaniels (CKCS) from the UK (2,4,5). The history of exercise and excitement-induced "collapse" was preceded by a "deer-stalking" action with the head held close to the ground and the bottom high in the air. An increasingly stilted gait was observed until there was finally collapse to one side. An increase in extensor tone of the muscles of all four limbs was evident during the period of collapse. The time period required for recovery of limb function was usually about ten minutes. Mentation was not abnormal throughout the episodes. Electromyographic evaluation showed the muscles were electrically silent. While muscle biopsy specimens were not abnormal at the light microscopic level, rather non-specific ultrastructural abnormalities were observed. A similar syndrome has recently been identified in five CKCS dogs from the USA (three males, two females). Two of the dogs were from Ohio and one each were from Louisiana, Florida, and Illinois (See video clip). Age at clinical presentation ranged from two-seven months. These dogs showed very similar clinical presentations to the dogs originally described from the UK. Where evaluated, neither electrophyiological or histological abnormalities were identified. A syndrome described in humans, hyperekplexia, has a similar presentation in which hypertonia may be elicited by unexpected auditory, visual, or sensory stimuli including fright or stress. (3). These attacks may cause unchecked falling despite retention of consciousness. In some human cases, dramatic although not complete responses to the benzodiazepine drug clonazepam, which enhances gamma-aminobutyric acid (GABA) neurotransmission, have been described. Low levels of CSF GABA have been reported (1). Clonazepam has been used in two affected CCKS dogs with reports of approximately 70-80% improvement suggesting a problem with GABA neurotransmission. Investigators at the Comparative Neuromuscular Laboratory at the University of California, San Diego are interested in being notified of any other possibly affected dogs.
2. Herrtage ME, Palmer AC. Episodic falling in the Cavalier King Charles Spaniel. Veterinary Record 1983;112:458-459. 3. Shiang R, Ryan S, Ya-Zhen Z et al. Mutations in the alpha 1 subunit of the inhibitory glycine receptor cause the dominant neurologic disorder, hyperekplexia. Nature Genetics 1993;5:351-358. 4. Wright JA, Brownlie SE, Smyth JBA et al. Muscle hypertonicity in the cavalier King Charles spaniel - myopathic features. Veterinary Record 1986;118:511-512. 5. Wright JA, Smyth JBA, Brownlie S, Robins M. A myopathy associated with muscle hypertonicity in the Cavalier King Charles Spaniel.
J Com Path 1987;97:559-565.
|