NEUROMUSCULAR CASE OF THE MONTH - MAY 2006

Hereditary myotonia in an Australian Cattle Dog
Contributed by Dr. Brad Hanna, Roberto Poma & Andrew Bendall
Ontario Veterinary College, University of Guelph
Guelph, ON, Canada

Clinical History
A 3-year-old FS Australian Cattle Dog was presented to the referring veterinarian with a life-long history of muscle hypertrophy and stiff gait. Physical examination revealed marked hypertrophy of the skeletal musculature, particularly in the limbs and neck. The owners reported that the dog would fall at the onset of exercise and stay down for several seconds, after which it was able to exercise stiffly. The dog was referred to the University of Guelph for diagnostic workup.

Physical and Neurological Examination
A neurological examination revealed generalized muscle stiffness. Following rest the dog was stiff in all limbs, moving with a “bunny hop” gait for the first 2 or 3 steps. Stiffness subsided but did not disappear with exercise. Dimpling of the triceps and anterior tibial muscles was clearly visible on percussion of the muscle bellies. Congenital myotonia was suspected and further testing done.

Diagnostic Testing
Serum biochemical abnormalities included a mild elevation of creatinine, AST and ALT, and high CK. Electromyography of the gastrocnemius, tibialis cranialis, biceps femoris, triceps, extensor carpi radialis and biceps muscles revealed diffuse, typical myotonic discharges. Histological examination of a semitendinosus muscle biopsy showed a moderate variability in myofibre size with scattered atrophic fibres having a round shape and numerous subjectively hypertrophic fibres. Fibre type grouping was not observed. Small numbers of necrotic fibres were present undergoing phagocytosis. Intramuscular nerve branches were normal in appearance and no other abnormalities were noted. Myofibre hypertrophy without other significant pathological abnormalities is consistent with myotonia and not with forms of muscular dystrophy.

Clinical Course
The dog was left untreated. At four years of age her owners reported that the muscle stiffness had progressed, but that the dog did not appear to be in pain at any time. When chasing a kicked soccer ball from a standstill following a short period of rest, the dog’s muscles would stiffen severely after a few strides so that she either rolled sideways with limbs rigidly extended or performed an involuntary summersault (see online supplementary video).

Comments
The history, neurological abnormalities, electromyographic findings, and histopathology results were consistent with a diagnosis of myotonia. We subsequently identified a novel mutation in the skeletal muscle chloride channel gene CLCN1 in this dog. This mutation truncates the protein near the carboxy terminus, with the loss of 88 amino acids. The affected dog is homozygous for this mutation, which we have not identified in any unrelated dog. A DNA-based blood test is now available through the Animal Health Laboratory at the University of Guelph.

References
Finnigan DF, Hanna WJB, Poma R, Bendall AJ. A novel mutation of the CLCN1 gene associated with Myotonia Hereditaria in an Australian Cattle Dog. J. Vet. Int. Med. In press – Vol. 21(3), May 2007.

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