Myotonia Congenita in Dogs and Cats

Myotonia congenita, a developmental condition, is thought to be an inherited autosomal recessive trait in Chow Chows and Miniature Schnauzers. A similar condition has been observed in several other dog breeds (eg, Labrador Retrievers, Australian Cattle Dogs, Border Collies) as well. Myotonia congenita has also been described in six kittens; the kittens were from separate litters, but the queens were related (1).

The defining feature of myotonia congenita is sustained muscle contraction after voluntary movement has stopped. Abnormal sarcolemmal chloride channels create a hyperexcitable muscle membrane, which results in potassium ion accumulation that sustains muscle contraction.

Clinical signs of myotonia congenita first appear when puppies and kittens begin to ambulate. A stiff gait is typical, and because the pelvic limbs are more affected than the thoracic limbs, "bunny-hopping" is common. Kittens frequently snag their nails on carpeting, and startling them may cause them to fall laterally with their limbs hyperextended. Generalized muscle hypertrophy is typical, and pressing on the muscle can leave an indentation (called a "myotonic dimple").

Clinical signs often appear worse after a period of rest and can be exacerbated by cold temperatures.

Diagnosis of myotonia congenita is by signalment, clinical evaluation, and electromyographic testing. DNA testing by PCR assay is available for Miniature Schnauzers and Australian Cattle Dogs.

Treatment can include membrane-stabilizing agents to help manage clinical signs: procainamide, carbamazepine, tocainide, nifedipine, and mexiletine have all been used in dogs with myotonia congenita. Treatment with these agents, if the dog responds, is lifelong. In cats, treatment by way of environmental modifications is preferred. Such modifications include providing the following: a warm, low stress environment; gentle activity (small amounts of activity can reduce stiffness); rugs or mats to use on slippery floors; and soft food for easy chewing.

Myotonia congenita is not a progressive disease. The prognosis for long-term survival is good; however, the prognosis for sustained clinical improvement is guarded.

For other degenerative/developmental myopathies in dogs and cats, see the Overview for this chapter.