This rare disease, more aptly termed "myotonia congenita," affects the muscles by allowing the continuous contraction of muscle fibers. A painful, hereditary condition, myotonia is the result of chloride channel malformations in dogs, though a small number of cats have also been found to suffer it. The inability of the chloride channels (which allow for proper conduction of the complex electrical impulses from the nerves to the muscles) to do their work means constant contraction of voluntary muscles (like that of most of the skeletal system).
Myotonia congenita, as its name denotes, has been determined to be a recessive trait in certain breeds. This means that dogs with one copy of the aberrant chloride channel gene are carriers and affected dogs have both copies of these genes within their DNA.
In some cases, myotonia has been found to be "acquired," that is, dogs with certain conditions may also suffer chloride channel disturbances by virtue of their primary disease. Cushings disease has been associated with myotonia in dogs, though only rarely.
Symptoms and Identification
Dogs with myotonia exhibit a stiff gait, experience trouble when rising, often suffer swollen tongues and may have difficulty swallowing. Young dogs start to show symptoms as early as of a few weeks of age.
Muscle biopsy and electromyography (or "EMG," a study of electrical impulses in the muscles) have been shown to be helpful in diagnosing the problem, but any dog presenting muscle hyperexcitability (as determined by the above symptoms or specific neurological tests of the muscles) should be tested for it. A DNA test pioneered by the University of Pennsylvania is currently available to screen for both carriers and diseased dogs.
The following breeds are predisposed:
Procainamide, a drug used to treat heart arrhythmias, is an option that has been found to reduce the symptoms in many affected dogs. Apart from this approach, nothing has been found to be effective, including (to this date) treatment of the underlying disease in those who have "acquired" (non-congenital) forms of myotonia. Yet few dogs will ever be free of symptoms altogether and many are euthanized.
The cost of diagnosis is the largest expense owners will face. For muscle biopsies and neurologist-facilitated EMGs, the costs may range from the high hundreds to the low thousands of dollars. The cost of the drug itself is not usually considered a major factor.
Prevention of congenital myotonia is achieved primarily through testing of affected breeds and removing carriers or affected patients from the breeding pool through spay and neuter. A campaign is currently underway by the University of Pennsylvania to test all miniature Schnauzers intended for breeding to completely eliminate this problem in this breed.
1. C. Vite, F. Cozi, M. Rich, A Klide, S. Volk, and R. Lombardo, Myotonic Myopathy in a Miniature Schnauzer: Case Report and Data Suggesting Abnormal Chloride Conductance Across the Muscle Membrane, J Vet Intern Med 1998:12:394-397.
2. C. Vite, J. Melniczek, D. Patterson, and U. Giger, Congenital Myotonic Myopathy In the Miniature Schnauzer Is Inherited As An Autosomal Recessive Trait. Journal of Heredity 90(5): 578-580, 1999.
3. T. Rhodes, C. Vite, U. Giger, D. Patterson, C. Fahlke, and A. George, A Missense Mutation in Canine ClC-1 Causes Recessive Myotonia Congenita in the Dog. FEBS Letters 456(1): 54-58, 1999.
4. American Journal of Veterinary Research; October 2002, Vol. 63, No. 10, Pages 1443-1447doi: 10.2460/ajvr.2002.63.1443.