Myotonia congenita is an inherited muscular disease found in Miniature Schnauzers.

There have been reports of a newly identified disorder in Miniature Schnauzers that has been variously labeled as “myotonia,” “myotonia congenita,” and “myotonic myopathy.” This article refers to the disorder as myotonia congenita, the term the researchers at the School of Veterinary Medicine of the University of Pennsylvania are currently using, based on the clinical signs, the cause, and the evidence that the disease is inherited.

Most of the information contained in this article comes from Dr. Charles Vite at the University of Pennsylvania. Dr. Vite has done extensive work on the disorder and has been of immense assistance in providing information to us. Dr. Vite and his associates have made remarkable progress in a very short time in developing a treatment for myotonia. Dr. Vite, Dr. Urs Giger of the University of Pennsylvania’s Josephine Deubler Genetic Disease Testing Laboratory, and other colleagues at the University of Pennsylvania and Vanderbilt University, likewise made great strides in a very short time. They have identified a mutated gene responsible for causing the disorder, and have developed a DNA test capable of determining whether a dog is affected with myotonia, a carrier, or “clear” of myotonia. Click here for information about testing your dogs.

Symptoms
Myotonia congenita is characterized by the delay of skeletal muscle relaxation following the cessation of a voluntary activity. The delay in skeletal muscle relaxation is not accompanied by pain or cramping.

Affected animals have significant excessive growth of the muscles (muscle hypertrophy). They also have a stiff, stilted gait that improves with exercise. Difficulty in swallowing, excessive salivation and an abnormal bark are often present. All affected Miniature Schnauzers examined so far at the University of Pennsylvania also have overshot upper jaws (maxilla) and abnormal dentition, but that is not typical of myotonia in other breeds of dogs and in other species.

Causation and Inheritance
Dr. Vite reports that the researchers have documented that the disorder is inherited as a simple autosomal trait. A single mutation in the gene encoding the chloride channel for skeletal muscles appears to be responsible for the clinical signs seen. Affected dogs are homozygous for this mutation, i.e., they inherited the mutated gene from both parents. Carrier dogs are heterozygous for the mutation, i.e., they inherited the mutated gene from one parent and a normal one from the other parent. The research on mode of inheritance is based on five litters examined at the University of Pennsylvania and two additional litters reported to the researchers that they were unable to examine.

Incidence
We do not know how many litters there have been to date with affected puppies. The disorder does not seem to be widespread as yet. So far, the researchers at the University of Pennsylvania have examined 10 affected dogs in the Philadelphia area. We are advised that there have been some affecteds elsewhere as well.

Treatment
There is no known cure for the disorder. Dr. Vite reports that procainamide (a drug used in humans for heart arrhythmia) administered at a dose of 40mg/kg every 8 hours results in some clinical improvement, allowing the animals to walk with greater ease and to eat without difficulty. He and his colleagues are working actively to develop therapies that may improve the life of affected dogs.

Information for Veterinarians
Dr. Vite has kindly agreed to speak with veterinarians who are treating Miniature Schnauzers they believe to be affected with myotonia congenita. We ask breeders not to contact Dr. Vite directly because doing so will absorb excessive amounts of his time.

Breeders of affected puppies may wish to refer their veterinarians to the following three articles:

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.

Recommendations Regarding Breeding
The Health Committee and Drs. Vite and Giger recommend that all Miniature Schnauzers be tested before being bred. Click here for further details on how to test your Miniature Schnauzer for Myotonia.

This article was prepared by the Health Committee of the AMSC based on information received from Dr. Charles Vite and Dr. Urs Giger. Dr. Vite reviewed the information contained herein for accuracy.

Charles H. Vite, DVM, is a Diplomate of the ACVIM in Neurology and practices at the School of Veterinary Medicine of the University of Pennsylvania.

Urs Giger, PD Dr.med.vet. FVH, is a Diplomate of the ACVIM and of the ECVIM (Internal Medicine), and is the Charlotte Newton Sheppard Professor Of Medicine, and Chief, Section of Medical Genetics, at the School of Veterinary Medicine of the University of Pennsylvania.

Copyright, American Miniature Schnauzer Club, Inc., 1999, 2000.

Last Updated: March 2, 2000