Myotonia Congenita in Dogs

Dogs with myotonia congenita will often experience abnormal muscle stiffness and difficulty getting up, as the disease impacts their muscles through repeated contraction of muscle fibers. There is a great deal of laboratory and clinical proof showing that the anomaly causing the condition is located in the muscle cell membrane. This disease can impact dogs, along with humans, horses and goats. Myotonia may also present as an acquired condition. It may be associated with Cushing’s disease, and should your dog be diagnosed with that condition, treatment for it may resolve their symptoms of myotonia.

Myotonia congenita is a rare condition causing muscle hypertrophy in dogs as well as noticeable stiffness upon rising to a standing position.

Dogs that have myotonia congenita will demonstrate clinical signs at a few weeks of age. These include:

• Muscle stiffness
• A strange gait (like a bunny hop)
• Enlarged tongue which may make it more difficult to swallow
• Craniofacial structure anomalies
• Furrowing on several muscles when they are struck

Types 

There appear to be several types of membrane lesions that can lead to the condition of myotonia. In the form that impacts goats, along with at least one form that impacts humans, the disease can be linked to a deficiency in chloride being able to penetrate across the muscle membrane. The deficiency will cause an electrical instability in the membrane that will lead the muscle fiber to have self-generated action potentials after any regularly produced action potential. It is due to these repetitive extra potentials that stiffness develops. 

In the form that affects dogs, horses (and the more severe cases in humans), chloride conductance appears typical; however, the secondary repetitive action potentials take place, leading to the development of muscle stiffness. 

While myotonia congenita is an inherited condition, myotonia can also be acquired, often as a result of a primary condition that your dog is experiencing.

Myotonia congenita is an autosomal recessive disease that can be seen in Cavalier King Charles Spaniels, Chow Chows, Rhodesian Ridgebacks, Australian Cattle dogs, Jack Russell Terriers, Miniature Schnauzers, and Staffordshire Bull Terriers. Chloride channels typically allow proper conduction of electrical impulses from the nerves to the muscles. In cases of myotonia congenita, the chloride channels are lacking this ability. The disease is caused by a myocyte chloride conductance abnormality, which leads to constant contraction or a delay in the relaxation of your dog’s voluntary muscles.  

Some dogs develop myotonia as a secondary condition to a primary disease (for example, Cushing’s disease, an infectious disease or an immune-mediated condition). In these cases, the primary disease that your dog is experiencing may be contributing to chloride channel disturbances.

The veterinarian will diagnose your dog based on the clinical signs he is displaying and the findings of an electromyography (EMG). A muscle biopsy would show mild and nonspecific changes in the muscle.

Should your dog be diagnosed with myotonia, particular if your veterinarian suspects the condition has been acquired, rather than congenital, further evaluation will likely be conducted to be sure that the myotonia is not being caused by another issue that your dog is experiencing.

Treatment options for myotonia include quinidine, mexiletine, and procainamide. All three of these are medications that stabilize the membrane and have been found to reduce the symptoms in dogs with this condition. If it is determined that your dog has Cushing’s disease or another condition that may be leading to the myotonia he is experiencing, the veterinarian will want to treat that condition first. In some cases, the treatment for this other condition may resolve the symptoms of myotonia that are present in your dog. Should there be no resolution or if it is determined that your dog does not have another issue, your veterinarian will look at treating the myotonia with the medications listed above. With medication, your dog will have a good prognosis.

If your dog has been diagnosed with myotonia congenita, you will want to administer the medication recommended by your veterinarian and alert him to any changes you notice in your dog’s condition. Should your dog be diagnosed with Cushing’s disease or another primary condition, it is important to follow the instructions of your veterinarian in order to help the condition of your dog to become stabilized. Ideally, resolving the condition will resolve his myotonia. Unfortunately, that is not always the case. Should he not have a contributing condition or if treatment for the condition does not resolve his myotonia, your veterinarian will recommend treatment specifically for myotonia. It is likely that symptoms will not be eliminated with medication but will be improved. To ensure the best outcome for your dog, you will want to work closely with your veterinarian and attend follow up appointments as necessary.