Skeletal Muscle Disorder (Myotonia) in Dogs

The chloride ion channels within muscle cell membranes in healthy animals act as conduits for the electrical signals that direct muscles to action, mainly to contract and relax. The muscles in a dog with myotonia congenita have defective chloride channels that block those signals. Muscles that cannot relax become stiff. This rigidity occurs in episodes that last less than a minute while the muscle is slowing relaxing. During this time, an affected dog may shake, shiver, and be unable to move or get up, and injuries can occur from falling. Increased muscle stiffness can be seen from inactivity, while relief can come through exercise.

Myotonia congenita is the medical term for the rare genetic skeletal muscle disorder in which a gene mutation prevents a muscle’s ability to relax after contracting. Muscles become hyperexcitable and can contract easily, but become abnormally stiff and slow moving to relax. While this condition does not seem to do any damage to the muscle itself, muscle rigidity can cause falling, injuries, an inability to stand, and can lead to a fatality due to a progressive deterioration. Treatment can control symptoms, but symptoms often reoccur.

Symptoms of a skeletal muscle disorder begin in puppies a few weeks of age. They include:

• Stiffness in limbs
• Muscle stiffness, especially after vigorous exercise
• Increase in muscle size
• Stiff gait
• Bunny hop gait while running
• Falling due to stiffness
• Shaking or shivering during episodes
• Inability to move during episodes
• Difficulty standing
• Prominent muscles in shoulders and thighs
• Enlarged tongue
• Stiffened tongue when touched
• Peak-shaped lower jaw
• Hair loss

The cause of myotonia congenita is believed to be an inherited autosomal recessive or dominant gene that causes a mutation in the chloride channel. There have been 80 mutations discovered in the chloride channel gene, referred to as CLCN1. These cause problems in the electrical conductivity in the channel, compromising skeletal muscle function by temporarily preventing the muscle from relaxing after contraction. Affected dogs have two copies of the mutated gene, while dogs with one copy are considered carriers. It is also theorized that this condition could be related to or caused by hyperadrenocorticism or Cushing’s disease.

While myotonia mainly affects Miniature Schnauzers, other breeds reported with the condition include:

• Australian Cattle Dog
• Maltese mixed breeds 
• Chow Chow 
• Staffordshire Bull Terrier 
• West Highland Terrier
• Great Dane 
• Jack Russell Terrier
• Samoyed
• Labrador Retriever
• Poodle
• Cocker Spaniel
• Ridgeback

Diagnosis is based on symptoms and the results of testing. Palpation of limbs can show an increase in muscle tone. A blood test can be used to detect myotonia in Miniature Schnauzers and Australian Cattle Dogs. A muscle biopsy can detect muscles changes, including fiber size, fiber atrophy, fatty infiltration, muscle degeneration, and necrosis of the tissue. An EKG is used to analyze electrical muscle discharges, called myotonic discharges, and can confirm a diagnosis of myotonia.

Other tests can include a urinalysis, X-rays, ultrasounds, neurological testing for cranial nerve abnormalities, and an adrenocorticotropic hormone stimulation test. Hyperadrenocorticism and Cushing’s disease are often tested for as they could occur concurrently, or be the underlying cause.

Treatment usually begins by correction of any underlying problem. If hyperadrenocorticism is present, it can be treated with trilostane. Treatment for an endocrine condition can sometimes help symptoms. If an endocrine issue is not related, or treatment elicits no change in symptoms, is can be a sign that the success of therapy will be minimal.

Prescribed medications will attempt to stabilize muscle fibers, and can include phenytoin, quinidine, and procainamide. Trilostane may be used alone, or in addition to phenytoin. L-carnitine supplementation may help to alleviate muscle weakness in metabolic cases.

Myotonia can persist despite treatment, and symptoms can remain present or worsen. In some cases, myotonia can eventually lead to other health complications and death.

Even with treatment, myotonia can progress to an inability to stand and subsequent deterioration, which can be fatal. Treatment can give adequate control, but more often results in a progression of symptoms. 

Prevention for genetic diseases through selective breeding is critical, and a DNA test is available that can detect the defective gene in affected or carrier Miniature Schnauzers and Australian Cattle Dogs. Currently, there are no tests available for other breeds.