Hereditary Muscle Disease (Non-inflammatory Myopathy in Labrador Retrievers) in Dogs

Labrador Retrievers can inherit a muscle disease called Centronuclear Myopathy (CNM). This is a non-inflammatory disease that causes weakness and deficiency in type II muscle fibers. On a cellular level, it is characterized by an excessive number of internalized nuclei in the muscle cells. First discovered in 1976, CNM has since been studied enough to identify that it is inherited as an autosomal recessive trait in Labrador Retrievers, especially black and yellow labs. A similar disease has also been found in Great Danes and Bouviers Des Flandres. Animals carrying the mutation will typically start to show symptoms between 8-12 weeks of age, but occasionally onset can be delayed as late as six months. The most notable sign is an abnormal gait or a “bunny-hopping” tendency in the back legs. Dogs also have generalized muscle weakness that often becomes more remarked with cold temperatures or excitement. The disease gets progressively worse over the first year of the dog’s life, but after about twelve months of age symptoms will taper off and some animals may even see improvement. There is no cure, but many Labradors with mild CNM can live a normal life although they may be somewhat weaker and have a reduced ability to exercise. Very severe symptoms can be difficult to manage and rarely dogs may develop megaesophagus, weakness and overstretching of the esophageal muscles, as a complication.

Inherited muscle disease can cause weakness and abnormal gait in Labrador Retrievers. This is called centronuclear myopathy. It usually becomes apparent in the first year of a dog’s life.

These are some of the sign you might see in a Labrador puppy with CNM.

• Abnormal (bunny-hopping) gait
• Arched back
• Abnormal stance
• Flexion of the neck and cervical spine (ventroflexion)
• Stunted growth
• Atrophied or shrunken muscles
• Generalized muscle weakness (worse with cold, exercise, or excitement)
• Decreased exercise tolerance
• Difficulty swallowing and aspiration pneumonia (megaesophagus)

Types

There is only one type of CNM. Other inherited conditions in Labrador Retrievers that could produce similar symptoms include Exercise Induced Collapse (EIC), which is related to a neurological condition and usually takes the form of an attack during strenuous exercise, and X-linked myotubular myopathy which is found only in males and has more severe symptoms.

CNM is inherited as an autosomal recessive trait, so affected animals will carry the gene from both parents. Dogs inheriting the disease from only one parent will be asymptomatic, but they can still pass CNM on to their offspring.

CMN is diagnosed symptomatically and through ruling out other diseases that could cause similar symptoms. The veterinarian will physically examine your dog and take blood and urine tests to evaluate for infection. Creatine kinase may be slightly elevated on a blood test. Since this is a hereditary condition, your dog’s breed, age, and family history will all be important.

Definitive diagnosis is made with electromyography and muscle biopsies. Electromyography will examine muscle strength during stimulation, especially in the front limbs and jaw muscles. Dogs with only mild symptoms may show little or no abnormality with electromyography. Microscopic examination of a sample obtained through a muscle biopsy will reveal weakness and atrophy in the fibers and as well, an increase in the cells’ central nuclei. A muscle biopsy is an invasive procedure that involves inserting a hollow needle in the muscle to obtain a tissue sample. DNA testing can also reveal whether Labrador Retrievers carry the mutation that causes CMN. This can be an important diagnostic test before an animal is bred, especially if one or more sibling has CMN.

There is no effective treatment for CMN. Keeping warm temperatures in your house can help to reduce the severity of symptoms. L-carnitine supplements have also been found to increase muscle strength in some dogs and may be recommended to help manage the condition. Clinical signs may also be reduced with diazepam, however, this medication will have side effects with long-term use. Some dogs show a marked improvement as they reach adulthood, so treatment may only be necessary in the first year of life.

Mild cases of CMN are usually manageable by reducing exercise and eliminating cold temperatures. Many dogs with CMN still make good pets although some lifestyle changes may be necessary. Very severe symptoms could limit your dog’s life, especially if megaesophagus develops as a complication. This can sometimes be managed with diet change and improved feeding posture, but aspiration related pneumonia becomes an increased risk. Very severely affected dogs could need to be euthanized, but this is rare with CMN.