CNM used to be called Hereditary Myopathy of the Labrador Retriever (HMLR), Autosomal Recessive Myopathy (ARMD), Type II deficiency Myopathy, Labrador Muscular Myopathy, among other names. However, CNM is distinctly different from Dystrophic Myopathy that is found in a variety of breeds as well as being very different from other diseases with myopathy in the name, such as cardiomyopathy.

A scientific article on the disease and the identification of the causal mutated gene can be read by clicking here to visit the Oxford Journals.

CNM is a disease where the muscles of the Labrador do not develop properly. The mutation is present in two copies from birth in affected pups. Affected pups have missing tendon reflexes as well as less weight gain than others by four weeks of age. Caution should be used however to not use these symptoms as a defining diagnosis for CNM, since they could be caused by other factors.

At birth, affected puppies are indistinguishable from their control littermates but from two weeks of age, a progressive significant weight loss is observed. At one month of age, the absence of tendon reflexes is noticed and used as an early and reliable diagnosis. The age of onset of the disabling phenotype varies between 2 to 5 months, with an awkward gait and a decreased exercise tolerance, associated with a generalised muscle weakness.

The more obvious symptoms of CNM appear when a pup is between twelve and twenty weeks of age. The pup will begin to stumble and fall when trying to walk. There are difficulties in swallowing since the muscles in the esophagus are affected. Food can then gets into lungs causing pneumonia. The symptoms are typically worse in cold weather. The pup will never recover from this disabling disease. The symptoms remain for the entire life of the dog.

A number of veterinarians have not had the opportunity to see or diagnose an affected CNM pup. This is because sometimes a breeder might be not familiar with CNM. They might think it is a unique birth defect or caused by some external factor such as poison of some type. Other times breeders might not want others to know that there is CNM in their pedigrees.

There is no cure for the disease. Medications do not improve the condition since there are missing muscle fibers that will never appear.

CNM is a simple recessive trait. The table below describes its inheritance patterns following the classical Mendelian's laws.