(Originally posted October 25, 2012 - Updated November 18, 2022)
Centronuclear Myopathy (CNM) is an inherited congenital disorder that causes muscle dysfunction.
In a nutshell, CNM in Labradors causes muscle wasting which makes moving normally difficult. Some dogs have severe symptoms while other dogs are only mildly affected.
In the past, it's been called generalized muscle weakness, polyneuropathy, muscular myopathy, muscular dystrophy, hereditary myopathy.
Breeds affected are Labrador Retrievers, plus doodles and other Lab mixes.
Here is a video of a Labrador suffering from acute CNM.
To develop a genetic test, researchers collected samples from 7,426 Labradors living in 18 countries. They found 80 dogs from various countries had two copies of the mutated gene. These dogs all showed muscle wasting symptoms. Of the 1,172 dogs that had one copy of the mutated gene, none showed symptoms which showed that CNM in Labradors is caused by a simple recessive gene.
Dogs with two normal genes are normal. Dogs with one normal gene appear normal, but could pass the mutated gene to their puppies. Dogs with two mutated gene are affected and will develop muscle wasting.
Currently researchers believe the mutated gene is the result of a single mutation. Also that the gene spread rapidly through the Labrador population because many people bred to a small group of popular sires about 60 years ago.
CNM also affects people. Locating the mutated gene in Labradors is helping scientists study the disorder in people.
Symptoms generally develop between two to five months of age and include gait abnormalities, generalized weakness, tiring easily and muscle wasting. CNM can affect both sexes and all three recognized colors.
Puppies seem normal at first, but as they grow they develop generalized muscle weakness and abnormal postures. Another sign is an odd "bunny hop" with the rear legs. Often they can't exercise much and may collapse in colder weather.
Here is a video of another Lab having a particularly tough day although she's determined to get her bumper.
Affected dogs will never develop normal muscles. You can see the difference most clearly in the chest and rear legs.
There is no cure. However some dogs, with their owner's help, have learned how to overcome most symptoms. This dog is doing quite well.
According to his owners, "As a puppy, he couldn't hold his head up, control his tongue, hold a tug toy, walk for more than a few feet without having to crawl, or climb stairs."
PREVENTING CNM IN LABRADORS
CNM is caused by a recessive gene which means a dog can have one copy of the gene and appear normal. If that dog is bred to another dog that appears normal, but has the same recessive gene, some of the puppies may be affected with CNM.
CNM affected dogs should not be bred as all of their puppies will carry the gene. Because of the muscle dysfunction, it may be difficult for a male to breed naturally and even harder for a female to carry and whelp puppies.
Genetic testing is the best way to prevent producing a dog with CNM. Many laboratories offer the testing, but my favorite is DDC. They are available to answer questions, can help order the correct tests and are reasonably priced. They can also provide the swabs needed to collect DNA.
To collect your dog's DNA, make sure he hasn't eaten or had anything to drink for at least an hour. You may need to put a leash on your dog and/or have a helper handy to keep him still. The collection doesn't hurt, but some dogs may object anyway.
Wash your hands and pull out one swab at a time. Lift your dog's lip on the side of his muzzle and insert the swab between his lip and gums. Swirl several times and remove. Place the swab back in the plastic wrap. Repeat on the other side of his muzzle and then a third time on one side or the other.
Write your name, your dog's name, breed and registration number on an envelope and place the swabs inside. Seal and place the envelope inside another envelope, seal, address and mail.
If you are collecting samples from more than one dog, be sure to keep their swabs separate and wash your hands between collections.
And, no, CNM is not the same as Exercise Induced Collapse (EIC), although both disorders are genetic. CNM is a muscle weakness whereas EIC is a neurologic dysfunction.