Muscular Dystrophy – X-Linked in Dogs
In the simplest sense, muscular dystrophy is defined as any degenerative muscular disorder that can be attributed to faulty "nutrition" of muscle cells, fibers and tissues. The muscular dystrophies in domestic dogs are a group of somewhat rare, progressive and degenerative hereditary disorders that involve an abnormality in, or a deficiency of, certain proteins that are critical to coordinated movement, among other things. These are called cytoskeletal proteins. There are a number of different types of muscular dystrophy in mammals. In veterinary medicine, the types seen most frequently are canine X-Linked muscular dystrophy (CXMD), Becker's muscular dystrophy (BMD) and autosomal recessive muscular dystrophy (ARMD). ARMD is especially prevalent in Labrador Retrievers and was formerly called type 2 muscle fiber deficiency. BMD is a fairly mild form of muscular dystrophy that has been reported in the Japanese Spitz. Canine X-linked muscular dystrophy, which is strikingly similar to Duchenne muscular dystrophy in people, has also been called Golden Retriever muscular dystrophy (GRMD) because of its prevalence among male Golden Retrievers. It is probably the most widely seen form of the disease seen in veterinary practices.
Causes of Canine X-Linked Muscular Dystrophy
Canine X-linked muscular dystrophy (CXMD) is a genetic disorder that is caused by an abnormality in a particular location on a particular gene located on the X chromosome. In this disease, certain molecules called cytoskeletal proteins are either absent or are present in insufficient amounts. The cytoskeletal protein most commonly deficient in dogs with CXMD is dystrophin. This is the same protein that is lacking in humans who have Duchenne muscular dystrophy. The genetic abnormality in both people and dogs with this particular disease is caused by the absence of the "Duchenne gene transcript," which is the portion of the gene on the X chromosome that is responsible for coding the production of dystrophin. Dystrophin and other cytoskeletal proteins are essential to normal muscle function and must be present in sufficient amounts to maintain the structural integrity of muscle tissue. They play a key role in the ability of muscle fibers to contract. When muscles lose their contractile capabilities, they cannot work properly, if they can work at all.
Prevention of Canine X-Linked Muscular Dystrophy
Because CXMD is a genetic disorder, most authorities recommend that responsible breeders remove affected animals and their parents from the breeding population. Some experts also suggest not using the siblings of affected dogs for breeding purposes. Other than close management of breeding practices, there is no reported way to prevent dogs from developing X-linked muscular dystrophy.
Owners of dogs with CXMD should tell the breeder of their pet about its condition, so that the breeder has an opportunity to adjust his or her breeding program appropriately. Breeders cannot take steps to eliminate CXMD from the gene pool unless they are aware that it has shown up in one or more of their puppies.
How Canine X-Linked Muscular Dystrophy Affects Dogs
It is difficult to speculate about how a dog that has a severe muscle disorder is affected by the disease. Many puppies born with canine X-linked muscular dystrophy (CXMD) are weak and die shortly after birth. If an affected dog lives, certainly its inability to eat and swallow normally must cause it some degree of frustration or distress. Moreover, aspiration pneumonia is unpleasant under any circumstances, with its associated coughing, sore throat, difficulty breathing and general malaise. The ability of a dog with CXMD to get around (ambulate) normally will be markedly limited throughout its life, which will prevent it from enjoying what most people think of as important companion canine experiences such as running, fetching balls, chasing squirrels, going on long walks and the like. However, in the absence of secondary respiratory difficulties, affected dogs will have lived with their condition since the time they were born and will know no other way of life. One might say that they won't know what they are missing, as long as they are comfortable, pain-free and loved.
Symptoms of Canine X-Linked Muscular Dystrophy
The common denominator among the different types of muscular dystrophies is progressive muscle weakness. Of the various forms of the disease in dogs, canine X-linked muscular dystrophy is the most severe. It mainly affects newborns and young puppies. Dogs with CXMD have stunted growth. Some breeders observe affected puppies having difficulty nursing starting from birth. By as early as 6 weeks of age, but more commonly by 10 to 12 weeks, owners of affected animals notice one or more of the following signs:
Difficulty swallowing (dysphagia)
Difficult grasping food (abnormal food prehension)
Excessive drooling (ptyalism)
Abnormal "bunny-hopping" gait (characteristic trait of CXMD)
Stiff, stilted movement
Outward-turned elbows (front limb abduction)
Inward-turned hocks (hind limb adduction)
Sagging of the back (sway-backed appearance; ventroflexion of the spine)
Muscle atrophy (muscle wasting; especially along the trunk and around the temples of the face)
Enlargement of the tongue (hypertrophy)
Problems eating and swallowing tend to be the first things that owners report in puppies with canine X-linked muscular dystrophy. By about 6 to 9 months of age, the signs of CXMD usually have stabilized. By this time, the puppy's esophagus (the muscular tubular organ that carries food and fluids from the mouth to the stomach), diaphragm (the muscular partition between the chest and abdominal cavities) and heart (also a muscular organ) can become affected by the progressive disease. Many puppies develop a condition called megaesophagus, in which the esophagus becomes abnormally stretched (dilated) and lacks the tone necessary for normal food passage. This can cause food to accumulate in loose pockets along the lining of the esophagus. When the diaphragm is also affected, the dog may develop secondary aspiration pneumonia as a result of: Coughing, Regurgitation, Difficulty breathing (dyspnea; respiratory distress).
When the heart muscle becomes weakened, the dog may develop signs of heart failure, including coughing, dyspnea and abnormal abdominal distension from the build-up of fluid. Heart failure is a very serious potential consequence of CXMD.
While the symptoms of this disease tend to stabilize by one year of age, unfortunately they usually begin to worse at some later time. This is a progressively degenerative disease that rarely stays static for long.
Dogs at Increased Risk
Canine X-linked muscular dystrophy is most commonly reported in male dogs – particularly male Golden Retrievers. However, it has also been seen in male Labrador Retrievers, Samoyeds, Rat Terriers, Miniature Schnauzers, Alaskan Malamutes, Rottweilers, Japanese Spitz, Irish Terriers, Samoyeds, Belgian Shepherds, Belgian Tervurens, German Shorthair Pointers, Groenendaeler Shepherds, Pembroke Welsh Corgis and Brittany Spaniels. It has even been reported in domestic shorthair cats. While males are the primary targets of CXMD, females can also be born with this disease if a female carrier of the genetic mutation is bred to a male that actually has the disease.
How Canine X-Linked Muscular Dystrophy is Diagnosed
Because canine X-linked muscular dystrophy (CXMD) almost always is obvious at an early age, it usually can be diagnosed without much difficulty. Of course, any "sick" puppy brought to a veterinarian will have a thorough physical examination. It probably will also have a urinalysis and routine blood work, including a complete blood count and a serum biochemistry profile. The results of the chemistry profile in dogs with CXMD will show dramatically elevated levels of creatine kinase (CK). CK is an enzyme found in skeletal muscle, cardiac (heart) muscle and smooth muscle; it is also found in tissues of the brain and nerves. CK levels in circulating blood rise after muscle damage. Puppies with CXMD will have elevated CK that can be detected by as early as 1 week of age. CK levels typically plateau at about 100 x normal by 6 to 8 weeks of age.
More advanced testing to confirm the diagnosis of CXMD may not be necessary, if the dog's clinical signs and CK levels are highly suggestive of the disease. However, several confirmatory tests are readily available to most veterinarians. Electromyography can be used to detect abnormal nerve conduction within certain muscle groups after a dog is about 10 weeks old. The attending veterinarian may recommend taking muscle biopsies for submission to a pathology laboratory. The pathologists can look at the tissue samples microscopically and evaluate them through a process known as histopathology. They can also perform a test called immunocytochemistry to look specifically for deficiencies in the levels of the important cytoskeletal protein, dystrophin. Dogs with CXMD will have very low to nonexistent levels of this protein in their muscles.
Chest films (thoracic radiographs/X-rays) may reveal megaesophagus and, if present, evidence of aspiration pneumonia. Both of these conditions frequently occur in dogs with X-linked muscular dystrophy.
Unfortunately, there are no proven treatments for dogs suffering from X-linked muscular dystrophy (CXMD). One of the key goals of managing dogs with this disease is to do whatever is possible to prevent them from developing pneumonia. Aspiration pneumonia often occurs in dogs with CXMD as a result of their inability to chew and swallow properly, and also because of the abnormal functioning of the muscles of their esophagus and diaphragm, which promotes regurgitation. If the dog does develop pneumonia secondary to muscular dystrophy, its veterinarian can prescribe appropriate antibiotics to treat that condition. Anecdotal reports suggest that administration of glucocorticosteroid medications may provide some relief to dogs with CXMD, although the reason for this association is unclear. Otherwise, there is little that an owner or a veterinarian can do to treat this ultimately fatal disorder.
The outlook for dogs with X-linked muscular dystrophy is guarded to grave in almost all cases. The prognosis is worse if the animal develops megaesophagus and/or heart failure – both of which are not uncommon with this disorder. Certainly, if the puppy is only mildly affected by CXMD and survives its first year of life - after which the symptoms of this disease typically stabilize - it may be able to have a good quality of life as an indoor pet for several years. However, because CXMD is a progressive and degenerative disease, the chances of long-term survival are grave.