Exercise intolerance in retrievers - Veterinary Medicine
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Exercise intolerance in retrievers
When a dog from one of the popular retriever breeds is brought to you because it collapsed while exercising or seems to tire easily, you need to sift through the many potential underlying causes. It could be anything from an out-of-shape weekend athlete to a dog with an inherited metabolic myopathy. Here are some of the conditions to consider.


VETERINARY MEDICINE


Polymyositis

Polymyositis is a generalized inflammatory myopathy presumed to have an immune-mediated basis.45 Large-breed middle-aged dogs are most commonly affected.49,50 Affected dogs are usually profoundly weak, reluctant to exercise, and may have a stiff, stilted gait.50 Conscious proprioception and limb reflexes are normal. Atrophy typically occurs in the appendicular muscles and the muscles of mastication.45,49,50 Muscle pain is uncommon.50 Dysphagia, megaesophagus, and regurgitation occur in about 10% of affected dogs.49 Elevated serum creatine kinase (CK) activity is seen in most affected dogs at rest, and even more dramatic increases are common following exercise.45 The diagnosis is based on clinical signs, CK measurement, electromyography, and muscle biopsy results consistent with inflammatory muscle disease.45,49,50 Attempts should be made to rule out tick-related diseases, Toxoplasma gondii, and Neospora caninum using serology and immunohistochemistry on muscle biopsy samples.49,50 Noninfectious polymyositis is most often seen as a primary immune-mediated disorder, but it can also occur as a component of systemic lupus erythematosus, as a paraneoplastic syndrome associated with lymphoma or other tumors, or as a complication of drug administration.49,50 Treatment of primary immune-mediated polymyositis consists of immunosuppressive therapy with a good prognosis for recovery in dogs without aspiration pneumonia.45,50 Marked muscle atrophy and fibrosis due to chronically untreated polymyositis may not resolve, so it is important to confirm the diagnosis and initiate appropriate treatment early in the course of disease.

Inherited muscle disorders


Table 5. Clinical Comparisons of Canine X-linked Muscular Dystrophy (CXMD), Centronuclear Myopathy (CNM), and X-linked Myotubular Myopathy (CX-MTM) in Retrievers
A number of breed-associated inherited myopathies have been described in retrievers. These disorders cause progressive muscle weakness and exercise intolerance that becomes apparent within the first six months of life. The inherited myopathies that have been described in retrievers include dystrophin-deficient muscular dystrophy, centronuclear myopathy, and, most recently, X-linked myotubular myopathy (Table 5).

Muscular dystrophy. The muscular dystrophies are a group of inherited myopathies characterized by progressive degeneration of skeletal muscle.45,51 A condition similar to Duchenne muscular dystrophy (DMD) in people is the most common form of muscular dystrophy in dogs.51,52 Affected dogs are missing a crucial cytoskeletal protein, dystrophin, in their skeletal and cardiac muscle. The dystrophin gene is located on the X chromosome, so dystrophin-deficient muscular dystrophy is an X-linked trait that is clinically apparent in male dogs, while heterozygous females are asymptomatic carriers.51,52 Canine X-linked muscular dystrophy (CXMD) has now been reported in many breeds, but it is best described in the golden retriever and has been reported in one male Labrador retriever puppy.51,53

The severity of clinical expression of CXMD in golden retrievers is variable.54 Puppies are often stunted even before weaning. Abduction of the elbows, a bunny-hopping gait, and difficulty opening the mouth may be noted.45 With time, affected puppies develop a progressively more stilted gait; exercise intolerance; a plantigrade stance; carpal hyperextension; atrophy of the truncal, limb, and temporalis muscles; and muscle contractures.45,51,52,54 Proprioceptive positioning and spinal reflexes are normal, but spinal reflexes may be difficult to evaluate once muscle fibrosis and joint contractures occur.45,51 Tongue hypertrophy is common, and most severely affected dogs develop pharyngeal or esophageal dysfunction.45,52,54 Dilated cardiomyopathy leading to cardiac failure may occur.52 Dysphagia, lingual hypertrophy, and inspiratory stridor were prominent features in the Labrador retriever puppy with CXMD.53


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Source: VETERINARY MEDICINE,
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