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.


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Inherited neurodegenerative disorders

Breed-related degenerative disorders affecting the nervous system have been described in many dog breeds, including retrievers. Clinical presentations and neurologic findings reflect the nervous system tissues affected. Littermates and closely related litters are often affected, suggesting a genetic cause. Diagnosis usually requires postmortem histologic examination of nervous tissues.

Sensory ataxic neuropathy is a disorder affecting young male and female golden retrievers in Sweden.77 Affected dogs are ataxic, have profound postural reaction deficits, and have reduced or absent spinal reflexes without muscle atrophy.77,78 Signs begin before 6 months of age and are slowly progressive. Sensory nerve conduction velocities are slowed, and histologic examination reveals a chronic progressive central and peripheral sensorimotor axonopathy that is most pronounced in the proprioceptive pathways.77 A causative mutation in the mitochondrial genome was recently identified.78

Central axonopathies have been described in Labrador retrievers and in golden retrievers.79,80 Affected Labrador retrievers show pelvic limb incoordination by 5 to 6 weeks of age, progressing to hypermetria and a severe spastic ataxia affecting all four limbs.79 Most pups are nonambulatory by 3 to 5 months of age. Severe degeneration of spinal cord white matter occurs and is most pronounced in the spinocerebellar tracts in the thoracic spinal cord, medulla, cerebellar peduncles, and cerebellum.79 The axonopathy and neuronopathy reported in golden retrievers also affects pups in the first few months of life, but the signs are of severe lower motor neuron dysfunction without ataxia.80 Pelvic limb weakness progressing to tetraparesis, severe muscle atrophy, and mild hyporeflexia but normal proprioceptive positioning reflect the involvement of motor neurons in the ventral grey columns of the spinal cord.80

A number of breed-associated degenerative polyneuropathies have been reported.81 Most of these cause progressive lower motor neuron dysfunction in young dogs, with severe tetraparesis, muscle atrophy, and hyporeflexia. In some breeds, concurrent laryngeal paralysis occurs.81 Diagnosis requires electrophysiologic evaluation of nerve function and nerve biopsy.

Acquired spinal cord disease

Progressive weakness or incoordination can result in reluctance to exercise or in an altered gait. Perform a complete screening neurologic examination in all dogs with a history of exercise intolerance, and use abnormal findings to localize a lesion. Retrievers are susceptible to a wide variety of acquired spinal cord disorders including fibrocartilagenous embolism, acute and chronic intervertebral disk disease, infectious meningomyelitis, granulomatous meningoencephalitis, neoplasia, and degenerative myelopathy.13,82 Medical evaluation may include screening for infectious disease and systemic neoplasia, radiographs of the spine, and advanced imaging such as myelography, computed tomography (CT), or MRI.13,82 Genetic DNA tests for degenerative myelopathy are becoming available for a number of breeds, including Chesapeake Bay and golden retrievers (Orthopedic Foundation for Animals, Columbia, Missouri: http://offa.org/dnatesting/).

Weakness and ataxia resulting from chronic spinal cord compression can worsen with exercise, particularly if compression exists in the cervical region where excessive motion occurs during walking and running. Intervertebral disk protrusion, articular cysts, neoplasia, or the vertebral malformations and soft tissue proliferation associated with cervical spondylomyelopathy (canine wobbler syndrome) can cause spinal cord compression.13,82

Pain may cause the reluctance to exercise in some dogs. Painful disorders of the spine may include diskospondylitis, intervertebral disk extrusion or protrusion, vertebral body neoplasia, nerve root neoplasia, polyarthritis, and meningitis.13

Acquired peripheral neuropathies

Polyneuropathies result in muscular weakness that may manifest as reluctance to exercise or exercise intolerance. These disorders typically cause generalized lower motor neuron signs that include flaccid muscle weakness or paralysis, marked muscle atrophy, decreased muscle tone, and reduced or absent reflexes.81,82 Chronic polyneuropathies can be seen in association with metabolic disorders such as hypothyroidism and diabetes mellitus.39,40,82,83 They can also occur as a paraneoplastic syndrome in dogs with insulinoma or other cancers.82

Polyneuritis that is steroid-responsive can be seen as an idiopathic immune-mediated disorder or in association with systemic lupus erythematosus. Demyelinating polyneuropathies with no known etiology and no effective treatment also occur. Suspect a polyneuropathy based on clinical findings, and confirm the diagnosis with electrodiagnostic testing and nerve biopsy. Treatment and prognosis depend on the underlying cause.82

DIAGNOSTIC EVALUATION

Diagnostic evaluation of a retriever with exercise intolerance will vary based on the description of the event or events, the physical examination findings at rest, and initial diagnostic test results. When examination at rest reveals abnormalities such as pallor, stridor, muffled heart sounds, a cardiac arrhythmia, or severe muscle atrophy, the diagnostic approach required to further investigate the underlying problem becomes obvious. When no abnormalities are identified at rest, systematically evaluate the dog to rule out metabolic, respiratory, and cardiac causes of exercise intolerance before considering muscular, neuromuscular, and neurologic disorders that require specialized testing. Once the cause of exercise intolerance has been determined, strategies for treatment or management can be developed.

Editors' note: Dr. Taylor is a patent owner of the genetic test for EIC and receives a portion of the proceeds from this test.

Kevin L. Cosford, DVM, MVetSci, DACVIM
Susan M. Taylor, DVM, DACVIM
Department of Small Animal Clinical Sciences
Western College of Veterinary Medicine
University of Saskatchewan
Saskatoon, SK S7N 5B4 Canada


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Source: VETERINARY MEDICINE,
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