Exercise-induced collapse in Labrador Retrievers
Q: Please provide an updated review on exercise-induced collapse in Labrador Retrievers.
A: Dr. Susan M. Taylor gave an excellent lecture on "Exercise-Induced Collapse in Labrador Retrievers: An Update" at the 2008 American College of Veterinary Internal Medicine Forum in San Antonio. Here are some relevant points:
A syndrome of exercise-induced collapse (EIC) in Labrador Retrievers was first described in the veterinary literature in 1993. During the last 15 years, EIC has been recognized with increasing frequency by veterinarians, dog trainers and owners.For years veterinarians evaluated affected dogs for episodes of collapse, and speculated that their episodes were due to heat intolerance, low blood sugar, cardiac arrhythmias or possibly metabolic myopathies.
Now that it has been established as a specific disorder, it is evident that EIC is the most common reason for exercise intolerance and collapse in young, apparently healthy Labrador Retrievers.
Affected dogs can tolerate mild to moderate exercise, but occasionally become ataxic and collapse after five to 15 minutes of participation in trigger activities involving strenuous exercise.
A comprehensive investigation has been ongoing for nearly a decade, involving investigators from the University of Saskatchewan, the University of Minnesota and the Comparative Neuromuscular Unit at the University of California.
A survey of owners of Labrador Retrievers with presumed EIC was performed. To participate in the survey, dogs had to be purebred and registered, have experienced at least three episodes and veterinary evaluation must have ruled out systemic and cardiac causes of exercise intolerance or collapse. Over 350 completed surveys were received, and 225 met the survey criteria.
Clinical and laboratory evaluation of 14 dogs with EIC at rest and during and following completion of a standardized strenuous-exercise protocol was compared with findings from 14 Labrador Retrievers without EIC performing the same exercise.
Physical examination, neurologic examination, thoracic radiographs, ECG, echocardiography, CBC, biochemical profile and acetylcholine receptor antibody were performed on each dog at rest. Plasma and urine carnitine were measured.
Dogs retrieved a soft plastic dummy repeatedly for 10 minutes or until a gait abnormality was apparent. Exercise and recovery were videotaped. At the conclusion of exercise, rectal temperature, pulse rate and character, limb reflexes, arterial blood gas, plasma lactate and pyruvate and a serum biochemistry panel were performed.
Sequential re-evaluation of temperature, pulse rate and character, lactate and pyruvate and limb reflexes continued for 120 minutes following termination of exercise. Muscle biop-sies were collected on the day after exercise for analysis.
Who gets EIC?
Most, but not all, affected dogs are from field-trial breedings. Signs first become apparent in young dogs between 5 months and 5 years of age. Labradors of all colors (black, yellow, chocolate) are affected. Littermates and other related dogs are commonly affected. Dogs with EIC usually are described as being extremely fit and muscular with an excitable temperament and lots of retrieving drive.
Affected dogs can tolerate mild to moderate exercise, but occasionally after five to 15 minutes of strenuous exercise they develop weakness, apparent incoordination and then collapse.
Early gait changes include a rocking or forced gait, carpal hyperextension and excessive pelvic-limb abduction when turning. The abnormalities are progressive, and dogs that remain ambulatory typically walk with a flexed (crouched) rear limb gait characterized by long strides, excessive limb abduction during turning and a basewide stance.
Owners describe the rear limbs as "floppy" and the gait as wobbly or "loose." Some dogs are affected in all four limbs, resulting in collapse and inability to rise during a severe episode; some lose all voluntary motor ability. Some recumbent dogs exhibit markedly increased forelimb extensor tone. Hypermetria and loss of balance are common, particularly during recovery.
Gait, posture and balance abnormalities suggest neurologic dysfunction during the episode. Most dogs are mentally normal, but some appear dazed or confused during an episode. Dogs do not seem to be in pain during or after the collapse, and usually are anxious to continue participating in the activity. Patellar reflexes are absent after exercise in collapsing dogs, and do not reappear until five to 30 minutes after exercise. After 10 to 30 minutes of rest, the dogs return to normal.
Activities associated with inducing collapse episodes in EIC-susceptible dogs include "fun" retrieves, retrieving bumpers or birds on land as part of a training exercise or field trial, excited play with other dogs and upland hunting.
Factors perceived by owners as contributing to the likelihood that an EIC-susceptible dog will collapse on a given day or during a particular exercise include extreme excitement associated with the trigger activity, stress during formal training and excessive environmental heat and humidity.