Introduction

• A metabolic disease usually affecting athletic and working dogs.
• Results from extreme muscular work to the point that the muscle cells are damaged.
• Cellular damage creates secondary problems ranging from minor muscle pain to death.
• Few references in canine veterinary literature although well documented in equine medicine, and has recently been a topic in the human literature.
• Mostly seen in Sled Dogs, Racing Greyhounds, bird dogs, coursing dogs and Field and Trial competitors.
• Greyhound and Sled Dog veterinarians in Sports Medicine deal with this problem much more commonly than the general practitioner.
• In general veterinary practice condition seen occasionally in the pet dog that has escaped and run away.
• This problem is also seen by the wildlife veterinary practitioner. A condition called capture myopathy is seen in animals that have been stressed too greatly during the capture process. This is a common cause of death related to the capture process.

Pathogenesis

Etiology

• Two scenarios produce its occurrence in the athletic and working dog.

Exercising unfit dogs

• Relatively unfit dogs taken to a race or trial and forced to run several heats over a short period of time.

Conditioned dog running above its normal activity level

• At the racetrack this is seen when there is a problem with the stopping area of the lure system. If the Greyhound ends up running further than the normal race distance (ie twice around the track), rhabdomyolysis is a common result. This syndrome is sometimes described as "running the back off the dog, because the muscles most affected by the problem are the lumbar muscles groups. The name comes from the dramatic reduction in the size of the lumbar muscle groups after one of these bouts.
• Sled-dog myopathy is a term used in the sled dog world and can be a result of either predisposing scenarios. The literature states that it is usually seen in the first 400 km of a race. A correlation can be seen to blood CK levels Blood biochemistry: creatine phosphokinase. In the Iditarod sled dog race, the CK levels were reported to be elevated in the first 400 miles of the race, but returned to normal by the end of the race. Weather or terrain may present unforeseen conditions impacting the pre-race conditioning program.
• Also seen in dog escape where the dog runs free.

Predisposing factors

• Two additional environmental and managerial circumstances seem to predispose Greyhounds to exertional rhabdomyolysis.
  • One is the tendency of highly-strung Greyhounds to become very tense and excitable prior to running.
  • The other is a hot and humid climatic condition during traveling, or in pre-race kenneling, where climatic factors are not controlled.

Acute syndrome

• This form of the problem will present itself following a race or performance event. Predisposing factors are stated above.
• Highly strung dogs:a highly strung dog that is barking or is panting excessively will develop respiratory alkalosis from loss of CO2.
• Kidneys try to compensate by excreting bicarbonate into the urine to lower the blood pH.
• This is detrimental for the dog because the bicarbonate is needed to offset the lactic acid buildup produced by the performance event. In this case the dog begins the event in an acidic state, it will go into a more acidic state as the event progresses.
• Acidosis Acid base imbalance results in muscle cell membranes dysfunction, leakage, and disruption.
• Hot and humid environments:when the conditions are hot and humid it is difficult for dog to dissipate heat and panting only adds to the humidity of the dog's environment.
• As the humidity goes up, the ability to lower body temperature by evaporative cooling from the respiratory system decreases - also the increase in the rate of respiration is accompanied by an increased amount of salivation. In this case respiratory alkalosis combined with the elevated body temperature predisposes the dog to rhabdomyolysis.

Subacute syndrome

• When a muscle cell contracts there is an outflow of potassium ions from the cells.
• These potassium ions act upon the blood vessels causing vasodilation of the arterioles and the capillary bed to increase blood flow. This helps dissipate the heat produced with exercise, flush out waste metabolites, and increase the inflow of oxygen, glucose and bicarbonate buffers.
• If the potassium levels within the cells decrease below a critical threshold then there is an insufficient outflow of potassium items on muscular contraction to stimulate vasodilation.
• Some dogs subjected to a stressful performance schedule suffer a relative potassium deficiency. This deficiency and the related failure to induce vasodilation leads to a retention of the heat produced by exercise and cell ischemia.

Pathophysiology

Muscular activity overview

• During exercise the three end products of muscular activity are:
  • Heat - carried away from the muscles by the blood flow.
  • Energy output - produces work.
  • Metabolic acids - neutralized by buffers in the muscle and the blood stream.

Muscular over-activity consequences

• Excessive workload results in enormous production of hydrogen ions in the form of lactic acid. Energy derived from the largely anabolic breakdown of glucose and glycogen breakdown within the muscle cells results in high lactic acid levels.
• Production of lactic acid and associated hydrogen ions exceeds the neutralizing ability of the intracellular buffers of the muscles. This reduces efficiency of the cell wall enzyme system.
• Water is drawn into the cells, which subsequently swell to the point of cell membrane disruption.
• Swelling of the muscle cells puts direct pressure on the small vessels that supply their blood and oxygen resulting in a decrease in essential blood flow to the damaged muscle fibers.
• Reduced blood flow also reduces heat dispersion, aggravating the existing process of destruction.
• Muscle cell contents of proteins and electrolytes leak into the general circulation to be ultimately filtered and excreted by the kidney.
• One of the muscle proteins that leak out with cell disruption is myoglobin. Excessive amounts of myoglobin will form a precipitate or jelly-like mass in the acidic urine produced by the kidney Myoglobinuria. This tends to block the kidneys and causes them to fail. As a result, death is common within 48 hours from acute kidney failure Kidney: acute kidney injury (AKI).

Diagnosis

Treatment

Prevention

Outcomes