Equine Exertional Rhabdomyolysis - Causes and Process of ER

Causes and Process of ER

Beyond a highly probable hereditary factor, there does not seem to be a single cause that triggers ER in horses. Exercise is seen in every case, but exercise is always accompanied by another factor. It is likely that several factors must act together in order to cause an ER attack.

Other possible factors include:

• The overfeeding of non-structural carbohydrates (grain and pellets, for example)
• Poor conditioning or fitness, sudden increase of workload
• The work of a horse after a period of rest, if the concentrate ration was not reduced
• Electrolyte or mineral imbalances, especially seen with potassium
• A deficiency in selenium or vitamin E
• Imbalance of hormones, including the reproductive hormones in nervous fillies and mares and thyroid hormones in horses with hypothyroidism
• Wet, cold, or windy weather conditions

The more factors that are present, the greater the likelihood that the horse will develop ER. However, the most common cause of ER is an imbalance between the animal's diet and his workload, especially when he has a high-grain diet.

ER occurs when there is an inadequate flow of blood to the muscles of an exercising horse. The muscle cells, lacking in oxygen, begin to function anaerobically to produce the needed ATP. The anaerobic work creates a buildup of waste products, acid, and heat. This subsequently alters the cell by preventing the cell's enzymes from functioning and the myofilaments from efficiently contracting. The cell membranes may then be damaged if the horse is forced to continue work, which allows muscle enzymes and myoglobin to leak into the bloodstream.

The body builds up a store of glycogen from converted carbohydrates in muscle cells. Glycogen, a fuel used by muscles for energy, is depleted during work and restocked when a horse rests. Oxygen-carrying blood metabolizes glycogen, but the blood can not flow fast enough to metabolise the excess stored glycogen. The glycogen that is not metabolized aerobically (by the oxygenated blood) must then be metabolized anaerobically, which then creates the cell waste products and heat, and ER has begun. A horse on a high-grain diet with little work collects more glycogen in its muscles than it can use efficiently when exercise begins, which is why horses on a high-grain diet are more likely to develop ER.

Proper conditioning can help prevent ER, as it promotes the growth of capillaries in muscles and the number of enzymes used for energy production in muscle cells. However, improvement in these areas can take several weeks. Thus, ER is more common in horses that are only worked sporadically or lightly, and in horses just beginning an exercise regimen.

A common misconception is that ER is caused by the buildup of lactic acid. Lactate is not a waste product for a cell, but a fuel, used when the cell's oxygen supply is insufficient. Lactate does not damage a cell, but is rather a byproduct of the true cause of cell damage: inadequate blood supply and altered cell function. Lactate naturally builds up in an exercising horse without harming the muscle cells, and is metabolized within an hour afterward.

The pain is caused by the inadequate blood flow to the muscle tissue, the inflammation from the resulting cell damage, and the release of cell contents. Muscle spasms, caused by the lack of blood to the muscle tissue, are also painful.