R.E.R. Recurrent Exertional Rhabdomyolysis. a.k.a. Exertional Myopathy. Tying Up.

An intermittent from of Tying Up causing painful muscular contractions similar to those seen in P.S.S.M. but arising from a different mechanism. RER affects mainly Thoroughbreds and Standardbred horses. Current theory is that susceptible horses store excess calcium inside muscle cells in the membranes of the sarcoplasmic reticulum. Normally, calcium is released from here and enables the muscle to contract whilst stimulating energy metabolism. In RER horses the release of calcium during exercise can be excessive, causing persistent contraction of affected muscle with disruption of the normal energy systems. This is followed by loss of mitochondria (energy centers within the cell) and eventual disruption of the cell walls with loss of cell contents into the circulation. Essentially the disease stems from abnormal calcium storage and ion flows within the muscle cells. No specific gene mutation has yet been identified though a genetic link is strongly suspected. Chronic high stress also seems to be an important trigger, possibly by affecting calcium regulation inside the muscle cell.

RER horses often suffer repeated episodes of the condition and there are several factors which predispose to an attack:

Age – 2-3 year old’s most commonly.  Sex – 65% are fillies

Diet – most affected horses tied up whilst on high grain / high NSC diets (<5kg grain per day)

Temperament – more likely to be of a nervous temperament

Exercise – Thoroughbreds are more likely to tie up during galloping training. Standardbreds after 15 minutes jogging. Eventers prior to the cross country phase of competition. Attacks are more likely after rest days, especially if the ration was not adjusted downwards for the lack of work.

Lameness – Attacks are more likely in horses which have suffered lameness (this may be related to time off fast work)

Management of RER. As stress is an important triggering factor the calmer these horses are kept, the better. Keep them in a quiet stall at the end of the row away from noise so far as possible. Take them out for exercise first rather than last to avoid excitement in the anticipation of going out. Avoid rest days altogether or turn them out on days off. Deal with any painful conditions such as lameness or stomach ulcers to reduce pain mediated stress.

Despite the difference from P.S.S.M., dietary management for RER horses also revolves around reducing reliance on starch (grain) for energy and substituting with fat and fiber. Ideally, NSC should account for less than 25% of energy with another 25% coming from fat and the rest from low NSC fiber sources like beet pulp and soy hulls. This is because diets high in fat rather then starch energy produce calmer horses with lower insulin and glucose responses and lower levels of the stress hormone cortisol, all of which reduce the triggers causing an attack. Low starch compounded concentrate feeds are available, often with high fat content, to which vegetable oil or rice bran can be added to boost energy density in the ration. A balancer pellet or mineral / vitamin supplement may be needed where concentrate rations are <3kg/Day along with electrolyte supplementation. Cautious carbohydrate loading by introducing limited grain for 2-3 days pre- race to boost glycogen stores may work with some horses where performance is sub optimal.

Consistent fast work is the key to prevention and should resume as soon as possible following an attack unless there has been significant myoglobinuria (coffee coloured urine), preferably within 24-48 hours. Inappropriate exercise for the starch energy intake is the most frequent cause, coupled with stress factors acting as triggers.

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