‘Tying up’, also known as Azoturia, Monday Mornings Disease or Recurrent Exertional Rhabdomyolysis (RER) is the most common muscle disorder in horses, frequently limiting performance in sport horses of varying breeds. Tying-up is basically muscle cramps, the largest muscles in the horse (back and hindquarters) are most often affected by a combination of different (it is not fully understood) mechanisms, leading to a buildup of lack of muscle oxygenation, lactic acid and muscle cell death.

‘Tying-up’ usually affects horses in a high level of work that are rested for 1 or more days and still fed a high carbohydrate:low fat diet. It most often occurs after 20-30 mins of work during the first exercise following a period of rest. However, it may also occur as a result of increasing intensity of work or unfit horses undergoing prolonged periods of exercise. Any breed of horse can be affected. It most often occurs amongst younger horses and affects mares more than males. Some horses experience only one or two isolated cases, whilst others suffer repeat episodes which subsequently limits their athletic potential.

Stress, excessive sweating, lack of drinking before and after work or not travelling well will cause electrolyte imbalances or disturbances which predispose a horse to ‘tying-up’. As can a diet high in cereals (as these contain a high potassium:sodium ratio) or deficient in certain minerals and vitamins. Some blood lines are also prone to producing horses that regularly ‘tie-up’ so there is a suspected genetic component too.

Depending on the severity of the episode, horses will demonstrate varying clinical signs from a mild discomfort and stiff gait to a very stiff gait and refusal to move or even in severe cases recumbency due to the pain. Most horses will have firm painful muscles in the gluteal, hamstring and back area, some horses will develop muscle swelling. Tying up is very painful and horses may become distressed and anxious, they may increase their respiratory rate, sweat or even show colic like behaviour.

If you are out riding away from your horse’s stable and your horse ‘ties-up’, phone for help. The horse should be transported by lorry or trailer to limit any further muscle damage. Once the horse is in a stable, keep them warm with plenty of rugs and offer water.

Diagnosis is made based on clinical signs and a blood test measuring muscle enzymes and kidney parameters. The enzymes are released from inside muscle cells when they are damaged. Myoglobin is a product also released by damaged muscle cells. This can discolour the urine brown or orange, potentially causing kidney damage if severe enough.

Immediate treatment depends on the severity of the episode but the aim is to provide pain relief, reduce further muscle damage and protect the kidneys. If mild; encourage the horse to drink (restoring electrolyte balance) and if the horse is comfortable you can walk them around to stop them stiffening up further, but do this with caution! If the horse is very painful, reluctant to move or recumbent, do not try to move the horse! This may lead to further muscle damage. Call your vet and allow them to provide pain relief and anti-inflammatories. The vet may also give acepromazine (ACP), a drug that calms the horse down and causes the blood vessels to dilate, increasing blood supply to the muscles. If the horse is dehydrated there is a risk of kidney damage so the horse may require rehydration by oral fluids or intravenous (IV) fluids. Other drugs are available to help stabilise the muscles and protect from further damage.

Prevention is better than cure and so if you have a horse prone to tying up it is important to provide a high-fat (bran and oils), low-carbohydrate (grains) diet, ensure your horse is properly warmed up and cooled down before and after work and exercise the horse every day, as one or more days of inactivity seems to preclude ERE episodes.

Vitamin E and selenium supplements can also be of benefit, as can antioxidants and other drugs and herbal supplements which your vet can inform you about.

Some horses suffer from chronic ER, these horses require further investigations to define why they continue to ‘tie-up’, this can involve repeat blood tests, exercise tolerance tests, and muscle biopsies.

Chris Baldwin, BVetMed, MRCVS