Tying Up is a syndrome and a term used to describe a group of common muscle disorders in the horse. It is one of and controversial syndromes in the horse, with much speculation over its causes and mechanisms. The typical signs of a horse that is tying up are muscle fatigue, pain, cramping and reluctance to move. Other names that have been used include Monday morning disease, azoturia, myopathy, myositis and rhabdomyolysis. It is a condition that has been known throughout history and described in 19th-century literature. In a veterinary text from 1883, the clinical signs were described as “sweating and trembling, scarcely able to turn in the stall, the muscles of the back and loins in a state of spasm, tail quite stiff” – this is still a relevant.
Over the years, researchers have learned much about this condition, and we now understand that tying up is not one disease but several different ones. One of the key researchers is Dr Stephanie J Valberg, DVM, PhD, who has spent years researching this very interesting but frustrating condition. Here, we are going to look at muscle anatomy and physiology, some of the different types of tying up, clinical signs and management.
Horses are supreme athletes and, as a result, have developed a large muscle mass for their endeavours, be it speed, endurance, jumping, working cattle or elegant dressage movements. It is the muscles that create movement by pulling on the bones to operate the joints that allow their incredible feats. Minor or intermittent disruptions in muscle function can have major impacts on an equine athlete’s performance.
Diagram 1 above shows the main muscle groups of the horse. Fun fact: there are approximately 700 muscles in the horse, the largest muscles of which are those of the back and the hindquarters.
Muscles are composed of groups of muscle fibres – essentially, within a muscle there are fibres that are stacked on top of one another and are all parallel. Within those fibres are smaller units called myofibrils, which are the functional units of muscles and are microscopic in size. Diagram 2 shows the muscle fibre structure. The myofibrils are composed of two main protein filaments – actin and myosin which lie against one another and interdigitate with each other with little finger-like projections from the actin molecule. The filaments slide back and forth during exercise to allow the muscle to stretch and shorten.
A sarcomere is the basic building block of myofibrils and is the cellular structure responsible for muscle fibre contraction. When the myofibrils are extended, the muscle is relaxed, and when they are interdigitated more, the muscle is contracted.
Without getting into the depths of molecular biology during muscle contraction, key molecules involved include acetylcholine (a neurotransmission chemical), adenosine triphosphate (ATP) sodium and calcium. Glycogen is the key molecule for the energy production required for muscle activity and is stored in the muscles at a higher concentration than that of people.
The clinical signs are usually associated with exercise; they can vary in severity and include:
Severe cases may result in: Recumbency, Myoglobinuria – dark wine-coloured urine, Kidney failure
Usually horses that are in work and being exercised are prone to tying up – this includes racehorses, showjumpers, endurance horses, eventers, cutting and campdrafting horses and dressage horses. All breeds are susceptible, but certain types of tying up are more prevalent in Quarter Horses, Thoroughbreds, Arabs, Morgans, Warmbloods and Draft horses. Some of the conditions have been shown to have a genetic predisposition.
The diagnosis of tying up is based on history and clinical examination. Diagnostic tests that may be performed include:
The diagnostic tests utilised will depend upon the clinical case and the type of tying up that may be implicated.
Creatine Kinase (CK)
Short term muscle enzyme used an indicator of recent muscle tissue damage. It peaks at 6 – 12 hours. Levels over 400 IU / L are significant.
Aspartate Transaminase (AST)
This enzyme may remain elevated for days to weeks following an episode. Levels over 1000IU/L are significant.
There are many predisposing factors implicated in tying up, but generalised ones are:
Exertional Rhabdomyolysis (ER) is the term currently in favour for this “tying up” group of conditions – exertional pertaining to exercise, and rhabdomyolysis referring to the changes that occur in the skeletal muscle (breakdown or necrosis of the muscle).
Sporadic Exertional Rhabdomyolysis occurs as a singular episode (irregular or random). Usually, these horses have no previous history of tying up, and all breeds of horses are susceptible to this type. The most common causes are exercise levels that exceed the horse’s state of training and dietary imbalances, particularly diets high in soluble carbohydrates and low forage. Other predisposing factors are infectious respiratory disease, exhaustion, dehydration, dietary deficiencies of sodium, vitamin E or selenium and calcium – phosphorous imbalances.
Once the horse has recovered, changes in management in terms of diet and exercise usually prevent this type of tying up from happening again.
Chronic Exertional Rhabdomyolysis. Some horses have repeat or recurrent episodes of tying up or rhabdomyolysis, even with light exercise. There have been 4 types of the chronic form identified (on the basis of muscle biopsies and genetic testing). These types of horses often don’t respond well to prolonged rest.
The management and treatment very much depend on the severity of the episode and the underlying cause. Your veterinarian will instigate treatment appropriate to the individual case.
Tying up is an umbrella term for a group of diseases affecting the muscles can vary in severity but are usually associated with exercise and include painful, cramping muscles, anxiety, reluctance to move, sweating and high respiratory and heart rates. The cases can be sporadic or chronic. If your horse ties up, then the key to preventing further muscle damage is to stop exercising the horse, put it in a confined area, and to call your veterinarian.
