Congenital/developmental disorders

Hyperkalemic periodic paralysis (HYPP) (Fig. 8.1)

This is a disease of Quarterhorses and their crosses. To date it has only been traced to descendants of the stallion ‘Impressive’. It is transmitted as an autosomal dominant trait. Most cases are seen in 2–3-year-old well-muscled males but cases have been seen in foals as young as 4 months.

Heterozygotes and homozygotes show clinical signs, with signs often being more severe and detected earlier in the latter. The most common sign is muscle fasciculations. This can be followed by muscle spasms, weakness and recumbency. Death can occur due to cardiac or respiratory failure. Increased respiratory rates are common during attacks. The pharyngeal and laryngeal muscles can also be affected resulting in stridor or dyspnea.

Attacks may be precipitated by stressors such as transport, showing, change in weather and general anesthesia. Hyperkalemia is common during an attack although episodes without hyperkalemia have been reported.

Diagnosis and treatment

• Definitive diagnosis is by genetic testing. This can be performed on whole blood or hair root. The American Quarter Horse Association (AQHA) will accept HYPP test results only if performed through a licensed laboratory. Testing kits for non-registration purposes are available from the AQHA.

• Diagnosed subjectively by clinical signs and signalment.

• Post-mortem samples that can be collected if a horse has died during a suspected attack are: hair samples for DNA testing and aqueous humor for potassium concentration.

• Mild cases can be treated with light exercise, by feeding a readily absorbable source of carbohydrate (oats), and/or with acetazolamide (3 mg/kg orally).

• For severe cases, the following treatment is recommended: intravenous administration of 5% dextrose with sodium bicarbonate (1–2 mEq/kg) and intravenous administration of 23% calcium gluconate (0.2–0.4 ml/kg ) diluted in 5% dextrose (4.4–6.6 ml/kg).

Myotonia congenita (Fig. 8.2)

Myotonia congenita is a rare condition of Quarterhorse (QH) and QH cross foals that results in periods of involuntary muscle contractions following stimulation or start of exercise. Affected animals usually show clinical signs within the first year and often have well-developed musculature and a pelvic limb stiffness/lameness. The well-muscled appearance is similar to that seen in HYPP-affected animals.

Only the skeletal muscle is involved in myontonia congenita which means that progression of clinical signs is not seen beyond 12 months of age. A separate condition known as myotonic dystrophy has also been reported in a QH foal. This condition progresses to severe muscle atrophy and involvement of other organs.

Glycogen branching enzyme deficiency

This is an autosomal-recessive glycogen storage disorder that affects neonatal Quarterhorses or Paint horses and has been identified in aborted fetuses. As a result of the specific mutation certain tissues such as the brain, liver, cardiac and skeletal muscle cannot store and mobilize glycogen to maintain normal glucose hemostasis. Approximately 8% of both Quarterhorses and Paint horses are carriers of GBED. In one study 2–4% of aborted fetuses were homozygous for GBED. The condition is likely underdiagnosed due to the lack of readily available testing and the similarity in clinical signs between this and many other neonatal conditions.

The most frequently encountered clinical signs are weakness, hypothermia, hypoglycemia, flexural limb deformities and ventilatory failure. None of these signs is pathognomonic and all are easily confused with other conditions.

Polysaccharide storage myopathy (PSSM) (Figs. 8.3 & 8.4)

This disorder is characterized by higher glycogen concentrations and abnormal granular amylase-resistant inclusions in skeletal muscle. There are two types of PSSM: type 1 PSSM refers to horses with a specific glycogen synthase 1 gene (GYS1) mutation. This is commonly found in Quarterhorses, draft horse breeds and their crosses; type 2 PSSM is not related to a mutation in GYS1 and occurs in Quarterhorses and a large number of Warmblood breeds.

The clinical signs seen are somewhat breed-dependent:

• Quarterhorses. The most common signs are firm, painful muscles, stiffness, fasciculations, weakness, sweating and reluctance to move. The hindquarters are most commonly affected but other muscles can also be involved. Signs of pain can be severe and long-lasting. Signs can be seen with minimal work especially in horses that have been rested for several days or are on a high-grain diet. There is no specific association to gender, body type or temperament. Average age of onset of clinical signs is 5 years but signs have been seen in horses as young as 1 year of age. Serum CK and AST activities are often persistently high in affected horses with median values reported at 2 809 and 1 792 U/L, respectively.

• Draft horses. Many affected horses do not have clinical signs. Rhabdomyolysis and myoglobinuria can be seen in horses that are fed high-grain diets, exercised irregularly or have little turnout. Signs can also be seen with general anesthesia. Again there is no gender predilection and the mean age of onset of clinical signs is 8 years. The median CK and AST levels in affected horses are reported to be 459 and 537 U/L, respectively.

• Warmbloods. Warmbloods may have either type 1 or type 2 PSSM depending on the crosses from which they are derived. The most common clinical signs noted are painful, firm back and hindquarter muscles. Overt rhabdomyolysis is not that commonly seen. The mean age of onset is 8–11 years of age and the median CK and AST activities are 323 and 331 U/L, respectively.


• Horses with PSSM should ideally have some form of turnout available.

• Exercise regimens should only be implemented after there has been adequate time allowed for dietary changes (2 weeks). The duration and not just the intensity of exercise is important and exercise should be gradually introduced and consistently performed. It is also important to minimize the number of days without some form of exercise.

• Dietary management is very important in affected horses and should first consist of reaching the ideal body weight for the horse. Once this has been done the dietary modifications should combine reducing glucose load and providing fat as an alternative energy source. There are now a wide variety of commercially produced diets available for horses with PSSM. The basic dietary principle is that less than 10% of digestible energy (DE) be provided in the form of starch and at least 13% in the form of fat.

Recurrent exertional rhabdomyolysis (RER)

There are many causes of exertional rhabdomyolysis in horses. The term recurrent exertional rhabdomyolysis is used to describe one form that appears to have a heritable basis in Thoroughbred horses. The specific cause has not however been identified although it is believed that there is abnormal regulation of intracellular calcium in skeletal muscle. Clinical signs are more common in fillies especially those with a nervous temperament. These signs include hindlimb pain and lameness, high respiratory rate, sweating, a reluctance to move and colic signs such as repeated pawing. These signs may last several hours without treatment. Clinical signs are more frequently seen in horses fed high-grain diets and concurrent lameness. The type of exercise is also important with episodes being more common in racehorses restrained to a slow canter, while post-race episodes are infrequent. In some cases episodes can be seen without any exercise or after transportation and in these cases may be more related to temperament and nervousness associated with anticipated work or racing.


• Evaluation of serum CK and AST levels is used by many to identify poor performance in association with subclinical episodes of RER. In doing such tests it is important to be consistent with regard to sampling times. The ideal is 4–6 hours after exercise but in many cases due to yard management this is not possible and then samples should be routinely collected at a standardized time in relation to the previous day’s work.

• If serum muscle enzymes are normal but RER is still believed to be a cause of poor performance an exercise test can be performed with samples taken before and 4–6 hours after exercise. Exercise for such tests should consist of 15 minutes of collected trotting. A 3–4-fold rise in serum ck activity is indicative of subclinical muscle damage.

• While there is currently no genetic testing available for RER horses it is an area of active research and such a test may be available in the near future.


• The goals of therapy for specific episodes of exertional rhabdomyolysis are relieving pain and anxiety, replacing fluid and electrolyte losses and maintaining renal function.

• Non-steroidal anti-inflammatories are frequently used to relieve pain and in mild cases may be the only treatment necessary. However, they should be cautious or combined with intravenous fluid therapy in dehydrated horses.

• Severely affected horses, dehydrated horses or horses with myoglobinuria should receive intravenous fluids. Hyperkalemia can occur in some horses and can be treated by the administration of isotonic sodium chloride. Some horses will be hypocalcemic and will require supplementation with calcium.

• Other drug therapies include dantrium sodium (4 mg/kg PO q 4–6 h) which decreases muscle contracture and may help prevent further muscle necrosis. Methocarbamol (a muscle relaxant) (5–22 mg/kg IV slowly) produces variable results perhaps as a reflection of the dose used. Corticosteroids are advocated by some especially if the horse is recumbent. DMSO has been used as an anti-oxidant, anti-inflammatory and osmotic diuretic. Tranquilizers may help relieve anxiety and the perpetuation of an episode.

Feb 27, 2017 | Posted by in EQUINE MEDICINE | Comments Off on Muscle

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