Muscle cramping is a painful condition that arises from hyperactivity of motor units caused by repetitive firing of the peripheral and/or central nervous system. The origin of the cramp in most cases is believed to be the intramuscular portion of the motor nerve terminals. Most muscle cramps are also accompanied by fasciculations in the same muscle and normal serum CK activity. Muscle cramps can be induced by forceful contraction of a shortened muscle, by changes in the electrolyte composition of extracellular fluid, and by ear tick infestations. In contrast, muscle contractures, like those seen in exertional rhabdomyolysis, are painful muscle spasms that represent a state of muscle contracture unaccompanied by depolarization of the muscle membrane. Muscle contractures are invariably accompanied by markedly increased serum CK activity.
Muscle cramping in endurance horses is most frequent in hot, humid weather. Horses may lose fluids at a rate of up to 15 L/hr in the form of sweat and develop remarkable deficits in sodium, potassium, chloride magnesium, and calcium. Clinical signs of electrolyte derangements include muscle stiffness and periodic spasms of muscle groups. In addition, exhausted horses are often dull, depressed, and clinically dehydrated with increased heart and respiratory rates and persistently increased body temperature. Synchronous diaphragmatic flutter may be seen in association with cramping. Affected horses do not generally develop myoglobinuria or have marked increases in serum CK and AST levels.
Mild muscle cramping is self-limiting, and the signs abate with rest or light exercise. However, exhausted horses with metabolic derangements require immediate treatment, including plasma volume expansion with oral or IV isotonic polyionic fluids and cooling (using water and fans). Because most horses with this condition are alkalotic, administration of solutions containing sodium bicarbonate is contraindicated. Daily direct addition of 2 oz of sodium chloride and 1 oz of potassium chloride to the feed is recommended for horses with recurrent cramping, in addition to electrolyte supplementation before and after endurance rides.
Hypocalcemia is a relatively rare disorder in horses that has also been referred to as lactation tetany, transport tetany, idiopathic hypocalcemia, and eclampsia. Clinical signs, diagnosis, and treatment are discussed elsewhere (see Hypocalcemic Tetany in Horses). In addition to hypocalcemia, a metabolic alkalosis, hypomagnesemia/hypermagnesemia, and hyperphosphatemia/hypophosphatemia may be present and need correction before a return to normal function is seen. Relapses do occur.
Synchronous diaphragmatic flutter is due to firing of the phrenic nerve in synchrony with atrial depolarization, causing the diaphragm to contract with each heartbeat. This occasionally produces an audible thumping sound. Inciting causes include endurance exercise, hypocalcemia, hypoparathyroidism, digestive disturbances, and repeated administration of calcium-containing fluids to performance horses. Synchronous diaphragmatic flutter may be a singular occurrence or a chronic recurring problem. The most consistently reported metabolic derangement is low serum ionized calcium concentrations usually associated with hypochloremic metabolic alkalosis. Metabolic alkalosis may alter the ratio of free to bound calcium (increasing calcium binding to protein and decreasing ionized calcium), which possibly induces diaphragmatic flutter.
Most horses undergo rapid remission of signs when given calcium solutions IV. Although hypomagnesemia is often present with synchronous diaphragmatic flutter, horses do not respond to magnesium supplementation unless calcium is administered concurrently. Response to therapy is also reflected by improved mental status, return of appetite, and gut motility. For horses with chronic diaphragmatic flutter, providing chloride, potassium, sodium, calcium, and magnesium during prolonged exercise may help reduce fluid losses and the metabolic alkalosis. Alternative approaches involve reducing dietary calcium for a few days before competition in horses prone to diaphragmatic flutter. This reduction in dietary calcium may stimulate the endocrine homeostatic mechanisms and increase osteoclastic activity. Limiting alfalfa hay, which has a relatively high calcium concentration, may be indicated in chronically affected horses.
Otobius megnini infestations in the ear canal can produce remarkably painful intermittent muscle cramps not associated with exercise that last from minutes to a few hours and often resemble colic. Horses may fall over when stimulated. Between muscle cramps, horses appear to be normal. Percussion of triceps, pectoral, or semitendinosus muscles results in a typical myotonic cramp. Horses have increased serum CK, ranging from 4,000 to 170,000 IU/L. Numerous ear ticks can be identified in the external ear canal of affected horses. O megnini is found in the southwestern USA. Without treatment, the spasms continue; however, local treatment of the ear ticks using pyrethrins and piperonyl butoxide results in recovery within 12–36 hr. Acepromazine may be helpful to relieve painful cramping.
“Shivers” is a spastic condition of the hind- and occasionally forelimbs of horses that is usually only evident when horses are backing or having their feet picked up. It is most common in adult draft horse breeds, Warmbloods, Warmblood crosses, and Thoroughbreds >16.3 hands tall. The condition is characterized by periodic, involuntary spasms of the muscles in the pelvic region, pelvic limbs, and tail that are exacerbated by backing or picking up the hindlimbs. The affected limb is elevated, abducted, and may actually shake and shiver; the tail head is usually elevated concurrently and trembles. When more severely affected animals are backed up, the hindlimb is suddenly raised, semiflexed, and abducted with the hoof held in the air for several seconds or minutes. The tail is elevated simultaneously and trembles. After a variable period of time the spasms subside, the limb is extended, and the foot is brought slowly to the ground. Some horses will refuse to pick up their hindlimbs and are very difficult to shoe. Suggested causes include genetic, traumatic, infectious, and neurologic diseases, although the exact etiology is unknown. The condition in draft horses is usually progressive and eventually debilitating; in Warmbloods and Thoroughbreds, it usually has less impact on performance and progresses more slowly. There are no known treatments, but avoiding stall rest and keeping horses fit appears helpful.