Muscle cramping is a painful condition that arises from hyperactivity of motor units caused by repetitive firing of the peripheral nervous system or CNS. 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 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/hour 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 occur in association with cramping. Affected horses do not generally develop myoglobinuria or have marked increases in serum CK and AST activities.
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 in Horses
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 Hypocalcemic Tetany in Horses Hypocalcemic tetany in horses is an uncommon condition associated with acute depletion of serum ionized calcium and sometimes with alterations in serum concentrations of magnesium and phosphate... read more ). In addition to hypocalcemia, metabolic alkalosis, hypo- or hypermagnesemia, and hyper- or hypophosphatemia may be present and need correction before a return to normal function occurs. Relapses do occur.
Synchronous Diaphragmatic Flutter
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 administered 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 treatment 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 decrease fluid losses and the metabolic alkalosis. Alternative approaches involve decreasing dietary calcium for a few days before competition in horses prone to diaphragmatic flutter. This decrease in dietary calcium may stimulate the endocrine homeostatic mechanisms and increase osteoclastic activity. Limiting alfalfa hay, which is relatively rich in calcium, may be indicated in chronically affected horses.
Ear Tick–associated Muscle Cramping
Spinose ear tick (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 activity, ranging from 4,000 to 170,000 U/L. Numerous ear ticks can be identified in the external ear canal of affected horses. Spinose ear ticks are found in the southwestern US. Without treatment, the spasms continue; however, local treatment of the ear ticks using pyrethrins and piperonyl butoxide results in recovery within 12–36 hours. 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. The exact etiology is unknown; however, a focal degenerative lesion in the deep cerebellar nuclei has been identified in severe cases. Horses with shivers do not show hyperflexion of the hind limbs at a trot. As the condition advances, hyperflexion may occur for a few forward steps at a walk.
Shivers should be distinguished from standing hyperflexion, a condition in which horses back up normally but have difficulty holding up the hind limb for the farrier due to spasms. In addition, shivers should be distinguished from stringhalt, which, unlike shivers, manifests quite consistently at a walk and trot and variably with backing up.
Shivers is most common in adult draft horse breeds, Warmbloods, Warmblood crosses, and Thoroughbreds >16.2 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 hind limbs. For horses with shivers hyperextension, horses have an extended hind- and forelimb when backing up. For horses with shivers hyperflexion, the affected limb is elevated, abducted, and may actually shake and shiver; the tail head is usually elevated concurrently and trembles when the horse is made to back up. When more severely affected animals are backed up, the hind limb is suddenly raised, semiflexed, and abducted with the hoof held in the air for several seconds or minutes. The tail is usually elevated simultaneously and trembles. After a variable period, the spasms subside, the limb is extended, and the foot is brought to the ground. Some horses will refuse to pick up their hind limbs and are very difficult to shoe.
The condition of shivers hyperflexion or hyperextension is usually slowly progressive. There are no known treatments; however, body work, avoiding stall rest, and keeping horses fit appear helpful.