Inherited problems of the musculoskeletal system are less common in horses than in many other species. Underlying viral infections and toxic causes may lead to congenital (birth) defects in foals but are rare. Most of the problems that arise are related to specific genetic conditions.
Contracted flexor tendons are probably the most common abnormality of the musculoskeletal system of newborn foals. The condition is inherited, and the positioning of the foal while in the uterus may affect its severity..
At birth, the pastern and fetlocks of the forelegs and sometimes the carpal joints (knees) are flexed to varying degrees due to shortening of the associated muscles. A cleft palate may accompany this condition in some breeds of horses. Slightly affected animals bear weight on the soles of the feet and walk on their toes. More severely affected animals walk on the front of the pastern and fetlock joint. If not treated, the surfaces of these joints become damaged, and the joints can become infected. This may lead to a rupture of the common digital extensor tendon.
Mildly affected animals recover without treatment. In moderate cases, your veterinarian may administer medication and apply a splint to force your horse to bear weight on its toes. Care must be taken that the splint does not limit blood circulation, or tissues in the foot could be damaged. For a horse with a moderate case, your veterinarian might demonstrate how to help your horse stretch. By manually moving a horse’s legs to extend these joints, you can assist in stretching the ligaments, tendons, and muscles. Severe cases require the surgical cutting of one or both flexor tendons. A cast may also be needed in some cases. Extreme cases may not respond to any treatment.
Glycogen is a complex carbohydrate normally stored in the liver and muscles. The body converts it to glucose (sugar) as a source of energy. Animals with glycogen storage diseases cannot metabolize glycogen properly. They may become progressively weaker until they are unable to rise from a lying position. To date, 5 of 11 types of glycogen storage diseases found in humans have also been found in animals, including horses.
This syndrome, more commonly associated with calves, has also occasionally been seen in foals. It is characterized by the “locking” of limbs in abnormal positions, and it can make foaling abnormally difficult for the mare. Affected foals may have other abnormalities, including hydrocephalus (water on the brain), resulting in an enlarged head, cleft palate, and abnormalities of the spine. The condition may be lethal, but some mildly affected animals recover completely. In some types of the syndrome, the muscle fiber dysfunctions might be the primary disorder. Most often, the syndrome has its roots in a nervous system disorder. The muscular and joint problems begin when muscles are no longer served by healthy nerves.
Osteochondrosis (osteochondritis dissecans) most commonly occurs in young horses during periods of rapid growth. It is caused by a number of factors and can have a genetic component. Damage to the joints occurs when the animal is growing at its fastest, and the stress on the immature skeleton is greatest. The damaged cartilage may become detached and float loosely in the joint cavity, where it can cause inflammation and further interference with proper bone formation.
Digit malformation is a congenital defect occasionally found in horses. In its most common form, the second or fourth splint bone develops into a complete lower limb and foot (polydactyly). One or all 4 limbs can have the condition.
Improper formation of the navicular bone can also occur in foals. This can result in an underdeveloped navicular bone or a navicular bone that is divided into 2 or 3 sections.
Angular limb deformities may be present at birth (congenital) or acquired later in life. In these skeletal defects, a portion of a limb is bent inward or outward at an abnormal angle from the midline of the body early in the newborn’s life. Angular limb deformities may be caused by abnormal positioning of the limb while in the womb, a thyroid hormone deficiency (hypothyroidism), trauma, a poorly formed or loosely jointed limb, or inappropriate growth of the carpal or tarsal and long bones. One to 4 limbs may be affected.
The carpus (knee) is the bone affected most often, but the tarsus (hock) and fetlocks are occasionally involved. Most foals have no signs, but lameness and soft-tissue swelling can accompany severe deformities. A limb with very slight deviation may be regarded as normal. Foals with inappropriate growth of the carpal or tarsal bones or excessively loose joints often become lame as the deformity worsens. Your veterinarian may diagnose the condition by a thorough examination of the limb and x‑rays.
Treatment will be determined by the severity of the condition and the tissues affected. Foals that are only mildly affected may improve on their own without treatment. In cases where joints are excessively loose, a cast or a splint may be required. Such limb support may be required for up to 6 weeks, and restricted exercise will be necessary to maintain tendon and ligament tone.
Surgery may be necessary if the growth plates have been disturbed. These surgeries must be performed before the growth plates close (as early as 2 to 4 months of age for some joints). Success depends on the continued growth and development of the bones. Examinations and x-rays should be used to determine if the condition is improving or further surgery is required.
Without treatment, the outlook for recovery from severe deformity of the carpus is poor, as it can lead to degenerative joint disease. However, with early detection, careful evaluation, and proper surgery, most foals respond favorably.
Dwarfism is the lack of appropriate growth, resulting in a smaller horse. All or part of the body may be affected. A deficiency in growth hormone causes all body parts to be small and in proportion. Abnormal thyroid hormone levels can affect growth in only some parts of the body. This results in a foal with a large head, silky hair coat, floppy ears, and overbite ("parrot mouth"). Your veterinarian can diagnosis dwarfism based on the appearance of your horse and blood tests.
Although defects of the spine are uncommon in foals, 4 types are possible. Congenital scoliosis, an S-bend of the spine, is encountered occasionally. It is often difficult to assess the severity of the deformity with just a physical examination. X-rays provide a better view of the condition. Even in more severe cases, there is rarely any obvious abnormality in gait or the ability to move. Mild cases sometimes completely correct themselves.
Synostosis is fusion of a vertebra with a vertebra next to it. An x-ray is necessary for confirmation of synostosis, which is often associated with secondary scoliosis.
Swayback, known as lordosis, is a down-ward curving of the spine in the lower back. Congenital lordosis affects the spine of a horse whose vertebral joints fail to develop properly. In adult horses, degrees of acquired lordosis occur as the horse gets older. Kyphosis (an upward curving of the spine, also known as roach-back) is also occasionally seen. Both of these conditions contribute to back weakness. A veterinarian diagnoses the condition by thorough examination, often confirmed by x-rays that reveal an abnormal curvature of the vertebral column.
Other skeletal malformations include torticollis (twisted neck), wry nose (twisted nose), spina bifida (incomplete closure of the bones in the spine), and hydrocephalus (enlargement of the head due to abnormal accumulation of fluid within the skull).
Hyperkalemic periodic paralysis is a hereditary condition of Quarter Horses, Paint Horses, and Appaloosas, in which abnormally high levels of potassium in the blood produce intermittent episodes of muscle weakness or paralysis. In severe cases, the condition can lead to collapse and sudden death. All affected horses can be traced back to a Quarter horse sire named "Impressive" and should not be bred.
Glycogen branching enzyme deficiency may be a common cause of newborn death in Quarter Horses and related breeds. Affected horses cannot store sugar molecules in body tissues. The condition is fatal because heart, brain, and skeletal muscles are unable to function. Diagnosis can be complicated by the variety of signs that resemble other diseases of newborn horses. Signs of glycogen branching enzyme deficiency may include curving or bending limb deformities lasting for only a short time, stillbirth, seizures, respiratory or heart failure, and the inability to rise from a recumbent position. Your veterinarian may notice other abnormalities on blood tests of affected foals. The condition is inherited.
Also see professional content regarding congenital and inherited disorders of the musculoskeletal system in horses.