Angular Limb Deformities
In these congenital or acquired skeletal defects, the distal portion of a limb deviates laterally or medially early in neonatal life. In utero malposition, hypothyroidism, trauma, poor conformation, excessive joint laxity, and defective endochondral ossification of the carpal or tarsal and long bones have been implicated. One to 4 limbs may be affected, depending on the severity of the condition.
The carpus is affected most frequently, but the tarsus and fetlocks are occasionally involved. The deviation is obvious but varies in severity. A lateral deviation (valgus) of up to 6° of the distal portion of a limb may be regarded as normal. Most foals are asymptomatic, but lameness and soft-tissue swelling can accompany severe deviations. Outward rotation of the fetlocks invariably accompanies carpal valgus. Foals with defective ossification of the carpal cuboidal bones or excessive joint laxity are frequently lame as the legs become progressively deviated. Affected limbs must be palpated carefully to detect ligament laxity and specific areas that may be painful.
Diagnosis should include a precise determination of the site and cause for the deviation. The distal radial metaphysis, physis, epiphysis, or cuboidal bones may be the site of deviation. Radiography is helpful in detecting physeal flaring, epiphyseal wedging, and deformation of carpal bones. Mildly affected foals frequently improve spontaneously.
Treatment depends on the severity of the condition and tissues affected. Excessive joint laxity, with or without cuboidal carpal bone involvement, requires tube casts or splints. The fetlock and phalangeal region should not be included in the casts, which should protect the weak joint from trauma but allow restricted exercise to maintain tendon and ligament tone. Such limb support may be required for up to 6 wk.
Physeal and epiphyseal growth disturbances are also amenable to surgical correction through hemicircumferential transection and periosteal elevation of the distal radius on the concave side of the defect or through transphyseal bridging of the physis on the convex side. These surgeries must be performed before the physeal growth plates close (as early as 2–4 mo of age), and success depends on continued growth and development of the bones. Sequential examinations and radiographs are necessary to follow spontaneous improvement or to establish a need for surgery.
Without treatment, the prognosis for severe carpal valgus is poor. The conformational anomaly leads to early degenerative joint disease. Likewise, deformity of the cuboidal carpal bones contributes to a poor prognosis. However, with early detection, careful evaluation, and proper surgical treatment, most foals respond favorably.
Defects of the Spine
Defects of the spine include scoliosis, synostosis, and lordosis. Although all of these conditions are uncommon in foals, congenital scoliosis is encountered most frequently. On clinical examination, it is often difficult to assess the severity. A better appreciation of the condition can be obtained by radiographic examination. In mild cases, improvement is spontaneous and may be complete. Even in the more severe cases, there is rarely any obvious abnormality in gait or maneuverability. However, these foals frequently are not raised because they appear unlikely to be able to withstand being ridden or worked.
Another occasional congenital deformity is that of synostosis (fusion of vertebrae), which may be associated with secondary scoliosis. Radiography is necessary for confirmation.
Congenital lordosis (swayback) is associated with hypoplasia of the intervertebral articular processes. In adult horses, degrees of acquired lordosis and kyphosis (roachback) are occasionally seen, which contribute to back weakness. Diagnosis is based on the clinical appearance and can be confirmed by radiography, which reveals an undue curvature of the vertebral column, usually in the cranial thoracic region (T5–10) in lordosis and in the cranial lumbar region (L1–3) in kyphosis.
Hyperkalemic Periodic Paralysis
Hyperkalemic periodic paralysis (HPP) is a hereditary condition of Quarter horses that is the result of a genetic mutation in the skeletal muscle sodium channel gene. It is inherited as an autosomal dominant trait. Most affected horses are heterozygotes.
Polysaccharide Storage Myopathy
see Myopathies in Horses: Chronic Exertional Rhabdomyolysis.
Glycogen Branching Enzyme Deficiency
Glycogen branching enzyme (GBE) deficiency may be a common cause of neonatal mortality in Quarter Horses and related breeds that is obscured by the variety of clinical signs that resemble other equine neonatal diseases. Affected foals lack the enzyme necessary to store glycogen in its branched form and therefore cannot store sugar molecules. The disease is fatal; heart muscle, brain and skeletal muscles are unable to function. Clinical signs of GBE deficiency may include transient flexural limb deformities, stillbirth, seizures, respiratory or cardiac failure, and persistent recumbency. Leukopenia, high serum CK, AST, and γ-glutamyl transferase are present in most affected foals. Gross postmortem lesions are inconclusive. Muscle, heart, or liver samples contain abnormal periodic acid-Schiff-positive globular or crystalline intracellular inclusions in amounts proportional to the foal's age at death. Accumulation of an unbranched polysaccharide in tissues is suggested by a shift in the iodine absorption spectra of polysaccharide isolated from the liver and muscle of affected foals. Skeletal muscle total polysaccharide concentrations are reduced, but liver and cardiac muscle glycogen concentrations are normal. Several glycolytic enzyme activities are normal, whereas GBE activity is virtually absent in cardiac and skeletal muscle, as well as in liver and peripheral blood cells. GBE activities in peripheral blood cells of dams of affected foals and several of their half-siblings or full siblings are ~50% of controls. GBE protein in liver is markedly reduced to absent in affected foals. Pedigree analysis supports an autosomal recessive mode of inheritance.
Last full review/revision March 2012 by Russel R. Hanson, DVM, DACVS, DACVECC