Dwarfism in Horses
Dwarfism is the lack of appropriate growth, resulting in a smaller horse.
A dwarf horse can be proportionate or disproportionate. Proportionate dwarfism is a result of a deficiency in growth hormone, whereas disproportionate dwarfism is a result of abnormal thyroid hormone levels. The latter results in foals with musculoskeletal immaturity, characterized by delayed cuboidal bone development, large head, silky hair coat, floppy ears, and mandibular brachygnathia.
Determination and interpretation of either growth hormone assays or thyroid hormone function are not entirely developed or understood in horses, hence the importance of clinical diagnosis. Efforts toward characterizing thyroid function and growth hormone levels should be undertaken to prevent overdiagnosis of this condition.
Nitrates can impair thyroid gland function, and nitrate exposure has been proposed as a cause of congenital hypothyroidism and dysmaturity syndrome of foals. Excessive dietary nitrate intake has been confirmed in the dams of certain foals born with this condition.
Cervical Vertebral Stenotic Myelopathy (Wobbler Syndrome) in Horses
Cervical vertebral stenotic myelopathy (wobbler syndrome) is characterized by postnatal deformation of the cervical vertebrae, resulting in stenosis of the vertebral canal. In turn, this causes spinal cord compression and results in progressive proprioceptive deficits, primarily in the hindlimbs.
Thoroughbreds, Quarter Horses, and warmblood breeds have a genetic predisposition for the syndrome, and the condition is more common in males than females.
There are two pathological syndromes:
Cervical vertebral instability (dynamic compression) usually affects fast-growing horses and manifests as clinical disease (ataxia) at age 8–18 months.
Cervical static stenosis (static compression) tends to manifest later in life as a result of cervical osteoarthropathy (osteoarthritis of the cervical facet joints) and tends to be located more caudally in the neck (C5-C7).
Compression can happen anywhere in the spinal column between C2 and C7, most commonly at C3-C4 and C4-C5.
Diagnosis of both syndromes is by radiographic examination of the cervical spine and clinicopathological testing (including CSF) to rule out other neurological diseases. In cases of dynamic compression, radiographs will often be inconclusive, and CT myelography is indicated.
Treatment includes restricting protein intake to slow rapid growth in foals, vitamin A and E supplementation, restricting exercise, medical management of any cervical facet osteoarthritis (intra-articular medications such as corticosteroids or orthobiologics), or surgical fusion of the vertebrae at the affected segment.
Surgical fusion options include use of a kerf-cut cylinder or hollow, perforated stainless-steel dowel ("Bagby basket") implanted between the vertebrae, ventral locking compression plate fixation, and subtotal dorsal decompression laminectomy (less commonly performed due to high risk of cervical fracture).
Surgical complications are frequent and include infection, hemorrhage, fracture, implant failure, neck pain, seroma, and migration of implants.
Surgical success depends on the horse's temperament and use. Ataxia improves in most treated horses.
Other Skeletal Defects in Horses
Congenital skeletal malformations in horses include the following:
wry nose (twisted nose)
torticollis (twisted neck)
scoliosis (lateral deviation of the back)
lordosis (concave deviation of the back when viewed from the side)
kyphosis (convex deviation of the back when viewed from the side)
synostosis (fusion of vertebrae)
spina bifida (incomplete closure of the bones in the spine)
hydrocephalus (enlargement of the head due to abnormal accumulation of fluid within the skull)
Although all of these conditions are uncommon in foals, congenital scoliosis is encountered most frequently. Severity is often difficult to assess on clinical examination. A better appreciation of the condition can be obtained by radiographic examination. In mild cases, improvement is spontaneous and may be complete. Even in more severe cases, obvious abnormality in gait or maneuverability rarely occurs. However, because they appear unlikely to be able to withstand being ridden or worked, affected foals are frequently euthanized.
Another occasional congenital deformity is vertebral synostosis (fusion of vertebrae), which may be associated with secondary scoliosis. Radiographic examination 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 present and contribute to back weakness.
Diagnosis is based on 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 (see scoliosis radiograph and lordosis photograph).
Other, less common, skeletal malformations include incomplete closures of the bony spinal canal (cervical meningomyelocele, spina bifida) and hydrocephalus (abnormal accumulation of fluid in the cranial vault with resultant enlargement of the head).
Courtesy of Dr. Thomas Lane.
Courtesy of Dr. Thomas Lane.
Hyperkalemic Periodic Paralysis in Horses
Hyperkalemic periodic paralysis (HYPP) is marked by sudden attacks of paralysis which, in severe cases, may lead to collapse and sudden death. HYPP is an inherited missense mutation in the gene encoding the alpha chain of adult skeletal muscle sodium channels, resulting in increased sodium permeability across skeletal muscle cell membranes. (See image of HYPP.)
Courtesy of Dr. Sameeh M. Abutarbush.
HYPP can affect Quarter Horse, Paint, and Appaloosa progeny tracing back to the Quarter Horse sire named Impressive and must be eliminated from any reproductive program. Most affected horses are heterozygotes. A sequela of HYPP in horses that undergo general anesthesia is malignant hyperthermia syndrome, in which a progressive increase in body temperature, muscle rigidity, and metabolic acidosis lead to rapid death.
Myotonic Dystrophy-Like Disorder in Horses
In myotonic dystrophy-like disorder, a rare disorder of newborn foals, affected animals may struggle to stand after birth or have progressive stiffness and weakness beginning at about 1 month of age.
Quarter Horses, Thoroughbreds, and Standardbreds are sporadically affected.
Percussion of the muscle, particularly the gluteal muscle, results in muscle dimpling. Progressive weakness ultimately leads to euthanasia, and profound muscular hypertrophy with fibrosis and fat infiltration of the muscle is observed at necropsy.
There is no evidence the disorder is genetic.
Equine Polysaccharide Storage Myopathy
Equine polysaccharide storage myopathy (PSSM) is a glycogen storage myopathy that results in abnormal accumulation of glycogen in muscle cells. Two forms of PSSM are recognized: PSSM1 and PSSM2. While PSSM1 is caused by a known genetic defect in the glycogen synthase 1 gene (GYS1) and is autosomal dominant, the cause of PSSM2 is unknown.
Both forms can cause the following clinical signs:
exertional rhabdomyolysis ("tying up": sweating, lameness, reluctance to move)
progressive weakness
muscular atrophy
chronic pelvic limb lameness
In severe cases, an acute severe episode of rhabdomyolysis can cause extensive muscle necrosis, renal failure, or respiratory failure due to diaphragmatic muscle failure.
PSSM is most common in draft, warmblood, and Quarter Horse breeds, and genetic tests exist for PSSM1.
Affected horses can be managed with low-glucose diets and regular low-intensity exercise. Horses experiencing an episode of exertional rhabdomyolysis should discontinue exercise and be provided with fluid and electrolyte support.
Glycogen Branching Enzyme Deficiency in Horses
In Quarter Horses and related breeds, glycogen branching enzyme (GBE) deficiency may be a common cause of neonatal mortality that is obscured by 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 and high serum activities of CK, AST, and gamma-glutamyl transferase are present in most affected foals.
Gross postmortem lesions are inconclusive. Periodic acid–Schiff (PAS) stain to detect glycogen in tissues can be helpful. Muscle, heart, and liver samples lack the normal background staining indicating the presence of glycogen and contain abnormal PAS–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 iodine absorption spectra of polysaccharide isolated from the liver and muscle of affected foals. Skeletal muscle total polysaccharide concentrations are decreased; however, 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 approximately 50% of those of controls. GBE protein in liver is markedly decreased to absent in affected foals.
Pedigree analysis supports an autosomal recessive mode of inheritance.
