Some congenital defects (defects present at birth), are inherited from the parents, whereas others are caused by environmental factors (such as toxic plant ingestion, nutritional deficiencies, and viral infections). For many, the cause is unknown. Foals are born with a well-developed nervous system, and disorders may be recognizable soon after birth.
Birth defects of the nervous system are categorized according to the primary region of the nervous system affected: forebrain, cerebellum, spinal cord, peripheral nerve and muscle disorders, or multifocal disorders that include signs of more than one area. Many of these inherited disorders are rare or breed-specific, or both. A few of the more common disorders of each area are described below.
Juvenile epilepsy (also called idiopathic or familial epilepsy) occurs in young foals, particularly Arabians, up to 12 months of age. The foal can have seizures that result in head injuries or blindness. Foals may recover without treatment, but anticonvulsant medication is usually recommended for the first 1 to 3 months.
Narcolepsy occurs in several equine breeds, particularly Shetland ponies. Signs include excessive sleepiness or sudden attacks of paralysis during which the animal is limp and paralyzed but conscious. The animal is otherwise healthy. During narcoleptic episodes, rapid eye movements occur, and at the same time, the animal may also have muscle weakness or collapse.
Cerebellar disorders (defects in the cerebellum) usually result in a tremor and a lack of coordination in both the head and legs.
Cerebellar hypoplasia is a condition in which the cerebellum does not develop completely. The horse typically has a tremor that does not worsen as the animal matures. Hydrocephalus can also be found in animals with cerebellar disorder. Cerebellar abiotrophy was originally thought to be cerebellar hypoplasia in horses (see below).
Cerebellar abiotrophies develop when cells in the cerebellum start to die after the foal is born. This is an inherited condition seen almost exclusively in Arabian foals, with the onset of signs occurring from birth to 9 months of age. The mode of inheritance is recessive, so a "carrier" of the disease is not affected, but breeding of 2 carriers has a 25% chance of producing an affected foal. Signs are similar to those seen in severe cerebellar injury, including tremor and poor motor control. Young horses may show exaggerated action of the front legs and a wide-based stance. The signs get progressively worse over time.
Neuraxonal dystrophy is inherited in Morgan horses, and signs usually develop around 6 to 12 months of age. It occurs occasionally in the German horse breed Hafflinger. The legs become weak and movement can appear stiff, awkward, or uncoordinated. It is now thought that neuraxonal dystrophy in horses is the same as equine degenerative myeloencephalopathy (see below).
Equine degenerative myeloencephalopathy has been mainly associated with vitamin E deficiency, but it may be inherited in the Appaloosa and other breeds. The connections between the spine and the cerebellum degenerate, and results in a slowly progressive loss of coordination or paralysis of all 4 legs that starts as early as 7 months of age.
Cervical vertebral stenotic myelopathy (also known as cervical spondylomyelopathy or wobbler syndrome) may have some genetic basis in young, rapidly growing horses, particularly Thoroughbreds. Tennessee Walking Horses and Warmbloods also appear to be predisposed. Males are affected more commonly than females. Feeding excessive amounts of a rich diet is an important contributory factor, and the signs may be reversed in animals less than 9 months of age if the diagnosis is made early. Signs are caused by malformations that occur during rapid growth or injuries to the vertebrae in the neck, and usually appear between 6 months and 3 years of age. Diagnosis involvestaking x-rays or performing other spinal examinations to look for deformity of the neck vertebrae. Treatment usually involves surgery to decompress these areas and sometimes, vertebral stabilization with an implant placed in the vertebrae. The outlook for recovery is variable.
Occipitoatlantoaxial malformation is inherited in Arabian foals and may also be seen in foals of miniature horses. Signs include a progressive loss of coordination, partial paralysis of all 4 legs, and standing with an extended neck. Affected foals usually are partially paralyzed at birth, although other neurologic signs may not develop for several years. Diagnosis requires x-rays.
Spina bifida is seen in most species and usually results in dysfunction of the tail and anus, incontinence, and sometimes hind leg weakness.
Hyperkalemic periodic paralysis is an inherited disorder of Quarter Horses or Quarter Horse-derived breeds. Signs usually appear between the ages of 2 to 3 years and include episodes of muscle tremor and sometimes recumbency, both of which may be brought on by exercise. Blood potassium levels are increased during these episodes. Diagnosis involves testing for the causative gene and is supported by results of needle electromyography. Exercise, nutritional management, and drug treatment can lessen the severity and frequency of episodes, but it does not cure the disease.
Myotonia congenita is an inherited disorder that causes muscle rigidity, marked dimpling when tapping the muscle belly, and a stiff, stilted gait. Diagnosis is via an electromyogram.
Also see professional content regarding congenital and inherited anomalies of the nervous system.