Congenital and Inherited Disorders of the Nervous System in Dogs
Some congenital defects (defects present at birth), are inherited from the parents, while others are caused by environmental factors in the womb, such as nutritional deficiencies or some viral infections. For many, the cause is unknown.
Puppies are born with a nervous system that is not fully developed, and birth defects may not become apparent until they begin to walk. In some cases, evidence of an inherited disorder may not be seen until the dog has reached adulthood, even though the defect has been present since 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.
Forebrain disorders (defects in the cerebrum) often result in vision problems, changes in awareness or behavior, abnormal movements or postures, and seizures.
Hydrocephalus, commonly known as “water on the brain,” is an excess of fluid that puts pressure on the brain and may damage the cerebrum. This condition is not uncommon in puppies, especially in toy and brachycephalic breeds, such as Pugs, Bulldogs, and Bull Mastiffs. Hydrocephalus usually results in signs similar to those of a cerebral injury, and may worsen over time. However, some animals may not show any obvious signs. Blindness or impaired vision can also develop. Ultrasonography or magnetic resonance imaging (MRI) can confirm the diagnosis. This condition may be treated with corticosteroids, but surgery may be necessary in severe cases.
Idiopathic epilepsy refers to epileptic seizures of unknown cause. It may be inherited in certain breeds, including Beagles, Keeshonden, Irish Setters, Belgian Tervurens, Siberian Huskies, Springer Spaniels, Labrador Retrievers, Golden Retrievers, and German Shepherds. A diagnosis of idiopathic epilepsy depends on eliminating other causes of seizures, particularly structural brain abnormalities (such as hydrocephalus), encephalitis, or metabolic disorders such as hepatic encephalopathy.
Hepatic encephalopathy is usually caused by a birth defect that leads to blood vessel abnormalities within the liver, or in rare cases it may result from an enzyme deficiency in the liver. Breeds often affected include Miniature Schnauzers, Yorkshire Terriers, Cairn Terriers, Australian Cattle Dogs, Old English Sheepdogs, and Maltese Terriers. Nervous system signs are usually evident before the pup is 6 months old. Signs include “staring into space,” inappropriate barking or whining, aggression, and agitation. In advanced disease, depression, blindness, sudden jerking motions, stupor, coma, or seizures can be seen. Hepatic encephalopathy is diagnosed by using radiographic imaging techniques, such as computed tomography or ultrasonography.
Puppy hypoglycemia, or low blood sugar, is seen in toy breeds in the first 6 months of life. It seems to relate to a relative immaturity of the liver. This condition can usually be managed by feeding frequent meals of a commercial puppy food. The problem usually disappears as the animal matures.
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 animal typically has a tremor that does not worsen as the animal matures, and affected animals can be good pets. Hydrocephalus can also be found in animals with a cerebellar disorder.
Cerebellar abiotrophies develop when cells in the cerebellum age prematurely and degenerate. Signs are similar to those seen in severe cerebellar injury, including tremor and poor motor control. The signs get progressively worse over time.
Spinal cord disorders do not affect coordination of head movement but cause a loss of motor function and coordination in the legs or sense of position.
Congenital vertebral malformations involve the bones of the spinal column, called vertebrae. These malformations can cause damage to the spinal cord. They include hemivertebrae (shortened or misshapen vertebrae), block vertebrae (fused together), and butterfly vertebrae (cleft vertebrae). Hemivertebrae are most common in screw-tailed dog breeds, such as Pugs, Bulldogs, and Boston Terriers. Specialized imaging techniques such as computed tomography (CT) scanning may be necessary to determine whether a spinal defect can be corrected by surgery.
In caudal cervical spondylomyelopathy, also called wobbler syndrome, the spine in the neck area is deformed. The most commonly affected breeds include Borzois, Basset Hounds, Doberman Pinschers, and Great Danes. The condition may be inherited, and signs begin to show at a variety of different ages. Signs range from mild difficulty in walking to paralysis of all 4 legs. Affected dogs often keep their neck flexed awkwardly, and the neck may be painful. Surgery can relieve pressure on the spinal cord.
Atlantoaxial subluxation is most common in young toy or miniature breeds of dogs and is seen occasionally in large breed dogs as well. Signs usually develop within the first few years of life and consist of sudden or progressively worsening neck pain or difficulty moving. Signs can be mild or progress to paralysis of all 4 legs. Surgery is necessary to stabilize the dog’s condition, and the outlook for recovery is uncertain.
Peripheral nerve and muscle disorders can result in muscle weakness and awkward or uncoordinated movement similar to that seen in spinal cord disorders. In addition, they often cause a loss of reflexes and pain sensation, or wasting or withering of the muscles. Although there are a number of these disorders that may affect particular breeds, they are quite rare in most dogs.
Congenital deafness occurs most often in Dalmatians but has also been recorded in a number of other breeds, including Australian Shepherds, English Setters, Boston Terriers, and Old English Sheepdogs. The brain stem auditory evoked response test (see Laboratory Tests and Imaging) can identify deaf puppies at an early age.