Many of the nervous system disorders of dogs are due to congenital abnormalities affecting the head and the spine. They are mainly inherited conditions, but in some cases, they are acquired by the puppy as it develops in the womb. Viral infections in the mother as well as deficiency diseases during pregnancy can result in these defects in the puppy developing in the womb. The actual cause of certain congenital disorders has not been determined.
Since the nervous system of the puppy is immature at birth, these abnormalities typically appear a few weeks later when the puppy starts walking. Some genetic disorders become evident only when the puppy is fully grown even though they are congenital disorders.
Congenital nervous system disorders are grouped into various categories such as forebrain disorders, spinal cord disorders, peripheral nerve and muscle disorders etc., with respect to the area that is mainly affected. Some abnormal conditions are known as multifocal disorders because several areas are affected. Many of the congenital nervous system disorders are specific to certain breeds while others are more general in nature, affecting all kinds of breeds. Some disorders are rarer than others too.
Some of the commonly occurring nervous system disorders are:
The cerebrum, or the forebrain, is mainly involved in these disorders, resulting in problems that affect the eye. Other common symptoms include behavioral changes, clumsy movements and awkward postures. The dog may have seizures too.
Hydrocephalus In Dogs
This condition occurs when the cerebrospinal fluid builds up inside the skull, exerting pressure on the front portion of the brain. The name literally means ‘water in the brain.’ Brachycephalic dog breeds such as Bulldogs, Bull Mastiffs and pugs are more prone to hydrocephaly. The symptoms resemble the typical signs of cerebral injury, but this is a progressive condition that results in the steady worsening of the symptoms. But not all dogs with hydrocephalus display these typical symptoms.
Continued pressure on the cerebrum may lead to various levels of vision impairment and blindness. The disorder can be diagnosed by ultrasonograpy or MRI scans. Administration of corticosteroids may offer temporary relief, but surgical intervention is essential for most cases.
Idiopathic epilepsy In Dogs
When the dog suffers from epileptic seizures for no apparent reason, it is called idiopathic epilepsy. While the exact cause is not known, some breeds are found to be predisposed to it, so it is considered an inherited disorder in them. German Shepherds, Golden Retrievers, Labradors, Springer Spaniels, Irish Setters, Keeshonden, Siberian Huskies, Belgian Tervurens and Beagles belong to this category.
Idiopathic epilepsy is suspected when other probable causes that may result in seizures, such as encephalitis, hydrocephalus, hepatic encephalopathy and structural abnormalities, are found to be absent. Diagnostic tests of this disorder are geared towards eliminating these causes.
Congenital abnormalities in the blood vessels of the liver and the deficiency of an enzyme are the main causes of hepatic encephalopathy in dogs. Several breeds of dogs, such as Maltese Terriers, Yorkshire Terriers, Cairn Terriers, Old English Sheepdogs, Australian Cattle Dogs, and Miniature Schnauzers, are usually affected more often. The characteristic symptoms appear early in life. Some of them are agitation and aggression as well as whining and barking for no particular reason. The dog may stare into space, or become depressed. Tremors and seizures may be present. The dog may become blind in many cases, or it may fall into a stupor or coma.
Several tests such as ultrasonography, CT scan and MRI scan may help confirm a diagnosis of hepatic encephalopathy.
An abnormally low level of blood sugar, mainly occurring puppies of miniature breeds and toy dogs, is the main characteristic of puppy hypoglycemia. It is generally seen only until the puppy is 6 months old. The lack of maturity of the puppy’s liver could be the reason for this abnormality. Feeding the puppy frequently with food specially formulated for their age is the solution. As the puppy gets older, and its liver becomes more mature, puppy hypoglycemia gets resolved.
Abnormalities of the cerebellum result in characteristic symptoms of cerebellar disorders such as loss of coordination in the limbs and the head, and frequent tremors.
Incomplete development of the cerebellum is the main characteristic of this abnormality. Tremors are the main symptoms, but they don’t become worse with age. Hydrocephalus may be present with any of the cerebellar disorders. In spite of the symptoms, a dog with cerebellar hypoplasia may be able to live comfortably without any treatment.
Premature aging of the nerve cells that constitute the cerebellum, and their subsequent deterioration, are responsible for this disorder. This is a progressive condition, with symptoms indicative of extensive damage to the cerebellum. The dog may have severe tremors and loss of motor control.
Disorders of the Spinal Cord
These neurological disorders mainly affect the motor function and the positioning of the dog’s limbs. The leg movements are uncoordinated and awkward. Head movements of the dog are not affected by spinal cord disorders.
Congenital vertebral malformations
Congenital deformities in the vertebrae that make up the spinal column can result in spinal cord disorders. Several types of vertebral deformities occur. When the vertebrae are shortened, or have an abnormal structure, they are called hemivertebrae. When they are fused together, they are known as block vertebrae. Those with a cleft are named butterfly vertebrae. Screw-tailed breeds, including Boston Terriers, Pugs, and Bulldogs commonly have hemivertebrae. Surgical intervention is planned only after careful study of the vertebral structures through CT scans and other accurate imaging techniques because not all defects can be surgically corrected.
Caudal Cervical Spondylomyelopathy
This neuromuscular disorder is also known as wobbler syndrome. The affected dogs have spinal deformities in the neck. This condition is thought to be inherited, and it is more prevalent in Great Danes, Basset Hounds, Borzois, and Doberman Pinschers. The characteristic symptoms may not be apparent at birth, but they start to appear later at various ages. An awkwardly held neck, accompanied by pain in the area is a typical symptom. In mild cases, the dog may have just a little difficulty in walking, but severe cases result in the paralysis of limbs. All four limbs may be affected. Surgical intervention to relieve the pressure inflicted by the deformed spine on the spinal cord may help.
Though this condition can occur in large dog breeds, it commonly affects toy breeds and miniature dogs. The characteristic symptoms appear early in life with pain in the neck and movement difficulties. It progresses either gradually, or suddenly, until paralysis affects all the limbs of the dog. Early surgical intervention may help stem the progress of the disorder, but complete recovery is doubtful.
Peripheral Nerve and Muscle Disorders
Muscle weakness, as well as loss of coordination resulting in awkward movements, may indicate peripheral nerve and muscle disorders. The main symptoms of this condition resemble the characteristic symptoms of spinal cord disorders. They may be accompanied by other symptoms such as loss of muscle mass, loss of sensation of pain, and lack of reflex reactions. Even though peripheral nerve and muscle disorders are rare in most breeds of dogs, a few specific breeds are affected by them.
Other Birth Defects
Deafness as a congenital defect is seen in many breeds of dogs such as Boston Terriers, English Setters, Old English Sheepdogs and Australian Shepherds, but it is more prevalent in Dalmatians. The defect can be diagnosed in puppies quite early in life by conducting the brain stem auditory evoked response test.