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, brain stem, 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 cerebrospinal 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 omeprazole or 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. Blood tests may aid in diagnosis.
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, abnormal stance, and a lack of coordination in the head, torso, 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, except that the signs develop after birth and get progressively worse over time.
Brain stem disorders can result in dysfunction of the cranial nerves, weakness, and an inability to balance, walk, or stand correctly. Severely affected animals may appear dull or unaware of their surroundings.
Congenital vestibular disease appears to be inherited in German Shepherds, English Cocker Spaniels, and Doberman Pinschers. It results in permanent deafness and balance and posture dysfunction. There is no treatment, but animals can learn to compensate, improving their balance and posture.
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, neuromuscular junction (the point of contact between nerves and muscles), 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 several of these disorders may affect particular breeds, they are quite rare in most dogs.
Hypertrophic neuropathy of Tibetan Mastiffs is an inherited disease that causes rear leg weakness in Tibetan Mastiff puppies by 8 weeks of age. The condition can progress to weakness of all legs. The outlook is uncertain, and there is no treatment. Some puppies can regain the ability to walk but remain weak.
Alaskan Malamute polyneuropathy affects 10- to 18-month-old Alaskan Malamutes. These dogs typically have difficulty exercising, weakness in the rear legs that spreads to the front legs, muscle wasting, and, sometimes, paralysis of the vocal cords (laryngeal paralysis). There is no effective treatment. Signs do not worsen in some dogs, but most affected dogs have to be euthanized due to worsening of their disability.
Congenital laryngeal paralysis is vocal cord paralysis that occurs in Siberian Huskies, Rottweilers, Bull Terriers, and Bouvier des Flandres puppies less than a year old. Affected dogs have difficulty breathing and exercising. Veterinarians diagnose the condition by looking at the vocal folds while the dog is under sedation. Laryngeal paralysis can also occur with widespread peripheral nerve dysfunction in several breeds, including Dalmatians, Rottweilers, and Pyrenean Mountain Dogs (Great Pyrenees). Prognosis is guarded to poor.
Sensory neuropathy of longhaired Dachshunds causes incoordination, an impaired sense of paw position, and decreased sensation in 8- to 12-week-old longhaired Dachshund puppies. They can also have impaired digestive and urinary functions. There is no treatment, but these dogs can have a relatively good quality of life as long as they do not chew excessively on their bodies.
Sensory neuropathy of Border Collies causes incoordination, an impaired sense of paw position, and decreased sensation in 5- to 7-month-old Border Collies. There is no treatment, and unfortunately, the disease progresses to a point that most affected dogs have to be euthanized.
Sensory neuropathy of Pointers is seen in English Pointers in the USA and Shorthaired Pointers in Europe. When these puppies are less than 6 months old, they begin to chew excessively on their toes. Their ability to feel pain is reduced in the front legs and absent in the rear legs. There is no treatment, and the outlook is poor.
Signs of inherited polyneuropathy of Leonberger dogs begin when they are 1–9 years of age. The signs include weakness, difficulty exercising and breathing, and a change in the sound of the bark.
Musladin-Lueke syndrome is a condition that appears in Beagles shortly after birth. Muscle contracture and scarring (fibrosis) cause these puppies to look like they are walking on their "tip-toes." Affected dogs can have thickened ear cartilage and wide-set eyes. Seizures are also possible. There is no treatment, but the signs are usually not progressive.
Congenital myasthenia gravis is noticed in Parson Russell Terriers, Smooth-haired Fox Terriers, and Springer Spaniels when they are 5–10 weeks old. It has also been seen in 12- to 16-week-old Gammel Dansk Hønsehund dogs. These puppies are weak after playing and may regurgitate due to an enlarged esophagus. The outlook is uncertain, and puppies that are born with myasthenia gravis tend to do worse than older dogs that develop it.
Scotty cramp causes increased muscle tone in Scottish Terrier puppies that worsens with excitement, exercise, and poor health. The puppies have an exaggerated gait and arched spine severe enough to cause them to somersault when they run. Muscle relaxants can relieve these signs.
Congenital myoclonus of Labrador Retrievers (familial reflex myoclonus) causes muscle spasms and increased muscle tone in young Labs. These puppies have such rigid muscles that they may be unable to walk. The outlook is very poor.
Myotonia congenita is severe stiffness upon rising in Chow Chows, Staffordshire Terriers, Great Danes, and Miniature Schnauzers. The signs are similar to those seen in "fainting" goats that freeze and fall over when startled. The outlook is uncertain, but medications can significantly improve signs.
Labrador Retriever myopathy causes a stiff gait and muscle wasting in puppies as young as 3 months of age. An affected dog may not even be able to keep its head up. Exercise, stress, and cold temperatures worsen signs. Fortunately, the signs do not worsen after 6–8 months of age, and these dogs can be good pets.
Dermatomyositis of Collies and Shetland Sheepdogs occurs in puppies just a few months old. It affects Collies, Shetland Sheepdogs, Beauceron Shepherds, Pembroke Welsh Corgis, Australian Cattle Dogs, Lakeland Terriers, Chow Chows, German Shepherds, and Kuvasz dogs. It causes withering of the jaw muscles used for chewing and the muscles in the lower legs. This can lead to weakness, difficulty opening the mouth, and regurgitation due to an enlarged esophagus. Affected dogs also have skin inflammation on the face and legs. The signs, which are usually not severe, may come and go.
Glycogen storage diseases cause muscle weakness and difficulty exercising in young German Shepherds, Akitas, Lapland dogs, and English Springer Spaniels.
Mitochondrial myopathy has been seen in Clumber and Sussex Spaniels and in Old English Sheepdogs. Affected dogs have difficulty exercising and can collapse. The condition is diagnosed by examining tissue samples taken by biopsy.
Central core myopathy can cause weakness, muscle loss, trouble exercising, and collapse in young Great Danes. It has been reported in the United Kingdom. Signs begin at approximately 6 months of age. The outlook is poor.
Congenital megaesophagus describes an enlarged esophagus seen at birth. It is inherited in Wirehaired Fox Terriers and Miniature Schnauzers and might be inherited in German Shepherds, Great Danes, Irish Setters, Newfoundlands, Chinese Shar-Pei, and Greyhounds. Food gets trapped in the enlarged esophagus and is then regurgitated. These dogs can inhale this food, leading to pneumonia. The outlook is uncertain.
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.
Also see professional content regarding congenital and inherited disorders of the nervous system.