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.
Kittens 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 cat 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.
Kittens with hydranencephaly are born without a cerebrum. The mostly empty skull is instead filled with cerebrospinal fluid. The disorder has been described mainly in kittens who were exposed to feline panleukopenia virus (parvovirus) while in the womb. This can also cause brain stem malformations and cerebellar hypoplasia.
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. 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. Ultrasonography or magnetic resonance imaging (MRI) can confirm the diagnosis. This condition may be treated with medications, but surgery may be necessary in severe cases.
Hepatic encephalopathy is a neurologic syndrome caused by liver disease. It 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. Signs include “staring into space,” excessive drooling, inappropriate meowing or crying, aggression, and agitation. In advanced disease, depression, blindness, sudden jerking motions, stupor, coma, or seizures can be seen. Signs are usually noticed before the kitten is 6 months old. Hepatic encephalopathy is diagnosed by using imaging techniques, such as computed tomography or ultrasonography. Blood tests may aid in diagnosis.
In cerebellar hypoplasia, the cerebellum does not develop completely. This is caused by exposure of the kitten to feline panleukopenia virus (parvovirus) while in the womb. The kitten typically has a tremor that does not worsen as the cat matures, and affected animals can be good pets. Magnetic resonance imaging (MRI) is used to diagnose the disorder. Hydrocephalus or hydranencephaly can also be found in animals with cerebellar disorders.
Congenital vestibular disease has been reported in Burmese cats. The vestibular system, located within the brain and middle and inner ear, is responsible for maintaining balance. Kittens born with dysfunction of the vestibular system have impaired balance and posture and may be permanently deaf. There is no treatment, but animals can learn to compensate, improving their balance and posture.
Spina bifida occulta, which literally means "split spine," is a birth defect seen in Manx cats. In affected kittens the spine does not close properly during development in the womb. It is seen as a result of the spinal changes resulting in the tailless body type. Affected cats may not be able to control urination or defecation and may have weakness and decreased reflexes in the rear limbs. These cats may also be born with deformed vertebrae (called sacrocaudal dysgenesis). The outlook for severely affected cats is poor.
Neuropathy of hereditary hyperchylomicronemia (hyperlipidemia) is an inherited disorder that causes body-wide peripheral nerve damage in cats. Signs do not develop until the kitten is at least 8 months of age. This condition causes granules of fat to be deposited within nerves, and some studies show that the signs can be controlled by a low-fat diet.
Congenital myasthenia gravis occurs in kittens. The disease affects the connection between muscles and nerves. Affected kittens are weak after playing and may regurgitate due to an enlarged esophagus. Medications are available for treatment, but the outlook is uncertain. Kittens born with myasthenia gravis tend to do worse than older cats that develop it.
Hypokalemic myopathy of Burmese cats is a muscle disorder caused by low levels of potassium in the blood. It is an inherited disorder that causes episodes of weakness in the neck muscles. During these episodes, affected cats cannot lift their heads. The disease usually occurs when kittens are 3 to 4 months old. The condition is usually successfully treated by supplementing the cat's diet with potassium.
Muscular dystrophies can occur in male cats due to an inherited defect in the production of an essential muscle protein. Early on, affected cats display muscle stiffness and enlargement, difficulty swallowing, and weakness. As the cat ages, muscle wasting occurs, and the cat may be unable to lift its ankles from the ground. Veterinarians use blood tests and muscle biopsies to diagnose the disorder. The outlook is uncertain to poor, and there is no treatment.
Glycogen storage disease is an inherited disorder that causes glycogen (a type of sugar) to accumulate excessively in the cells of the body. The disease can cause muscle weakness and difficulty exercising in Norwegian Forest cats.
Congenital megaesophagus is an enlarged esophagus that is inherited in Siamese cats. Signs include frequent regurgitation and pneumonia. The outlook is uncertain.
Animals with multifocal disorders display signs consistent with more than one anatomical location.
Spongiform degenerative conditions have been described in Egyptian Mau and Burmese kittens. Tissue in the central nervous system deteriorates, causing incoordination, an abnormal gait, head tremors, muscle contractions, and behavior changes. The development of the disease is not understood, and the outlook is poor.
Lysosomal storage disorders are a rare group of conditions that result from incomplete breakdown of a protein, carbohydrate, or fat. Their byproducts build up within the body and can be toxic to cells. Signs usually, but do not always, develop early in life. The outlook is currently poor for all types of these disorders, but novel treatments are being studied.
Pendular nystagmus is an abnormal movement of the eyes seen in various breeds of Asian cats. The eye movement is similar to that of a pendulum on a clock. The condition does not appear to cause a problem for affected cats.
Congenital deafness occurs in cats, especially those with blue eyes and white fur. The brainstem auditory evoked response (BAER) test can be used to identify deafness in one or both ears.
Also see professional content regarding congenital and inherited anomalies of the nervous system.