Congenital defects of the CNS are, by definition, present at birth. Some congenital defects may be inherited, others may be caused by environmental factors (eg, toxic plants, nutritional deficiencies, viral infections); for many, the cause is unknown. In those animals born with a well-developed nervous system (foals, calves, lambs, pigs), the clinical signs of a congenital neurologic disorder may be recognizable at birth. Kittens and puppies are born with a less well-developed nervous system, and in those species, neurologic signs may not be apparent until they begin to walk.
In some inherited neurologic diseases, clinical signs of the disorder are not seen until the animal is an adult (eg, caudal fossa malformation syndrome/syringomyelia, neuronal ceroid lipofuscinosis), even though the defect has obviously been present since birth.
Congenital lesions can be categorized according to the primary region of the nervous system affected.
Forebrain disorders (cerebrum and thalamus) primarily result in clinical signs such as visual disturbances, changes in mental status or behavior, abnormal movements or postures, and seizures.
Cerebellar disorders usually result in an intention tremor, wide-based stance, and incoordination (dysmetria) of the head, trunk, and limbs.
Brain stem disorders may result in cranial nerve dysfunction, postural disturbances, or vestibular dysfunction. In some cases, brain stem disorders may also result in weakness and proprioceptive gait disturbances, with more severely affected animals exhibiting impaired consciousness.
Spinal cord disorders do not affect cerebral function or coordination of head movement but may produce weakness, motor dysfunction, or proprioceptive deficits of the limbs, including either dysmetria or reduced proprioceptive placing in more than one limb. (Also see Diseases of the Spinal Column and Cord.)
Neuromuscular disorders include diseases of the peripheral nerves, neuromuscular junction, or muscles. Disorders of these systems can result in signs of weakness and ataxia similar to those seen in spinal cord disease. In addition, they often cause disturbance of reflex function, pain sensation, or marked muscle atrophy. In some cases, these deficits are markedly episodic.
Multifocal disorders result in combinations of signs from more than one of these categories of neurologic deficits.