Overview of Congenital and Inherited Anomalies of the Nervous System in Large Animals

Congenital defects of the CNS are, by definition, present at birth. Some congenital defects are inherited; others are caused by environmental factors (eg, toxic plants, nutritional deficiencies, viral infections). For many of these defects, the cause is unknown.

In animals born with a well-developed nervous system (eg, foals, calves, lambs, piglets), the clinical signs of a congenital neurological disorder might be recognizable at birth. In animals born with a less developed nervous system (eg, kittens, puppies), neurological signs might not be apparent until the affected animal begins to walk.

In some inherited neurological diseases (eg, progressive ataxia of Charolais cattle, equine degenerative myeloencephalopathy), clinical signs of the disorder are not evident until the animal is an adult, 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.

• Brainstem disorders can result in cranial nerve dysfunction, postural disturbances, or vestibular dysfunction. In some cases, brainstem disorders also result in weakness and proprioceptive gait disturbances, and more severely affected animals exhibit impaired consciousness. No known brainstem disorders affect large animals.

• Spinal cord disorders do not affect cerebral function or coordination of head movement; however, they can produce weakness, motor dysfunction, or proprioceptive deficits of the limbs, including either dysmetria or decreased 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 of spinal cord disease. In addition, they often cause disturbance of reflex function, pain sensation, or substantial muscle atrophy. In some cases, these deficits are markedly episodic.

• Generalized disorders result in combinations of signs from more than one of these categories of neurological deficits.

Also see Congenital and Inherited Anomalies of the Nervous System in Small Animals.