A spinal cord abnormality, most of which are focal, is called myelopathy, and may cause somatosensory, motor, or autonomic dysfunction for parts of the limbs or torso connected to the affected spinal cord levels, as well as all the levels below. For example, a myelopathy about halfway up the spinal cord may cause abnormalities of both legs and the lower trunk, while a myelopathy near the top may affect all four limbs and the entire torso. The term sensory level is often used to refer to somatosensory loss in the bilateral dermatomes of the affected spinal cord level, as well as all the levels below; this pattern of somatosensory loss can be distinctive for myelopathy. Weakness of both legs is called paraparesis, and weakness of all four limbs is called quadriparesis; these patterns often occur with myelopathy. Most of the motor abnormalities of myelopathy are usually related to upper motor neuron axons descending through the affected spinal cord levels, but there may also be lower motor neuron abnormalities of the myotomes of the affected level because their somas may be involved there. Common autonomic abnormalities with myelopathy are loss of control of urination, defecation, and genital functions.
Many types of pathology often cause focal myelopathy syndromes, including mechanical, immune, neoplastic, infectious, and nutritional disorders. Mechanical disorders are common causes of myelopathy, including suddenly from traumatic spinal cord injury, or more slowly by compression from spondylosis. The autoimmune disorder multiple sclerosis commonly causes a fairly rapid focal demyelinating lesion of the spinal cord. Neoplasia, particularly the spread of cancer from other tissues into the spine, often compresses the spinal cord. The nutritional disorder vitamin B12 deficiency and the infectious disorder human immunodeficiency virus can both cause parts of the spinal cord to slowly degenerate.