This is the most common cause of compression of the spinal cord/or nerve roots. The vertebral bodies are separated by the intervertebral disc,each consisting of an outer rim of cartilage,the annulus fibrosus, and a central core of soft gelatinous material,the nucleus pulposus.
Prolapse of a disc is herniation of the nucleus pulposus,causing the annulus fibrosus and the posterior longitudinal ligament to protrude into the neural canal. It is most common in the lumbar region,usually below the level of the spinal cord,and therefore affects nerve roots only. If it occurs in the cervical region,the cord may also be compressed. Herniation may occur suddenly,typically in young adults during strenuous exercise or exertion,or progressively in older people when bone disease or degeneration of the disc leads to rupture during minimal exercise. The hernia may be:
- One sided,causing pressure damage to a nerve root.
- Midline,compressing the spinal cord,the anterior spinal artery and possibly bilateral nerve roots.
The outcome depends upon the size of the hernia and the length of the time the pressure is applied. Small herniations cause local pain due to pressure on the nerve endings in the posterior longitudinal ligament.
Large herniations may cause:
- Unilateral or bilateral paralysis.
- Acute or chronic pain perceived to originate from the area supplied by the compressed sensory nerve.
- Compression of the anterior spinal artery,causing ischaemia and possibly necrosis of the spinal cord.
- Local muscle spasm due to pressure on motor nerves.
