Prolapsed intervertebral disc

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A prolapsed intervertebral disc most commonly affects the 20-55 year age group, and is most often seen at the L4/5 and lumbosacral levels. It may also affect the cervical discs, particularly at C5/6 and C6/7. The thoracic discs are rarely affected. Acute tearing or chronic degeneration of the posterior lamellae of the anulus fibrosus allows deformation and herniation of the softer nucleus pulposus. The disc most often prolapses just lateral to the posterior longitudinal ligament and can compress one or two spinal nerves unilaterally. Much less commonly, the prolapse is central, in the midline posteriorly. The compression of neural structures may then be bilateral, affecting the cord itself or the whole cauda equina. If the damaged anulus ruptures completely, some of the nuclear tissue may escape into the vertebral and 'root' canals. This sequestrated material may migrate within the canals and cause nerve compression at spinal levels distant from that of the disc rupture. The disc material itself may have an irritative effect on the spinal nerve.


Regarding the anatomy of the vertebral canal and intervertebral foramen in relation to disc prolapse, it is important to understand that one or both of two spinal nerves and their roots may be affected by a single prolapse, depending upon the exact site of the prolapse in the horizontal plane. At the level of each disc and foramen, there are two spinal nerves (and their roots) to consider: these are the exiting nerve and the traversing nerve (Macnab & McCulloch 1990). The nerve usually affected at lumbar levels is the traversing nerve, which crosses the back of the disc on its way to become the exiting nerve at the level below. Thus a lumbosacral (i.e. L5/S1) disc prolapse usually compresses the S1 nerve. However, a prolapse may affect the exiting nerve at its own level.

At cervical levels, because the roots and nerve leave the vertebral canal almost horizontally, the prolapse usually affects the exiting nerve. This nerve will still bear the number of the vertebra below the affected disc, because cervical nerves exit the canal above the pedicle of their numerically corresponding vertebra.

Neurological presentation will include signs and symptoms of spinal nerve damage at the affected level. Thus pain and sensory loss will be dermatomal in distribution. Sensory changes usually precede motor loss.

Internal disruption of a lumbar intervertebral disc is more common than disc prolapse, and is now an increasingly recognized cause of back pain. Typically, the nucleus is decompressed and the inner lamellae of the anulus appear to collapse into it.

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