If the protrusion or the bulge does not press on any sensitive structures there are no symptoms, because the disc has almost no nerve supply of its own. If the bulge presses against one of the ligaments which bind the spine together, or against a nerve root, the pain can be intense. This condition, called a prolapsed disc, is what is popularly, but incorrectly, called a slipped disc. A disc cannot, in fact, slip out of place, because its fibres are knitted into the bone of the adjacent vertebral bodies.
Any disc may suffer a prolapse but, for reasons that are not well understood, those most frequently affected are the last two in the lumbar spine - the one that lies between the fourth and fifth lumbar vertebrae and the one between the fifth and the sacrum. One reason why discs at those sites become affected may be because those are the points of maximum movement for the lumbar spine.
The commonest direction for a prolapse is backwards and sideways. The symptoms arise from the irritation of the sensitive structures and the associated inflammation, similar to the inflammation following any injury. This develops over a day or so, and spreads to involve other tissues at the same vertebral level. It may cause back pain or sciatic pain (down the leg), depending on whether it affects the tissues of the spine or of the nerve root, or both.
Prolapsed discs resulting from violent injury are probably likeliest to occur in young people who are most apt to go in for strenuous games, ‘working out’, disco dancing, and so forth; young nurses also have a fairly high rate of disc injury, through lifting and turning patients.
In middle-aged people, however, it is gradual degeneration of the casing of the disc, rather than violent exercise, that is the commonest cause of disc prolapse. The fibrous outer casing of the disc gradually weakens and grows stiffer, developing cracks through which the nucleus can leak out.
As a small compensation, disc prolapse becomes much less common once middle age is past and this, too, is due to degenerative changes. The nucleus, losing much of its moisture, shrinks considerably, and though it may spread out, it is less apt to leak out of the casing. However, this change brings its own problems, as the disc loses some of its shock-absorbing ability.