It is often assumed that back pain is synonymous with disc herniation, but this is certainly an over- statement. Even the finding of a disc lesion on magnetic resonance imaging is not proof that it is causing the patient’s symptoms. Other mechanical causes of back pain include osteoarthritis of the facet joints and other degenerative changes in the spine; these can give rise to obstruction of the epidural veins, fibrosis in and around the nerve roots, and atrophy of the neurons within the nerve roots leading to a deafferentation syndrome. There are thus many possible secondary causes of pain but there is increasing awareness today that much chronic back pain arises centrally.
Changes, which may be long-lasting, occur in the dorsal horns of the spinal cord. The changes may spread so that the pain is perceived as coming from a much wider area and may help to explain the widespread sensitivity of the back and lower limbs often seen in patients with chronic back pain. Descending impulses from the brain can increase the activity of the cells in the dorsal horn, which helps to explain why psychological factors are important. The anterior cingulate cortex, thalamus, and lenticular nucleus are areas in the brain that are activated by pain. Although we do not yet know how acupuncture affects these central structures, it is plausible to suppose that they may do so.