Feature

A Pain In The Back

Most people suffer from lower back pain at some time, but the underlying causes remain unknown and the effects on the nervous system of treatments like manipulation are largely uninvestigated.

By Dr Noel Dawson

Eighty per cent of adults will suffer from lower back pain at some time in their lives. At any given point in time, up to 30% will be experiencing it.

It would be reasonable to expect that such a common problem would have attracted many biomedical researchers -- along with funding support -- with the objectives of discovering the basic causes and providing a sound scientific basis for treatment. So far, this does not seem to be the case.

Epidemiological studies, covering broad population groups, have revealed the surprising fact that there is little or no correlation between lower back pain and any detectable changes found on x-ray examination. Clearly, the problem is not an anatomical one, but a physiological one. That is, changes in function are the most likely explanation, not structural changes in the bones and muscles themselves. It is possible that these functional changes are in the excitability of the nerve cells that transmit information about pain, and which supply the so-called pain receptors in the bones and muscles of the back.

Treatment

People with lower back pain seek treatment from a variety of sources, such as rheumatologists, medical manipulators and manual therapists, a group of practitioners that includes physiotherapists, chiropractors and osteopaths. Not all of these practitioners agree on methods of treatment or philosophy, and they obviously differ in the types and emphases of the training that they have received.

We have been interested in finding out whether one form of manual therapy -- chiropractic manipulation -- has a measurable effect on the excitability of nerve cells in the spinal cord, that extension of the brain that fills much of the cavity of the spine.

To carry out the work, we needed to combine the practice of chiropractic manipulation with the techniques of neurophysiology, the science that seeks to understand the function of the nervous system. The team consisted of Bernadette Murphy, a graduate in both biology and chiropractic; John Slack, a physicist turned neuroscientist; and myself, a physiologist with a particular interest in human neurophysiology. Valuable practical support was given by David Murphy, an Auckland chiropractor, and the Hamblin Chiropractic Research Fund Trust.

Experimental Study

We used a reflex technique that allowed us to measure how excitable the cells were that supplied the soleus muscle in the lower leg. The nerve cells that control this muscle are found in the same part of the spinal cord as those receiving information from the lower back. We measured nerve cell excitability before and after manipulation of the sacroiliac joint in the pelvis.

The volunteer lay prone on a special chiropractic couch where the section underneath the pelvis was designed to drop in response to a manipulative thrust applied to the joint. The nerve supplying the soleus muscle was electrically stimulated through a small electrode applied to the skin at the back of the knee.

At low voltages, nothing happens. As the stimulus voltage is gradually increased, electrical activity begins to be detected by recording electrodes attached to the skin overlying the soleus muscle. This can be done because when a muscle contracts there is accompanying electrical activity.

At lower voltages, only nerve fibres that carry information into the spinal cord from stretch receptors in the muscle are stimulated. The nerve impulses produced in these fibres by the artificial electrical stimulus also travel up to the spinal cord.

These nerve fibres connect to cells in the spinal cord that send nerve impulses back to the soleus muscle. When nerve cell (1) stimulates the spinal nerve cell (2), the muscle contracts. This reflex activity is detected as electrical activity by the recording electrodes over the muscle.

If the excitability of the spinal nerve cell is changed by manipulation, this can be detected by a change in the magnitude of the electrical activity generated in the soleus muscle.

Thus, we have a cunning way of looking inside the nervous system of a conscious human volunteer without causing any pain. The technique is so innocuous that some volunteers fall asleep.

Effect of Manipulation

We were excited to find that when a volunteer's sacroiliac joint was manipulated, this was followed by a significant decrease in the excitability of nerve cells in the spinal cord. Other experiments in which the skin over the joint was ansthetised locally showed that the effect was not due simply to stimulation of pressure receptors in the overlying skin, from which some inhibition might have resulted.

Our observations lead us to propose that manipulation changes, by some means, the pattern of information coming in to the spinal cord from receptors in the manipulated joint and its surrounding muscles. We are beginning to have an inkling about two things -- first, the possible functional basis of back pain, and second, the way in which manual therapy might work.

Clearly, this work is only a beginning, and much more needs to be done on the important topic of how manual therapies might change function in the nervous system, not only in symptomless volunteers but also in people who complain of lower back pain.

One day, not only may we know why we have a pain in the back, but we may also have a better way of treating it.

Dr Noel Dawson is a senior lecturer in Auckland University's Physiology Department.