Last year in June, the Lancet medical journal contained a paper (Lancet 1995; 345: 1403-5) describing an investigation of the long-term health effects of diving. The study used magnetic resonance scans of the brain and spine to see whether diving could produce damage. These scans produce high resolution images of the brain, and discriminate between tissues with different fat and water contents.

The authors compared the scans of 52 amateur divers with those of 50 control subjects of similar ages who were not divers. They found that 27 of the 52 divers had a total of 86 lesions in their brains. This compared with only 14 lesions in the brains of 10 of the 50 control subjects.

In addition, 32 divers and 9 control subjects had degenerative changes of the cartilage disc (intervertebral discs) between the bones of their spine (vertebra) in their neck. Such degenerate changes are commonly called slipped (or prolapsed) discs. Slipped discs are most commonly troublesome when they occur in the low back not the neck. We then recognise their presence by pain in the back which goes down a leg (sciatica).

The differences between the two groups were statistically significant. The authors concluded that, even without occurrence of decompression illness, diving can produce damage to the brain and cause prolapsed (slipped) intervertebral discs. The authors also reasoned that both types of lesions in divers were the result of liberation of so-called silent gas microbubbles. They equated the brain lesions (called lacunar infarcts) on the scans to the lesions found at post mortems on the brains of professional divers.

Some divers are concerned about these findings - a concern compounded by the way the paper was reported in the press. Are these concerns justified The paper itself has come in for heavy criticism. Firstly, there is a serious problem with the methods used for the study. The sample of amateur divers was biased by self-selection. Those studied were the first respondents to a magazine article requesting volunteers for the project. Such a sample is likely to be biased by inclusion of individuals who felt that they may have suffered neurological problems - for example neurological decompression illness.

Indeed, one of the divers had previously suffered decompression illness. Is it therefore justified to use such a study to determine the effect of diving on people who have not had decompression illness In the divers there was no correlation between the number of brain lesions and any one of the six indicators of diving experience which might be expected to correlate with gas load.

Furthermore, it is difficult to accept the authors assertion that the brain lesions on the scans equate with lacunar infarcts found at post mortem examination of divers. Similar scans of commercial divers in another study showed no excess of lesions on magnetic resonance scans compared with control individuals - yet we know that lacunar infarcts are as common, if not more common, in commercial divers.

It also seems unlikely that the brain and intervertebral disc lesions could have resulted from the same mechanism. The reason for this is the different gas handling properties of brain and cartilage. Brain is a highly perfused tissue with a high fat content. Its half-times of uptake and elimination of nitrogen are fast, and the nitrogen content when saturated is high. On the other hand, the cartilage of intervertebral discs is one of the least well perfused tissues, has a low fat content, long uptake and elimination half-times for nitrogen and a low dissolved nitrogen content.

It seems highly improbable that two such dissimilar tissues would be affected by an identical mechanism. Instead, if other studies confirm that the disc lesions are more frequent in amateur divers than non-divers, it is likely that they result from another mechanism, such as the abnormal posture divers maintain while swimming underwater and the effects of lifting heavy diving gear around.

At present, these findings do not confirm that diving (as opposed to decompression illness), can damage either your brain or the intervertebral discs, but they do highlight the need for further research on this subject.