Brain Damage in Divers?

Are one-third of all divers at risk for dangerous brain lesions? A team of German researchers thinks so.

-By Sam Shelanski, M.D.

As any lovesick teenager will tell you, affairs of the heart can mess with your mind. But it has taken until now for medical research to prove it. Following up on the June 1995 study in the medical journal The Lancet, which showed an increased number of abnormal magnetic resonance imaging (MRI) findings in the brains of divers, researchers in Germany have identified a select group of divers who they suggest are at higher risk.

The new German study, published this March in British Medical Journal, finds that divers with a patent foramen ovale (PFO) - a heart defect found in as much as one-third of the adult population - are at increased risk of suffering damage to the brain from gas bubbles in the bloodstream.

What is a Patent Foramen Ovale?

The foramen ovale is an opening between the right and left sides of the heart. A membrane over this opening causes it to act like a one-way valve during fetal development, when the fetus receives oxygenated blood from the mother. The opening allows oxygenated blood to bypass the nonfunctioning fetal lungs, take the path of least resistance through the foramen ovale from the right side of the heart to the left, then be pumped to the rest of the fetus's body. Immediately upon birth, the infant's hemodynamics change in such a way that causes the membrane to be held in place over the foramen ovale, closing the hole and routing blood to the lungs. In most cases, the membrane eventually attaches permanently to the heart wall.

However, studies have shown that in about 30 percent of the population, the foramen ovale never completely closes, but remains open, or patent. If the foramen is patent, there remains the potential for blood to flow from the right chamber to the left across the heart without passing through the natural filter of the lungs. This abnormal blood flow pattern occurs when the pressure on the right side of the heart rises and becomes equal to or exceeds the pressure on the left. The most common situation under which this pressure change occurs is during a Valsalva maneuver, a technique frequently employed by divers to aid in equalization of the ears and sinuses.

How Could This Affect Divers?

It has been known for some time that bubbles can form in the bloodstream of divers without producing symptoms of decompression sickness (DCS). Normally these so-called "silent bubbles" form in the venous bloodstream and are filtered by the lungs before they have a chance to spread to the rest of the body. However, in a diver with a PFO, the bubbles could potentially bypass the lungs and flow directly to the brain, where they could cause what are essentially mini-strokes. Over time, enough of these small strokes could cause neurological or cognitive deficits.

The earlier study in The Lancet postulated that these silent bubbles were the underlying cause of the brain lesions found in test subjects, but the study did not address the issue of how these bubbles got from the right side of the heart to the left. In this latest study, Michael Knauth and others at the University of Heidelberg looked at the Lancet study and noticed that only a few of the divers accounted for the vast majority of the abnormal findings. Noting that other studies have suggested the presence of a PFO may predispose a diver to Type II (neurological) DCS, Knauth and his group set out to see if there was a link between a diver having a PFO and also having abnormal MRI findings.

What They Found

The first thing Knauth discovered was that brain lesions occurred in only 13 percent of the divers studied, whereas the Lancet study found abnormalities in over 50 percent of subjects. This difference is likely due to a more sensitive MRI method employed in Knauth's study which more accurately identified areas that were truly abnormal. In contrast, the Lancet study may have erroneously counted many false positives.

The second important finding was that divers without a PFO were just as likely to have an abnormal MRI as those with a PFO. However, the number of abnormal findings differed significantly between those with and those without a PFO. All the divers with abnormal scans but without a PFO had no more than a single brain lesion. Yet 23 percent of divers with lesion scans and with a large PFO had multiple lesions. No attempt was made in this study to correlate the MRI results with any observable neurological deficit.

What Does This Mean For You?

Should you be reaching for the phone right now to call your doctor and schedule tests to find out if you have a PFO? Probably not. Let's look at some reasons why.

First, none of the divers in the study had fewer than 160 dives. The divers with multiple lesions averaged slightly over 750 dives each. Yet they averaged only 11 abnormal MRI findings each, or about one for every 75 dives. Clearly, brain lesions are not something that occur every dive. However, you should not dismiss this study because you dive only a few times each year. There is always the possibility that the damage could occur.

Second, the data from the study appear to be an issue of concern primarily to those who dive frequently. It is possible to argue that active divers - instructors, divemasters, researchers, photographers, writers and avid sport divers - should be evaluated for the presence of a PFO. Once they know whether or not they are at possible increased risk due to a PFO, they can make more informed decisions about future diving.

But I do not think that even this step is necessary because the results from the study may already be essentially moot. To explain: the ongoing Doppler study being conducted by the Divers Alert Network has already indicated that the silent bubbles indicted as the cause of these brain lesions can essentially be eliminated if a diver performs a safety stop at 15 feet for three to five minutes.

Therefore it is unlikely that anyone will be faced with a choice between continuing to dive and undergoing open-heart surgery to close a PFO. Instead, it may be that altering dive profiles in ways that we feel confident decrease bubbling - not so deep, not so long, and a slightly longer safety stop - may be enough. Other changes, including the increasing availability and use of enriched air mixtures, may also prove to help reduce the incidence of venous bubbling and, by extension, the brain lesions which may result in those divers with a large PFO.

I suspect that if this study were to be done again in 10 years, looking only at divers who began the sport after the introduction of slower ascent rates and safety stops, it is unlikely they would find any difference between those with, and those without, a PFO.

Questions for Dr. Shelanski should be addressed to RSD, Dive Medicine, 6600 Abercorn St., Suite 208, Savannah, GA 31405; e-mail: RSDmgzn@aol.com.