Long Term Diving Effects
By Fred Bove, M.D., Ph.D.
Since the mid 1980s there have been reports of unusual findings from the MRIs of patients with no evidence of a brain disorder. These studies suggested that certain bright objects seen on MRI scans indicated that the brain had sustained injury in the past. Some of these early studies were done on divers, and the study authors suggested that divers had a high incidence of such injuries.
However, these studies did not compare the divers to other people, and when appropriate control studies were done, the MRI findings were the same in people with no exposure to diving. When examining workers from different occupations, a relation was found between the incidence of these abnormalities and previous head trauma, excess alcohol use, cigarette smoking, a history of other medical illness and age greater than 35 years. More recent papers described an increase in the incidence of the abnormalities in divers compared to nondivers. The divers were older, and some also showed abnormalities in the cartilages of the neck. To date, MRI studies in divers have been inconclusive because of difficulty in matching divers to nondivers.
The Heart and DCS: We have been discussing the impact of a heart structure-the Foramen Ovale, part of the wall between the two upper chambers of the heart (right and left atria)-on risk for decompression sickness. Before birth, this area is an open hole in the wall between the atria that allows oxygen-rich blood to flow from the placenta to the fetus. It is closed by a flap of tissue at birth. The flap grows into the wall of the atria to form a permanent closure, but in about a third of people there is incomplete closure (called a Patent Foramen Ovale or PFO), and the potential for blood to leak from the right to the left atrium.
This leak is usually of no consequence unless there are blood clots or bubbles in the blood of the right atrium that can flow across the PFO, enter the arteries and block blood flow to an organ or tissue. Of most concern is blocking blood vessels to the brain, as this will result in a stroke.
We have established that the presence of a PFO will increase the risk for decompression sickness from about two cases in 10,000 dives to four cases in 10,000 dives. Either risk is quite small, and should not be a concern in sport divers. We have also established that many cases of exceptionally severe decompression sickness are related to the presence of a PFO. A few commercial divers, who were prone to decompression sickness to the point where their careers were threatened, were able to prevent their recurrent decompression sickness by having the PFO closed. We do not recommend being tested for a PFO if you are a sport diver because of the low risk for decompression sickness and the lack of evidence that it is a problem in sport diving.
Recently we have seen interesting data that link the PFO to the MRI bright objects. The data suggest that MRI lesions in the brain are more frequent in divers with a PFO.
How do we react to this information? First, there are no studies that show any relation between these MRI objects and brain function, behavior or performance. If bubbles are present in the veins after diving, they will flow to the right atrium and across a PFO if one is present. In the absence of a PFO, these bubbles would ordinarily be trapped in the lung and would not cause symptoms of decompression sickness unless their numbers were large.
Small amounts of bubbles that cross the PFO into the arteries would be more likely to flow to the brain and might in some circumstances cause these MRI objects to form in the absence of any symptoms. The study relating a PFO to MRI abnormalities is inconclusive. The authors did not compare their data to nondiver control subjects, ages were not matched and previous medical histories were not explored. Besides diving, there are other factors that relate to the MRI findings.
A Solution: We know from experiments done as early as 1908, that the greater amount of nitrogen in the tissues, the more likely you are to develop bubbles and decompression sickness. Keeping dives to reasonable depths and times will minimize the risk for bubble formation, and eliminate concern for a PFO effect. This means keeping well within the no-deco limits, limiting depth in sport diving to no deeper than 130 feet (100 feet is even better), avoiding more than three dives a day and avoiding rapid ascents. This list is applicable to all sport divers under all circumstances.
We have no data that any type of long term harm occurs from sport diving (unless you have a diving accident), but as we continue to gather data from well conducted clinical studies, we may find a population of divers who should exercise special precautions to avoid long term harm. We emphasize again that studies to date are inconclusive regarding the cause of "lesions" found on MRI scans in normal people.
In the future, knowledge that a diver is free of excess risk should not result in abandoning well-established safe diving practices. You can find more about diving medicine at our website: www.scubamed.com.