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Professional divers as well as almost all persons who develop bends for whatever reason are presently monitored automatically by brain imaging. One of the results of these selected studies is that, in divers having brain lesions, a great majority occur in a relatively small fraction of the group. Since most gas bubbles in divers with bends affect the venous system and right heart output, this finding led to the question of how the emboli reached the arterial blood of the brain. Current studies suggest that brain lesions accompanying decompression illness especially affect persons with a patent foramen ovale (PFO), but the data were limited.
The current study directed its efforts to finding whether brain lesions in sport divers occurred with any frequency and, if so, whether PFOs showed a high association with the development of brain lesions. The study included 91 experienced sport divers, all having performed a minimum of 161 dives. Four candidates were excluded on the basis of previous neurologic illness. Evaluation included examination employing a 1.5 T MR scanner and applying the FLAIR technique to improve MRI differentiation between widened perivascular spaces and abnormal brain lesions. All subjects were also studied to detect air bubbles in the middle cerebral artery. The test technique applied consisted of transcranial sonography of the middle cerebral artery following injection of a contrast agent containing microbubbles a) with the person at rest, and b) during the performance of a Valsalva maneuver.
Sixty-seven men and 20 women with an average number of 565 dives completed the tests. Twenty-five subjects showed a right-to-left sonographic atrial shunt, hemodynamically high in 13. Forty-one brain lesions were detected in 11 of the 87 divers: seven who did not have an atrial shunt had one lesion each, whereas four divers showing right-to-left shunts had 34 lesions. One "shunt diver" had a single lesion; the remaining three totaled 33 lesions. Overall, the prevalence of multiple brain lesions in the 25 divers who had a PFO was significantly higher (P = 0.02) than in the 62 divers who lacked PFOs.
The BMJ, commenting on this article, indicates that while it is well-known that divers may sometimes suffer acute cerebral damage or death, little is known about silent brain injuries that may arise during either asymptomatic decompression or in association with the bends. Deep water divers earn their livings seeking exotic foods in several areas of the world, and many sporting divers seek new discoveries at ever-increasing depths. Functional spinal injuries are well-recognized in decompression sickness, and retinal fluorescent studies in asymptomatic divers have identified vasculopathies presumed to be reflections of what the brain may endure (Polkinghorne, et al. Lancet 1988;2:1381-1383).
The physics of diving should be known to all, especially neurologists, but it remains an astonishing fact that only a few understand the risk of deep diving without proper equipment. Dives to the depth of about 23 feet increase the external pressure to the same degree as the decreased pressure at the top of Mt. Everest. Apparently, nitrogen-induced narcosis can also develop at roughly 95 feet of depth.
The authors comment that a previous study by Reul et al (Lancet 1995;345:1403-1405) found almost a double number of sport divers suffering brain lesions compared to the present study. A major difference from the Reul et al group was Knauth et al’s use of the FLAIR technique, which minimizes identifying perivascular signals on MRI films as being abnormal. Nevertheless, Todneu et al (EEG Clin Neurophys 1991;79:322-329) and Rinck et al (J Magn Reson Imaging 1991;1:293-299), using low Tesla MRI machines, found more clinical and EEG abnormalities in professional divers than were detected in the present study on MRI.
The findings of Knauth et al’s study, the inconsistency of other reports in identifying important neurologic abnormalities in sport divers as well as professional ones, and the steady increase of this sport all suggest the need for a much more intensive and methodical study of the brain risk under decompression circumstances. fp