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Odds Ratio Meta-Analysis and Increased Prevalence of White Matter Injury in Healthy Divers.

INTRODUCTION
Increased white matter hyperintensities (WMH) on magnetic resonance imaging (MRI) brain scans of high altitude aircrew and altitude chamber workers indicate that exposure to low ambient pressure (hypobaria) promotes white matter injury. If associated with frequent decompression stress then experienced divers should also exhibit more WMH, yet published case-control studies are inconsistent. This meta-analysis evaluated the prevalence of WMH in healthy divers and controls.


METHODS
Eligible studies compared experienced divers (or hyperbaric workers) without neurological decompression illness with nondiving controls, identified from multiple database searches and reference list reviews. Studies were scored for sample size, recruitment bias, control matching, MRI sensitivity, and confounding factors before grading as low, medium, or high quality. Meta-analysis of odds ratios (OR) with 95% confidence intervals (CI) was conducted on all data using a random effects model and repeated after exclusion of low-quality studies.


RESULTS
There were 11 eligible studies identified. After data adjustment to exclude diving accidents, these encompassed 410 divers and 339 controls, of which 136 (33%) and 79 (23%), respectively, exhibited WMH (OR 1.925, 95% CI 1.088 to 3.405). Excluding four low-quality studies eliminated meta-analysis heterogeneity, with 98 of 279 divers (35%) and 44 of 232 controls (19%) exhibiting WMH (OR 2.654, 95% CI 1.718 to 4.102).


CONCLUSIONS
Results suggest that repeated hyperbaric exposure increases the prevalence of white matter injury in experienced healthy divers without neurological decompression illness. This is consistent with reports of increased WMH in asymptomatic altitude workers and an association with intensity of dysbaric exposure.