ObjectiveWe investigated the role of right-to-left shunt with standardized transcranial Doppler ultrasonography in a large population of divers referred for symptoms of decompression illness.DesignCase series compared with a control group.SettingMilitary teaching hospital, hyperbaric unit.PatientsPatients were 101 consecutive divers with clinical evidence of decompression illness and a control group of 101 healthy divers.InterventionSpecification of the type of decompression illness involved and detection/evaluation of right-to-left shunt by standardized transcranial Doppler. The degree of right-to-left shunt was defined as major if the number of high-intensity transient signals in the middle cerebral artery was >20.Measurements and Main ResultsWe evaluated the odds ratios by logistic regression analysis with vs. without right-to-left shunt for subjects with cochleovestibular symptoms, cerebral decompression illness, spinal decompression illness, and Caisson sickness. Of the 101 divers presenting with decompression illness, transcranial Doppler detected a right-to-left shunt in 59 (58.4%), whereas control subjects demonstrated a right-to-left shunt in 25 cases (24.8%; odds ratio, 4.3; 95% confidence interval, 2.3–7.8;p= .09). When a right-to-left shunt was detected, the right-to-left shunt was major in 12 of 25 patients in the control group and in 49 of 59 patients in the decompression illness group (odds ratio, 8.7; 95% confidence interval, 4.2–18.0;p< .001). Within the decompression illness group, the proportion of major right-to-left shunt was 24 of 34 (odds ratio, 29.7; 95% confidence interval, 10.0–87.2;p< .0001) in the cochleovestibular subgroup, 13 of 21 (odds ratio, 24.1, 95% confidence interval, 6.8–86.0,p< 0.0001) in the cerebral decompression illness subgroup, ten of 31 (odds ratio, 3.9; 95% confidence interval, 1.5–10.3;p< .01) in the spinal decompression illness subgroup, and two of two (odds ratio, 1.1; 95% confidence interval, 0.2–5.7;p= .9) in the subgroup of divers with Caisson sickness.ConclusionBased on our results, we conclude that major right-to-left shunt was associated with an increased incidence of cochleovestibular and cerebral decompression illness, suggesting paradoxical embolism as a potential mechanism.