Allocation Concealment in a Blinded, Randomized Trial of Hyperbaric Oxygen for Post-Concussive Symptoms

Publication Type
Conference Paper
Year of Publication
Churchill, Susan; Weaver, Lindell; Wilson, Stefanie; Lindblad, Anne; Deru, Kayla
Undersea and Hyperbaric Medical Society Annual Scientific Meeting 2017
Date Published
Naples, FL
Undersea and Hyperbaric Medical Society
BIMA; brain injury; HBO2; hyperbaric oxygen; post-concussive symptoms; TBI


In randomized trials, allocation concealment (blinding) can minimize bias. We report concealment results in a randomized, double-blind clinical trial of hyperbaric oxygen (HBO2) for persistent post-concussive symptoms (BIMA*).


BIMA randomized 71 military service members at yhree sites to receive forty 60-minute sessions of HBO2 (1.5 ATA, oxygen) or sham (1.2 ATA, air) chamber sessions over 12 weeks. Participants were blind to pressure and breathing gas. At 13 weeks, participants completed a two-question written survey asking whether they thought they received HBO2 or sham sessions and why. Blinded study staff completed a similar questionnaire for each participant.


Seventy participants (99%) completed the questionnaire (HBO2 N=36, sham N=34). Forty-one (59%) were unsure of intervention, balanced between HBO2 (N=21) and sham (N=20) groups (HBO2 N=36, sham N=34). Four participants in each group thought they received HBO2, while 10 sham and 11 HBO2 participants believed they received sham. The blind was protected (P=0.99). Of eight participants who believed they received HBO2, seven (three in HBO2, four in sham) cited symptom improvement, while 15 (six in HBO2, eight in sham) believed they received sham due to lack of symptom improvement. Of the other six participants who believed they received sham, five (three HBO2, two sham) cited lack of in-chamber neurological symptoms, one (HBO2) listed gas smell, and one (HBO2) listed pressure change. One HBO2 participant selected HBO2 because he wore a hood in the chamber. No participant based their conclusions on interactions with chamber or study personnel. Site principal investigators selected “not sure” for every participant. Study coordinators were unsure of allocation in 75% of participants and correctly guessed allocation in eight (12%) (P=0.74).


Sham pressurization protected the blind in this trial. Participants based allocation assumptions on outcome and could not discern intervention arm by pressure, smell, taste or gas flow. * BIMA is the Brain Injury and Mechanisms of Action of Hyperbaric Oxygen (HBO2) for Persistent Post-Concussive Symptoms after Mild Traumatic Brain Injury (mTBI) (BIMA) Study.