Introduction: It has been suggested that application of CPAP may lead to an immediate intraocular pressure (IOP) increase through a transmission of positive pressure to the intrathoracic space which would then increase venous pressure in the ocular circulation. Reduced venous return is also believed to be responsible for IOP elevation when lying down. Aim: We aimed to determine whether a brief exposure to CPAP increases IOP and whether any potential IOP changes correlate with the CPAP level. Methods: We conducted a daytime experiment with 3 groups of participants: primary open-angle glaucoma patients (POAG) established on IOP-lowering treatment, newly diagnosed untreated POAG patients and control subjects without glaucoma. IOP measurements were first performed in a sitting position and repeated 30 min later in a supine position. CPAP was then applied at 4 pressure levels (6, 10, 13 and 16cmH2O). Each pressure level was applied for 30 min in a random order. IOP measurements were taken repeatedly at each CPAP level in both eyes. Results: A total of 47 participants were recruited: 21 treated POAG, 8 untreated POAG and 17 control subjects. Baseline supine IOP was significantly higher than IOP measured in the upright position (1.7+2.2mmHg, p=0.00). There was no significant change in IOP across all CPAP levels and in relation to the supine measurements off CPAP (p=0.11). The three groups did not differ in their responses to CPAP (p=0.2). Conclusions: CPAP does not cause IOP increase in wakefulness when set within the most commonly used therapeutic pressure range. CPAP induced nocturnal IOP increase may occur through mechanisms other than simple mechanical pressure transmission.
