In a previous article, we found that elevation of asymmetric intraocular pressure (IOP) in the lateral decubitus position was associated with asymmetric visual field (VF) loss in patients with glaucoma. We also found that the number of patients preferring the worse-eye-dependent lateral decubitus position was significantly higher than that of patients preferring the better-eye-dependent lateral decubitus position; the better eye and the worse eye were defined based on the mean deviation value determined by Humphrey visual field analysis. In considering these results, we speculated that preferred sleep position, with postural IOP, might be a contributory factor to glaucoma manifestation. So in the present study, we endeavored to verify our hypothesis concerning the association between preferred sleep position and asymmetric VF loss in a larger subject group.

Dr. Punita Kumari recommended the use of age-matched controls and the ruling out of other risk factors for glaucoma. However, in our study, we investigated the relationship between the laterality of the preferred sleep position and the laterality of the worse eye in the same patient. In this way (each patient’s 2 eyes, obviously sharing the same age and cardiovascular comorbidities), we sought the possible cause of asymmetry, irrespective of age or cardiovascular disease. We agree that both age and cardiovascular disease influence ocular dynamics and, further, we think that there might be contributory factors other than the habitual sleep position that led to VF loss asymmetry in glaucoma. For example, because the short posterior ciliary arteries constitute the main blood supply to the optic nerve head and the surrounding choroid, the hemodynamics of these vessels are believed to be important in glaucoma pathogenesis. Galambos and associates reported that in healthy subjects, these arteries showed almost perfect compensation of flow velocity and insignificant change in resistivity in response to sitting-to-supine postural change; in patients with both normal-tension glaucoma and primary open-angle glaucoma, by contrast, the blood-flow velocity increased significantly, indicating an impaired postural-change compensation. This is considered to be one of the most likely mechanisms for IOP elevation associated with postural change.

Our present study suggests that the glaucoma patients’ habitual sleep positions might be associated with greater VF loss. In order to confirm this hypothesis, simultaneous measurement of postural IOP, ocular perfusion pressure and ocular blood flow in each eye of each subject, along with investigation of the preferred sleep positions, will be necessary as part of a prospective long-term follow-up study.