The idea that different head and body positions may affect intraocular pressure (IOP) and the reaction of a glaucomatous eye to therapy has become popularized. To that end, this prospective randomized study examined the effect on IOP of varying the neck position of normal subjects and found that IOP was lowest when subjects sat with their neck in the neutral position.
IOP was elevated in all other head and body positions compared with the position used for typical clinical measurements. The results also indicate that lateral decubitus positions may result in a small increase in IOP in the lower eye.
Subjects were 24 healthy volunteers between the ages of 19 and 47 years. In the sitting position, IOP was measured in neutral neck position, neck extension and neck flexion. In recumbent positions, IOP was measured in the supine position, and right and left lateral decubitus positions. In all of these positions the order of measurements was randomized. The authors note that no previous studies have investigated the effects of body position in a randomized fashion to eliminate the effects of measurement sequence.
All IOP measurements were performed with pneumatonometry. Subjects maintained each position for a minimum of five minutes in order to allow IOP to reach a steady state.
The mechanisms for changes in IOP related to various neck and body positions have been ascribed to possible factors, such as changes in episcleral venous pressure (EVP) resulting from venous compression that may occur during neck flexion or extension. Choroidal vascular engorgement that may occur in the recumbent position could be another cause for changes in EVP.
The alterations in IOP found in this study suggest areas of caution that need to be further investigated in glaucoma patients. The findings may warrant warnings for glaucoma patients, such as regarding neck flexion or extension during yoga or while sleeping in a lateral decubitus position. However, since the study's participants were young and healthy, the results cannot necessarily be extrapolated to older healthy subjects or glaucoma patients.
The authors say that IOP elevation in the dependent eye while in lateral decubitus positions should be investigated as a potential contributor to the development of asymmetric or unilateral glaucoma, as should IOP elevation during neck flexion and extension as a potential risk factor for glaucoma. Further research is also necessary to elucidate the effect of head and body position on glaucoma parameters, such as ocular perfusion pressure and retrolaminar cerebrospinal fluid pressure.