Several studies have investigated IOP changes with body position, but all are limited by periodic, single time point IOP measurement techniques available in the clinic, and therefore do not capture the complete time course of IOP change with body position change. Krieglstein et al.
2 have shown that IOP changes with different body positions, and there is a nonlinear relationship between IOP increase and body position from 60° semi-upright tilt to 30° head-down tilt when using a Perkins applanation tonometer and the Alcon Pneumatonograph (Alcon, Fort Worth, TX, USA). In these studies, IOP is measured by using snapshot, single time point measurements, and no information on the time course is presented other than measurements that are taken approximately 3 minutes apart. Lee and colleagues
9 have taken measurements 5 minutes and 30 minutes after changing position, using Tonopen XL to study the effect of the LDP on IOP in healthy young subjects. Subjects switched from the seated to supine to lateral decubitus position and exhibited significant changes in IOP between the dependent eye in LDP versus supine position. Lee et al.
10 have also studied the effects of different sleeping postures on IOP and ocular perfusion pressure (OPP) in 20 healthy young subjects. They have found a 1.4 mm Hg difference between eyes in the LDP and similar results in the prone position with head turned to the side, also noting that IOPs are higher in the prone position with head turned versus LDP. They also report IOP in the LDP with different head positions,
11 and a difference of 1.0 and 1.3 mm Hg between eyes is found in the head-neutral LDP. Finally, they have studied the effect of LDP on IOP in patients with untreated and treated glaucoma and report differences ranging from 1.2 to 1.6 mm Hg between eyes in the LDP.
12,13 Hwang and colleagues
14 have found more pronounced results and report that the mean differences in IOP between the eyes in the LDP range from 2.9 to 4.1 mm Hg in 20 patients, a greater difference between eyes than reported in other studies, potentially due to the duration spent in the LDP (5 to 150 minutes) or possibly due to large interpupillary distances of 7.0 ± 0.4 cm, creating a slightly larger hydrostatic column difference between eyes. They conclude that IOP is higher in the dependent eye and that IOPs in anesthetized patients are higher than in alert patients. Carlson and colleagues
15 have measured changes in IOP in response to changes in body position (±15° and ±50° tilt from horizontal) while also looking at aqueous turnover as measured by fluorophotometry. They conclude that while aqueous formation is relatively insensitive to IOP, IOP changes 2.4 ± 1.2 mm Hg for ±15° from horizontal and 11.2 ± 2.7 mm Hg for ±50° from horizontal (mean ± SD), further demonstrating the significant changes in IOP with different body positions. Malihi and Sit
16 have shown that IOP is lowest when measured while sitting with the neck in the neutral position. All other head and body positions result in an elevation of IOP, compared with the seated, head-neutral position typically used in clinical practice to measure IOP. Furthermore, this study also confirms a difference between eyes in the LDP. Eklund et al.
3 have studied the postural influence on simultaneously measured IOP and intracranial pressure (ICP) and report similar results on IOP changes with body position in the sitting and supine positions. They have calculated the trans–lamina cribrosa pressure difference (TLCPD) in different body positions by subtracting ICP from IOP. ICP (in millimeters of mercury) is lowest in the sitting position (−0.8 ± 3.8), resulting in the largest TLCPD as compared to the supine and head-down tilt positions (19.8 vs. 12.3 [supine] and 6.6 [head-down tilt]).
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