The study was planned and conducted in accordance to the principles governing clinical research as set out by our Institutional Review Board and the Declaration of Helsinki (1989). A signed informed consent was obtained from the subjects after explanation of the nature and possible consequences of the study. Twenty-one healthy young volunteers, male and female, ranging in age from 24 to 45, were recruited from Mayo Clinic employees and students, and local area residents. Subjects were given a complete dilated eye examination and an updated medical history was performed. Specific exclusion criteria for subject participation included: IOP greater than 22 mm Hg in the seated position, evidence of glaucomatous optic neuropathy, poor fixation, nystagmus, tropias, phorias, recent infection, corneal scarring preventing reliable tonometry, evidence of pigment dispersion, narrow angles, history of trauma or surgery, systemic use of beta blockers or steroids, diabetic eye disease, uveitis, high (>6 D) myopia, or high (>4 D) hyperopia. Subjects with chronic medical conditions (eg, hypertension) were allowed to participate as long as their medical condition had been under good control over the preceding 12 months. Subjects also needed to have good cervical neck malleability to be able to meet the physical demands of the protocol, which necessitated 4 minutes of neck extension.
On the day of the experiment, subjects were asked to maintain a regular schedule and normal activities. This included avoiding excess caffeine intake, large deviations from normal sleep cycle, or exposure to systemic medications with IOP-modifying potential.
Subjects were seated in standard ophthalmologic examination chairs and reclined slightly to 70° from the horizontal in a quiet room with subdued lighting for approximately 5 to 7 minutes and the patient's neck was extended until the anterior aspect of the cornea was level with the floor. Anesthesia was achieved by instilling 3 to 4 drops of proparacaine 0.5%. The contralateral eye was patched in a semi air-tight fashion with an adhesive eye patch to reduce evaporative losses and drying of the cornea (
Fig. 1). The patient was asked to keep their eyes closed before the start of measurements. When ready, the patient was asked to open their eyes and to observe a dim fixation target that was placed on the ceiling of the room, roughly 6 feet from the patient's eyes. The right eye was always tested first, followed by the left.
Baseline IOP was measured using a pneumatometer (Model 30 Classic; Reichert, Inc., Depew, NY). Constant weight tonography (
Fig. 2) was then performed using an electronic Schiotz tonographer designed by the Mayo Clinic Division of Engineering. The measurement head of an electronic Schiotz tonography consists primarily of a linear variable differential transformer (LVDT), which is a device that converts linear movement of the tonometer tip to an electrical signal. Our device used the original Schiotz tonometer head from a commercially available unit (Berkeley Bio-Engineering, Inc., San Leandro, CA), modified with a lighter more flexible cable, which was then attached to an LVDT Signal Conditioner. This allowed for the conversion of the signal to DC voltage and digitalization for transfer via USB to a computer (Dell 620 laptop; Dell Inc., Round Rock, TX). Tonography measurements were undertaken with a 5.5 g weight with a 4-minute tracing (
Fig. 3). The procedure was then repeated on the contralateral eye, with patching of the already tested eye.
After sitting measurements were completed, the patient was allowed to rest for a minimum of 30 minutes to allow for the eye to reestablish a state of equilibrium. This time interval was based on an assumption of exponential decay of IOP during tonography and an exponential recovery after removal of the weight.
16 We conservatively estimated that IOP would recover half as fast as the IOP decay, and thus estimated that 30 minutes would provide at least 5 half-lives for recovery assuming a 5 minute measurement. The patient was then placed in the supine position. A minimum of 5 minutes was allowed to elapse once the patient was in the supine position before starting measurements, allowing for autonomic, postural, and hormonal changes to stabilize.
17 Previous research has shown that little change occurs between 2 minutes and 5 minutes after a position change.
11 However, we selected a 5 minute interval as a conservative measure to ensure steady state. Baseline IOP for the supine position was measured, followed by constant weight Schiotz tonography in each eye using the protocol described above.
Data from the tonography tracing were collected electronically and was exported to a standard spreadsheet (Excel; Microsoft Corp., Redmond, WA) file. Data were fitted to a second order polynomial to help determine the adequacy of the collected sample. If tracings were erratic with multiple unusual spikes, rises, and falls, they were to be deemed inadequate for the study. Experience from previous studies using this method has shown that these tracings generally show little correlation to the derived curve of best fit. Tracings with a poorly fitted curve may occur for a variety of reasons including patient tenseness and anxiety, uncontrollable tendency to fall asleep, incidental interruptions such as a sudden wave of intestinal peristalsis, or technician error. Our protocol required poor tracings to be excluded from the final analysis. However, all tracings were adequate and none were excluded from the final data analysis. Values for the initial steady state IOP and the outflow facility were determined based on standard tables and normograms.
18
Based on our previous research from a similar subject population,
19 outflow facility would need to decrease by 52% in the supine position to account for the measured IOP change, assuming sitting EVP of 9 mm Hg and constant uveoscleral outflow rate. However, outflow facility is unlikely to be the only cause of postural IOP change. Therefore, we designed our study to detect a 20% change in facility between the sitting and supine positions, which is the amount of change required to account for non-EVP related IOP variations based on the results of Weinreb et al.
8 Our study, based on 42 observations from 21 subjects, had a power of 82%, α = 0.05, assuming a normal supine outflow facility to be 0.27 ± 0.11 μL/mL/mm Hg, based on our previous research.
19 Since data from both eyes were included as individual measurements, statistical significance was determined using Generalized Estimating Equation (GEE) models. GEE models allow for analysis of paired data where a correlation may exist between the pairs without having to explicitly define a model for the origin of their dependency. Statistical significance was assumed for
P < 0.05.