Arthur J. Sit

Circadian variation of aqueous dynamics in young healthy adults.

PURPOSE Recent research indicates that intraocular pressure (IOP) does not decrease significantly during the nocturnal period, although aqueous humor flow decreases by 50% or more at night. This study was undertaken to investigate whether changes in outflow facility, episcleral venous pressure, or uveoscleral flow at night could account for the nocturnal IOP. METHODS Sixty-eight eyes of 34 healthy subjects (age, 18-44 years; mean, 29) were studied. Aqueous humor flow rate, IOP, and outflow facility were measured with pneumatonometry, anterior chamber fluorophotometry, and Schiotz tonography respectively, in each eye during the mid-diurnal (2-4 PM) and mid-nocturnal (2-4 AM) periods. Nocturnal IOP, flow rate, and outflow facility were compared to the same variables during the diurnal period. Mathematical models based on the modified Goldmann equation were used to assess the conditions under which these results could be reconciled. RESULTS Supine IOP decreased slightly from 18.9 +/- 2.7 mm Hg in the mid-diurnal period to 17.8 +/- 2.5 mm Hg in the mid-nocturnal period (mean +/- SD, P = 0.001). Aqueous flow rate decreased from 2.26 +/- 0.73 to 1.12 +/- 0.75 microL/min (mean +/- SD, P < 0.001). There was a nonsignificant trend toward a nocturnal decrease of outflow facility (diurnal, 0.27 +/- 0.11 microL/min/mm Hg; nocturnal, 0.25 +/- 0.08 microL/min/mm Hg; mean +/- SD, P = 0.13). CONCLUSIONS Outflow facility measured by tonography does not decrease enough during the nocturnal period to compensate for the decreased aqueous humor flow rate. Modeling results indicate that the experimental results could be reconciled only if nocturnal changes in episcleral venous pressure and/or uveoscleral flow occurred.

Aqueous Humor Outflow Facility by Tonography Does Not Change with Body Position

PURPOSE Intraocular pressure (IOP) varies with body position and previous research has indicated that most, but not all, of the variation in IOP is due to changes in episcleral venous pressure (EVP). Positional changes in other aqueous humor dynamic parameters may contribute to the change in IOP. The purpose of this study was to investigate the variation of aqueous humor outflow facility with body position changes. METHODS Healthy volunteers, aged 24 to 45 years old, were recruited for this study. Constant weight tonography was performed using a modified electronic Schiotz tonometer in two positions: seated position, 70 degrees from horizontal with neck extended until the cornea was level with floor; and supine position. A minimum of 30 minutes was allowed between the two measurements. Tonography data were fitted to second order polynomials and values for the initial steady state IOP and the outflow facility were determined using standard tables and normograms. IOP was measured using pneumatonometry. RESULTS Forty-two eyes from 21 subjects were studied. IOP in the sitting and supine positions were 17.8 +/- 1.7 mm Hg and 19.9 +/- 1.6 mm Hg, respectively, and were significantly different (P < 0.001). The mean outflow facility in the sitting and supine positions were 0.30 +/- 0.31 microL/mL/mm Hg and 0.28 +/- 0.09 microL/mL/mm Hg, respectively, and were not significantly different (P = 0.37). CONCLUSIONS Aqueous humor outflow facility measured with electronic Schiotz tonography does not vary significantly between the supine and sitting positions.

Intraocular Pressure After 2 Hours of Small-Diameter Scleral Lens Wear

Objectives: Compression of episcleral veins or deformation of tissue in the Schlemms canal beneath the landing zone of scleral lenses could elevate intraocular pressure (IOP). We examined the effect of 2 hr of small-diameter scleral lens wear on IOP. Methods: Twenty-nine participants, 29 ± 6 years old (mean ± SD) who experienced no history of eye disease or scleral lens wear, were included in the study. Each participant was fitted with a 15-mm Jupiter scleral lens on one eye (study eye). Intraocular pressure was measured in both eyes by pneumatonometry centrally on the cornea and peripherally on the sclera. The lens was then placed on one eye and was worn for 2 hr. Intraocular pressure was remeasured immediately after lens placement, at 1 and 2 hr of lens wear, and immediately after lens removal. Intraocular pressure after removal of the scleral lens was compared with IOP before placing the lens and to IOP in the control eye using paired t tests. Results: Immediately after removing the scleral lens, mean central IOP in the study eye (13.9 ± 3.1 mm Hg) was not different from mean central IOP in the control eye (13.5 ± 2.2 mm Hg, P = 0.4) or in the same eye before lens wear (13.6 ± 1.9 mm Hg, P = 0.6). There were also no differences in IOP measured peripherally at 2 hr of lens wear (P = 0.8). Conclusions: Neophyte scleral lens wear of a 15-mm scleral lens for 2 hr does not increase IOP in healthy eyes.

In Vivo Noninvasive Measurement of Young’s Modulus of Elasticity in Human Eyes: A Feasibility Study

Purpose: Abnormal ocular biomechanical properties may be important for understanding the risk of glaucoma. However, there are no clinical methods for measuring standard material properties in patients. In this feasibility study we demonstrated proof-of-principle for a novel method, ultrasound surface wave elastography (USWE), to determine the in vivo Young’s modulus of elasticity of corneas in normal human eyes. Methods: In total, 20 eyes of 10 healthy subjects (mean age, 51.4±7.2; ±SD; range, 43 to 64 y) were studied. A spherical-tipped probe (3-mm diameter) was placed on closed eyelids and generated a gentle harmonic vibration at 100 Hz for 0.1 second. Wave speed propagation in the cornea was measured by USWE, and Young’s modulus was calculated from the wave speed. Associations between Young’s modulus and intraocular pressure (IOP), age, central corneal thickness, and axial length were explored by the Pearson correlation. Statistical significance was determined by using generalized estimating equation models to account for possible correlation between fellow eyes. Results: Mean IOP was 12.8±2.7 mm Hg. Mean wave speed in the cornea was 1.82±0.10 m/s. Young’s modulus of elasticity was 696±113 kPa and was correlated with IOP (r=0.57; P=0.004), but none of the other variables (P>0.1). Conclusions: USWE is a novel noninvasive technique for measuring ocular biomechanical properties. Corneal Young’s modulus in normal eyes is associated with IOP, consistent with measurements in cadaver eyes. Further work is needed to determine elasticity in other ocular tissues, particularly the sclera, and if elasticity is altered in glaucoma patients.