Abstract
Purpose :
Impaired aqueous humor outflow facility is the primary mechanism for most cases of elevated intraocular pressure (IOP), the leading risk factor for the development of glaucoma. However, as crucial as outflow facility is to the development and treatment of glaucoma, there are very limited means to measure outflow facility. We have developed an applanation-based tonographic method to measure outflow facility based on the IOP decay curve over time when a weighted platen is placed on the cornea. In this pilot study, we compare outflow facility measurements in eyes with and without glaucoma using applanation tonography.
Methods :
Patients presenting to the Duke Eye Center for routine care were recruited for applanation tonography. Briefly, applanation tonography was performed using a custom prototype that was rested under gravity on the anesthetized, supine eye for a total of two minutes while the applanation mire was continuously imaged. Recordings were analyzed to extract the IOP decay curve as a function of time based on the applanation mire diameters and the reference tonometer tip diameter. The IOP decay data were then fit to a mathematical model derived from Friedenwald’s relationship and Goldmann’s equation to extract the best estimate of outflow facility.
Results :
Nine POAG eyes of 8 patients and 8 control eyes of 8 patients were included. Mean age (71.6 ±7.1 vs 62.5 ±15.1 years), IOP (15.4 ± 2.4 vs 16.8 ± 2.0 mmHg) and CCT (540.1 ± 11.9 vs 544.4 ± 74.6 µm) for POAG vs control eyes did not differ significantly (p = 0.12, 0.23, and 0.90, respectively, t-test). The outflow facility for 9 POAG eyes and 8 control eyes were successfully calculated. The outflow facility was reduced in POAG eyes (0.08±0.17 µl/min/mmHg) compared with control eyes (0.17±0.17 µl/min/mmHg) but did not reach statistical significance (p=0.28, t-test). A comparison of the incidence of measurable (non-zero) outflow facility between the 2 groups demonstrated a trend towards significance with 6 of 8 eyes in the control group with a measurable outflow facility, compared to 3 of 9 eyes in the POAG group (p = 0.08, chi-square) (Figure).
Conclusions :
Preliminary measurements of outflow facility using applanation tonography are promising. Future studies will include validation against ex vivo eyes where outflow facility is determined by perfusion and comparison to Schiøtz tonography.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.