Evaluation of an Alternate Method of Outflow Facility Calculation from Tonography

Abstract

Purpose : The IOP decay data from pneumatonography can be captured digitally and used to calculate outflow facility(C). This study evaluates an alternate method of calculating C from tonography data. The interobserver and intraobserver variability of both methods also were evaluated.

Methods : In a prospective study of both eyes of 20 male Dutch belted rabbits, 2-min pneumatonography was performed by two observers at the same time of the day, on 3 different days (Day 1, 3 & 5). Supine IOP was recorded at the start of the procedure. A pneumatonometer with an attached 10gm weight was applied to the eye for 2 min. At the end of the 2 minute period, IOP was recorded again after removing the weight. All data from the IOP decay curve were digitally captured at 40 Hz. Measurements were performed on both eyes on day 1 by observer 1 and on day 3 by observer 2. On day 5 all measurements were performed on the right eyes by observer 1 and on the left eyes by observer 2. The C was calculated using two different methods.
Method 1 (conventional): Regression line was fit to the IOP decay curve and used to predict the starting and ending IOP.
Method 2 (alternate): The un-weighted IOPs just before and just after the 2 minute tonography procedure were used instead.
These values in conjunction with the pressure-volume data for the rabbit eye were used to calculate C.

Results : C calculated using the conventional method was (0.16±0.10, 0.17±0.11 & 0.14±0.05) and that using the alternate method was (0.16±0.09, 0.15±0.10 & 0.12±0.07) µl/min/mmHg on day 1, 3 & 5 respectively. Using Bland Altman method 95% limits of agreement between the 2 methods was -0.14 to +0.17 µl/min/mmHg (Fig 1a). The difference between the 2 methods was ≤0.05 µl/min/mmHg in 50% of eyes and ≤0.10 µl/min/mmHg in 77% of eyes (Fig 1b). C calculated using the alternate method was 0.16±0.09 µl/min/mmHg for observer 1 and 0.15±0.10 µl/min/mmHg for observer 2 and was similar to the conventional method. The inter-observer and intra-observer agreement on separate days was poor with either calculation method.

Conclusions : At a group level, tonography has excellent inter and intra-observer repeatability. However the repeatability at the individual eye level remains poor. This may be due to measurement uncertainty or day-to-day variability in the variable itself. The alternate method of outflow facility calculation provides a good means of making the calculation with fewer assumptions.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.