Abstract
Purpose: :
To assess dynamic IOP response time (DRT) in vivo using a rapid IOP increase and to longitudinally compare ocular volumetric rigidity changes in normal NHP eyes to that in eyes with EG.
Methods: :
DRT, was simultaneously measured in both eyes of six adult NHPs at baseline (BL) (n=3 pre-laser tests) and every 2 weeks post-unilateral laser (PL) to the trabecular meshwork as follows. Two needles were inserted into the anterior chamber, with one connected to a saline reservoir for manometric IOP control and the other connected to a transducer so as to dynamically record IOP during a rapid, stepped IOP increase from 10 to 30 mmHg. Retinal Nerve Fiber Layer Thickness (RNFLT) was also assessed by Spectral Domain Optical Coherence Tomography (Spectralis, Heidelberg Engineering, GmbH) at each session. The data for each DRT, test were analyzed as follows: the recorded IOP response to the step increase in manometer pressure was fit with a simple exponential rise to derive the parameters X0 (a brief delay) and tau (a time constant). For each eye, DRT parameters from all BL measurements were averaged to obtain a single BL estimate for each eye and DRT parameters from all PL measurements (from those PL sessions in which SDOCT RNFLT was below 85 µm; i.e. ~15% below average normal of BL) were averaged to obtain a single PL estimate. Repeated measures ANOVA with Bonferroni post hoc testing (Prism v5) was used to evaluate effects of time (BL vs PL) and treatment (EG vs CTL).
Results: :
IOP at the follow-up time points was 17.5 mmHg higher, on average, in EG eyes than in fellow CTL eyes. RNFLT was significantly thinner in EG eyes than at BL (-37% ± 13%, p = 0.001). The effects of time (p= 0.006, p=0.03) and time-treatment interaction (p=0.01, p=0.008) were significant for both the X0 and tau parameters. In the EG eyes, X0 (23% ± 15%, p < 0.01) and tau (31% ± 15%, p < 0.01) were both significantly increased from baseline.
Conclusions: :
In EG eyes, DRT following an acute IOP challenge increases as compared to both their own pre-glaucomatous BL and to their fellow contralateral control eyes. The results suggest that chronic elevation of IOP has caused the EG eyes to have an altered ocular volumetric rigidity. It’s speculated that as EG develops, a change in either the shape (volume) and/or in the material properties of the eye occurs and that an IOP challenge is one method to assess these changes.
Keywords: intraocular pressure • outflow: trabecular meshwork • anterior chamber