Does IOP Variability Post–Trabeculectomy Cause Progressive Loss of Optic Nerve Head Rim Area? Results From the Moorfields (More Flow) Surgery Study

Abstract

 

Purpose:

 

Raised intraocular pressure (IOP) is a major risk factor in the progression of structural optic nerve head (ONH) changes. It has been shown that IOP control is most successful following trabeculectomy. The purpose was to examine the effects of IOP variability post trabeculectomy on the progression of ONH changes.

 

Methods:

 

As part of the follow up of the Medical Research Council Moorfields Eye Hosp (MoreFlow) study, patients underwent ONH imaging with the Heidelberg Retina Tomograph (HRT; Heidelberg Engineering GmbH) preoperatively, at 3months then annually post–operatively. IOP was measured pre– and 6 times in the first 3 months post–operatively, & 3 times per year thereafter. Images were analyzed using a 320µm reference plane. Linear regression of sectoral RA over time was performed (HRT pre–defined sectors). Variability was estimated from the residual standard deviation (RSD). Subjects were classified as having high or low variability based on a 50^th centile RSD cut–off. Change was defined as a significant slope >1% of baseline RA/year in any sector (p<0.01, low variability series and p<0.001, high variability series). Patients were classified as improving (I), no change (NC) and progressing (P). Variance of IOP measurements over the follow up period was used as a measure of IOP variability. Mean and variance of IOP between week 1 & week 12 (T1) and between week 1 & the final disc imaging visit (T2) was calculated. IOP variance of each of the 3 disc classification groups at T1 and T2 was compared using the F–test.

 

Results:

 

204 eyes of 204 patients were included. Disc analysis classified discs as 11 I, 28 P, 165 NC. Results for each group are tabulated below. Only mean IOP at final visit differed significantly between groups (Student t–test, p<0.01 level) . Variance in IOP was significantly greater in the progressors at both time intervals.  

 

Conclusions:

 

The analysis suggests that both degree of reduction and variability of IOP play a role in the progression of structural ONH changes following trabeculectomy.

 

Keywords: intraocular pressure • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • clinical (human) or epidemiologic studies: outcomes/complications