Retinal nerve fiber structure versus visual field function in patients with ischemic optic neuropathy. A test of a linear model

Abstract

Purpose: To test a linear model relating the regional loss in retinal nerve fiber (RNFL) thickness to the corresponding regional loss in sensitivity with data from patients with previous anterior ischemic optic neuropathy (AION).

Design: Case-control study.

Participants: Twenty-four individuals with AION and 20 with normal vision were tested. The time since the AION attack ranged from 5.2 months to more than 20.3 years (median, 2.95 years).

Methods: Eyes were tested with standard automated perimetry (SAP) and with optical coherence tomography (OCT), both RNFL thickness scans. The average RNFL thickness of the inferior and superior disc sectors was plotted against the average total deviations (linear units) of the corresponding superior and inferior arcuate field regions, and a linear model was fitted. According to the model, the RNFL thickness R=s(o)T+b, (1), where T is the relative SAP sensitivity loss (on a linear scale; e.g., for -3 dB, T = 0.5), s(o) is the RNFL thickness attributable to axons in the healthy or normal state (T = 1.0), and b is the residual RNFL measured when all sensitivity and axons are lost.

Main outcome measures: Optical coherence tomography RNFL thickness and SAP sensitivity.

Results: The data from the AION patients resembled the data from glaucoma patients previously tested and were described by the linear model. For patients with SAP losses of more than -10 dB in the arcuate region, the RNFL thickness provided an estimate of residual RNFL thickness, b. The median value of b (45.5 microm) was similar to the value for patients with glaucoma. It varied among individuals (range, 30.4-63.3 microm), showing a very weak correlation with patient's age (r = 0.30) and the time since the AION episode (r = 0.26), but an excellent correlation (r(2) = 0.94; P<0.01) with the value of s(o), estimated from the unaffected eyes.

Conclusions: The relationship between a structure (OCT RNFL thickness) and function (SAP sensitivity loss) is the same for patients with AION and glaucoma and can be approximated by a simple linear model. The model may provide a framework for identifying those patients with ganglion cell axons that are malfunctioning but are alive.

Figure 1

Static automated perimetry (SAP) (A) total deviations plots and (B) optical coherence tomography (OCT) retinal nerve fiber (RNFL) profiles for a patient with anterior ischemic optic neuropathy (AION) of the right eye (OD); the left eye (OS) was unaffected. Schematic showing the Garway-Heath et al map of (C) SAP regions to (D) disc sectors. The superior arcuate field region (red) maps to the inferior RNFL arcuate sector from 271° to 310°, and the inferior arcuate field region (blue) maps to the superior RNFL arcuate sector from 41° to 80°. INF = inferior disc sector; NAS = nasal disc sector; SUP = superior disc sector; TEMP = temporal disc sector.

Figure 2

A, Graph demonstrating the retinal nerve fiber (RNFL) thickness of the inferior disc sector as a function of superior visual field loss for the 24 affected eyes and 19 unaffected eyes of the patients with anterior ischemic optic neuropathy (AION), as well as for 20 control eyes and 15 eyes with glaucoma. B, Graph demonstrating the RNFL thickness of the superior disc sector as a function of inferior visual field loss for the same groups in A. In both panels, the curve is the fit of the linear model. See text for details. dB = decibels; NYC = New York City.

Figure 3

A, Graph demonstrating residual retinal nerve fiber layer (RNFL) thickness, estimated from eyes with arcuate field losses resulting from anterior ischemic optic neuropathy (AION) of more than −15 dB in Figure 2, plotted against the age of the individual. Arrows, 2 oldest patients. B, Graph demonstrating residual RNFL thickness, as in A, plotted against the time since the last AION episode. Arrows, 2 oldest patients. C, Graph demonstrating residual RNFL thickness, as in A, plotted against the RNFL thickness of the unaffected eye.