A comparison of visual field sensitivity to photoreceptor thickness in retinitis pigmentosa

Abstract

Purpose: To explore the relationship between visual field sensitivity and photoreceptor layer thickness in patients with retinitis pigmentosa (RP).

Methods: Static automated perimetry (central 30-2 threshold program with spot size III; Humphrey Field Analyzer; Carl Zeiss Meditec, Inc., Dublin, CA) and frequency domain optical coherence tomography (Fd-OCT) scans (Spectralis HRA+OCT; Heidelberg Engineering, Vista, CA) were obtained from 10 age-matched normal control subjects and 20 patients with RP who had retained good central vision (better than 20/32). The outer segment (OS+) thickness (the distance between retinal pigment epithelium [RPE])/Bruch's membrane [BM] to the photoreceptor inner-outer segment junction), outer nuclear layer (ONL), and total retinal thickness were measured at locations corresponding to visual field test loci up to 21 degrees eccentricity.

Results: The average OS+ thickness in the control eyes was 63.1 +/- 5.2 microm, varying from approximately 69 microm in the foveal center to 56 microm at 21 degrees eccentricity. In patients with RP, OS+ thickness was below normal limits outside the fovea, and thickness decreased with loss in local field sensitivity, reaching an asymptotic value of 21.5 microm at approximately -10 dB. The ONL thickness also decreased with local field sensitivity loss. Although relative OS thickness was linearly related to visual field loss at all locations examined, a slightly better correlation was found between the product of OS and ONL thickness and visual field loss.

Conclusions: In patients with RP with good foveal sensitivity, the OS thickness and the product of OS thickness and ONL thickness (assumed to represent the number of photoreceptors) decreases linearly with loss of local field sensitivity. In general, in regions where perimetric sensitivity loss is -10 dB or worse, the OS+ thickness approaches the thickness of the RPE/BM complex.

Figure 1.

A fundus image overlaid with an example of 30-2 visual field test locations and the extent of the four volume scans of the Fd-OCT.

Figure 2.

Examples of visual field 30-2 and Fd-OCT line scan through the fovea from a normal subject (A) and a patient with RP (B).

Figure 3.

OCT thickness parameters from the vertical midline as a function of retinal eccentricity, with total retina (left), OS+ (middle), and ONL (right). Shaded region: the mean ±2 SD of normal thickness; colored symbols: the 20 patients with RP.

Figure 4.

OCT thickness parameters as a function of TD. Shown are total retina (left), OS+ (middle), and ONL (right) for all retinal locations along the horizontal and vertical meridians. X, normal subjects (n = 10); open and filled symbols: patients with RP (n = 20).

Figure 5.

(A) Plot of normalized OS thickness as a function of TD for all retinal locations along the vertical and horizontal meridians from a single patient. (B) The same data as shown in (A), with the TD expressed in linear units and data are shown only up to −10 dB. (C) Plot of normalized ONL thickness squared as a function of TD for all retinal locations along the vertical and horizontal meridians from a single patient. (D) The same data as shown in (C), with the TD expressed in linear units and data are shown only up to −10 dB. (E) Plot of normalized product of OS thickness and ONL thickness as a function of TD for all retinal locations along the vertical and horizontal meridians from a single patient. (F) The same data as shown in (E), with the TD expressed in linear units; data are shown only up to −10 dB. Solid line: the simple linear model; dashed line: linear regression line for the data in the right panels.

Figure 6.

(A) Plot of product of normalized ONL thickness and OS thickness as a function of TD in dB for all retinal locations. Solid line: the prediction of a linear model, is the same as that shown in Figure 5. (B) The same data as shown in (A), with the TD expressed in linear units; data are shown only up to −10 dB.

Figure 7.

Plot of normalized product of OS and ONL thickness as a function of TD for the different retinal locations, shown separately. Solid line: the prediction of a linear model (the same model is used in all plots and is the same as shown in Figs. 5 and 6).