Mechanisms of cone sensitivity loss in retinitis pigmentosa

Abstract

Purpose: To explore the mechanisms of cone sensitivity loss in retinitis pigmentosa by combining two-colour perimetry with threshold versus intensity (tvi) testing.

Methods: Seven subjects with autosomal recessive retinitis pigmentosa and 10 normal subjects were recruited and underwent perimetric testing of one eye using 480- and 640-nm Goldman size V targets presented under scotopic conditions (no background illumination) and against a white background ranging in luminance from -1.5 to 2 log cd m^-2 in 0.5 log cd m^-2 steps. Data were fitted with tvi functions of the form logT = logT₀ + log ((A + A₀)/A₀)ⁿ, where T is the threshold, T₀ is the absolute threshold, A is the background intensity, A₀ is the 'dark-light' constant and n is a gain constant.

Results: Reliable tvi functions could not be obtained within the region of the visual field corresponding to loss of the ellipsoid zone on optical coherence tomography. At fixation, changes in both T₀ and A₀ were observed, consistent with a d₁ mechanism loss, which resulted in an upwards and rightwards shift of the tvi function. Losses at [±3°, ±3°] demonstrated changes in T₀, consistent with a d₃ mechanism loss, resulting in an upwards translation of the tvi curve.

Conclusions: Although the absolute cone threshold was elevated at each location, shifts in the tvi function (so-called d₁ mechanism loss) at fixation minimise threshold elevation in the presence of white adapting backgrounds, such as those typically employed in standard two-colour perimetry. At more peripheral testing locations, changes in threshold occurred independent of background luminance (so-called d₃ mechanism loss). These findings suggest that backgrounds which selectively adapt rods while maintaining cones at, or near, absolute threshold may be preferable to conventional two-colour perimetry for assessing loss of cone sensitivity, especially at the point of fixation.

Keywords: cone sensitivity; retinitis pigmentosa; rod–cone dystrophy; selective perimetry; threshold versus intensity; two‐colour perimetry.