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 log T = log T 0 + log (( A + A 0 )/ A 0 ) n , where T is the threshold, T 0 is the absolute threshold, A is the background intensity, A 0 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 T 0 and A 0 were observed, consistent with a d 1 mechanism loss, which resulted in an upwards and rightwards shift of the tvi function. Losses at ±3°, ±3° demonstrated changes in T 0 , consistent with a d 3 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 d 1 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 d 3 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.