Increment Thresholds in Retinitis Pigmentosa: Association With Retinal Thickness.

PURPOSE

To provide a comprehensive analysis of increment thresholds under different background illuminance conditions and to define the association with retinal structure in individuals with retinitis pigmentosa (RP).

METHODS

Ten subjects with RP (35.3 ± 14.4 years) and 10 visually-normal individuals (28.3 ± 7.3 years) participated. Increment thresholds were measured along the horizontal meridian of the visual field (10° nasal to 10° temporal) using short-wavelength Goldmann size III targets under dark-adapted (DA) conditions and across a broad range of background illuminance levels (up to 50 cd/m2). The relationship between threshold and background illuminance level was described using a standard "threshold-versus-illuminance" (TvI) model to derive absolute threshold (T0), slope (n), and "dark-light" (A0). Retinal thickness was quantified by OCT for comparison to the TvI data.

RESULTS

Mean threshold for the 10 RP subjects was elevated for all background illuminance levels, with the greatest abnormalities under DA conditions. Analysis of variance indicated significantly elevated T0 and A0 for the RP subjects, but n did not differ significantly from the controls. Two subgroups of RP subjects were identified: d1 patients had equal elevations in T0 and A0, whereas d3 patients had elevated T0 with little or no elevation in A0. All d1 patients had normal foveal thickness, whereas all d3 patients had foveal thinning.

CONCLUSIONS

Cone sensitivity loss in the d1 subgroup may be attributed to reduced quantal absorption, whereas cone sensitivity loss in the d3 subgroup may be better explained by a reduced cone density, consistent with their marked retinal thinning.