Origin of tapetal-like reflexes in carriers of X-linked retinitis pigmentosa.

PURPOSE To determine the origin of the tapetal-like reflex (TLR) in carriers of X-linked retinitis pigmentosa. METHODS Spectral fundus reflectance of carriers of X-linked retinitis pigmentosa was measured and compared with that of normal subjects. The influence of visual pigment was determined by measuring the density difference, that is, the difference between the logarithmically scaled spectra of a bleached and a dark-adapted retina. In addition, fundus reflectance maps at 514 nm were made with a scanning laser ophthalmoscope in a bleached and a dark-adapted condition. Finally, the optical Stiles-Crawford effect was measured to determine the angular sensitivity of the TLR. RESULTS The tapetal-like reflex showed its spectral fingerprint in a high reflectance for wave-lengths smaller than 600 nm. The density difference measured at retinal sites of TLR was significantly larger than in normals, but the shape of the spectrum was similar. The optical Stiles-Crawford effect showed a similar peakedness for all retinal positions. However, in the TLR region, the amplitude of the directionally dependent part was increased dramatically. For the foveal region, no differences were observed. CONCLUSIONS An increase in reflectance of the outer segments of the photoreceptors in heterozygotes compared to normals can explain both the high-density difference and the large amplitude of the Stiles-Crawford effect. An earlier suggestion that only cones contribute to the TLR is unlikely.