Image analysis of the tapetal-like reflex in carriers of X-linked retinitis pigmentosa.

PURPOSE: To increase understanding of the tapetal-like reflex (TLR), a unique retinal feature in carriers of X-linked retinitis pigmentosa (XLRP).
METHODS: Color fundus photographs of XLRP carriers were digitized at high resolution. A mathematical model of the imaging system was used to restore the digital retinal images. TLR was separated from the retinal background with an automated segmentation method. Mathematical morphology was used to estimate directional properties. Images from serial photos were registered and compared to study temporal progression.
RESULTS: Quantitative analysis of well-focused funduscopic images show point-like unit reflexes forming the TLR. The average unit reflex is circularly symmetric with a diameter of approximately 8.5 microns and has a maximum reflectance 40% higher than that of the neighboring nonreflex retina. Two or more unit reflexes form small elongate patches that can cluster together into larger patches. Both smaller and larger patches have a strong preferential direction toward the fovea. Comparison of images taken 23 years apart in one patient and 3 years apart in another patient show no detectable changes in the size and location of the reflexes.
CONCLUSIONS: The pattern of reflexes at high and low resolution suggests that the TLR represents an X-inactivation mosaic. Based on the size of the unit reflexes, the authors speculate that the cone photoreceptors participate in the TLR. The stability of the reflex over more than two decades questions the longstanding assumption that the TLR is a stage of the retinal degeneration.