Nature of the visual loss in observers with Leber's congenital amaurosis caused by specific mutations in RPE65

PURPOSE: To characterize visual losses associated with genetic mutations in the RPE65 gene that cause defects in the RPE-specific isomerase, RPE65. RPE65 is an important component of the retinoid cycle that restores 11-cis-retinal after its photoisomerization to its all-trans form. The defects investigated here cause Leber's congenital amaurosis (LCA2), an autosomal, recessively-inherited, severe, congenital-onset rod-cone dystrophy.

METHODS: Vision was assessed in nine patients and 10 normal controls by measuring: (1) long-wavelength sensitive (L-) cone temporal acuity (critical flicker fusion frequency or cff) as a function of target illuminance, and (2) L-cone temporal contrast sensitivity as a function of temporal frequency at a fixed-target illuminance. Measurements were made by modulating either a 650-nm light superimposed on a 480-nm background or the red phosphor of a color monitor on a background produced by the monitor's blue phosphor.

RESULTS: RPE65-mutant observers have severely reduced cffs with shallower cff versus log illuminance functions that rise with a mean slope of 4.53 Hz per decade of illuminance compared with 8.69 Hz in normal controls. Consistent with the cff differences, RPE65-mutant observers show losses in temporal contrast sensitivity that increase rapidly with temporal frequency.

CONCLUSIONS: All RPE65-mutant observers have consistent and substantial losses in temporal acuity and sensitivity compared with normal observers. The losses can be characterized by the addition of two sluggish filters within the mutant visual pathway, both filters with a time constant of 29.5 ms (i.e., low-pass filters with cut-off frequencies of 5.40 Hz).