Chromatic pupillometry dissects function of the three different light-sensitive retinal cell populations in RPE65 deficiency

The aim of the study was to objectively characterize the function of rods, cones, and intrinsic photosensitive retinal ganglion cells (ipRGCs) in patients with RPE65 mutations by using two published protocols for chromatic pupillometry, and to correlate the data with the clinical phenotype. The study group comprised 11 patients with RPE65 mutations, and for control purposes, 32 healthy probands and 2 achromats. A custom-made binocular chromatic pupillometer (Bino I) connected to a ColorDome Ganzfeld stimulator was used to assess changes in pupil diameter in response to red (640 nm) and blue (462 nm) light stimuli. Light intensities, stimulus duration, and background varied depending on the protocol used. Results were compared to the clinical phenotype, that is, visual field (Goldmann perimetry), best corrected visual acuity, and full-field stimulus testing (FST). No significant differences in any of the pupil response parameters were observed in intraday or intervisit variability tests. Pupil responses to rod-weighted stimulation were significantly diminished in all RPE65 patients. Pupil responses to cone-weighted stimuli differed among RPE65 patients and did not always correlate with residual visual field and cone sensitivity loss in FST. Pupil responses to ipRGC-weighted answers were slightly but significantly diminished, and the postillumination pupil response was significantly increased. Chromatic pupillometry represents a highly sensitive and objective test to quantify the function of rods, cones, and ipRGCs in patients with RPE65 mutations.