The Melanopsin, Cone and Rod Components of the Pupil Light Reflex and Color Discrimination in Glaucomatous Optic Neuropathy

Abstract

Purpose: To evaluate in glaucoma patients the contribution of the inner and outer retina (melanopsin expressing ganglion cells, and cone and rod photoreceptors, respectively) to the pupillary light reflex (PLR) and to investigate its relationship to color discrimination

Methods: 70 eyes from 23 primary open angle glaucoma patients and 15 healthy age-matched control participants were studied. All were submitted to a complete ophthalmological exam. Glaucoma severity was graded based on visual field performance (24-2 SITA, Humphrey Field analyzer II) using the Hodapp-Anderson-Parrish (HAP) classification, into mild (27 eyes), moderate (7 eyes) or severe defects (10 eyes). PLR was measured with an eye tracker (View Point, Arrington); stimuli were presented in a Ganzfeld (Q450, Roland). Pupil responses were measured monocularly, to 1 s blue (470 nm) and red (630 nm) flashes with -3, -2, -1, 0, 1, 2, and 2.4 log cd/m² luminances. The normalized peak amplitude (PA) of the PLR and the amplitude of the sustained response at 6 s (SR) after light offset were measured. Color discrimination was evaluated with the Cambridge Colour Test (CCT) (Cambridge Research Systems). ANOVA for repeated measures with Fisher post hoc test was used for comparison among groups.

Results: The PA for blue stimuli was smaller for patients with moderate and severe glaucoma compared to controls at most intensities (p<0.05). The red PA was significantly (p<0.05) reduced in mild, moderate and severe glaucoma patients, respectively at -2 log cd/m²; from -2 to 2 log cd/m² and at -1, 1 and 2 log cd/m². Relative to controls, the SR for blue stimuli was smaller at 0 log cd/m² in moderate glaucoma (p=0.025) and at 2.4 log cd/m² in severe glaucoma (p=0.0034). Color vision losses were found in patients with mild and severe glaucoma relative to controls for the tritan axis of the CCT and for ellipse area (p<0.05). Ellipse area was inversely related to amplitude of the SR in severe (p<0.0025) and mild (p=0.05) glaucoma.

Conclusions: Patients with severe glaucoma had reduced rod, cone and melanopsin (SR) contributions to the PLR compared to controls while in moderate glaucoma only the rod and cone responses were affected. Curiously, there was an increased melanopsin response in mild glaucoma relative to controls. Chromatic discrimination losses correlated with melanopsin response decrement in mild and severe glaucoma patients.

Keywords: 649 photoreceptors: visual performance • 668 pupillary reflex • 471 color vision