Basic physiological principles of the chromatic pupil light reflex testing.

Physiological principle of cPLR testing in canine eyes (image is provided by Dr Sinisa Grozdanic, DVM, PhD, Dipl ACVO). Blue light of 480 nm wavelength activates short and medium-long wave length cone opsins, melanopsin and rhodopsin. The red light of 630 nm wavelength can’t activate short wave length cone opsin and melanopsin. Since red light of 630 nm wavelength is activating the far edge of the rhodopsin and medium-long wave cone opsin sensitivity spectrum, pathological change in these specific photoreceptors (rods and medium-long wavelength cones which are dominant photoreceptors in the canine retina) may result in abnormal electrical response, which can be evident as an incomplete PLR or PLR with a presence of pupil escape, particularly after the red light illumination.