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Effects of Tobacco Smoking on the Kinetics of the Pupillary Light Reflex: A Comparison between Smokers and Non-Smokers

Background: The time course of the pupillary light reflex (PLR) is determined by the successive activation of parasympathetic and sympathetic innervations of the iris, latency and amplitude reflecting parasympathetic activity and recovery time showing mainly sympathetic activity. Objective: To determine the effects of tobacco cigarette smoking on the PLR in smokers after an abstinence period of at least 12 h. Methods: Ten smokers (mean 15.7 cigarettes/day) and 10 non-smokers participated in a randomised, non-intervention controlled, cross-over study that included a parallel control group. Smokers underwent two sessions with a time interval between 3 and 8 days; two recordings were taken at each session, separated by 20 min: session 1, without smoking, and session 2, smoking 3 cigarettes within a 30-min period. Non-smokers underwent one session; two recordings were taken separated by 20 min. At each recording, in both groups, PLR was elicited with four light flashes of increasing luminance. Results: The relationship between PLR parameters and light intensity was linear in each subject. The slope of the regression line for relative amplitude increase versus intensity was significantly flatter in abstinent smokers than in non-smokers (p = 0.033); the slope returned significantly after smoking (p = 0.043). No other significant effects were obtained. Conclusions: Kinetic parameters of PLR provide a sensitive pharmacological test to detect cholinergic neurotransmission manipulation effects, as they seem to detect changes in moderate smokers after 12 h of abstinence, and their reversal on return to smoking. These results suggest an enhancement in the suppression of the parasympathetic oculomotor reflex arc rather than a facilitation of the sympathetic drive to the iris.