Affiliations: Institut für Medizinische Psychologie,
Ludwig-Maximilians-Universität München | Augenklinik, Campus Innenstadt,
Ludwig-Maximilians-Universität München | Institut für Allgemeinmedizin, Klinikum rechts
der Isar, Technische Universität München
Abstract: Objectives: The study aimed to investigate whether placebo
interventions that induce expectations of a miotic or a mydriatic drug effect
can affect pupil size and accommodation, and in the case of such effects, to
investigate underlying autonomic mechanisms. Methods: Fifty healthy adults were
randomly assigned to one of five groups and were administered eye drops into
their right eye: Twenty participants received saline or pilocarpine drops in a
double-blind design and were informed about the miotic effects of pilocarpine,
a further 20 participants received saline or tropicamide drops in a
double-blind design and were informed about the mydriatic effects of
tropicamide, and ten participants served as controls and knowingly received
saline eye drops. Pupil size and amplitude of accommodation as well as blood
pressure (systolic, diastolic) and heart rate were assessed immediately prior
to the intervention as well as at regular intervals during the 45 minutes
following the intervention. Following the experiment, participants were
questioned regarding subjective treatment effects. Results: Neither of the two
placebo interventions which aimed to decrease or increase pupil size actually
affected pupil diameter or amplitude of accommodation as compared to the
control intervention. Similarly, blood pressure and heart rate were not
impacted by the two experimental placebo interventions. Subjective symptoms,
such as itching of the eye or blurred vision, were more frequently reported by
participants in the two experimental placebo groups than by controls. Conclusion: Results suggest that eye functions controlled by the autonomic
nervous system, such as pupil size and accommodation, cannot be influenced by
placebo interventions based on expectancy manipulation.
