Eye (2017) 31, 615–619 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved 0950-222X/17 www.nature.com/eye 1,2 1 3 Caffeine intake is S Abokyi , J Owusu-Mensah and KA Osei CLINICAL STUDY associated with pupil dilation and enhanced accommodation Abstract Purpose It is purported that caffeine, an Introduction autonomic stimulant, affects visual Pupillary response and accommodation are performance. This study sought to assess important physiological processes of the eye that whether caffeine intake was associated with affect vision. Ocular accommodation is a blur- changes in pupil size and/or amplitude of driven reflex and results in focusing of images accommodation. onto the retina.1 The pupil regulates retinal Patients and methods A double-masked, illumination to optimize visual perception. crossover study was conducted in 50 healthy Other visual functions such as contrast subjects of age range 19 to 25 years. sensitivity, color perception, and visual acuity Subjects were randomized to treatments are influenced by pupil size and such that subjects consumed either 250 mg 2 caffeine drink or vehicle on separate days. accommodation. The iris and ciliary muscles, Amplitude of accommodation was measured which are directly responsible for the manifest by the push-up technique, and pupil size structural changes in pupillary response and using a millimeter ruler fixed to a slit accommodation respectively, receive autonomic 1 lamp biomicroscope in dim illumination innervations from both the mesencephalic Department of Edinger–Westphal nucleus and hypothalamic Optometry, School of (5 lux). Amplitude of accommodation – Allied Health Sciences, center in the diencephalon.3 5 Parasympathetic and pupil size were taken at baseline, College of Health and and at 30, 60 and 90 min time points post innervation of the iris sphincter muscle results in Allied Sciences, University treatment. Repeated measures one-way reduction of pupillary diameter, whereas of Cape Coast, Cape ANOVA and paired t-test were used in sympathetic innervation of the iris dilator Coast, Ghana muscle increases pupil diameter. However, the analyzing data. 2 accommodative state is largely controlled by School of Optometry, Results Amplitude of accommodation and Hong Kong Polytechnic pupil size after caffeine intake were parasympathetic innervation of the ciliary University, Hong Kong significantly greater than vehicle (Po0.001) muscle, although sympathetic innervation plays at each time point. Consumption of the a complementary role in relaxing 3School of Optometry, 6 caffeine beverage was associated with accommodation. University of Alabama at significant increases in amplitude of Caffeine, a psychoactive drug, is commonly Birmingham, Birmingham, AL, USA accommodation and pupil size with time consumed in various forms and in variable o amounts by people worldwide. The absorption (P 0.001). Amplitude of accommodation Correspondence: rose from 12.4 (±2.2 D) at baseline to 15.8 of caffeine into the bloodstream after oral intake S Abokyi, School of (±2.6 D) at 90 min. Similarly, pupil size is very efficient, and widely distributed to all Optometry, Hong Kong increased from 3.4 (±0.4 mm) at baseline to organs of the body including the nervous system Polytechnic University, 4.5 (±0.72 mm) at 90 min. Consumption of because of its hydrophobic properties.7,8 Kowloon, Hong Kong Tel: +852 5108 6048. Interestingly, caffeine has wide variations in its vehicle was not associated with increase in E-mail: samyomah22@ 9 amplitude of accommodation or pupil size nature of action on biological tissues, an yahoo.com or sabokyi@ucc. with time. attribute that makes it worth of study. edu.gh Conclusion Pupil size and accommodation Because of the increased awareness of the are affected after ingestion of caffeine. wide spread effects of caffeine on the nervous Received: 19 September This study suggests caffeine may have some system,8 scientists have sought to investigate for 2016 fl Accepted: 8 November in uence on visual functions. ocular and visual changes associated with 2016 Eye (2017) 31, 615–619; doi:10.1038/eye.2016.288; caffeine intake. This is because ocular tissues Published online: published online 16 December 2016 including the iris muscles, ciliary muscle, 16 December 2016 Effect of caffeine on pupil size and accommodation S Abokyi et al 616 smooth muscles of the eyelids, lacrimal gland, and any potential examiner bias. The second treatment was choroidal and conjunctival blood vessels receive received a week after the first as this was necessary for the autonomic innervation.10 Although results of studies may washout phase of caffeine. Pupil size and amplitude of be inconsistent, caffeine intake has been associated with accommodation were measured at 30, 60, and 90 min post changes in ocular parameters including tear secretion, treatment. All measurements were taken on the same eye – intraocular pressure, and macular perfusion.11 13 that was randomly selected per subject at the start of the Recently, some researchers have found significant study. Two masked examiners took all measurements, changes in some vision-related tasks associated with one measured amplitude of accommodation and the other caffeine intake.14,15 We therefore hypothesized that pupil diameter. caffeine could affect visual performance through changes in pupil size and/or accommodation. Measurement of subjective amplitude of accommodation The monocular amplitude of accommodation was Materials and methods measured through distance spectacle correction. Ethical consideration Spectacles were worn at 15 mm vertex distance. The ’ Ethical approval was obtained from the Institutional subject s attention was directed to the 6/9 row of letters Review Board of the University of Cape Coast. The on a movable target of the Royal Air Force (RAF) ruler at research protocol followed the tenets of the Declaration of 40 cm and instructed to keep the letters clear. Target was Helsinki for use of human subjects, and informed consent moved slowly at 1 cm/s toward the nose and subjects was obtained from all subjects before participation. were to report when the letters become and remain blurred. The distance from the target to spectacle plane was measured in cm and referred to as the near point of Sampling procedure accommodation. The linear distance was converted into Fifty healthy subjects of either sex, aged 19 to 25 years and diopters by dividing the near point of accommodation in ’ weighing 59 to 65 kg, were sampled from the student cm into 100. This represents the subject s accommodative population of the university. Subjects underwent a amplitude. preliminary eye examination before recruitment. The eligibility criteria were refractive correction of spherical Pupil size equivalent r ± 0.50 D and best spectacle corrected visual acuity of 6/6 or better. Persons excluded were those Pupil size was measured using a ruler under a slit lamp having systemic or ocular health diseases, microscope in a dimly lighted room (5 lux) with the face accommodative anomalies or pupillary defects, as well as illuminated from below and pupils in a semidilated state history of habitual ingestion of caffeine-containing as recommended for pupillary measurement. The subjects fi beverages in order to eliminate tolerance effects due to were made to xate a distant target (6/9 letter of the chronic use. Subjects were not supposed to have Snellen chart). To measure pupil size, the ruler was consumed any caffeine at least a week before the study. positioned horizontally such that the zero calibration on the rule laid tangential and vertical on the temporal margin of the pupil. The horizontal pupil diameter was Experimental protocol read off the vertical calibration that laid tangential to the A double-blind, placebo-controlled, randomized inner margin of the pupil. crossover experimental design was conducted. Subjects were allowed to sit in the examination room for 15 min Repeatability of measurements after which baseline measurements for pupil size and accommodation were taken. The value recorded for pupil The two masked examiners, who are experienced size and amplitude of accommodation was the average of clinicians, measured amplitude of accommodation or three closest measures out of five taken per subject. pupil size of subjects, using the push-up and mm ruler Subjects were then randomized to treatments on respectively, on two separate occasions. Repeatability of separate days such that each received either 250 mg each measurement (pupil size and accommodation) taken caffeine (NuSci Caffeine Anhydrous Pure Powder, per examiner was evaluated using intraclass correlation HerbStore, Walnut, CA, USA) dissolved in a 250 ml coefficients (ICCs) and 95% limits of agreement (LoAs) to lemon drink or same volume of placebo with reversal of set the limits within which any change detected could be the treatment on the second visit. This was done to ensure due to the repeatability of the measurement. The ICC and examiners were masked to the treatments to eliminate LoA for each subjective measurement were recorded Eye Effect of caffeine on pupil size and accommodation S Abokyi et al 617 (pupil size: +0.91, ± 0.6 mm; amplitude of accommodation: +0.82, ± 1.4 D, respectively). Data analysis Statistical analysis of the data was done using Graphpad prism (GraphPad Software Inc., San Diego, CA, USA). Data was presented as mean (± SD). The independent t-test was used to compare the amplitude of accommodation and pupil size between males and females. Repeated measures one-way ANOVA was used in evaluating the effect of time for each treatment. Paired t-test was used to compare amplitude of accommodation and pupil size between caffeine treatment and placebo treatment at specific time points. The threshold for statistical significance was set at a probability of 5%. Results Fifty healthy subjects (29 males and 21 females) of mean age 22.2 ( ±1.9) years and weighing 59 to 65 kg (mean ± SD: 62.7 ± 2.4 kg) participated in the study. At baseline, amplitude of accommodation and pupil size were 12.4 ± 2.2 D (range: 8.3–20.0 D) and 3.4 ± 0.4 mm (range: 2.7–4.1 mm) respectively.