Original Research

Baseline pupil size predicts dilation rate with 1% tropicamide in a young African population

Michael Ntodie, Enoch Eduaful, Carl H. Abraham, Osbert D. Antwi, Khathutshelo P. Mashige

Received: 14 Oct. 2024; Accepted: 01 Sept. 2025; Published: 11 Nov. 2025

Copyright: © 2025. The Author(s). Licensee: AOSIS.
This work is licensed under the Creative Commons Attribution 4.0 International (CC BY 4.0) license (https://creativecommons.org/licenses/by/4.0/).

Abstract

Background: The use of ophthalmic topical agents is critical to the success of several ophthalmic diagnostic procedures, particularly posterior segment examination.

Aim: This study investigated the relationship between age, gender, and anterior segment biometry and pupillary dilation dynamics using 1% Tropicamide ophthalmic solution in a young African population.

Setting: The study took place at the School of Optometry eye clinic, University of Cape Coast, Ghana.

Methods: A pre-test–post-test study design was utilised, involving 51 participants of Black African descent with dark irides, classified according to the Franssen reference set. Baseline anterior segment biometric parameters, including central corneal thickness (CCT), anterior chamber depth (ACD), and pupillary diameter (PD) of one randomly chosen eye, were measured using the Cirrus High Definition-Optical Coherence Tomography (HD-OCT) system. Following baseline assessment, a drop of 1% tropicamide was instilled in a randomly selected eye of each participant. Subsequent PD measurements were obtained at five-minute intervals over a 30-minute period using the Cirrus HD-OCT. All data collection took place in a dark room set up.

Results: The mean baseline pupillary diameter (PD) was 4.31 ± 0.67 mm, with a total change of 2.00 ± 0.73 mm over 30 minutes. No significant correlations were found between the rate of pupillary dilation and age, anterior chamber depth (ACD), or central corneal thickness (CCT) (P > 0.05). However, the rate of dilation showed a significant negative correlation with baseline PD (r = –0.54, P < 0.001).

Conclusion: Smaller baseline pupils dilated faster, indicating baseline pupil size may predict individual dilation dynamics for diagnostic or therapeutic applications.

Contribution: The results of this study provide data that could optimise the use of tropicamide 1% for pupillary dilation in young African populations.

Keywords: biometry; pupil; dilation; African; ocular.

Introduction

Assessment of ocular health with the use of diagnostic procedures in eye care often require employing mydriatic agents.^1,2 Tropicamide 1% ophthalmic solution is a commonly used mydriatic agent which is effective in achieving adequate pupillary dilation prior to examining the posterior segment of the eye.^3,4,5 A parasympatholytic drug, tropicamide blocks muscarinic receptors in the iris sphincter muscle, leading to pupil dilation. Tropicamide is known for producing short-acting mydriasis and cycloplegia, with dilation typically occurring within 20–35 min.^5,6,7 Although the general efficacy and safety of tropicamide is well-established,^8,9,10 an understanding of how individual ocular characteristics influence the dilation process, particularly the rate of dilation, remains unclear, particularly in the African population with dark irides.

Anterior segment ocular biometry, which includes measurements of the anterior chamber depth (ACD), corneal curvature and thickness (CCT) and pupil diameter (PD), provides critical insights into the structural aspects of the anterior eye. These biometric measures, which vary among populations, can affect the efficacy and effectiveness of topically applied ophthalmic drugs.^10,11

Previous studies have highlighted the influence of certain anterior segment biometric parameters, such as central corneal thickness, on the efficacy of topical ocular hypotensive agents.^10,11 In addition, several studies have examined the impact of mydriatic agents on ocular biometry. For instance, Haider et al. (2022) reported a statistically significant difference in ACD measurements before and after pharmacological dilation.¹² Similarly, Hashemi et al. observed a significant reduction in crystalline lens thickness and a corresponding increase in ACD following the instillation of 1% cyclopentolate.¹³ Despite these findings, the relationship between anterior segment biometric parameters and the rate of pupillary dilation remains underexplored, particularly in African populations, whose ocular biometric profiles differ considerably from those of other racial groups.^2,14,15

Age and gender are two important demographic factors which influence ocular biometry findings and response to cycloplegic agents.^16,17 Age and gender differences in biometric measures and cycloplegic effects, may reflect underlying differences in anatomy and neurophysiological functions.¹⁸ However, it is unclear whether these two factors exert main or interactive effects on pupillary dilation dynamics.

The rate of pupillary dilation is not only a reflection of drug efficacy but may also provide insights into underlying autonomic function and ocular physiology.¹⁹ Understanding this relationship is particularly relevant in African populations, where unique ocular anatomical features may influence clinical outcomes. The African iris colour because of melanin pigmentation has been reported to contribute to a relatively lower response of the ciliary muscles to cycloplegia.^7,20,21

Limited literature exists on the association between anterior segment biometry and pupillary dilation rate, particularly among African populations. Such knowledge could be useful for optimising mydriatic protocols and improving diagnostic procedures in this population. For example, knowledge of the relationship between anterior segment biometry and pupillary dilation could help easily distinguish normal physiological variations from pathological presentations during diagnosis. Moreover, understanding the dynamics of pupillary dilation is particularly relevant in paediatric and geriatric populations, where variations in autonomic function and physiology may lead to differing responses to mydriatic agents – a valuable information to optimise scheduling and reduce waiting times.^22,23 In addition, evaluating the rate of dilation could aid in identifying patients who might require adjusted dosages, alternative or combined mydriatic agents to achieve adequate pupil dilation.

The aim of this study was to investigate whether some anterior segment biometric measures and demographic characteristics of participants influence pupillary dilation with tropicamide 1% ophthalmic solution in a young African population.

Research methods and design

Study design

The study utilised a pre-test–post-test design to investigate the effects of age, gender and anterior segment biometry on pupil dilation when using 1% tropicamide ophthalmic solution.

Setting

Study participants were recruited through the School of Optometry and Vision Science, University of Cape Coast eye clinic. The eye clinic provides eye care to the university community and neighbouring communities.

Population and sampling strategy

A minimum sample size of 64 participants was calculated using a mean rate of pupil dilation of 0.06 mm/min from a previous work,²⁴ statistical power of 80% and an alpha level of 0.05. However, 51 participants (male participants: 28, female participants: 23) of black African descent aged 18–74 years, who consented to participate in the study were purposively sampled. These participants presented with distance visual acuity (VA) of 0.0 LogMAR or better, and were visually normal. Participants were predominantly university students who visited the eye clinic for examination, with no remarkable systemic diseases and no recent history of ophthalmic medication use. Participants with systemic conditions such as hypertension, diabetes mellitus were excluded. Similarly, participants on systemic medication, those with ocular diseases such as glaucoma and pupillary abnormalities were also excluded.

Data collection procedure

Eye examination included distance VA (with the LogMAR chart at 6 m) and near VA (N notation at 40 cm), ocular health assessment of the anterior and posterior segments using the slit lamp and a direct ophthalmoscopy respectively.

The right and left eye were labelled and concealed, and by simple random selection by participants, the eye to be used for measurement was obtained. Baseline anterior segment biometric measurements on the randomly selected eye were obtained using the Cirrus HD OCT 500 model (Carl Zeiss Meditec, Dublin, CA, United States). Anterior segment biometric measures included ACD, CCT, and baseline pupil diameter. All biometric measurements were taken in a dark room setup with no light source except that from the optical coherence tomography (OCT) instrument. Prior to biometric measurements, participants were appropriately set up before the OCT device to ensure comfort and easy of data acquisition. Participants were allowed to dark-adapt for approximately 20 s. Baseline pupil diameter and other biometric measures were obtained using the anterior segment OCT prior to instillation of the tropicamide 1% ophthalmic solution. Pupillary diameter changes were measured at 5-min intervals to assess the dynamic response to tropicamide 1% instillation using the anterior segment Cirrus HD 500 OCT. The average of three readings was taken and recorded for the pupil diameter and other anterior segment biometric measures. A drop of tropicamide 1% (Tropicam 1% ophthalmic solution) was then instilled on the randomly selected eye of each participant. Prior to the measurement of the biometric parameters and instillation of the tropicamide, each participant’s iris pigmentation was classified using the Frannsen’s photographic reference set.²⁵

Data analysis

Data analyses were carried out using Statistical Package for Social Science (SPSS, IBM Corp., Version 26.0, Armonk, NY, United States). Baseline measures were normally distributed (Shapiro-Wilk P > 0.05), therefore parametric statistics were employed for analyses. Participants’ characteristics and baseline measures were summarised using descriptive statistics and data were presented as mean (±s.d.) and percentages. Differences between variables were evaluated using an independent sample t-test. Pearson’s and partial correlations were employed to assess the associations between continuous variables. Repeated measures using analysis of variance (ANOVA) were used to analyse differences in pupil diameter over time. Statistical significance was set at P < 0.05. Pupillary diameter at each 5 min interval was measured for the 30 min duration as D1, D2, D3, D4, D5 and D6 respectively. The change in pupil diameter for each 5 min interval was calculated as: (D_5min, D_10min, D_15min, D_20min, D_25min, D_30min)

D_5min = D1 – D0, D_10min = D2 – D1, D_15min = D3 – D2, D_20min = D4 – D3, D_25min = D5 – D4, D_30min = D6 – D5.

Participants with baseline PD of less than ≤ 4 mm and > 4 mm were categorised as small PD and large PD respectively.

Ethical considerations

The study was conducted in accordance with the tenets of the Helsinki Declaration and commenced after obtaining approval from the Institutional Review Board Secretariat of the University of Cape Coast (reference number: UCC-IRB/CHAS/2022/71). Prior to the commencement of data collection, participants were informed of the purpose of the study, and signed consent forms were obtained. Data were collected at the School of Optometry and Vision Science Optometric Clinic at the University of Cape Coast. All data collected from participants were anonymised and secured electronically on a password-protected computer.

Results

Demographics and baseline characteristics

A total of 51 visually normal subjects participated in the study. The mean age of participants was 26.57 ± 9.63 years (18–74 years), with more male participants (n = 28; 55%) than female participants (n = 23; 45 %). The mean age for male and female participants were 25.32 ± 4.11 years and 28.09 ± 13.62 years respectively, and there was no significant difference in age between male and female participants (Independent sample t-test, t_(25.3) = −0.94, P = 0.36). The mean score for all participants on the Frannsen’s photographic reference set was 22.22 ± 1.27. Scores above 20 are within the darkly pigmented iris region.

The mean baseline pupillary diameter measured before the instillation of 1% Tropicamide solution was 4.31 ± 0.67 mm (range: 3.15 to 6.12 mm, see also Figure 1, panel A). A total of 18 participants (n = 18) had ≤ 4 mm baseline PD (mean 3.6; s.d. 0.29).

FIGURE 1: Line graph (with error bars) showing (a) pupil diameter at each interval of measurement and (b) change in pupil diameter over a 30 min duration.

The mean CCT and ACD were 538.22 ± 30.97 µm and 3.08 ± 0.42 mm, respectively. There were no statistically significant gender differences in mean CCT (t_(46.3) = 1.36, P = 0.18) and mean ACD (t_(48.9) = 0.15, P = 0.88). Table 1 shows descriptive summary of all biometric measures.

Repeated pupillary diameter measures

The pupil diameter differed significantly across the duration of measurement (Repeated measure ANOVA: F_(6,300). = 248.87, P < 0.0001, partial η2 = 0.88, Greenhouse-Geisser correction applied). An average change in pupil diameter (a change of 0.29 mm) was measured after the first 5 min after the instillation of the tropicamide among all the participants, and the pupillary diameter on average changed significantly across the six measurement intervals (Figure 1, panel B).

Moreover, the average total change in pupil diameter (as the difference between the pupil diameter at 30 min after the instillation of tropicamide and the baseline pupil diameter) for all participants was 2.00 ± 0.73 mm. The average rate of change in pupil diameter over the 30 min measurement duration was 0.07 ± 0.02 mm/min. There was no statistically significant difference in dilation rate between male participants and female participants (Independent sample t-test, t_(48.4) = −0.34, P = 0.74). The rate of dilation significantly increased after each interval with the lowest and highest rate observed at the first 5-min and 30-min interval, respectively (see Table 1).

Relationship between age, baseline biometric measures and pupillary dilation

The linear relationships between the anterior biometric measures, demographics, and pupillary dilation rate are shown in the scatter plots of Figure 2 and Figure 3. A Pearson’s correlation was run to determine the relationship between biometric measures (ACD, CCT, and baseline PD), age and pupillary dilation rate (Figure 2). There was no statistically significant correlation between the age of participants and baseline biometric measures (ACD: r = 0.06, P = 0.88, CCT: r = 0.02, P = 0.66 and baseline PD: r = −0.23, P = 0.11). Similarly, there was no significant relationship between ACD and baseline PD (r = −0.10, P = 0.47) and between CCT and baseline PD (r = 0.09, P = 0.54). However, there was a weak negative correlation between ACD and CCT (r = −0.29, P = 0.04), Figure 3.

FIGURE 2: Scatter plot showing the relationship between the rate of change in pupillary dilation with (a) anterior chamber depth (r = –0.01, P = 0.92), (b) age (r = 0.26, P = 0.06), (c) with central cornea thickness (r = –0.11, P = 0.44) and (d) baseline pupillary diameter (r = –0.54, P < 0.0001).

FIGURE 3: Scatter plot showing the relationship between central cornea thickness and anterior chamber depth (r = –0.029, P = 0.04).

There was no significant correlation between CCT and rate of dilation (r = −0.11, P = 0.44), and between ACD and rate of dilation (r = −0.01, P = 0.92). However, there was a negatively moderate association between baseline PD and rate of dilation (r = −0.54, P < 0.0001). Age (r = 0.26, P = 0.06), weight (r = 0.35, P = 0.07) and Iris colour (r = −0.14, P = 0.32) had no influence on the rate of pupillary dilation respectively.

A partial correlation was run to determine the relationship between rate of pupillary dilation and baseline PD while controlling for age and weight. The negative correlation between pupillary dilation rate and baseline PD remained statistically significant (r = −0.49, P = 0.0003).

Discussion

This study reports for the first time, the relationship between age, gender, and some anterior segment biometric parameters and pupillary dilation using a sample from a predominantly young African population with mostly dark irides as revealed by mean score from the Frannsen’s reference set. Understanding the relationship between age, gender and anterior segment biometry, and the rate of pupillary dilation can aid clinicians in optimising ocular health assessments and improving diagnostic accuracy.

The study revealed that the baseline pupil diameter, measured under scotopic conditions was associated with the rate of pupillary dilation. Participants with a baseline PD of at least 4 mm demonstrated a significantly faster rate of dilation (0.08 mm/min) compared to those with a baseline PD greater than 4 mm (0.05 mm/min). Perhaps, variation in the density of sphincter and dilator muscle fibres in small and large pupils, and their differential response to the mydriatic agent could partly explain the observed difference in rate of dilation.²⁶ An understanding of the relationship between baseline PD and pupillary dilation rate can be a useful information for the clinicians to anticipate the time required to achieve adequate pupil dilation, allowing for more efficient scheduling of procedures such as fundus examinations and other diagnostic tests requiring mydriasis. A previous study done by Kung et al., comparing the rate of pupil dilation between 1% tropicamide and 2.5% phenylephrine reported a dilation rate of 0.18 mm/min for 1% tropicamide, which is notably faster than the rate observed in the present study (0.063 mm/min).²⁷ This discrepancy may, in part, be attributed to differences in participant characteristics. All participants in the current study were Africans with dark irides, whereas the prior study involved Asian participants. It has been well-documented that individuals with darkly pigmented irides tend to exhibit slower mydriatic responses because of greater pigment binding of the drug and reduced penetration of the mydriatic agent.^20,21

All participants achieved at least a 6 mm dilation within the 30 min duration despite the differences in baseline PD. This suggests that while the rate of dilation varies, the ultimate dilation capacity of the pupil in response to tropicamide 1% is generally sufficient despite different baseline PDs. The onset of mydriasis within 5 min – 10 min intervals as observed in this study is consistent with a study done by Kyei et al. which investigated the onset and duration of cycloplegic action of 1% cyclopentolate and tropicamide 1% combination.¹⁷ They reported a change of 0.5 mm change in pupil diameter between 5 min – 10 min after instillation which is comparable to the 0.63 mm change observed in this study. Furthermore, the present study found that clinically meaningful pupil dilation typically commenced between 5 min and 10 min following the instillation of tropicamide, with dilation progressively increasing over the study period. Notably, the change in pupil diameter during the first 5 min was relatively modest, suggesting a brief latency phase before the drug’s full pharmacological effect became apparent.²⁷

In the present study, gender was related neither to the baseline PD nor to the rate of pupillary dilation. The lack of gender difference in pupillary dilation is consistent with previous studies, which suggest that ocular pharmacodynamics is more strongly influenced by anatomical and physiological factors than by gender alone.^16,17 Similarly, age, anterior chamber depth, and central cornea thickness were not related to the rate of pupillary dilation. These results are consistent with data from an unpublished study involving Caucasian participants,²⁴ where age, iris characteristics and ocular biometry were unrelated to the rate of pupillary dilation. It is possible that these factors exert a less substantial influence on the rate of pupillary dilation compared to other variables such as genetic predisposition and individual differences in autonomic nervous system function. In addition, both the present study (n = 51) and the Caucasian sample study (n = 31) employed relatively small sample sizes, which may have limited the statistical power to detect significant relationships between the variables.

A limitation of this study is the limited age range of subjects and a small sample size, which could affect the statistical power of some of the study findings. Future studies should focus on expanding the study to include a broader age range, particularly recruitment of geriatric participants, where anatomy (pupil sizes), autonomic function and physiology might differ and yield interesting findings. In addition, future studies should explore the impact of other biometric parameters, such as intraocular pressure (IOP), axial length and lens thickness on pupillary dilation.

Conclusion

The present study revealed that participants with small pupils achieved a faster rate of dilation compared with those with larger pupils, which may reflect differences in neurophysiological activity. This finding has implications for clinical practice and patient management, including predicting patients’ responses to dilation which could minimise delays, adjusting drug doses based on pupil sizes and improved diagnostic accuracy in pupillary response-based assessments.

Acknowledgements

The authors thank the staff at the University of Cape Coast, School of Optometry Eye Clinic, for their technical support.

The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.

M.N. conceptualised the study, contributed to the design of the study protocol, coordinated data collection, carried out data analysis, drafted the initial manuscript and approved the final article. E.E. contributed to the design of the study protocol, reviewed and revised the article. C.H.A. contributed to data analysis, reviewed and revised the article. O.D.A. contributed to design of study protocol, data analysis, drafting of initial manuscript and subsequent revision. K.P.M. contributed to the design of the study protocol, reviewed and revised the article.

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

The data sets generated and/or analysed during the current study are available from the corresponding author, M.N., on reasonable request.

The views and opinions expressed in this article are those of the authors and are the product of professional research. The article does not necessarily reflect the official policy or position of any affiliated institution, funder, agency or the publisher. The authors are responsible for this article’s results, findings and content.

References