Iris color influences the rate and degree of pupillary response to dilating agents. Darkly pigmented irides do not dilate as fast or as well as light-coloured irides. Achieving adequate mydriasis has diagnostic and therapeutic implications; for instance, the visualization of the retinal periphery for the staging or diagnosis of retinal disorders. Mordi et al. documented a clear potentiating effect of proparacaine on tropicamide-induced mydriasis in blue—green and hazel—brown irides. However, while the blue—green pupils remained at 90% of maximal dilatation for a longer duration, the duration was not prolonged in brown—hazel eyes. This demonstrates that iris color affects the response of the pupil to mydriatics, being less responsive as the iris color darkens. On the other hand, Ghose reported a significant increase of 3.62 + 0.75 mm in pupil size in dark irides pre-treated with lignocaine 4%, when dilated with tropicamide drops, suggesting that pre-instillation of an anesthetic could indeed potentiate tropicamide-induced dilatation in darkly pigmented irides.(Ogun, et all)

This controversy therefore prompted this present study to determine and quantify the potentiating effect of a single drop of amethocaine, a topical anesthetic, on the mydriasis, produced by tropicamide, a commonly used dilating agent, in healthy young African adults, with darkly pigmented irides. Dr Olufunmilola A. Ogun discusses the background and findings of her Ophthalmology and Eye Diseases paper, Effect of Amethocaine on Tropicamide-induced Mydriasis in Dark Irides.

How did you become interested in studying the effect of amethocaine on the mydriasis in darkly pigmented irides?

I work in a busy teaching hospital set-up in West Africa. As a trainee (resident doctor) I noticed that we often had difficulty dilating patients for fundus examination in our outpatient clinics and many times, we found it difficult to achieve and maintain maximal mydriasis in our patient’s eyes during cataract surgery despite the use of both phenylephrine and mydriacil (tropicamide) drops. Dilatation in the outpatient clinics was often suboptimal (less than 6mm) even after 40 minutes waiting time. This made clinics slow, and patients reluctant to wait for dilated fundus examinations. I was surprised because all the textbooks describe pupil dilatation as a fast and simple procedure as an adjunct to the clinical exam. However, this was becoming a frustrating experience in our clinics.

I began asking myself the question, “Is there nothing we can do to make dilatation faster and better?”  I did a literature search on means of potentiating pupillary dilatation and learnt that the prior instillation of amethocaine was reported to improve the rate and magnitude of pupil dilatation in light coloured irides. I also learnt of the considerable controversy that exists regarding this effect in darkly pigmented irides.

What was previously known about dilating agents on dark pigmented irides?  How has your work in this area advanced understanding of it?

My work has shown using a sensitive measuring technique that indeed prior instillation of amethocaine appears to enhance the mydriatic ability of tropicamide. The mechanism of this potentiation is still not clearly elucidated and perhaps electron microscopy in laboratory experiments may throw more light on the proposed theory that amethocaine causes corneal epithelial changes which enhance the penetration and absorption of tropicamide.

What do regard as being the most important aspect of the results of your research?

My work confirms that dark irides are difficult to dilate.

What was the greatest difficulty you encountered in conducting this studying?

The main limitation I encountered in conducting the study was the small number of subjects.

Dr Ogun is a Consultant Ophthalmologist (Neuroophthalmology subspecialty) and Associate Lecturer at the University College Hospital, Ibadan. Her paper Effect of Amethocaine on Tropicamide-induced Mydriasis in Dark Irides is available now to download, share and comment on.

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