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Does iris color influence functions of the eye? A new study says yes!

Philippe Valmaggia (Institute of Molecular and Clinical Ophthalmology Basel (IOB) and Department of Ophthalmology, University Hospital Basel, Switzerland), leading scientist of the study “Iris color matters – a contractility analysis with dynamic volume-rendered optical coherence tomography pupillometry” explains the aim and main results of this research project.

What was the aim of you and your colleagues’ study “Iris color matters – a contractility analysis with dynamic volume-rendered optical coherence tomography pupillometry”?
The aim was to analyze the natural variability in pupillary contractility. For this we used three-dimensional images acquired with optical coherence tomography, also known as its abbreviation OCT. We were mainly interested how age, sex and iris color affects the contractility.

What exactly does pupillary contractility mean? 
Pupillary contractility describes the changes in volume of the pupil. It is formed by the iris - the colored part of your eye – which consists of two sheets of smooth muscle with contrary actions that allow the iris to expand and to contract. These muscles control the size of the pupil and determine how much light reaches the sensory tissue of the retina. The pupil is thus a regulator of incoming light. In darkness for example, our pupils are wider which helps us to better recognize objects, in bright daylight our pupils are narrower to prevent getting blinded.

Where did you get the data for the study?
The data was already collected a couple of years ago by Peter Maloca. With our ophthalmic imaging and OCT group at IOB, we compared the eyes of healthy individuals with the ones of people who suffered from Pseudoexfoliative Glaucoma or Non-Arteritic Anterior Ischemic Optic Neuropathy (NA-AION). In this previous study, we were able to validate this new measurement technique for the pupil with three-dimensional OCT. We were able to measure a difference in the pupillary contractility between the healthy individuals and the people affected by one of the two eye diseases. What we did not analyze then was the natural variability that is the difference in contractility among the healthy subjects. 

So the basis for your study was the data collected from the healthy individuals? 
Yes - we had most of the data on these individuals available already, for example their pupil measurements, their age and their sex. What we gathered now in addition for this study was information about their iris color for our statistical analysis. 

And how can the pupillary contractility be measured in 3D?
The pupil is, as every structure in the body, three dimensional. Besides being a gate for light, it is also a space filled with a fluid known as aqueous humour. With our novel method that we presented in our paper, we measured the pupil first illuminated with a daylight lamp, which is called the photopic state, and then in darkness by covering the subjects with a blanket, which is called the scotopic state. By comparing these measurements, we can basically quantify the volume of aqueous humour in the pupil at a given moment, calculate the contractility and make a comparison between our subjects. 

What are your key findings?
We found that iris color does indeed have an effect on contractility, as does age. Regarding the iris color we found that more pigmented eyes have a higher contractility. This means that the pupils of brown irises had a higher contractility than green and blue irises. Also, we confirmed that the contractility of the pupil decreases with increasing age.

Does this mean that people with brown eyes have a better vision than those with green or blue eyes?
With our method, we so far found differences in the pupillary contractility between different patient groups and that there is also a variability in healthy subjects. Potential clinical implications of our findings have to be further explored and this could indeed include comparisons regarding visual functions such as the visual acuity or contrast sensitivity. In general, there are of course eye diseases that influence the pupillary contractility, and we believe that three-dimensional measurements can be very helpful in the future to better understand these conditions.

And what was your favorite part about conducting this study?
What fascinated me most, was that with OCT we can almost image every structure of the eye in 3D and see what different parts of the eye really look like. And I really appreciate the open-minded atmosphere at our Ophthalmic Imaging and OCT group. On one side, OCT enables to answer highly relevant clinical questions directly affecting the treatment decision – and hence to benefit patients every day. On the other side, I really like to work curiosity driven and can investigate topics such as if the pupillary contractility is affected by iris color. 

Read the full paper here: