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Relationship Between the Degree of Iris Pigmentation and Corneal Sensitivity to a Cooling Stimulus

Ntola, Anna M., PhD*; Nosch, Daniela S., BSc, MSc, PhD, MCOptom, DipTP(AS), FBCLA, FEAOO; Joos, Roland E., PhD; Murphy, Paul J., BSc, MBA, PhD, FCOptom, FAAO, FBCLA, FEAOO‡,§

doi: 10.1097/ICO.0000000000001933
Clinical Science

Purpose: To explore the relationship between the degree of iris pigmentation and corneal sensitivity threshold (CST) on a variety of different ethnicities, using the air-jet noncontact corneal aesthesiometer and by applying a consistent method of subject iris pigmentation classification.

Methods: A total of 200 subjects (mean age 23.7 ± 3.1 years, 127 women) participated in this clinical cross-sectional study: 100 whites, 40 Asians, 40 Chinese, and 20 Afro-Caribbeans. CST was assessed within the central cornea using a noncontact corneal aesthesiometer, and the degree of iris pigmentation of each subject was noted according to the Seddon method using a set of graded photographs of iris pigmentation (grades 1–5). Inclusion criteria were absence of ocular disease including dry eye, no contact lens wear, and no use of artificial tears. Statistical testing between ethnicities was made by the pairwise t test with Holm adjustment, and a linear model was set up to analyze the effects of ethnicity and iris grade.

Results: A moderate trend for increasing CST with increasing iris pigmentation grade for all ethnicities was observed (R = 0.46; P < 0.0001), with CST changing from 0.66 ± 0.16 mbars for grade 1, 0.74 ± 0.18 mbars for grade 2, 0.86 ± 0.31 mbars for grade 3, 0.85 ± 0.32 mbars for grade 4, and 1.08 ± 0.40 mbars for grade 5. This correlation was stronger within the white group, representing the only ethnicity with all iris pigmentation grades (R = 0.50; P < 0.0001).

Conclusions: There is a moderate relationship between corneal sensitivity and the degree of iris pigmentation, with sensitivity increasing as iris pigmentation decreases. This relationship is stronger within whites.

*Specsavers Opticians Serpentine Green, Peterborough, United Kingdom;

Institute of Optometry, University of Applied Sciences (FHNW), Olten, Switzerland;

School of Optometry & Vision Science, University of Waterloo, Waterloo, Canada; and

§School of Optometry & Vision Sciences, Cardiff University, Cardiff, United Kingdom.

Correspondence: Daniela S. Nosch, BSc, MSc, PhD, MCOptom, DipTP(AS), FBCLA, FEAOO, School of Engineering Institute of Optometry, University of Applied Sciences and Arts Northwestern Switzerland (FHNW), Riggenbachstrasse 16, 4600 Olten, Switzerland (e-mail:

The authors have no funding or conflicts of interest to disclose.

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Received October 05, 2018

Received in revised form February 01, 2019

Accepted February 08, 2019

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