Estimating the prevalence of heterochromia iridum from high-resolution digital yearbook portraits

Abstract

Heterochromia iridum (HI), or complete heterochromia, is a condition where an individual's irises are of two different colors.1 It may be congenital, with or without a syndromic relationship, or acquired through injury, medication, or other means.2 Immaterial of its origin, HI is generally thought to be harmless and easily diagnosable. For these reasons, HI receives little attention in the medical literature, especially when it is not associated with a more concerning medical condition or cause. For example, a PubMed search on June 19, 2021 returned just 16 results for HI and complete heterochromia among its 32 million citations, and all 16 articles headlined HI's relationship with a syndrome, abnormality, or disease.3 Not surprisingly, a precise measure of HI's prevalence remains an open question, although it is considered rare. While it is possible HI is benign and presents no real medical risk, its association with troubling underlying medical conditions “warrants investigation".4 Moreover, it is difficult (if not impossible) to study potential correlations between HI and other more serious diseases without a better understanding of its prevalence. To the best of our knowledge, the most expansive attempt at estimating HI's prevalence was undertaken by Stelzer, who personally examined over 25,000 Viennese people between 1 and 80 years old in the late 1960s.5 Notably, he only considered cases without a presumed pathological cause, and he reported an observed prevalence of 0.06313%.5