Ethnic Variations in Interpupillary Distance May Influence Strabismus Trends

A global meta-analysis of 243 studies involving more than 128,000 subjects found that interpupillary distance (IPD) varies significantly across ethnic populations and may influence horizontal strabismus patterns. Researchers reported that populations with wider IPDs generally had lower esotropia/exotropia ratios, while narrower IPDs were associated with higher ET prevalence, supporting a possible anatomical and genetic contribution to strabismus development. The above map from the study shows IPD and EX/ET ratios throughout the Americas (Greenland is included because of its proximity). The size of the symbols denotes the size of the cohort examined. The ranges of the IPDs and ET/XT ratios are color-coded in a heat map as indicated. The multiple ethnicities in North and South America are indicated with different symbols, as indicated in the lower right-hand corner, along with the pooled estimate of the ET/XT ratio for five different ethnicities. See the study itself for additional maps (Europe, Middle East, Africa, Asia/Oceania). Photo: Zehra Z, et al. Clin Ophthalmol. May 20, 2026. Click image to enlarge. Interpupillary distance (IPD) is thought to differ between ethnicities, which could explain horizontal strabismus patterns. Some populations have shown more esotropia than exotropia, while others show the opposite. Large-scale data on this parameter has not been systematically explored, until now. A new study mapped the mean IPD side-by-side with strabismus patterns in different populations based on studies from 61 countries to determine whether a larger IPD is associated with a lower esotropia/exotropia ratio. The results were recently published in Clinical Ophthalmology. Researchers performed a systematic review and meta-analysis of 243 studies on IPD and mapped the data geographically. Only data from adults and teenagers 14 years or older were included, except for Down syndrome cohorts where adult IPDs are reached at an earlier age. These criteria ultimately yielded 128,100 subjects in 288 cohorts. The researchers then compared the regional distribution of the mean IPD with the distribution of horizontal strabismus patterns, which was used to map the relative frequency of esotropia and exotropia according to 301 population-based studies. The mean IPD ratios of major ethnicities were then analyzed by meta-regression analyses to determine an association between the two parameters.According to the study, IPDs are larger in Western Africa, South India, East Asia, in Latinos/Hispanics, in Native and African Americans, and smaller in Europe, North Africa, the Middle East, Northwestern India and in Inuit populations. Regression analyses revealed an association between the interpupillary distance and the esotropia/exotropia ratio with R2 values of 0.322 (major ethnicities) and 0.409 (populations at higher resolution).A summary of their results includes:Europe: Most of the mean IPDs in Central Europe range from ~ 60mm to 63mm, with larger mean IPDs (62-65mm) present in Eastern Europe, Eastern Turkey and the South Caucasus. The highest esotropia/exotropia (ET/XT) ratio (>3) was found in Western and Central Europe, with somewhat lower ET/XT ratios (1.25 to 3; 0.75 to 1.25) in the northern parts of Sweden, in Finland, Northern parts of Russia, in Portugal, and the Balkans.Africa: North and East Africa have populations with a narrower IPD of 61mm to 64mm, while West Africa has populations with the widest IPD, of 65mm to over 70mm. Most populations in West Africa have a low ET/XT ratio (<0.75), while most populations in North Africa (Arabs) and in East Africa (Sudan, Kenya) have more ET than XT with ET/XT ratios between 1 and 7.Middle East: Most studies from this region report a narrow (<63mm) IPD, including among Arabs, Jews, some Iranians and most Turks. In Eastern Turkey and Azerbaijan, the mean IPD is higher at 63mm to 67mm. Most populations in the Middle East have a higher ET/XT ratio (1.25 to 3), with the exception of the population in Iran, Azerbaijan and possibly Afghanistan. Nearly all studies (11/14) from Iran report a low ET/XT ratio (0.14 to 0.77). The large majority of studies from countries surrounding Iran report an ET/XT ratio between 1.48 and 5.Asia and Oceania: The studies from the Northwest of the Indian subcontinent mostly showed a narrow IPD of less than 61mm, while Central India and especially the South of India had larger IPDs of 61mm to over 65mm. The studies from East Asia report IPDs of 61mm to 63mm in China, and larger IPDs in Japan and Korea, mostly 63mm to over 65mm. For Oceania, studies reported on multiple ethnicities, Caucasians or indigenous populations (Maoris and Samoans). Caucasians had an IPD in the same range as those in the Americas (61mm to 62mm), similar to Maoris, while Samoans had a somewhat wider IPD (63.1mm).The lowest ET/XT ratios (<0.25) were reported in East Asia, including China, Nepal, South Korea and Japan. On the Indian subcontinent, most of the populations in the Northwest showed larger ET/XT ratios (1.14 to 6.25), while nearly all populations in the Central, Southern and Eastern regions had lower ET/XT ratios (0.11 to 0.69). Populations in Thailand, Malaysia and Singapore also had a lower ET/XT ratio. In Australia, New Zealand and other parts of Oceania, most indigenous populations had lower ET/XT ratios (0.09 to 1), while Caucasians had similar ratios as in North America (1.09 to 3.33), and populations of East Asian ancestry had lower ET/XT ratios, similar to those in East Asia (0.22 to 0.63).Americas: The five major ethnicities (Caucasian, Latino/Hispanic, African American, Native American, Inuit/Eskimo) show a large diversity of mean IPDs. Inuit populations have a very narrow IPD of 59mm to 60mm, while Native Americans have a wide IPD of over 65mm. Most studies on Caucasians in North America report an IPD similar to Caucasians in Europe. African Americans have a wider IPD, of 64mm to 68mm, which is similar to Africans from West Africa. In South and Central America, most studies on Latinos/Hispanics report an intermediate to wide IPD of 63mm to 64mm.ET/XT ratios depended on ethnicity. Nearly all studies on Native Americans report a low ET/ XT ratio (0.07 to 1), based on 11 studies from Alaska to the Amazon. On the other hand, Inuit (Eskimos) have a high ET/XT ratio of 1.67 to 3. Caucasians in the Americas have an ET/XT ratio of 2.25 (that’s lower than for Caucasians in Europe (4.35). African Americans have an ET/XT ratio of 1.71. Latinos/Hispanics in the Americas have an ET/XT ratio of 0.92. People with Down syndrome have a high ET/XT ratio (40.43).The authors wrote in their study, “Our regression analysis provides evidence at the ecological/population level that the orbital anatomy plays a role in determining the predominant type of horizontal strabismus. Thus, the higher risk or tendency in certain populations towards either ET or XT is largely genetically determined. The R2 values of 0.320 for major ethnicities and 0.410 for populations at higher resolution show a moderately strong association between the IPD and the ET/XT ratio, indicating that the IPD explains about 41% of the ethnic variation in the ET/XT ratio.”Limitations noted by the researchers include that not all cohorts are ethnically “pure” or homogeneous. “There has been substantial mixing of populations, especially due to the migrations within Africa, migrations from Europe to North and South America, and within Oceania,” they wrote in Clinical Ophthalmology, adding that several populations (Native Americans, Inuit/Eskimos and indigenous populations in Oceania) lacked data. The authors concluded, writing, “The significance of our work is that it provides a better understanding of horizontal strabismus, its development and evolution, and greater appreciation for diverse strabismus patterns; this is important for planning of vision care and risk factor awareness.”Click here for the source. Zehra Z, Hagen MM, Wang L, Von Bartheld CS. The interpupillary distance differs between ethnicities and associates with horizontal strabismus patterns: Evidence from a systematic review and meta-analysis. Clin Ophthalmol. May 20, 2026. [Epub ahead of print.] This article was developed by the editorial staff in conjunction with experts in the field. In the process, AI may have been among the editorial tools used to meet the goals of human editors, who approved all content.