Does Ethnic Variation of the Orbit Determine the Prevalence of Horizontal Strabismus?

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The prevalence of horizontal strabismus differs dramatically between studies for reasons that are not clear. Our preliminary data show that ethnicity is a major factor: some populations have much more esotropia than exotropia, while other populations have much more exotropia than esotropia. Older literature suggested that differences in the interpupillary distance may be responsible for ethnic differences in strabismus, but this has never been systematically explored. Because of the largely undefined ethnic variations, the global prevalence of strabismus is currently unknown. Furthermore, the prevalence within the same ethnicity has never been longitudinally compiled and compared. This lack of information about local and global prevalence makes it impossible to determine trends and to compare them with trends of major risk factors and thus define underlying causes or predispositions for strabismus. This impedes the planning of health care that will be needed in the future. Here, we propose to solve these problems by conducting secondary data analyses. Aim 1: Compile ALL relevant studies and publish a systematic review of the prevalence of horizontal strabismus throughout the world, taking into account ethnic variations and methodology bias. This will provide a much-needed reference guide and allow to estimate the true global prevalence. Aim 2: Examine the distribution of three orbital parameters that are ethnically distinct: interpupillary distance, proptosis, and interorbital width, and discern which parameters (or combination of parameters) best explains the ethnic variation of horizontal strabismus prevalence and patterns. This will provide an evidence-based framework to understand the current ethnic variations and their evolutionary history. Aim 3: Determine the trends (over the last 50-150 years) in prevalence of horizontal strabismus and compare them with local trends for major risk factors of strabismus. This will identify effective risk factors in local populations – which is relevant for planning purposes and for targeted health care initiatives. Our multidisciplinary team of investigators will move the field forward by establishing population-based prevalence numbers for each of the major ethnicities. This is important to provide a comprehensive reference work for future studies. The true prevalence in different countries and ethnicities is important for health care planning purposes. The second aim will inform which key anatomical features of the orbit underlie ethnic differences, with evolutionary, developmental and clinical implications. This will provide a novel conceptual framework of the impact of orbital parameters for strabismus. Finally, consideration of a much larger number of studies and populations throughout the world (~500 vs. ~30 in previous reviews) will allow us to define trends in prevalence and to compare such trends with those of major risk factors of strabismus within those same populations. This will inform which risk factors (e.g., maternal smoking, Cesarean section) likely affect prevalence rates in distinct populations.