Binocular Vision Anomalies in Scotland at Age 3.5-5.5 Years: An Epidemiological Study

Scotland has comprehensive vision screening at age 3.5-5.5 years, with ~85% participation (40,000-50,000 episodes annually). Orthoptists deliver the screening, including presenting vision and tests for binocular vision anomalies (BVAnom). The aims were to investigate (1) changes in the prevalence of BVAnom, 2013-2022 and (2) whether less comprehensive screening of solely presenting vision would detect BVAnom.

Data from eight Scottish Health Boards were available for 2013-2014, 2014-2015, 2015-2016, 2020-2021 and 2021-2022. Binocular vision tests included cover test, ocular motility (OM) and additional tests (near point of convergence, 20Δ base out, pass/fail stereopsis). Data were analysed to determine the prevalence of various BVAnom and adequacy of screening if based solely on vision.

From 2013 to 2022, there was a statistically significant increase in prevalence of exotropia (including intermittent; r² = 0.983, p = 0.001) and of any strabismus (including intermittent; r² = 0.887, p = 0.02), with strabismus prevalence ~2% in 2020-2022. Prevalence of OM anomalies remained stable (r² = 0.364, p = 0.28). The prevalence of BVAnom for each year studied, consecutively, was 3.02, 3.78, 3.83, 4.87, 4.89% (r² = 0.930, p = 0.008). If vision screening had been confined to presenting vision, 342-512 cases of BVAnom would have been missed each year, increasing over time (r² = 0.934, p = 0.007).

In a large population of children in Scotland aged 3.5-5.5 years, the prevalence of BVAnom is increasing, especially exotropia. Many cases of BVAnom would not be detected by solely assessing presenting vision, highlighting the benefits of including binocular vision tests in vision screening. It is recommended that vision screening is repeated during the school years.