The app for strabismus assessment (Figure 1) has been validated and evaluated previously in a clinical setting by comparing it with standard clinical methods, including the modified Thorington test, the cover test with prism, and synoptophores [12]. When the app was used in this study, the automated cover test mode was used, in which one of the subject’s eyes was covered while the computer vision algorithm kept tracking the status of both eyes, and the app automatically took a snapshot of both eyes with a flash as soon as the covered eye was uncovered. This mode allowed the examiners to perform cover/uncover or alternating-cover tests. In this study, the cover/uncover test was performed. The app calculates ocular alignment using the Hirshberg method and gives results in prism diopters (PDs) [13].

The app for refractive error measurement (Figure 1) has been validated previously and evaluated in a clinical setting by comparing it with standard clinical methods, including autorefractors and noncycloplegic subjective refraction [14]. The app estimates refractive error by measuring the far point distance for discerning 20/20 Tumbling E letters [14] or the 20/20-equivalent clock dial chart [16]. When Tumbling E letters are used, the result is spherical equivalent (SE) power in diopters (D), and when the clock dial chart is used, the results give spherical power and cylindrical power based on 2 far points for perpendicular lines. In this study, the Tumbling E letter stimuli were used first to determine the SE, and then clock dial chart was used to estimate cylinder power around the SE range.

Participants first underwent visual acuity testing at a 5-m distance from a standard Tumbling E chart (Figure 2). There are 5 letters on each line from 20/40 downwards. Participants were given 0.02 log units credit for each extra letter read. Participants who had visual acuity worse than 20/20 in at least one eye were tested further for refractive error with the refraction app (Figure 1). Near phoria was measured with the strabismus app (Figure 1) approximately 40 cm from the phone while the participants were instructed to fixate on the phone flash. The vision measurement with apps was performed by personnel without optometry expertise. All vision tests were performed under a noncycloplegic condition.

A total of 73 fourth and fifth graders in Meigu County, Sichuan, China, who attended a charity summer camp in 2023 were screened for vision as a part of the charity program. All children lived locally and were members of the Yi people, an ethnic minority group in China. The per capita gross domestic product in Meigu County is about US $2270, 18% of the Chinese national average. Being in a low-income mountainous area, most of the children had never undergone vision testing due to very limited access to local eye care services. They needed to be taught how to report the orientation of the letter E on the eye chart.

The vision screening was one component of the charity summer camp program. Consent for the vision test was included in the summer camp consent form. Participants received free tutoring classes, meals, and school supplies during the 2-week summer camp. Other than that, they were not compensated specifically for participating in the vision screening. This paper reports the results of a secondary data analysis. The vision measurement data, with individual identifiable information removed, were shared by the charity program. This research received approval with exemption of consent from the Institutional Review Board of Massachusetts Eye and Ear. The study was conducted in accordance with the tenets of the Declaration of Helsinki.

By using 2 mobile apps designed for vision testing, ocular alignment and refraction measurements were successfully and easily incorporated in school vision screening. The testing results provided much richer information than conventional vision screening, which normally does not include the 2 tests. Based on this particular screening, the vision issues among the rural-area, school-aged children were primarily uncorrected refractive error and amblyopia.

While myopia prevalence rates are very high in East Asia, including China, where in some economically developed cities, it is even above 80% among school-aged students [19], the majority of the fourth and fifth grade students in this screening had emmetropic vision (uncorrected visual acuity better than 20/20). Ample outdoor activity time and a light burden of study requiring the use of near vision, as we witnessed and surveyed during the summer camp, are likely the main reasons for the large differences between these students and their urban counterparts. Nevertheless, 4% (3/73) of the children mainly had refractive error (patients P1, P2 and P5 in Table 1). This rate is very similar to the prevalence (4.1%) found in the rural child population in India [20]. In addition, there were 2 students (2.7%) with manifest strabismus (patients P3 and P4 in Table 1). This rate is within the range of strabismus prevalence (2.06%-3.9%) among children in Taiwan in the years 2014 to 2019 [21]. As for the phoria of the nonstrabismic students in this study (mean phoria 0.2-PD esophoria, SD 4.1 PD; IQR 2.9-PD esophoria to 2.2-PD exophoria), it is similar to the normative range found among 879 elementary school students by Lyon et al [22] using the modified Thorington test (mean 1-PD exophoria, SD 4 PD; IQR 2-PD esophoria to 2-PD exophoria).

The consistency with epidemiology findings from other large population-based vision screening studies suggests that the screening results in this study are probably reliable. A difference from previous screening studies, though, is that this screening study used smartphone-based strabismus and refractive error tests, while most vision screening studies were conducted by professionals using conventional optometry methods. For instance, in the screening study by Dandona et al [20], strabismus was assessed using a penlight and refractive error was measured using a streak retinoscope and handheld autorefractor; both measurements were made by ophthalmic professionals. Our study provides a proof of concept that vision screening tests, including for strabismus and refractive error, can be performed using smartphone apps by persons without optometry expertise. A further study to develop the use of this simple and portable approach to mass vision screening is warranted.

One limitation of this study was that the vision test results were not verified by clinical gold standard testing due to the nature of the education charity program. However, it should be noted that the main focus of this study was not to evaluate measurement accuracy, as previous evaluation studies have already addressed that aspect. The difference from our previous evaluation is that this study involved a sample that included people with and without eye conditions, while previous studies deliberately selected patients with a wide range of conditions.

Thanks to their affordability, ubiquity, and validity, smartphone-based vision screening tools have the potential to play a pivotal role in mass vision screening efforts, particularly in areas where access to eye care is limited. These innovative tools may impact eye health care by providing a cost-effective and convenient solution for screening, identifying, and addressing vision issues, especially in underserved communities.

One example of such technology is the Peek Acuity app (Peek Vision), which has made significant strides in the field of mobile vision screening. This app has been deployed to screen tens of thousands of individuals in some studies in Africa [9,23,24]. Its success in large-scale vision screening efforts underscores the scalability and effectiveness of smartphone-based apps. By offering an efficient way to measure visual acuity, the Peek Acuity app has empowered health care providers to reach more people, making it a useful tool for addressing visual impairment.

Another photoscreening app, GoCheck Kids (Gobiquity Inc), is primarily designed to assess for amblyopia risk factors [11,25]. By analyzing eccentric photorefraction red-reflex images, the app does not require patients to respond, and it therefore can be used to examine preverbal children. The app has been tested in clinical trials by comparing it with its counterparts among dedicated screening devices, such as iScreening [26]. It is reported by the company that developed GoCheck Kids that 4 million children have been screened [27].

Considering the successful deployment of these smartphone apps, we expect that a mobile technology–based vision screening approach, operated by personnel with basic training, will be scalable and will complement service programs provided by ophthalmic clinicians and dedicated screening devices (eg, iScreen, Spot Vision Screener). The mobile app approach is particularly suitable for low -resource countries and could also be used for screening in classrooms. Compared to Peek Acuity and GoCheck Kids, the screening apps assessed in this study can measure refractive error and strabismus quantitatively rather than flagging for risk factors (GoCheck Kids) or not measuring these most common conditions at all (Peek Acuity).

Our future investigations will aim to assess the positive and negative predictive values of these apps in large-scale vision screening programs, building upon the feasibility demonstrated in this preliminary study.