Portable acuity screening for any school: validation of patched HOTV with amblyopic patients and Bangerter normals

Visual acuity screening in early elementary school age is an important safety net for children with undiagnosed or persistent amblyopia that has not yet been detected by a pediatrician in the medical home. In public schools, this task is usually performed by the school nurse. Students in private schools and in home schools often lack the deliberate, scheduled vision screening of a nurse. Acuity screening is an important task and there have been several attempts to simplify this important task including making it available in locations where a computer monitor is available [1].

The Alaska Blind Child Discovery (ABCD) has attempted to eliminate amblyopia by conducting statewide photoscreening of younger children [2] and by enhancing patched monocular acuity screening for older children [3]. Two devices were developed; a quick, portable acuity chart and rolls of patches designed to facilitate affordable, clean, monocular testing. Until now, the devices lacked calibration against the industry standard visual acuity (E-ETDRS). PEDIG has done this with their EVA acuity program so we empoyed their calibration scheme in this study [4].

We validated an inexpensive Flip-card, and a free- download acuity screening tool on known amblyopic patients. Industry standard for young children employs surround HOTV optotypes [5] whereas the standard for older children is the Electronic- Early Treatment of Diabetic Retinopathy Study (E-ETDRS) protocol [6]. Since HOTV optotypes are not copyright protected, they can be available for free internet download. The exhaustive validation of a screening device and calibration of an acuity test often takes a large number of patients with a wide range of disease severity. A Bangerter foil is a vinyl film with irregular surface creating graded amounts of blur (http://​www.​bernell.​com/​product/​3118/​).We used Bangerter foils on normal subjects to do a comparative validation and calibration confirmation study.

This cross-sectional study of patients with amblyopia, and then normals is conducted by the Alaska Blind Child Discovery with Institutional Review Board through Providence Hospital. It is compliant with HIPAA and the Declaration of Helsinki. Parents provided informed consent. Three acuity tools are compared: 1) a plastic flip card set, 2) a free computer download Adobe Acrobat file are compared with 3) the E-ETDRS protocol delivered on a calibrated M&S acuity system (Niles, Illinois). The primary outcomes were calibration of the HOTV flip card against the E-ETDRS standard and validation as a screening tool for amblyopia. The secondary outcome was how the device performed with normal subjects with Bangerter foil blur.

For the 31 amblyopic patients—aged 10.7 ± 3 years, range 6–19 years, a regression between their HOTV threshold logMAR acuity and the E-ETDRS in their worse amblyopic eye is given in Fig. 3. The etiology for these treated, residual amblyopic patients was refractive (anisometropia) in 15, strabismic in 4 and mixed mechanism in 12. Eleven were right eyes. Twelve were girls. The tested eyes had mean ± SD sphere of +1.88 ± 3.4 D with cylinder of +1.38 ± 1.57 D. The spherical equivalent of the tested amblyopic eyes was +3.42 ± 2.51 D. Just two of the 8 patients with astigmatism greater than 2 diopters had an oblique axis (> 10° from axial). Linear regression of HOTV versus E-ETDRS showed good correlation (r [2] = 0.88) with HOTV = 0.97 (E-ETDRS) -0.04 (Fig. 3).

Five normal teen females provided critical line surround HOTV, then threshold HOTV with five different Bangerter blurred goggles, then with a + 1 trial lens and additionally in their natural state (“none”). The normal subjects all corrected to better than 20/20. Visual acuity in each Bangerter blur condition, the +1 trial and in natural state was repeated on a subsequent day within 1 month with E-ETDRS. A regression of logMAR acuity with critical line identified is shown in Fig. 4.

For the 31 amblyopia patients, mean ± S.D. HOTV acuity was 0.43 ± 0.30 logMAR compared to E-ETDRS of 0.48 ± 0.29 logMAR. For the Bangerter blurred girls, the mean ± S.D. logMAR acuity for HOTV was 0.40 ± 0.25 and for E-ETDRS 0.23 ± 0.18. The difference between E-ETDRS and HOTV for the 31 amblyopic patients was 0.05 (95% C.I. -0.11, 0.25). The difference between E-ETDRS and HOTV for the Bangerter blurred normal subjects was 0.17 (95% C.I -0.03, 0.40).

A critical line screening (the pdf download) correctly sorted all but one (94%) of the amblyopic patients within 1 logMAR lines of 20/40 E-ETDRS. This one case was 20/80 E-ETDRS passing 20/40 screening HOTV. Critical line screening correctly (within one logMAR line) identified E-ETDRS acuity in the Bangerter-blurred normals in 34 of 35 (97%) of cases. The positive predictive value of the 20/40 HOTV screening was 93% for the amblyopic patients and 74% for Bangerter blurred normal subjects. The ROC curve for the amblyopic patients and the Bangerter blurred normals is given in Fig. 5.

Using the HOTV and E-ETDRS scales, the Bangerter foil goggles produced a degradation in visual acuity shown in Table 1. Demographics of the amblyopic children are given in Table 2.

This paper addresses two questions: 1) are the surround HOTV flip card and the paper critical line “box” valid for school screening? and 2) can blur by Bangerter filter enhance acuity screen calibration and validation?

The American Academy of Pediatrics (AAP) publishes an evidence-based, age- approriate series of vision screening techniques aimed at detecting amblyopia early when therapy is most effective. Instrument-based screening (photoscreening) can accurately detect amblyopia risk factors in toddlers, but it is preferrable for the instrument referral criteria to be deliberately set for high specificity (catch the most severe cases but not over-refer mild cases) [7]. In order for early specific screening to work [8], more sensitive methods for amblyopia detection must be applied in older children their first school years. A sensitive test makes sure no child with substantial amblyopia can inadvertantly pass the screening. School nurses therefore need an acuity screening tool that will not miss a case of amblyopia worse than 20/40. AAP recommends patching the non-tested eye so no child can peek.

The school nurse is hoping to quickly find children with amblyopia—ideally on school entry—so the visiual disorder will not result in permanent, lifetime vision impairment. School nurses should also identify those children with acuity consistent with good school performance (ie better than 20/30). It is less critical for the school nurse to differentiate between acuity of 20/80, 20/100 and 20/150—under each circumstance that student would need referral. In both the amblyopic patients and the Bangerter-blurred teens, a strong correlation was found for acuity in the better ranges. As a result, the ABCD surround HOTV flip chart—employing the critical line or threshold protocols—and the free downloaded critical-line folded “box” should serve their purpose well. Parents of home-schooled children can, and should, screen their students if this has not been done in a recent pediatrician, well-child examination. For private schools lacking a school nurse, parents, staff, or even upper classmen could perform screening for the early elementary grades. We found the critical line screening to take less than 90 s while threshold patched surround HOTV took 3–4 min. E-ETDRS took 4–7 min in the teens and the older, treated amblyopia patients.

In the amblyopic patients using critical-line screening, two patients passing had worse E-ETDRS acuity (20/50 and 20/80). One patient who failed 20/40 critical line actually had 20/40 E-ETDRS acuity. The advantage of critical line screening is that it takes much less time than threshold screening. The deficiency of critical line/matching is the possibility of correctly guessing. The chance a patient can guess 3 of four matching HOTV optotypes correctly (13/256) is about one in 20. The sensitivity of our test would increase by requiring 4 of 5 correct at critical line; only about 1 in 39 could pass by guessing. This would lengthen the time of testing somewhat.

For the normal girls, the Bangerter foil goggles provided substantial range of blur. The HOTV acuity mean was a line worse than the E-ETDRS, while for the amblyopic patients, the mean HOTV threshold acuity was almost a logMAR line better. The Bangerter blurred patients performed less consistently for higher levels of blur. It may be that the effect on acuity as a result of treated amblyopia may differ from that of Bangerter blur [9].

The Pediatric Eye Disease Investigator Group (PEDIG) compared a repeat, computer-presented surround HOTV patched protocol (EVA) to E-ETDRS and found similarities to our study [4]. The HOTV slightly over-estimated visual acuity by 0.68 logMAR especially for those with amblyopia and less for eyes better than 0.3 logMAR. We found 0.5 lines with our amblyopic patients and 1.7 lines with Bangerter blurred normals. The E-ETDRS was found reliable in older children [6]. Crowded Kay picture acuity has been correlated with ETDRS [10]. The VIPS study found non-copyrighted HOTV and copyrighted LEA Symbols similar for a large number of young, pre-school Headstart students; however, the critical line for HOTV is one logMAR line larger than for LEA Symbols [11].

Bangerter and convex lenses have been used to compare acuity as early as 1984 [12]. Our Bangerter foils were placed on color-coded swim goggles to assure non-peeking and to enable proper identification of each blur severity. The goggles themselves may have added a non-uniform amount of blur evidenced by the non-linear degree of acuity with Bangerter foil power. Future efforts may employ a better form of no-peek frame for the Bangerter foils. The PEDIG group assessed the degree of acuity degradation due to strong Bangerter filters on the spectacles over the sound eye of patients in a clinical trial [13, 14]. They found that Bangerter 0.2 decreased acuity 5.1 lines while Bangerter 0.3 decreased acuity 4.8 lines. In comparison, we found Bangerter 0.3 on goggles to reduce acuity 5.6 lines. Similar to PEDIG, we did not find a linear relationship between acuity degradation and weaker Bangerter stated power (Table 1) [15]. We also suspect our different swim goggles may independently have had variable effect on acuity—but this does not adversely effect a validation / calibration study in which we seek to degrade normal acuity graded amounts equally for comparison between two threshold and one acuity critical line testing methods.

We did not complete an expensive large, multi-center trial of low pre-screening prevalence preschool children. Weaknesses of our study are the lack of a population-based screening with three acuity charts age-appropriate pre-schoolers with confirmatory exams on each- ther referred and the pass- with sufficient numbers to yield a large number of referrals from which to calibrate each tool, and to determine sensitivity and specificity as a screening tool. We did not check test re-Test reliability. We housed the Bangerter foils in color-coded swim goggles- and these may have variably degraded acuity. We included children older than pre-school age. Multi-center, population –based vision screening validation is prohibitively expensive for many of the manufactureres, and we lacked the millions of federal dollars needed to carry out a more ideal, multi-year study.

We saved time and money. Strengths of our study were comparing methodology with PEDIG acuity test calibration [4], the demonstration of a novel method for attaining a larger number of paired comparisons by variably blurring cooperative normal subjects, and the presentation of a receiver operator characteristic curve comparing our relatively inexpensive clinical trails to calibrate and validate pediatric vision screening devices.

A recent proposed set of guidelines suggest 20/50 screening for children age 3 and 20/40 for children aged 4 and 5 years old [16]; the Flip card can accomplish this though the pre-printed instruction follow existing AAP guidelines.

These affordable, portable surround HOTV screening methods seemed to perform well on amblyopia patients. These critical line screening methods are inexpensive and can be quickly used by school nurses, private school staff and home school parents with a small chance of missing a child with amblyopia. Using critical line with 4 out of 5 needed to pass instead of 3 of 4 would improve sensitivity. Bangerter blur may be a way to facilitate acuity validation using normal subjects to approximate more profound levels of amblyopia.