Testing depth of suppression in childhood intermittent exotropia

Abstract

A test of suppression was developed to provide a standardized approach to detecting and grading density of suppression in children with intermittent exotropia when manifestly exotropic. This new Office Suppression Test is a three-step procedure to grade suppression on a 4-point scale (from 0 for “negligible suppression” to 3 for “dense suppression”). The test was performed in 57 children 3–13 years of age with intermittent exotropia (distance angle of 16Δ−35Δ, with spontaneous tropia) during enrollment in a randomized trial. Of the 57 children, 51 could complete testing: 28 (55%) had dense suppression, 12 (24%) had moderate suppression, 5 (10%) had mild suppression, and 6 (12%) had negligible suppression. In a subgroup of 20 untreated children, suppression was evaluated again at 8 weeks. There was moderate agreement between suppression scores at baseline and at 8 weeks (weighted κ = 0.65 [95% CI, 0.45–0.84]).

Children with intermittent exotropia do not typically report diplopia, presumably because in most cases it is suppressed.^1,2 Nevertheless, suppression is not routinely measured in clinical practice, despite the fact that treatments such as patching and vision therapy aim to remediate suppression as a means of improving exodeviation control. While designing a pilot randomized clinical trial (RCT) evaluating relieving base-in prism treatment for intermittent exotropia, we identified the need for a simple, easily administered test to document the presence and density, or depth, of suppression. In the present study, we report this new Office Suppression Test, which was administered as part of the randomized trial.

Methods

The pilot randomized controlled trial (www.clinicaltrials.gov: NCT03998670) was funded by the National Eye Institute and conducted according to the tenets of the Declaration of Helsinki by the Pediatric Eye Disease Investigator Group (PEDIG). The study protocol and Health Insurance Portability and Accountability Act–compliant informed consent forms were approved by the institutional review boards of participating study centers. A parent or guardian gave written informed consent for each child; written assent was obtained for children ≥7 years of age.

Study Participants

The major inclusion criteria for the trial were as follows: (1) age 3 to <13 years; (2) intermittent exotropia of 16^∆−35^∆ at distance and 10^∆−35^∆ at near by prism and alternate cover testing; (3) distance exotropia control of 2.0 points or worse (mean of 3 assessments) on a 0 (phoria) to 5 (constant tropia) scale,³ with at least one score ≥3 (ie, spontaneous tropia). Clinical and demographic data for the 57 enrolled children are in eTable 1.

Office Suppression Test

Prior to suppression testing, the care-provider showed the child printed illustrations of double versus single targets. If the child was unable to identify one versus two targets, testing was recorded as “unable,” and the test was not completed.

The Office Suppression Test was designed with input from orthoptists, optometrists, and ophthalmologists, utilizing existing testing equipment. The density of suppression while tropic is graded on a 4-point (0 to 3) scale (Figure 1). For children not spontaneously exotropic at 6 meters, an exotropia must be induced by (1) briefly covering, and then uncovering one eye, (2) placing a septum (postcard-sized occluder) between the eyes, or (3) placing a loose base-out prism in front of one eye (smallest magnitude necessary to induce a tropia). Any of these maneuvers (order not standardized) can be used to ensure that the exotropia is manifest throughout the assessment.

FIG 1.

Three steps of the Office Suppression Test procedure, grading suppression on a four-level scale (0, 1, 2, 3). All testing is to be performed while the exodeviation is manifest and the patient is fixing at 6 meters.

The three-step suppression test was administered as follows. First, after ensuring tropia (using the methods described above), a single 20/50 optotype was viewed at 6 meters, in normal room illumination, and the child was asked whether 1 or 2 letters were seen. If 2 letters were reported, suppression was classified as “negligible” (scored 0), and testing ended. If 1 letter was reported (indicating suppression), testing proceeded to the next step. In the second step, ensuring tropia, a single-point light source was viewed at 6 meters while a red filter was placed over the habitually fixing eye in normal room illumination. If the child reported seeing 2 lights (one white and one red), suppression was classified as “mild” (scored 1), and testing ended. If the child reported 1 light, testing proceeded. In step 3, room lights were dimmed, and the child was again asked to view the light at 6 meters with a red filter over the preferred eye. If 2 lights were reported, suppression was classified as “moderate” and scored 2, whereas if 1 light was reported, suppression was classified as “dense” and scored 3. See Figure 1.

The frequency of suppression scores (0–3) was calculated across all 57 children at baseline. The stability of suppression scores was analyzed using the weighted kappa statistic in a subgroup of the 29 children undergoing no active treatment (per randomization assignment), 20 of whom had testing at both baseline and at 8 weeks.

Results

Of the 57 children, 51 (89%) completed suppression testing at baseline. Six (all aged 4 or 5 years) were unable to understand the test. Seven of 51 (14%) required base-out prism to maintain exotropia throughout testing. Of the 51 with suppression scores, 28 (55%) had dense suppression (score 3), 12 (23%) had moderate suppression (score 2), 5 (10%) had mild suppression (score 1), and 6 (12%) had negligible suppression (score 0).

In the subgroup of 20 children undergoing no treatment who had suppression assessed at both baseline and 8 weeks (Table 1; 9 unable to be tested at baseline and/or 8 weeks). There was moderate agreement between baseline and 8-week suppression scores (weighted κ = 0.65; 95% CI, 0.45–0.84).

Table 1.

Suppression scores at baseline and at 8 weeks in 20 patients with intermittent exotropia not undergoing treatment

Suppression score at baseline	Suppression score at 8 weeks
	0 (negligible)	1 (mild)	2 (moderate)	3 (dense)
0 (negligible)	2	1	0	0
1 (mild)	2	0	1	0
2 (moderate)	1	2	2	1
3 (dense)	0	0	0	8

Discussion

We report a simple test of suppression, enabling standardized detection and grading of suppression during the tropic phase of intermittent exotropia. Using this test, approximately half of evaluated children had dense suppression.

Several tests are available for assessing suppression in the clinic, but there is currently no gold standard. Popular approaches include diplopia awareness (step 1 of our test) or using the Worth 4-Dot test. Some clinicians attempt to grade the density of suppression using the neutral density filter bar (10 gray filters), the Bagolini filter bar (16 pale pink to dark red filters), or the Sbisa bar⁴ (no longer available). These bars, designed to progressively reduce illumination to the fellow eye,⁴ are infrequently used in the United States. In addition, there are reported inconsistencies regarding filter density and light transmittance between filters and bars⁵ and poor repeatability.^4,5

In creating a new suppression test, we aimed to provide a quantitative, ordinal assessment of suppression density that required minimal equipment. The Office Suppression Test shows promise for assessing suppression in intermittent exotropia and possibly other types of strabismus.

We evaluated stability of suppression scores in a small number of untreated children, and although there was moderate agreement over 8 weeks, there is a need to more rigorously evaluate test–retest reliability, repeating measures the same day, and across participants with mild to dense suppression.

There are limitations to our study. We did not compare the Office Suppression Test to filter bars or other tests for suppression. In addition, we did not compare suppression scores with control or magnitude of deviation because we excluded those with mild intermittent exotropia. It would be helpful to study potential associations in a larger population including the full range of intermittent exotropia severity. Also, we did not mandate that the same examiner conduct baseline and repeat testing (stability analysis); thus, agreement may have been affected by interobserver differences.