Purpose
The course of intermittent exotropia and response to surgery may depend on whether there is underlying monofixation. The purpose of this study was to report the prevalence of sensory monofixation in intermittent exotropia using different stereotests and to determine the risk of misclassifying monofixation based on a single administration of each test.
Design
Retrospective case review of children with intermittent exotropia.
Methods
Forty-four children were identified for whom Preschool Randot, Frisby, and Titmus stereoacuity were measured at a single examination. Ninety-two children were identified with near stereoacuity measured on 2 sequential visits (Preschool Randot, n = 73; Frisby, n = 66; and Titmus, n = 40). Monofixation was defined as stereoacuity worse than previously published age-referenced normal thresholds, bifixation was defined as at least 40 arc seconds, and uncertain was defined as within normal range for age but worse than 40 arc seconds.
Results
In children measured by all 3 tests on the same visit, sensory monofixation occurred in 36% using Preschool Randot, in 48% using Titmus, and in 55% using Frisby ( P > .1 for each comparison). There was poor agreement between Frisby and Preschool Randot when classifying monofixation in individual patients ( P = .05). In children measured on sequential visits, misclassification occurred in 5% with Preschool Randot, in 13% with Titmus, and in 23% with Frisby (Preschool Randot vs Frisby, P = .005).
Conclusions
Classification of monofixation depends on the stereotest used. Regardless of the stereotest, there is a risk of misclassifying monofixation on a single assessment. Potential misclassification needs to be considered in clinical practice and in study design.
Intermittent exotropia (XT) often is characterized by bifoveal fixation (bifixation) and normal near stereoacuity. Nevertheless, a proportion of patients with intermittent XT demonstrate reduced near stereoacuity and have been described as having monofixation. The course of intermittent XT and response to surgery may depend on the presence or absence of underlying monofixation (Morrison D, et al. Sensory outcomes after surgery for intermittent exotropia, Paper presented at the American Association for Pediatric Ophthalmology and Strabismus Annual Meeting, San Francisco, California, April 17-21, 2009).
When establishing the presence of monofixation in intermittent XT, Worth 4-dot and 4-diopter base-out testing often are confounded by the intermittency of the deviation. Consequently, a diagnosis of monofixation relies primarily on the assessment of near stereoacuity. Ideally, a single measure of stereoacuity would accurately represent a patient’s sensory status, but we previously reported marked test–retest variability of stereoacuity testing in children. Recently, we noted that this variability may confound the diagnosis of monofixation in intermittent XT. There are a number of factors that make a single measure of stereoacuity difficult to interpret: (1) stereoacuity thresholds may differ solely depending on the test used, (2) stereo thresholds in children with intermittent XT demonstrate poor test–retest reliability, (3) there are a wide range of normal thresholds in young children, and (4) dichotomizing the results of any continuous measure carries the risk of misclassification. Despite these potential problems, clinicians often still use a single measure of near stereoacuity to define monofixation. The aims of this present study were to report the prevalence of monofixation in children with intermittent XT using 3 different tests of near stereoacuity—the Preschool Randot test (Stereo Optical, Inc, Chicago, Illinois, USA), the Frisby test (Frisby Stereotest Ltd, Sheffield, England), and the Titmus test (Stereo Optical, Inc, Chicago, Illinois, USA)—and to describe the risk of misclassifying monofixation with each test.
Methods
Children with intermittent XT (basic, true, or pseudodivergence excess types) at a single institution were identified using a clinical database; children with convergence insufficiency type (near angle more than 10 prism diopters [pd] more than distance) were excluded, as were children with developmental delay or a sensory or paralytic form of exotropia. The stereotest(s) used on a specific examination depended on practitioner preference. We defined 2 patient cohorts: one cohort with all 3 stereotests used on a single examination to compare tests directly and a second cohort with sequential examinations using the same test to evaluate misclassification.
Stereoacuity Testing
Stereoacuity was assessed with the subject’s refractive correction (if worn). The Preschool Randot measures stereoacuity from 800 to 40 seconds of arc, the Frisby was administered to measure from 400 to 40 seconds of arc, and the Titmus circles measure from 800 to 40 seconds of arc. Testing started with the largest disparity, and inability to identify correctly the target with the largest disparity was recorded as nil stereo. For Preschool Randot and Frisby, a pass on each level was defined as 2 correct responses, and for the Titmus circles, there was only one target at each level. Testing continued until a level was failed. The stereo threshold was recorded as the smallest disparity identified correctly.
It has been suggested that measurable stereopsis should be present for a diagnosis of monofixation to be made, even if only at the level of the Titmus fly (3000 seconds of arc). Nevertheless, we included nil stereoacuity as indicating sensory monofixation in intermittent XT because all patients showed motor fusion at near.
Patients
Prevalence of Sensory Monofixation Using Different Stereotests on a Single Examination
Forty-four children with intermittent XT (age range, 3 to 16 years; median age, 6 years) were identified who had completed all 3 stereotests (Preschool Randot, Frisby and Titmus) at a single examination. If there was more than 1 examination with the 3 stereoacuity tests, the earliest examination was included for analysis. The median angle of deviation by prism and alternating cover test (PACT) was 20 pd (range, 12 to 50 pd) at distance and 12 pd (range, 10 pd esodeviation to 40 pd exodeviation) at near. Forty-two of 44 patients (95%) had visual acuity of 20/40 or better in each eye; 2 of 44 patients had visual acuity of 20/60 in 1 eye.
Prevalence of Sensory Monofixation Confirmed at a Second Examination and Misclassification of Monofixation
Ninety-two children (age range, 2 to 17 years), including 37 from the above analysis, met the following criteria: (1) 1 or more of the 3 stereoacuity tests performed at 2 sequential examinations, (2) visual acuity normal at each examination (20/40 or better in each eye and ≤ 2 lines of interocular difference), and (3) no treatment between examinations (first time refractive correction, occlusion, exercises, over-minus lenses, and surgery). For each stereotest, the first pair of examinations meeting these criteria were included. Seventy-three of 92 patients had consecutive measures using Preschool Randot, 66 using Frisby and 40 using Titmus. Sixty-six of 92 (72%) patients provided data for more than 1 stereotest, 21 (23%) of whom provided data for all 3 stereotests, but only 6 (7%) provided data from the same examinations. Patient characteristics (age, angle of deviation, time between examinations) were compared between stereotest groups.
Statistical Analyses
Patients were allocated to 1 of the following 3 stereoacuity designations for each stereotest at each examination: monofixation, bifixation, or uncertain. Sensory monofixation was defined as any stereoacuity value worse than the lower 95% confidence interval (lower limit of normal) in age-referenced normal populations. Because there are no clear definitions of bifixation, we performed 2 separate analyses with 2 commonly accepted definitions: first at least 40 arc seconds ( Table 1 ), and then at least 60 arc seconds ( Table 2 ). Stereoacuity values worse than the bifixation threshold of 40 or 60 arc seconds but still within the normal range for age ( Tables 1 and 2 ) were classified as uncertain because it is not possible to determine whether they represent bifixation or monofixation. The width of the uncertain category changes with age because the range of normal stereoacuity values changes with age ( Tables 1 and 2 ).
| Age (yrs) | Preschool Randot (Arc Seconds) | Titmus (Arc Seconds) | Frisby (Arc Seconds) | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Bifoveal | Uncertain a | Monofixation b | Bifoveal | Uncertain a | Monofixation b | Bifoveal | Uncertain a | Monofixation b | |
| 3 | 40 | 60–400 | 800–nil | 40 | 50–140 | 200–nil | 40 | 60 | 70–nil |
| 4 | 40 | 60–200 | 400–nil | 40 | 50–80 | 100–nil | 40 | 60–nil | |
| 5 | 40 | 60–200 | 400–nil | 40 | 50 | 60–nil | 40 | 60–nil | |
| 6 | 40 | 60–100 | 200–nil | 40 | 50 | 60–nil | 40 | 60–nil | |
| 7 | 40 | 60 | 100–nil | 40 | 50–nil | 40 | 60–nil | ||
| 8 to 17 | 40 | 60 | 100–nil | 40 | 50–nil | 40 | 60–nil | ||
a Stereoacuity values worse than 40 arc seconds, but still within normal range for age.
b Stereoacuity worse than previously published age-referenced normal thresholds.
| Age (yrs) | Preschool Randot (Arc Seconds) | Titmus (Arc Seconds) | Frisby (Arc Seconds) | |||||
|---|---|---|---|---|---|---|---|---|
| Bifoveal | Uncertain a | Monofixation b | Bifoveal | Uncertain a | Monofixation b | Bifoveal | Monofixation b | |
| 3 | 40–60 | 100–400 | 800–nil | 40–60 | 80–140 | 200–nil | 40–60 | 70–nil |
| 4 | 40–60 | 100–200 | 400–nil | 40–60 | 80 | 100–nil | 40–60 | 70–nil |
| 5 | 40–60 | 100–200 | 400–nil | 40–60 | 80–nil | 40–60 | 70–nil | |
| 6 | 40–60 | 100 | 200–nil | 40–60 | 80–nil | 40–60 | 70–nil | |
| 7 | 40–60 | 100–nil | 40–60 | 80–nil | 40–60 | 70–nil | ||
| 8 to 17 | 40–60 | 100–nil | 40–60 | 80–nil | 40–60 | 70–nil | ||
a Stereoacuity values worse than 40 arc seconds, but still within normal range for age.
b Stereoacuity worse than previously published age-referenced normal thresholds.
For the 44 patients with all 3 stereotests measured on a single examination, the rate of monofixation was compared between tests using the Fisher exact test. Agreement between tests was analyzed by comparing proportions classified as monofixation and not monofixation (bifixation combined with uncertain) using the McNemar test. Exact 95% confidence intervals (CIs) were calculated for each proportion.
Among the patients with stereoacuity testing on 2 separate examinations, those with subnormal stereoacuity at both examinations were classified as having confirmed monofixation and those with bifixation at each examination were classified as having confirmed bifixation. Patients with monofixation at the first examination but bifixation at the second examination were designated as misclassified monofixation. Bifixation at the first examination but monofixation at the second examination was defined as possible deterioration. Proportions in each category (confirmed monofixation, confirmed bifixation, misclassified monofixation, and possible deterioration) were compared between stereotests using the Fisher exact test. For each stereotest, patients were compared for age, angle of deviation, and time between examinations using general estimating equations with contrast statements to account for some patients being included for more than 1 stereotest.
Results
Prevalence of Sensory Monofixation Using Different Stereotests on a Single Examination
With bifixation defined as at least 40 arc seconds, the rate of sensory monofixation was comparable between tests: Preschool Randot, 36% (95% CI, 22% to 52%); Titmus, 48% (95% CI, 32% to 63%); and Frisby, 55% (95% CI, 39% to 70%; Table 3 ; P > .1 for each comparison). When classifying patients as either with or without monofixation, there was good agreement between Titmus and Frisby ( P = .3; Table 4 ) and between Titmus and Preschool Randot ( P = .1; Table 4 ), but agreement was poorer between Frisby and Preschool Randot ( P = .05; Table 4 ).
| Fixation Status | Preschool Randot (n = 44) | Titmus (n = 44) | Frisby (n = 44) | |||
|---|---|---|---|---|---|---|
| Bifixation ≤ 40 Arc Seconds | Bifixation ≤ 60 Arc Seconds | Bifixation ≤ 40 Arc Seconds | Bifixation ≤ 60 Arc Seconds | Bifixation ≤ 40 Arc Seconds | Bifixation ≤ 60 Arc Seconds | |
| Monofixation a | 36% b | 36% b | 48% b | 45% b | 55% b | 32% b |
| Bifixation c | 32% | 50% | 43% | 48% | 45% | 68% |
| Uncertain d | 32% | 14% | 9% | 7% | None | None |
a Stereoacuity worse than previously published age-referenced normal thresholds.
b The rate of monofixation was comparable across tests using a threshold of at least 40 arc seconds for bifixation ( P > .1 for each comparison) and using a threshold of at least 60 arc seconds for bifixation ( P > 0.2 for each comparison).
c At least 40 arc seconds or at least 60 arc seconds.
d Worse than 40 or 60 arc seconds but within normal range for age.
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