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Sarah R Hatt, David A Leske, Laura Liebermann, Jonathan M Holmes; Quantifying Change in Control in Intermittent Exotropia. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4090.
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© ARVO (1962-2015); The Authors (2016-present)
Quantification of “control” in intermittent exotropia (IXT) remains an ongoing challenge. We have previously described a quantitative office control scale (score: 0 phoria to 5 constant tropia), reporting its variability, and subsequently proposed using the mean of 3 measures of control during an office examination (a “triple control score”). In the present study, we first defined test-retest variability of the triple control score and then applied derived thresholds to a separate longitudinal cohort of children with stable intermittent exotropia to evaluate the performance of the proposed measures.
To define test-retest variability of change in control scores, we re-analyzed data from a previous study of 12 children with IXT with 8 triple control scores across 2 days. We calculated 95% repeatability coefficients for distance and near. We then applied these thresholds to a separate cohort of 23 children with stable IXT assessed by triple control scores at visits at least 8 weeks and no more than 2 years apart (median 36 weeks, range 12 to 88). IXT was defined as stable based on parental history, physician assessment, no intervening treatment and no prescribed treatment at their second exam. An alternative threshold of at least 1 unit change in control score was also evaluated. The proportions of patients exceeding 95% repeatability coefficients, and at least one unit change, were calculated.
95% repeatability coefficients were 2.15 at distance and 1.57 at near. Of the 23 children in the longitudinal cohort, 1 (4%) exceeded the 95% repeatability coefficient at distance (improved) and 4 (17%) at near (1 improved, 3 worse). Applying the criterion of at least one unit change, 10 (43%) exceeded at distance (4 improved, 6 worse) and 9 (39%) exceeded at near (2 improved, 7 worse).
Using a triple control score, assessed during a clinic examination, may provide a representative measure of control in IXT, but there is still marked short-term test-retest variability. Applying 95% repeatability coefficients results in very little misclassification of worsening or improving, but the thresholds are large. Using a lower threshold of at least one unit change, results in troublesome misclassification. Control of IXT remains challenging to quantify.
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