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S. R. Hatt, D. A. Leske, B. G. Mohney, J. M. Holmes; Variability of Control in Intermittent Exotropia. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2842.
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Control of intermittent exotropia (IXT) may be defined as the proportion of time the deviation is manifest and the ease of re-establishing fusion after dissociation. Control scales allow control to be quantified but it is unclear whether a single measure adequately represents an individual patient. Using an established Clinic Control Scale rating phoria from 0 to 2 and tropia from 3 to 5 (Mohney and Holmes, Strabismus 2006;14:147-150), we evaluated agreement between 2 examiners and assessed minute to minute variability of the condition.
Distance and near control were independently assessed by 2 examiners in 17 patients with IXT (median age 8 years; range 2 to 33 years). Examiner 1 assessed control for each patient and was observed simultaneously by examiner 2 (time point 1). Within 2-5 minutes examiner 2 assessed control and was observed simultaneously by examiner 1 (time point 2). This yielded a total of 68 simultaneous assessments, 34 at time point 1 (17 near and 17 distance) and 34 at time point 2 (17 near and 17 distance). Agreement between examiners was analyzed using the Kappa test. The threshold for defining change in control between time points 1 and 2 was defined as the maximum difference in control scores between examiners. The frequency and severity of change between time points 1 and 2 was then analyzed.
There was almost perfect agreement between examiners for near (k=0.95) and for distance fixation (k=0.94). Disagreements occurred in only 5 (7%) of 68 assessments and differed no more than one level. There were no instances when one examiner recorded a phoria (0-2 on scale) while the other recorded a tropia (3-5 on scale). Four (23%) of 17 patients showed a change of >1 level between time points 1 and 2: 1 (6%) of 17 changed from tropia to phoria for distance, 3 (18%) of 17 changed from phoria to tropia for near.
The Clinic Control Scale has a high level of inter-observer agreement and therefore provides a reliable measure of control at a given time point. Nevertheless, in approximately a quarter of patients, control of IXT varied within minutes from phoric to spontaneously tropic and vice versa, suggesting that a measure of control at a single time point may be insufficient to categorize severity in an individual patient.
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