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Alexander Pogrebniak; Judging When Eyes Look Straight: Inter- and Intra-observer Variation in the Fixing Eye Prism Test. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4091.
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The fixing eye prism test (FEP test) can be useful for evaluating the amount of correction desired when strabismus occurs in blind or poorly seeing eyes. It also has applicability when corneal opacity or pupil eccentricity complicates use of the Krimsky test. The FEP test can be used during distance or near fixation. The test places successive amounts of prism over the fixing eye while assessing the position of the non-fixing eye. The prism measurement is recorded when the desired (most socially functional) position of the non-fixing eye is achieved. This study was undertaken to explore inter- and intra-observer variation in the fixing eye prism test using a simulated deviation.
Fifteen clinical staff members of a pediatric ophthalmology practice used the fixing eye prism test to measure a simulated 30 prism diopter (PD) left esotropia and 40 PD left exotropia, both during distance fixation. The deviation was simulated by having one staff member for all trials fixate on an eccentric target to generate a known deviation. The observers were instructed to record in masked fashion a prism measurement once they deemed the non-fixing eye to be straight.
The simulated 30 PD esotropia was measured most often as 25 esotropia (mean 25.9 ±5.3 SD, 95% confidence interval 23.2-28.5) with an acceptable ending deviation of 12 PD esotropia to 5 PD exotropia. The simulated 40 PD exotropia was measured most often as 35 exotropia (mean 34.3 ±4.6 SD, 95% confidence interval 32.0-36.6) with an acceptable ending deviation of 15 PD exotropia to orthotropia. The same observer looking at the same eye in each case judged straight to be relatively more exotropic when starting with an exotropic versus an esotropic deviation (mean disparity 9.8 PD ±6.0 SD, 95% confidence interval 6.8-12.8).
Mild underestimation of the deviation was most common among a range of measurements. The values yielded by the FEP test when repeated may indicate an acceptable range of postoperative alignments in patients with poor fusion.
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