The effects of varying examiner position on alternating cover test results.

Background: Although the alternate cover test is a widely accepted test of ocular alignment, clinical ambiguity exists with regard to the specific location of the examiner. This study was an attempt to replicate the findings of a previous study that compared two examiner positions and their effect on heterophoria measurement at 40 cm. Methods: Fifty-seven adult subjects (mean age, 24.0 ± 2.88 years) with normal binocular vision were measured for heterophoria posture using the alternate cover test while the examiner position was varied between midline and offset 30° to the right of patient midline. Results: A significant difference between midline and offset position of 1.3 Δ was found (p < 0.0001). The results show that a greater exophoric measurement was seen when the examiner performed the test in the 30° offset position. Discussion: A 1.3 Δ difference in measured phoria associated with examiner position is consistent with, though smaller than previously reported. Why the examiner position should influence the phoria is possibly related to proximal awareness or prismatic measurement error Given the small magnitude of the effect found in this study, its relevance for patients with normal binocular vision is slight, though examiner position may contribute to variability in cover test measures between examiners or on test-retest.