Background: Although the alternate cover test is a widely accepted test of ocular alignment, clinical ambiguity exists regarding 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 heterophoric measurement at 40cm.
Methods: Fifty-seven adult subjects (mean age, 24.0 +I-2.88 years) with healthy binocular systems were measured for heterophoric posture during the cover test while the examiner position was varied between midline and 30 degrees to the right of patient midline.
Results: A significant difference between midline and offset position of 1.3A was found (P < 0.001). The results show that a greater exophoric measurement was seen when the examiner performs the test in the offset position.
Discussion: A 1.3A difference in examiner positions demonstrates a statistically significant difference between examiner positions. This difference, it seems, is related to proximal awareness or prismatic measurement error. Given the small magnitude of the effect, its importance in clinical care is probably most relevant in patients with high phorias or intermittent strabismus. Examiner position may also contribute to variability in cover test measures between examiners or on test-retest.
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