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Thesis

Effect of examiner position and prism orientation on near point alternating cover test procedure

1 May 2003

Abstract

This study was designed to resolve uncertainty concerning whether the position of the examiner is associated with variability in the results of the nearpoint alternate cover test (NACT). Two previous studies have shown that when the examiner is positioned to the side of the patient, rather than directly in front of the patient, the NACT shows greater exophoria in the range of 1.3 to 4.4 prism diopters. In the current study, we sought to determine if proximal cues or effective prism power might be the source of the apparent exo shift associated with examiner position. NACT was performed on 52 subjects, with each subject measured under three conditions. 1. Examiner directly in front of the subject, measuring prism held in the frontal position. 2. Examiner 30 degrees right of subject, measuring prism held in the frontal position. 3. Examiner 30 degrees right of subject, measuring prism rotated 30 degrees toward the examiner. Results showed a small significant increase in measured exophoria when the examiner administers the NACT from a position to the side of the subject. The magnitude of the increase, 0.98 prism diopter, was less than in the previous studies. Intentionally rotating the measuring prism had a nominal effect on the measured phoria value consistent with error induced due to effective prism power. It appears that examiner position does affect the outcome of the NACT, but that the magnitude of the effect may be only clinically significant with neutralization of large heterophorias. A change in proximal cues may contribute to the effect.


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