Influences of examiner position and effective prism power on nearpoint alternate cover test

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.

Three conditions of the NACT were performed on 52 young adults with normal visual function. Two measurements using the method of limits were made in each condition and averaged. In all conditions, the fixation target remained on the subject's midline at a distance of 40 cm.

Our results showed a small significant increase in measured exophoria when the examiner administered 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, probably due to the control for proximal effect that was used. 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 is not clinically significant. A change in proximal cues may contribute to the effect.