Phoria variation secondary to cover test technique at near

This study examined variations in near-point phoria measurements as a result of cover test technique. Two observing positions by the clinician were assessed: (1) a midline position, with the clinician directly behind the near-point accommodative target and 12) an off-midline position, so as to not be within the patient's central field of vision.

One hundred thirteen subjects (55 men and 58 women) were classified using the off-midline cover test technique. A second set of measurements was taken with the examiner on the midline. All other testing aspects were held constant.

No significant differences were observed between techniques for patients who were orthophoric or esophoric. Significant differences (P < 0.00001) were observed in subjects who were exophoric. The midline positioning of the examiner was associated with a decrease in the measured exophoria by an average of more than four prism diopters.

While either measuring position by the clinician is comparable for patients with esophoria or orthophoria, the findings suggest that larger amounts of heterophoria will be measured if the cover test at near is performed from an off-midline position when patients with exophoria are examined.