Validity of Techniques to Evaluate Near Phoria

Background: While a number of previous studies have investigated the repeatability of techniques designed to measure heterophoria, there have been few studies of their validity. Accordingly, the present study examined the ability of 3 standard techniques; Von Graefe (VG), Maddox Rod (MR) and Modified Thorington (MT) tests to quantify a known change in heterophoria. Methods:The study was performed on 30 young subjects using each of the 3 procedures listed above. Near (40 cm) heterophoria was quantified both without and with an additional base-out prism. Five possible values were available, namely 2, 4, 6, 8 and 9Δ. After a period of 24 hours, the heterophoria measurement was repeated using the same technique with one of these prisms added to the refractive correction. Results:The mean heterophoria value measured before the introduction of prism using the VG procedure was significantly more exophoric than the findings obtained using the other 2 techniques (p = 0.035). No significant difference was observed between the measured and predicted change in heterophoria following the introduction of the prism for the VG and MT procedures, but a significant difference was found with the MR technique (p0.001). Conclusions:The ability to measure a known change in oculomotor deviation was significantly poorer with the MR technique, when compared with the VG and MT procedures. Given that the mean pre-prism measurement was significantly more exophoric when using the VG procedure, we recommend that MT be adopted as the technique of choice for the subjective measurement of oculomotor deviations in the clinical setting