Double-masked randomised placebo-controlled trial of the effect of prismatic corrections on rate of reading and the relationship with symptoms.

Practitioners' decisions on when to intervene in decompensated heterophoria are often based on symptoms, which are in some cases an unreliable indicator of whether an intervention will be helpful. The aim of our study was to determine when prismatic corrections improve performance at a measure of dynamic visual function: the Wilkins Rate of Reading Test (WRRT). All participants manifested an aligning prism (associated heterophoria) on the near Mallett Unit of 0.5Delta or greater. There were 80 participants, of whom 58 had exophoria, 15 esophoria, and seven hyperphoria. The effect of the aligning prism on the WRRT was compared with a control lens using a double-masked randomised design. For exophoria, an aligning prism of 2Delta and above has a sensitivity of 67% and a specificity of 79% for improving performance at the WRRT by 5% or more. It is not possible from our data to achieve a good compromise between sensitivity and specificity for the other types of heterophoria. Patients in the horizontal (but not vertical) heterophoria groups had significantly more symptoms than a control group. The patients whose visual performance is improved by prismatic correction are not necessarily those who report the most symptoms. Our data suggest that exophoric patients of any age are likely to have improved visual performance with an intervention if they have an aligning prism of 2Delta or more, even in the absence of symptoms. We stress that although the Mallett Unit Fixation Disparity test provides useful information, its results need to be considered in the overall context of the patient's symptoms, lifestyle, and the results of other optometric tests. Although this study evaluated prismatic corrections, we note that interventions for decompensated heterophoria include not only prismatic corrections, but also eye exercises and refractive modification.