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

Abstract

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.

The Mallett Fixation Disparity Test: influence of test instructions and relationship with symptoms

“…(1974) showed that patients consistently preferred spectacles with prisms prescribed using the Mallett Unit rather than spectacles without a prism. Recent research suggests that prismatic correction prescribed on the basis of the Mallett Fixation Disparity Test may also bring about an improvement in rate of reading (O'Leary and Evans, in press). However, most clinicians would be reluctant to prescribe prisms on the basis of one test result alone (O'Leary and Evans, 2003), and Evans (2002, Appendix 3) suggested an algorithm for determining cases of decompensated heterophoria that require treatment.…”

Hoya Eyegenius®: New Method for Measuring and Correcting Fixation Disparity

“…Prismatic corrections have been successfully used in German-speaking countries to treat patients with asthenopic complaints (Günthert, 1980;Methling and Jaschinski, 1996;Schmitz, 1998;Schroth, 1998). Promising results regarding the improvement of reading speed and visual acuity have also been reported in British optometry (Jenkins et al, 1994;O'Leary and Evans, 2006). …”

Is the Aligning Prism Measured with the Mallett Unit Correlated with Fusional Vergence Reserves?

“…This finding is unsurprising in light of earlier research which documents that an aligning prism of ≥1 dioptres in pre-presbyopes and ≥2 dioptres in presbyopes at near is likely to be associated with symptoms attributed to decompensating heterophorias [7]. In fact correcting an aligning prism of ≥2 dioptres has been shown to significantly improve near binocular visual acuity [9], distance binocular visual acuity [22] or reading speed measured via Wilkins rate of reading test [10]. At near the only correlation which produced a small (weak) effect size with base in aligning prism (Exo FD) in both the symptomatic group (12.2%) and the entire group (9.2%) was Percival’s Value.…”

“…The Mallett Unit does not quantify fixation disparity itself, instead it measures the associated phoria which is the “aligning prism” that nullifies the subjective disparity [5]. Research has shown that patients with associated phoria report either decreased binocular cortical responses [6], symptoms attributable to decompensating heterophoria at near [7]–[8], reduced binocular visual acuity [9], reduced reading speed [10] and elevated contrast sensitivity thresholds [11]. Under conditions of tiredness [12] and low illumination [13] an increase in both the degree of associated phoria and the levels of reported symptoms has been documented.…”