A Clinical Evaluation of Fixation Disparity

2006

Fixation disparity is a minute ocular misalignment under conditions of binocular single vision and is typically detected in primary eye care practices in the UK using the Mallett Unit Fixation Disparity Test. This instrument creates natural viewing conditions, when the patient's binocular system is fused using both central and peripheral fusion locks. This allows the examiner to determine the minimum prism power that eliminates the fixation disparity: the associated phoria or aligning prism. The spherical power that eliminates the fixation disparity, the aligning sphere, can also be determined. The near Mallett Unit Fixation Disparity Test has been shown to have good sensitivity and specificity for detecting symptomatic heterophoria. Cases of decompensated heterophoria tend to have a fixation disparity and the aligning prism or aligning sphere is a good indicator of the correction that will render the heterophoria compensated. The purpose of this study was, for the first time, to investigate the effect of test instructions on the results of the Mallett Unit Fixation Disparity Test. In study 1, we surveyed and observed practitioners to determine the instructions that are typically used. In study 2, we compared results obtained with this ÔstandardÕ method of questioning with a more ÔspecificÕ form of questioning that has been suggested in the literature. The participants for study 2 were 105 patients aged 7-70 years who were randomly selected from those attending a community optometric practice. Significantly different results were obtained with the two sets of instructions. The specific form of questioning revealed more cases of fixation disparity and the results with this method showed a better correlation with symptoms. This only held for near vision: for distance vision, symptoms were not significantly correlated with the presence of fixation disparity. This agrees with previous work with the Mallett unit, which showed a significant relationship with symptoms only at near. We also found that patients with more severe symptoms had greater degrees of aligning prism. Our study supports previous work indicating that the Mallett unit is a useful tool for detecting symptomatic heterophoria at near. However, we found that the testing method is important: patients need to be asked not just whether the nonius strips are aligned but also whether one or both of the strips ever moves. More research is needed to investigate the significance of precise test instructions in other optometric and orthoptic tests.

Section: Discussionmentioning

confidence: 99%

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

Karania

2006

“…, 1989). It has however been shown that the amount of fixation disparity (deviation in minutes of arc) and the amount of aligning prism (associated heterophoria: the amount of prism needed to correct the fixation disparity) are much more reliable indicators of compensation (Mallett, 1974; Payne et al. , 1974; Sheedy and Saladin, 1978; Pickwell et al.…”

Section: Horizontal Heterophoriamentioning

confidence: 99%

Criteria for prescribing optometric interventions: literature review and practitioner survey

O'Leary

2003

The core function of optometrists is the prescribing of refractive corrections, yet a literature review revealed a lack of evidence-based research on criteria for determining when a refractive correction is required. The reported criteria used by practising optometrists were investigated using a questionnaire to survey prescribing habits for borderline hypermetropia, presbyopia, astigmatism, and horizontal and vertical heterophoria. Thirty-eight questionnaires were returned and the results analysed. We calculated the 'cut off' point above which the anomaly would be corrected over 50% of the time that it was encountered. There was a large variation for each category, but it was clear that the presence or absence of symptoms was an important factor for most optometrists when deciding whether to correct a small error. It was found that for symptomatic patients, most optometrists would correct an anomaly if it reached: +1.00 D of hypermetropia, a reading addition of +0.75 D for presbyopia, -0.75 DC of astigmatism, 1.5 prism dioptres (Delta) of horizontal aligning prism, and 1 Delta of vertical dissociated heterophoria. For asymptomatic patients, optometrists would not correct any of the hypermetropic anomalies or heterophorias that were specified in our questionnaire. However most would correct a presbyopic error of +1.50 D or above, or an astigmatic error of -1.50 DC or above, even in the absence of symptoms. These results were compared with previously published guidelines.

“…The prismatic correction required to eliminate a fixation disparity is much less than the dissociated heterophoria and has recently been termed the aligning prism (Rabbetts, 2000; Evans, 2002). A small double‐masked randomised controlled trial showed that prisms prescribed with the Mallett Unit were consistently preferred by patients to spectacles without prism and the Mallett Unit was stated to be a more useful tool for prescribing prisms than Sheard's criterion (Payne et al ., 1974). A recent larger randomised controlled trial found that base in prisms prescribed based on Sheard's criterion were not effective at improving symptoms (Scheiman et al ., 2005).…”

Section: Introductionmentioning

confidence: 99%

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

O'Leary

2006

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.