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

Abstract

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.

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

“…Future studies should therefore include a system which grades both the quantity and severity of symptoms using a validated symptom questionnaire. It is of interest to note however that despite these limitations the present data again supports the findings from earlier research which suggests that an aligning prism reported in the distance is not a good predictor of binocular vision symptoms suggestive of a decompensating heterophoria [7]–[8], [26]–[27]. In fact the only research to support the use of the Mallett Unit in the distance, reported that correction of the associated phoria resulted in an improvement the equivalent of one half of a line on the Snellen chart (approximately 0.05 logMAR) [22].…”

“…Both units contain a central fusion lock which is known to be important when measuring aligning prism [18]–[20]. All patients received instructions to carry out the test based on recommendations by Karania & Evans (2006) [8] as it has been shown that appropriate instructions result in more likelihood of picking up symptomatic patients. A prism bar cover test was performed using a Gulden prism bar (Gulden Ophthalmics) at a distance of 6 m and 40 cm to measure the heterophoria at distance and near respectively.…”

“…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.…”

The relationship between binocular vision symptoms and near point of convergence

“…[1] About 20% of patients who come to optometry clinics will have binocular vision anomalies or decompensated heterophoria. [2] Due to the high prevalence of binocular vision anomalies, a regular vision examination is advisable for identification of these disorders. Convergence function is evaluated with clinical tests to determine whether the visual system can provide the patient's vergence needs at near.…”

Nearpoint Phorias After Nearwork Predict ADHD Symptoms in College Students

“…The magnitude and direction of prism required to null apparent target movement was taken as the phoria value. Specifically, a prism bar was used to incrementally add prism power until the subject reported a reversal of movement direction (prism bar values were 1, 2, 4, 6,8,10,12,14,16,18,20,25,30,35,40, and 45 prism diopters). Prism power was then reduced until the subject reported a second reversal.…”