The significance of instructions and testing method on the Mallett fixation disparity test

Rajula Karania

Stress associated with the use of fusional convergence can result in asthenopia. Clinical Fixation Disparity detection is a useful diagnostic tool because it is related to fusional convergence effort.  Fixation Disparity (FD) occurs when the images of a binocularly fixed object do not stimulate exactly corresponding retinal points but still fall within panum’s fusional areas: the object thus being seen singly. FD is usually measured in minutes of arc and is very small, normally 5 to 15 minutes of arc increasing further away from the fovea. This is because the receptive fields in the foveal region are small but become larger in more peripheral areas of the retina.

The Mallett Unit can be used to detect the presence of a FD and allow the examiner to determine the prism power that reduces the FD to zero; this has been called the aligning prism. The Mallett fixation disparity test has been used since the 1960s, and is a standard clinical test.  As with all subjective tests, the precise instructions given to the patient are likely to be important, although this has not been researched.

The objective of this study was to determine the importance and effectiveness of appropriate questioning and technique when using the Mallett Unit and to identify which results best correlate with reported symptoms.

Initially the standard way in which FD testing is currently carried out by a random sample of optometrists was observed across a wide geographic region. 

Then, a random selection of patients (aged 7-70 years ) consulting the researcher for routine eye examinations, were subjected to FD testing using a conventional question set (as obtained from the survey of optometrists) and by a modified, more detailed procedure according to a flow chart published by Dr Bruce Evans.

A symptom survey was used to quantify symptoms, so that if the two sets of instructions produce different results then those that best correlate with symptoms can be determined.

Ms Karania has been awarded her MSc following successful completion of this research project.  She is currently writing a paper for publication.