Research

A Normative Study of the Near Mallett Unit Foveal Suppression Test and its relationship with other Binocular Vision tests

Introduction

This is a prospective, cross-sectional study of foveal suppression.  Suppression exists in both normal and abnormal binocular single vision.  It occurs to avoid symptoms such as diplopia, confusion or incompatible images. It can be divided into those affecting one eye or either eye, be constant or intermittent, and measured by its area, position, or density (Sidorov, 2001).

The near Mallett Unit is an instrument used widely in optometric practice in the UK. Since its introduction in 1964, there have been several versions available.  All near vision versions have included a polarised test for the ‘quantitative evaluation of foveal suppression’ (Mallett, 1997) as well as tests for aligning prism (to correct fixation disparity), dissociated heterophoria, and stereo-acuity. 

The fovea is the part of the retina with the highest concentration of cones and receives the highest resolution image. Therefore, incompatible images falling on the two foveae of two eyes due to sensory or motor disturbances will give rise to the most severe symptoms.  This will often be a stimulus to suppress one of the images.  Foveal suppression is known to be a compensatory strategy in some cases of decompensated heterophoria of relieving symptoms (Siderov, 2001). It is also often present in cases of binocular instability (Evans, 2001). It has been shown in subjects with uncorrected fixation disparity that true monocular acuities (i.e. where one eye is occluded) are better than monocular acuities measured under binocular conditions (Evans, 2002).

However, there is no research to indicate how deep this suppression (e.g. as measured by the near Mallet Unit) may be in relation to decompensation.  There is little in the current literature about the exact use of the foveal suppression test. There are also not any studies on its repeatability and the norms in general optometric use. 

It should be noted that the foveal suppression described above is sometimes a feature of decompensated heterophoria and binocular instability.

This light suppression of the foveal area is likely to be fundamentally different to the deeper suppression that can occur in strabismus. Although the Mallett foveal suppression test is primarily intended to detect foveal suppression in heterophoria, it is also likely to detect the more serious forms of suppression in strabismus. This increases the possibilities for using the test to screen for a range of binocular vision anomalies. Since the test is also a form of visual acuity test, it will also screen for a variety of refractive and pathological conditions.

Study 1

Using the polarised Mallett foveal suppression test, monocular acuities under binocular and monocular viewing conditions on the foveal suppression test will be measured for each subject.  Also, the amount of prism required to correct any fixation disparity (constant or unstable) in each eye will be measured. 

Study 2

The aim of this part of the study is to investigate in more detail the relationship between foveal suppression and other tests used in practice to assess binocular status. Further optometric tests using standard testing protocols will be carried out. Foveal suppression will be measured again to give an idea of the repeatability of this test on different days, and also using a method suggested by some authors of counting the smallest letters if they cannot be resolved. (Evans, 2002)

It is hoped that the study will determine:

All of this information will be useful to optometrists using this test as part of a binocular vision work up in primary care.

Publication arising from this research