The Near Mallett Unit Foveal Suppression Test: a cross-sectional study to establish test norms and relationship with other optometric tests.

When the binocular system comes under prolonged stress in decompensated heterophoria, a small suppression area can develop at the fovea. This foveal suppression (FS) may be an adaptation to eliminate symptoms. The near Mallett unit (NMU) is a hand-held instrument that contains a binocular status test for detecting and quantifying FS. We measured FS in a cross-sectional study of 131 patients to investigate the normal range of responses. A ratio scoring method was devised which considers the number of letters read with dichoptic viewing when binocularly fused (b) and under monocular (m) conditions, using the formula 100 x (m - b)/m. The 95th percentile value of the FS ratios was calculated as the limit of the normal range and this gave a value for the right eye (RE; 33%), left eye (LE; 43%) and total (38%). This result shows that statistically significant FS occurs when a patient can read approximately one line further in the test monocularly, compared to under fused dichoptic conditions. A statistically significant correlation was found between absolute values of horizontal aligning prism (associated heterophoria on the NMU) and FS ratio. Some limitations of the FS test and unexpected results became apparent during our research and an abnormal result on the test does not always indicate a binocular vision anomaly. Nonetheless, we feel that the test can provide useful information when the results are taken in the context of other clinical tests. Our results are used to develop recommendations and clinical guidelines for using the FS test, and to make suggestions for improvements in future designs of the test.