The relative afferent pupillary defect and a novel method of fusion recovery with the Worth 4-dot test

To examine the relation of the relative afferent pupillary defect and commonly used clinical tests of visual cortex binocular cell activity, namely, fusion and stereopsis.

Sixty-seven subjects with neuro-ophthalmologic disorders underwent measurement of relative afferent pupillary defect and assessment of binocular sensory status by fusion on the Worth 4-dot test and stereopsis on the Titmus stereo test. Recovery of fusion on the Worth 4-dot test and improvement of stereopsis were tested by placing increasing neutral density filters in front of the eye that did not have the afferent pupillary defect.

Recovery of fusion on the Worth 4-dot test occurred with use of neutral density filters (Worth 4-dot neutralization test). No improvement of stereopsis was observed. A significant positive association was identified between the measured afferent pupillary defect and the measured log filter required for fusion (r = .83, P < .001 by the Spearman rank correlation coefficient). In no case was a relative afferent pupillary defect observed when fusion without use of neutral density filters was present at baseline. For subjects with relative afferent pupillary defects, the measurements of the Worth 4-dot neutralization test were equal before and after pupillary dilation.

The relative afferent pupillary defect is a dynamic process that results from an imbalance of brightness sensitivity between the two eyes. Altering the latency ("timing") and amplitude of the light stimuli by use of neutral density filters can restore fusion on the Worth 4-dot neutralization test but does not alter stereopsis. The Worth 4-dot neutralization test may be a useful adjunct in the assessment of visual dysfunction.