The monocular vertical prism dissociation test

• PURPOSE: To determine if the monocular vertical prism dissociation test can differentiate between organic and nonorganic visual loss. • DESIGN: A prospective, single-masked observational study. • METHODS: Three institutional neuro-ophthalmology practices. Group 1 consisted of 30 normal controls. Group 2 included 30 patients with known organic visual loss. Group 3 contained 35 patients with suspected nonorganic monocular visual loss. Participants were asked to describe what they saw while viewing a single Snellen letter when a 4-prism diopter base-down prism was placed in front of their better eye. Outcome was measured by whether the participant sees one or two letters with the prism in place. • RESULTS: Vision-appropriate results were given by all members of Group 1 (two images) and by all members of Group 2 (one image). Two images were seen by 31 of 35 members of Group 3, indicating nonorganic visual loss. The other 4 subjects in Group 3 saw one image; each was subsequently found to have occult pathology. • CONCLUSION: The vertical prism test quickly differentiates organic from nonorganic monocular visual acuity loss.