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J.M. Holmes, S. Fawcett; Testing distance stereoacuity with the Frisby–Davis 2 (FD2) . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3493.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:The new Frisby–Davis 2 distance stereoacuity test (FD2) may be useful for evaluating binocular status in conditions where distance and near stereopsis differ, such as intermittent exotropia. We developed a modified staircase protocol, and compared monocular and binocular performance to evaluate monocular cues. A new revised presentation protocol is proposed, that allows measurement of distance stereothresholds free of monocular cues. Methods:95 patients with a spectrum of strabismic and non–strabismic conditions, ages 4 to 84 years, were evaluated. We developed a presentation protocol , consisting of a screening phase (single targets presented at descending disparity levels ), a test phase (starting at the last level passed in the screening phase to determine the smallest disparity where 2 of 2 targets are correctly identified), and a retest phase (starting at the first disparity level failed in the test phase). The final score was defined as the better of the test or retest phase. 66 patients were tested at 3m (which provides a broad range of disparity levels but also may provide monocular cues) and 29 patients were tested at 6m (which provides a small range of disparity levels but may minimize monocular cues). Near stereopsis was also tested using the Frisby and Randot Preschool Stereoacuity tests. Results:When tested monocularly at 3m, 28 of 66 (42%) patients passed the largest disparities (80–200 secarc). The 6m viewing distance did not eliminate monocular cues; 7 of 29 (24%) patients passed the largest disparities monocularly (40–50 secarc). If near stereoacuity was taken as the "gold standard," binocular FD2 testing only identified 7 of the 21 (33%) stereoblind patients. A new revised presentation protocol is proposed, incorporating binocular and monocular phases, to confirm, on each test, whether a level passed binocularly could be passed monocularly. Conclusions:The FD2 distance stereoacuity test is most useful when used at 3m where the range of disparities is 20 to 200 secarc, but is flawed by the presence of monocular clues. We propose a new test protocol, which includes binocular and monocular phases, allowing determination of true distance stereothresholds. Using our protocol, the FD2 may prove to be a useful clinical tool in the evaluation and management of strabismus.
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