Abstract
Purpose: :
Two commonly used categories of stereotests are "real depth" tests, such as the near Frisby (nF) and the distance Frisby–Davis 2 (FD2), and polaroid vectograph–"randot" tests, such as the near Preschool Randot (nR) and the new Distance Randot (dR). To determine whether there are systematic differences between these categories of tests, we evaluated their performance in a large cohort of patients.
Methods: :
262 patients in a strabismus practice with visual acuity of 20/40 or better in each eye, 0 to 80 ET, 0 to 60 XT, 0 to 25 HT, completed the nF, FD2, nR and dR stereotests as part of their routine evaluation. To eliminate potential bias of comparing tests with different minimum and maximum levels of stereopsis, results were re–scored as "fine" (20–60 secarc), "moderate" (75–200 secarc) or "coarse/nil" (400–nil) stereo. We compared the level of stereopsis measured by the 4 tests. We also calculated the percentage of patients whose thresholds on randot tests were finer than on real depth tests and vice versa, and the proportion of patients who had measurable stereopsis when the corresponding alternative test indicated no stereopsis.
Results: :
At near, patients appreciated finer disparities with the nF than the nR test (median "moderate" vs. "coarse/nil", p=0.01). At distance, patients appreciated finer disparities on the FD2 than the dR test (median "moderate" vs. "coarse/nil", p<0.0001). This difference was not due to more frequent false positives; only 1 patient (Duanes) with >20 pd deviation had measurable stereopsis with nF and FD2. Only 5% of patients had better stereopsis with nR than nF, and when nF was nil, only 1% had measurable stereopsis by nR. Likewise, only 1% of patients had better stereopsis with dR than FD2, and when FD2 was nil, only 2% had measurable stereopsis by dR. Of patients with no measurable stereopsis by nR, 20% had measurable stereopsis by nF. When patients had no measurable stereopsis by dR, 44% had measurable stereopsis by FD2.
Conclusions: :
The type of stereotest influences measurable thresholds and the results from different tests are not interchangeable. The choice of test should depend on the question being asked; nF and FD2 would be appropriate for determining presence/absence of stereopsis and best measurable stereopsis. The more rigorous randot tests would be appropriate for determining subtle or sensitive changes.
Keywords: binocular vision/stereopsis • depth • esotropia and exotropia