Abstract
ABSTRACT The Hess and the Harms screen test each have different testing distances. While the Harms screen test is usually performed at 2.5 m, the Hess screen test is performed at 0.5 m. The geometry of the closer testing distance of the Hess screen test requires an increase of the convergence angle by 6°. This study investigates the quantitative differences between the two frequently employed screen tests. Ocular deviation of 18 normal subjects and 36 patients with congenital or acquired paralytic or concomitant strabismus were assessed with a complete orthoptic examination including alternate prism cover testing at near (nPCT) and far (fPCT), as well as Hess and Harms screen testing. One-way ANOVA was used for statistical analysis. The Hess test recorded more overall exodeviation compared to the Harms test for patients (mean difference −3.50°, 95% limits of agreement (CI) = [−4.79, −2.21], p < .001), and controls (mean difference −1.78°, CI = [−2.99, −0.56], p = .004). For vertical deviations, there was no statistically significant difference between the two tests for patients (mean difference +0.75°, CI = [−0.41, +1.91], p = .251), and controls (mean difference −0.28°, CI = [−0.68, −0.11], p = 0.231). This study emphasizes the importance to consider the divergence bias when comparing the Hess to the Harms screen test, which is likely explained by the greater vergence demand dependent on the closer testing distance. The exodeviation shift tended to be more pronounced in patients than controls, which may imply that patients with strabismus have an impaired convergence drive.
Highlights
KEYWORDS Hess test; Harms test; tangent screen test; prism cover test; vergence; strabismus. Screen tests such as the Hess and the Harms test to map ocular deviation for the assessment of patients with double vision are popular in many countries, especially in Europe.[1]
On the other hand, offering a different stimulus to each of the foveae separately will lead to superimposition of the two separate images, termed “confusion.” Both the Hess and Harms haploscopic screen tests are based on the principle of confusion, which is induced by color differentiation of the two eyes
We found a significant exodeviation shift in the Hess test and nPCT compared to the Harms test for horizontal eye position in strabis mus patients, that appears related to the nearer viewing distance of both the Hess test and nPCT relative to the Harms test
Summary
Screen tests such as the Hess and the Harms test to map ocular deviation for the assessment of patients with double vision are popular in many countries, especially in Europe.[1] They are useful in the differential diagnosis of various clinical types of strabismus. The graphical representation helps to identify general patterns including comitant, restrictive, or paralytic strabismus, as well as the affected eye or the affected muscle.[2,3]. Misalignment of one of the foveae, i.e. by strabismus, will induce double vision. On the other hand, offering a different stimulus to each of the foveae separately will lead to superimposition of the two separate images, termed “confusion.” Both the Hess and Harms haploscopic screen tests are based on the principle of confusion, which is induced by color differentiation of the two eyes. Normal retinal correspondence is a prerequisite for such haplo scopic screen tests
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