Divergence bias in Hess compared to Harms screen strabismus testing

Area of Science:

• Ophthalmology
• Vision Science
• Orthoptics

Background:

• The Hess screen test and Harms screen test are common methods for assessing ocular deviation.
• These tests utilize different testing distances: Hess at 0.5 m and Harms at 2.5 m.
• The closer distance of the Hess test necessitates a greater convergence angle (6° increase).

Purpose of the Study:

• To quantitatively compare the ocular deviation measurements obtained from the Hess and Harms screen tests.
• To investigate the differences in exodeviation and vertical deviation assessments between the two tests.
• To understand the impact of testing distance on ocular deviation measurements.

Main Methods:

• Ocular deviation was assessed in 18 normal subjects and 36 patients with strabismus.
• A comprehensive orthoptic examination was performed, including near (nPCT) and far (fPCT) prism cover testing.
• Hess and Harms screen testing were conducted, with statistical analysis using one-way ANOVA.

Main Results:

• The Hess test revealed significantly greater exodeviation than the Harms test in both patients (mean difference -3.50°) and controls (mean difference -1.78°).
• No statistically significant differences in vertical deviations were found between the Hess and Harms tests for either group.
• The exodeviation shift was more pronounced in patients with strabismus compared to controls.

Conclusions:

• A divergence bias exists when comparing the Hess and Harms screen tests, attributable to the increased vergence demand at the Hess test's closer distance.
• Clinicians should consider this divergence bias when interpreting results from these two tests.
• The findings suggest that patients with strabismus may have an impaired convergence drive, as indicated by the greater exodeviation shift observed with the Hess test.