Hess vs. Harms vs. PCT measurements
Reviewed by Fiona Rowe

This study aimed to compare the results of the different tests in 36 strabismus patients and 18 healthy controls aged 6-81 years. Tests included alternate prism cover test, Hess screen and Harms screen. Because of glasses restrictions with the Harms screen, all tests were performed without glasses except for two participants whose visual acuity was poor. Strabismus cases included congenital / acquired cranial nerve palsy, comitant strabismus and mechanical strabismus. The Lang stereotest was negative in 11 but none had suppression. There was a significant difference for horizontal deviation with more overall exo deviation measured by the Hess than Harms for both strabismic and healthy participants. The same was found for Hess versus distance peripheral corneal thickness (PCT) for strabismus cases but not healthy controls. There was no difference for near measurements. No bias was found for vertical deviations. The exo shift appeared related to the nearer viewing distance of both the Hess and PCT relative to the Harms screen. The authors conclude it is important to consider vergence bias at different examination distances when comparing different test measurements. The Hess and Harms screens cannot be used interchangeably.

Divergence bias in Hess compared to Harms screen strabismus testing.
Dysli M, Fierz FC, Rappoport D, et al.
STRABISMUS
2021;29(1):1-9.