Hess screen test
Also called Hess chart, Hess screen, Lees screen, Hess-Lees screen, Hess test
A Hess screen test is a plot of where each eye is pointing in the diagnostic positions of gaze, made with one eye fixating at a time on a flat screen 50 cm from the patient.1,3 The test is used to diagnose and follow an incomitant strabismus.1,3
Red and green goggles dissociate the eyes: the fixating eye sees red target lights and the other eye sees a green pointer.2,3 Where the patient sets the green light marks where the non-fixating eye is pointing.2,3
An underacting muscle shrinks the plotted field of its eye, and by Hering's law the yoke muscle in the fellow eye overacts and enlarges that eye's field.2 The smaller chart therefore belongs to the eye with the palsy, and its greatest inward displacement points to the weak muscle.2 The test requires normal retinal correspondence and no suppression.3
More detail
Each square on the screen represents 5 degrees of eye rotation, with an inner field 15 degrees from primary position and an outer field at 30 degrees.1 The Lees screen is a variant that uses a mirror instead of colored goggles, and both produce a Hess chart.1,2 Plotting with each eye fixating in turn separates the primary from the secondary deviation, and charts repeated over weeks distinguish a recovering palsy from a stable one.2,3 A single Maddox rod measures one position of gaze, whereas the Hess screen measures all nine.
Sources (4)
- Kanski, Clinical Ophthalmology (2024), ch. 18
- Pickwell's Binocular Vision Anomalies, ch. 17 and glossary
- Eperjesi and Rundström, Practical Binocular Vision Assessment, ch. 8
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 12
Reviewed October 2026