Hess screen test
Also called Hess chart, Hess screen, Lees screen, Hess-Lees screen, Hess test
A test that plots where each eye is pointing in the diagnostic positions of gaze, with one eye fixating at a time, on a flat screen 50 cm from the patient, to diagnose and follow an incomitant strabismus1,3. Red and green goggles dissociate the eyes: the fixating eye sees red target lights and the other eye sees a green pointer, and where the patient sets the green light marks where the non-fixating eye is pointing2,3.
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, so the smaller chart belongs to the eye with the palsy and its greatest inward displacement points to the weak muscle2. The test requires normal retinal correspondence and no suppression3.
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 degrees1. The Lees screen is a variant that uses a mirror instead of colored goggles; both produce a Hess chart1,2. 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 one2,3. 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