Hess-Weiss (Lancaster red-green test)
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左眼（OS）
右眼（OD）
What it measures
Hess-Weiss is a red-green dissociation test of ocular motility, built on the same principle as the Hess screen and the Lancaster red-green test. It charts where each eye is directed as the fellow eye fixes targets across the positions of gaze, and shows the action of each extraocular muscle and how a deviation changes with the direction of gaze.
The output is two charts, one per eye, read like a printed Hess chart: the deviation in primary position, the underaction or overaction of each muscle in its diagnostic position, and the relative size of the two fields.
How it is administered
The patient wears red-green glasses and keeps both eyes open. The eye behind the red filter sees only the red targets and fixes them; the eye behind the green filter sees only the green pointer. The patient lays the pointer over each target in turn, so the pointer marks where the green-filtered eye is looking. The right eye is charted first, with the green filter over it, then the glasses are swapped for the left eye.
The inner field is tested at 15° (primary position and the eight surrounding positions). The outer field at 30° can be added, for 25 points per eye.
The test runs on any screen, from a laptop to a TV. On the chart scale, the chart fills the screen and angles are read off it, as on a printed Hess chart. With true angles, the screen width and viewing distance are entered, and the test checks that every target fits on that screen. The pointer is moved with a mouse, a touch screen, a keyboard or a TV remote.
The room is dimmed, the head is kept straight and still, and the viewing distance stays the same in every session.
Reading the result
The two charts are set side by side as the patient sees them: left eye on the left, right eye on the right, each square 5°.
The smaller field belongs to the eye with the primary underaction. The larger field belongs to the fellow eye, which shows the overaction of the yoke muscle. Where a field is pulled inwards in one direction, the muscle acting in that direction is underacting, and the matching corner of the fellow field is pushed outwards.
VisReed reports the primary-position deviation in prism dioptres (eso, exo, hyper, hypo), reads each muscle in its diagnostic position for that eye, and flags the smaller field. Two fields of similar size and shape indicate a comitant deviation; the more they differ, the more incomitant the deviation.
The report downloads with both charts side by side, ready to print, so charts from successive sessions can be compared. A small field that expands towards the fellow field across sessions is consistent with recovery of a paresis; a field that keeps its shape over months fits a restriction or a long-standing deviation.
When it is used
Hess-Weiss is used for recent-onset diplopia, to separate a paretic pattern from a restrictive one and to point towards the muscle involved. It follows third, fourth and sixth nerve palsies, and documents thyroid eye disease and orbital floor fractures.
It is also used before and after strabismus surgery or botulinum toxin, and to record incomitance before prescribing a prism. Charts that vary on repeat testing in one session can indicate fatigable weakness.
Limitations
Dissociation depends on the glasses and the screen: the red filter must block the green pointer and the green filter the red targets. Cheap glasses or a very bright screen can let one through; check this before testing.
On the chart scale, angles are read off the chart, not measured in space, so values are comparable between sessions on the same set-up. True angles need an accurate screen width and viewing distance, and a large screen for the 30° field.
Head position is not controlled on a screen: a tilt or a turn shifts the whole chart. The charts record what the patient reports, so they depend on cooperation and attention, and a dense amblyopia or suppression can prevent a reliable plot.
A screen version does not replace a Hess screen or a Lancaster screen in a controlled room, nor the full motility examination.
Validated status means this implementation is considered ready for clinical use on this platform. It does not mean a published study has shown equivalence to the physical screen.
この検査についてのよくある質問
What is the difference between the Hess and Lancaster tests?
Both dissociate the eyes with red and green filters and chart where each eye points. The Hess screen uses a fixed grid of red targets and a green pointer; the Lancaster screen uses red and green projected streaks. Hess-Weiss follows the same principle on a computer or TV screen.
How is a Hess chart read?
The smaller field belongs to the eye with the primary underaction, and the larger field shows the overaction of the yoke muscle. Each muscle is read in its diagnostic position, and the deviation in the primary position is given in prism dioptres.
Are red-green glasses needed?
Yes. The eye behind the red filter fixes the red targets, while the eye behind the green filter sees only the green pointer. Check on your screen that each filter blocks the other colour.
Can the test run on a TV?
Yes. The chart scales to any screen, and the pointer can be moved with a TV remote or a keyboard. With the true-angle option, the test checks that the 15° and 30° targets fit at the chosen distance.