Hess-Weiss

What it measures

Hess-Weiss charts where an eye localises a target across the nine diagnostic directions of gaze. It shows the relative action of the extraocular muscles and how a deviation changes with the direction the patient looks. It belongs to the Hess screen and Lancaster family of motility charts, and the name used on this site is Hess-Weiss.

The output is two plots, one per eye. They do not give absolute eye position in space. They give where the patient localises each target, the displacement from the true position being the deviation.

How it is administered

The right eye is charted first, with the left eye covered. The patient wears habitual distance correction, keeps the head still and square to the screen, and keeps the covered eye open behind the occluder.

A target appears at each of the nine positions in turn: primary position and the eight surrounding positions. The patient indicates where the target appears to be. The sequence then repeats for the other eye.

The working distance follows the setup shown on screen, and must stay the same in every session.

The clinician keeps the head straight, watches for a compensating head turn or tilt, and repeats any point inconsistent with its neighbours.

Reading the result

VisReed plots the nine points for each eye, reports an average deviation in prism dioptres, and breaks that deviation down per muscle as a horizontal and a vertical value.

The smaller plot belongs to the eye with the underacting muscle. The larger plot belongs to the fellow eye, which shows overaction of its yoke muscle. Where a corner of a plot is pulled inwards, the muscle acting in that direction is underacting, and the matching corner of the other plot is pushed outwards.

Two plots of similar size and shape indicate a comitant deviation. The more the plots differ, the more incomitant the deviation.

Nothing is stored, so change over time is followed by keeping the exported sheet with the record and setting two sheets side by side. A small plot that expands towards the fellow plot across sessions is consistent with recovery of a paresis. A plot that keeps its shape over months fits a restriction or a long-standing deviation. Read the plots with the rest of the examination.

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. Plots that vary on repeat testing in one session can indicate fatigable weakness.

Limitations

The classical Hess screen separates the eyes with red and green filters, so the deviating eye points while the fellow eye fixates. A version that charts each eye in turn under occlusion records monocular localisation instead, and the two are not interchangeable.

The dioptre figure comes from a fixed conversion: ten screen pixels are one prism dioptre, whatever the panel size, the pixel density and the viewing distance. It is comparable only between sessions run on the same display at the same distance, and never between devices.

Head position and ambient light are not controlled on a screen. A tilt or a turn shifts the whole plot.

The plots record what the patient reports, so they depend on cooperation and attention. Dense amblyopia or poor acuity in one eye gives a plot that reflects the acuity as much as the motility.

A screen version does not replace a Hess screen or a Lancaster arc 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.