HESS chart

Question 1

During testing

Answer 1

only one screen remains alight. By placing the mirror so it bisects the two eyes, only one eye is fixing at a time. This is the eye which is viewing the illuminated dots through the mirror. When performing the test you are therefore recording the movements of the fellow eye.

Question 2

Lees screen consists of

Answer 2

two identical opalescent glass screens at 90° to each other. The screens are bisected at 45° by a double-sided mirror. This enables dissociation of the two eyes. Each screen is divided into an inner field being 15° and the outer field 30° from the primary position. Each square on the grid represents 5° at a testing distance of 50cm.

Question 3

Perform test with

Answer 3

without the patient’s glasses, as the rim may obscure some of the dots and the lenses produce a prismatic effect when viewing eccentric points. Both eyes most be kept open at all times during the test and the patient’s head kept still

Question 4

Test method

Answer 4

 Switch on the chart and place the mirror at 45° to both screens. With your eye at patient’s eye level view the screen through the mirror. The virtual image from the mirror should be superimposed exactly on the blank screen.

 Sit the patient so that he/she faces the blank screen square on.

 Move the chin rest so that the patient is comfortable, but ensuring that the eyes are level with the centre dot, check this by viewing the patient from the side.

 To dissociate the patient’s eyes, the mirror must be positioned between them.

 Take the pointer with a filled circle at the end for yourself and give the other pointer with a circle at the end to the patient.

 Instruct the patient that you will be pointing to each dot on the lighted screen in turn, and the patient must point to the corresponding point on the blank screen.

 When the patient has located the spot, press the foot pedal to light up the patient’s screen, and record the position of his pointer relative to the dot onto your Hess chart.

 Do this for each spot on both the inner and outer field.

 When you have finished one eye, sit the patient back, switch the screen so the other side now becomes blank, and move the patient so he/she faces square on to the other blank screen.

 Check the mirror is aligned and begin the process again.

Question 5

What law does the Lees screen represent

Answer 5

Herring’s Law of equal innervation. However, the Hess chart must not be considered in isolation. It is supplementary to a full orthoptic report

Question 6

What is the HESS chart used for

Answer 6

 Diagnose the type of deviation – horizontal and vertical, but not torsion

 Indicate the size of the deviation

 Indicate the affected eye – the eye with the smallest field

 Indicate the muscle that is affected in nerve palsies – the muscle which has the greatest deviation from normal

 Plot the development of muscle sequelae and concomitance

 Provide a baseline measurement in conditions which may change over time – stability, progression, regression

 Aid in the differential diagnosis of neurogenic versus mechanical deviations

 Indicate the presence of an A or V pattern

Question 7

What are the contraindications of the HESS

Answer 7

 Abnormal Correspondence – this test is based on foveal / foveal projection

 Inability to fixate in primary position

 Variable angle deviation

 Suppression

Question 8

What is the PC HESS chart

Answer 8

This is a recent development from Thomson software. Instead of using a screen, the patient is dissociated by wearing red and green goggles and is positioned in front of the computer screen (distance depends on size of screen). The room lights are extinguished and a red and a blue circle appear on the screen (the right eye sees the red circle and the left eye sees the blue circle).

Question 9

Patient instructions for PC HESS Screen

Answer 9

One point at a time, the patient is instructed to move the blue circle with the mouse until it appears to be centred on the red circle.

Question 10

Deviations and PC HESS chart

Answer 10

Any deviation results in a misalignment of the circles. The colour of the circles is then reversed and measurements repeated with the left eye fixating.

Question 11

Results for PC HESS

Answer 11

Displayed on screen

Question 12

Results for HESS/ Lees screen

Answer 12

Use sheet provided and mark point seen and N/S for not seen points. Then connect the dots for each eye.