The Hess screen test is based on the

haploscopic principle.

It was popularized by Lyle, in particular

for diagnosing possible paretic or

paralytic conditions in patients with

normal correspondence.

Principles

Dissociation

Foveal projection

Hering’s and Sherrington’s Laws of

Innervation

To perform this test, a black cloth 3 ft wide by 31⁄2 ft long, marked out by a series of red lines subtending between them an angle of 5°, is used.

At the zero point of this coordinate system and at each of the points of intersection of the 15° and 30° lines with one another and with corresponding vertical and horizontal lines, there is a red dot.

These dots form an inner square of 8 dots and an outer square of 16 dots.

An indicator is provided consisting of three short green cords knotted to form the letter Y.

The end of the vertical green cord is fastened to a movable black rod 50 cm long.

The ends of the other two cords are kept taut by black threads that pass through loops at upper corners of screen to small weights at corresponding upper corners of the screen.

This arrangement enables the patient to move the indicator freely and smoothly over the whole surface of the screen in all directions.

The patient wears red-green goggles and is seated 50 cm from the screen, preferably with his or her head fixed in a headrest.

The patient now sees the red dots with one eye (fixing eye) and the green cords with the other (charted eye)

The patient is instructed to place the knot joining the three green cords over each of the red dots in turn.

It is advisable to start from point A then go to above point B then proceed clockwise from C to I.

The examiner marks the positions indicated by the patient on the small card with a reduced copy of the screen.

The points found by the patient are connected by straight lines and permit the examiner to determine which, if any, muscles react abnormally.

To change fixation, the red-green goggles are reversed with the red filter now in front of the left eye.

Hess Chart (position)

The basic principle of Hess chart is foveal projection therefore the higher field belongs to the higher eye.

This is opposite of diplopia charting where the higher image is of the lower eye.

Position of the central dot indicates whether the deviation is in primary position or not.

Hess Chart (size)

The variation in the size of the Hess chart of each eye is due to the Hering’s law.

Small field belongs to the eye with primary limitation of movement.

Underaction can be seen with the inward movement of the dots and therefore the whole curve.

Maximum displacement occurs in the direction of the affected muscle if the patient has presented early before the spread of comitance.

Overaction can be seen by noting the outward

displacement of the dots.

Maximum displacement occurs in the direction of the main

action of the overacting contralateral synergist in the

larger field.

If the inward and outward displacement is less marked,

secondary underactions and overactions are present as a

result of the development of muscle sequel.

Outer field should be examined for small underactionsand overactions which may not be apparent on the inner field.

A narrow field restricted in opposing directions of movement denotes a mechanical restriction of ocular movement.

Equal sized field denote either symmetrical limitation of movement in both eyes or a non paralytic strabismus

Hess Chart (Shape and

measurements)

Each small square on the grid subtends 5 degree at the working distance of 50 cm.

Therefore the amount of deviation can be calculated.

In primary position, the amount could be calculated by fixing either eye by the displacement of the pointer from the centre dots.

The amount of underaction and overaction can be calculated in the various positions and hence the amount of excursions can also be calculated.

Uses

which muscle is affected

degree of paresis

extent of development of muscle sequelae

differentiate recent onset/ longstanding

differentiate mechanical/neurogenic

measurement of deviation (each little square is 5o )

Assess change over time

which muscle to operate on

effect of treatment

The left chart is much smaller than the right.

Left exotropia – note that the fixation spots in the inner charts of both eyes are deviated laterally.

The deviation is greater on the right chart (when the left eye is fixating), indicating that secondary deviation exceeds the primary, typical of a paretic squint.

Left chart shows underaction of all muscles except the lateral rectus.

Right chart shows overaction of all muscles except the medial rectus and inferior rectus, the ‘yokes’ of the spared muscles.

The primary angle of deviation (fixing right eye –FR) in the primary position is −20°

The secondary angle (fixing left eye – FL) is −28°

•Right chart is smaller than the left.

• Right chart shows underaction of the superior oblique and

overaction of the inferior oblique.

• Left chart shows overaction of the inferior rectus and

underaction (inhibitional palsy) of the superior rectus.

• The primary deviation (FL) is R/L 8°; the secondary deviation

FR is R/L 17°.

• No difference in overall chart size.

• Primary and secondary deviation R/L 4°.

• Right hypertropia – note that the fixation spot of the right inner chart is

deviated upwards and the left is deviated downwards.

• Hypertropia increases on laevoversion and reduces ondextroversion

• Right chart shows underaction of the superior oblique and overaction

of the inferior oblique.

• Left chart shows overaction of the inferior rectus and underaction

(inhibitional palsy) of the superior rectus.

• Right chart is smaller than the left.

• Right esotropia – note that the fixation spot of the right inner chart is

deviated nasally.

• Right chart shows marked underaction of the lateral rectus and slight

overaction of the medial rectus.

• Left chart shows marked overaction of the medial rectus.

• The primary angle FL is +15° and the secondary angle FR +20°.

• Inhibitional palsy of the left lateral rectus has not yet developed.

a. Which is the abnormal eye?

b. Which muscle is underacting?

c. Which muscle is overacting?

d. Is this a long-standing palsy?

e. What is the diagnosis?

a. Which is the abnormal eye?

b. Which muscle is underacting?

c. Which muscle is overacting?

d. What is the diagnosis?

a. Which muscle is underacting?

b. Which muscle is overacting?

c. What is the diagnosis?