Hess chart. used in management of paralytic squint.

2.  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.

3. Principles
 Dissociation
 Foveal projection
 Hering’s and Sherrington’s Laws of
Innervation

4.  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.

5.  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.

6.  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.

7.  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.

8. 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.

9. 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.

10.  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.

11.  Outer field should be examined for small underactions
and 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

12. 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.

13. 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

14.  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.

15.  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°

16. •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°.

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.

18. • 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.

19. 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?

20. a. Which is the abnormal eye?
b. Which muscle is underacting?
c. Which muscle is overacting?
d. What is the diagnosis?

21. a. Which muscle is underacting?
b. Which muscle is overacting?
c. What is the diagnosis?