Strabismus, or squint, is a misalignment of the eyes where the visual axes are not parallel. There are several types of strabismus including pseudostrabismus (apparent squint), heterophoria (latent squint), and heterotropia (manifest squint). Heterotropia can be concomitant where the deviation is the same in all gazes, or incomitant where the deviation varies with gaze. Diagnosis involves tests like cover tests and Hess screen testing to determine the type and amount of deviation. Management depends on the type of strabismus.

1. STRABISMUS

2. • A misalignment of the visual axes of the two eyes is called squint or strabismus

3. CLASSIFICATION OF STRABISMUS
• Broadly
,strabismus can be classified as below:
I.Apparent squint or pseudostrabismus.
II.Latent squint (Heterophoria)
III.Manifest squint (Heterotropia)
1.Concomitant squint
2.Incomitant squint

4. PSEUDOSTRABISMUS
• In pseudostrabismus (apparent squint),the visual axes are in fact parallel,but the eyes
seem to have a squint:
• 1.Pseudoesotropia or apparent convergent squint may be associated with a prominent
epicanthal fold (which covers the normally visible nasal aspect of the globe and gives a
false impression of esotropia) and negative angle kappa.
• 2.Pseudoexotropia or apparent divergent squint may be associated with hypertelorism,a
condition of wide separation of the two eyes,and positive angle kappa

6. HETEROPHORIA
• Heterophoria also known as‘latent strabismus’,is a condition in which the tendency of
the eyes to deviate is kept latent by fusion.
• Therefore,when the influence of fusion is removed the visual axis of one eye deviates
away.
• Practically a small amount of heterophoria is of universal occurrence and is known as
‘physiological heterophoria’

7. TYPES OF HETEROPHORIA
• 1.Esophoria.It is a tendency to converge.
• It may be:
distance).
i.Convergence excess type (esophoria greater for near than distance).
ii.Divergence weakness type (esophoria greater for distance than near).
iii.Non-specific type (esophoria which does not vary significantly in degree for any
2.Exophoria.It is a tendency to diverge.
It may be:
i Convergence weakness type (exophoria greater for near than distance).
ii Divergence excess type (exophoria greater on distant fixation than the near).
iii Non-specific type (exophoria which does not vary significantly in degree for any distance).

8. • 3.Hyperphoria.It is a tendency to deviate upwards,while hypophoria is a tendency to
deviate downwards.
• 4. Cyclophoria. It is a tendency to rotate around the anteroposterior axis.When the 12
O’clock meridian of cornea rotates nasally
, it is called incyclophoria and when it rotates
temporally it is called excyclophoria

9. CONCOMITANT STRABISMUS
• It is a type of manifest squint in which the amount of deviation in the squinting eye
remains constant (unaltered) in all the directions of gaze;and there is no associated
limitation of ocular movements.
• T
ypes of concomitant squint :
1. Convergent squint (esotropia),
2. Divergent squint (exotropia),and
3. Vertical squint (hypertropia).

10. CONVERGENT SQUINT
• Concomitant convergent squint or esotropia denotes inward deviation of one eye .It can
be unilateral (the same eye always deviates inwards and the second normal eye takes
fixation) or alternating (either of the eyes deviates inwards and the other eye takes up
fixation,alternately).

11. DIVERGENT SQUINT
• Concomitant divergent squint (exotropia) is characterised by outward deviation of one
eye while the other eye fixates.

12. INCOMITANT SQUINT
• It is a type of heterotropia (manifest squint) in which the amount of deviation varies in
different directions of gaze.
• It includes following conditions:
1.Paralytic squint,
2.‘A’ and‘V’pattern heterotropias,
3.Restrictive squint

13. PARALYTIC STRABISMUS
• It refers to ocular deviation resulting from complete or incomplete paralysis of one or
more extraocular muscles

14. FORCED DUCTION TEST (FDT).
• It is performed to differentiate between the incomitant squint due to paralysis of
extraocular muscle and that due to mechanical restriction of the ocular movements.
• FDT is positive (resistance encountered during passive rotation) in cases of incomitant
squint due to mechanical restriction and negative in cases of extraocular muscle palsy

15. ‘A’AND ‘V’PATTERN HETEROTROPIA
• The terms‘A’ or ‘V’pattern squint are labelled when the amount of deviation in squinting
eye varies by more than 10° and 15°,respectively
,between upward and downward gaze.
• ‘A’and‘V’esotropia.
In‘A’ esotropia the amount of deviation increases in upward gaze and decreases in
downward gaze.The reverse occurs in‘V’esotropia.
• ‘A’and‘V’exotropia.
In‘A’ exotropia the amount of deviation decreases in upward gaze and increases
in downward gaze.The reverse occurs in‘V’ exotropia

17. RESTRICTIVE SQUINT
• In restrictive squint,the extraocular muscle is not paralysed but its movement is
mechanically restricted.

18. • 1.Duane’
s retraction syndrome
It is a congenital ocular motility defect occurring due to fibrous tightening of lateral or medial
or both rectus muscles.
• 2.Brown’
s superior oblique tendon sheath syndrome
It is congenital ocular motility defect due to fibrous tightening of the superior oblique tendon
• 3.Strabismus fixus
It is a rare condition characterised by bilateral fixation of eyes in convergent position due to
fibrous tightening of the medial recti

20. COVERTEST
• A cover test or cover-uncover test is an objective determination of the presence and
amount of ocular deviation.
• sufficient light in the room so that the examiner can clearly see the patient's eyes
• Sit approximately 30 cm away from the patient to ensure you are able to observe their
eye movement.
• Explain the purpose of the test to the patient,for example:"I am going to check how well
your eye muscles are working toget
• The cover/uncover part includes 1 eye being coveredher.

21. • The examiner observes:
• The movement of the uncovered eye taking up fixation
• The movement as well as position of the eye under the cover when the paddle is
removed
• Cover/uncover method looks for heterotropia

23. • The alternate CT has to ensure that one eye is dissociated at all times:
• The movement of the covered eye is recorded as the paddle is changed from one eye to
the other every 3 seconds while allowing the eye to take up fixation
• The alternate CT can't distinguish between manifest and latent deviations.
• (e.g.,the eye will move outward in the presence of esophoria)

24. ALTERNATE PRISM C OVER TEST
• The alternate prism cover test is similar to the alternate cover test,with the addition of a
prism held over one eye to quantify the misalignment.
• The examiner performs the alternate cover test while changing the prism over the
deviated eye until the misalignment is neutralized.
• The magnitude of the deviation is the measurement of the prism in prism diopters
required to neutralize the deviation.

25. MADDOX ROD TEST
• Patient is asked to fix on a point light in the centre of Maddox tangent scale at a distance
of 6 metres.
• A Maddox rod (which consists of many glass rods of red colour set together in a metallic
disc) is placed in front of one eye with axis of the rod parallel to the axis of deviation.
• The Maddox rod converts the point light image into a line.
• Thus,the patient will see a point light with one eye and a red line with the other
.
• Due to dissimilar images of the two eyes,fusion is broken and heterophoria becomes
manifest.

26. • The number on Maddox tangent scale where the red line falls will be the amount of
heterophoria in degrees.
• In the absence of Maddox tangent scale, the dissociation between the point light and red
line is measured by the superimposition of the two images by means of prisms placed in
front of one eye with apex towards the phoria

30. MADDOX WING TEST.
• Maddox wing is an instrument by which the amount of phoria for near (at a distance of 33 cm)
can be measured.
• It is also based on the basic principle of dissociation of fusion by dissimilar objects.
• The instrument is designed in such a way that,through its two slits,right eye sees a vertical white
arrow and a horizontal red arrow and the left eye sees a vertical and a horizontal line of numbers.
• The patient is asked to tell the number on the horizontal line which the vertical white arrow is
pointing (this will give amount of horizontal phoria) and the number on the vertical line at which
the red arrow is pointing (this will measure the vertical phoria).
• The cyclophoria is measured by asking the patient to align the red arrow with the horizontal line
of numbers

32. HIRSCHBERG CORNEAL REFLEX TEST.
• It is a rough but handy method to estimate the angle of manifest squint.
• In it the patient is asked to fixate at point light held at a distance of 33 cm and the
deviation of the corneal light reflex from the centre of pupil is noted in the squinting eye.
• Roughly,the angle of squint is 15o and 45o when the corneal light reflex falls on the
border of pupil and limbus,respectively

34. THE PRISM AND COVERTEST (PRISM BAR COVER
TEST I.E.,PBCT).
• Prisms of increasing strength with apex towards the deviation are placed in front of one
eye and the patient is asked to fixate an object with the other
.
• The cover-uncover test is performed till there is no recovery movement of the eye
under cover
.
• This will tell the amount of deviation in prism dioptres.
• Both heterophoria as well as heterotropia can be measured by this test.

36. KRIMSKY CORNEAL REFLEX TEST.
• In this test the patient is asked to fixate on a point light and prisms of increasing power
(with apex towards the direction of manifest squint) are placed in front of the normal
fixating eye till the corneal light reflex is centred in the squinting eye.
• The power of prism required to centre the light reflex in the squinting eye equals the
amount of squint in prism dioptres.

38. DIPLOPIA CHARTING.
• It is indicated in patients complaining of confusion or double vision.
• In it patient is asked to wear red and green diplopia charting glasses.
• Red glass being in front of the right eye and green in front of the left.
• Then in asemi-dark room, he is shown afine linear light from adistance of 4 ft.and
asked to comment on the images in primary position and in other positions of gaze.
• Patient tells about the position and the separation of the two images in different fields.

40. HESS SCREEN TEST
• The Hess screen is an electrically operated greyboard marked with a grid on which small red
lights are individually illuminated and a movable illuminated green indicator
.
• Each of the red lights can be switched on and off by inserting and removing a conductor plug
into the switchboard that is controlled by the examiner
.
• Each eye needs to be tested separately
.
• The test uses red/green complementary filters to create color dissociation which maximizes
the ocular deviation.
• The eye behind the green glass is the testing eye and the other eye with red glass is the
fixating eye.

41. HESS SCREEN TEST
• Hess charting is a diagnostic procedure to measure the deviation and the amount of
underaction and overaction of extraocular muscles.
• It helps to rule out ocular motility defects.
• Each Hess screen consists of a central dot,an inner square of eight dots,and an outer
square of 16 dots (15 degrees and 30 degrees from the primary position,respectively).
• Lees screen test is also a similar test to a Hess chart.But to achieve dissociation, a mirror
is used instead of R-G goggles.

43. • The Hess screen is an electrically operated greyboard marked with a grid on which small
red lights are individually illuminated and a movable illuminated green indicator
.
• Each of the red lights can be switched on and off by inserting and removing a conductor
plug into the switchboard that is controlled by the examiner
.

44. • Each eye needs to be tested separately.The test uses red/green complementary filters to
create color dissociation which maximizes the ocular deviation.
• The eye behind the green glass is the testing eye and the other eye with red glass is the
fixating eye.

45. HESS CHART PRINCIPLE
• The procedure employs the haploscopic principle which involves foveal projection and
dissociation of eyes.
• The dissociation of two eyes is achieved by the means of colors.
• The Hess chart is plotted based on Hering’s law of equal innervation and Sherrington’s
law of reciprocal innervation.

46. • The Hess screen test is performed at a 50 cm distance (distance between the patient’s
eye and the Hess screen) with each eye fixating turn by turn (right eye first and then the
left eye).
• The patient is asked to wear R-G goggles in such a way that the red glass is placed on
the right eye (fixating eye) first.
• The patient’s head should be erect and centered on the fixation spot with eyes in
primary position throughout the test. If he is unable to maintain this position, it is
necessary to hold the head in the correct position by another person.

47. • A green light projection pointer is given to the patient.Red fixation light is projected on
the screen and the patient is asked to superimpose the green light over the red.In
normal conditions,both the lights will superimpose.
• The goggles are then reversed so that red glass is in front of the left eye and the
procedure is repeated.The responses are analyzed by connecting the response marks
with lines.

48. HESS CHART INTERPRETATION
• The Hess chart position, shape, size, and measurement explain the type and amount of
deviation in a particular eye. So, the interpretation of the Hess chart is based on these
primary characteristics.

49. • The difference in the size of the Hess chart is governed by Hering’s law of equal innervation.A
smaller chart indicates the eye with the underacting muscle (primary deviation).Likewise,the
larger chart indicates the eye with the overacting muscle (secondary deviation).
• The smaller chart shows the greatest restriction in the main direction of action of
underacting muscle whereas the larger chart shows the greatest expansion in the main
direction ofaction of the yoke muscle.
• Inward displacement of the dots (and the curve) depicts the underaction of the muscle.The
eye cannot move far enough to plot in a normal position.Similarly,the outward displacement
of dots indicates overaction (or contracture).Due to excessive movement of the eye,the
dots are displaced beyond the normal position.