use of prism in optometry and ophthalmology

1. Prisms : Clinical use in Optometry &
Ophthalmic Equipment's.
Moderator : Presenters:
Sanjeeb kr. Mishra Anita Poudel
Bipin koirala
MMC , IOM

2. Quote of the day…..

3. Presentation layout :
 Introduction
 Basic prism optics
 Prismatic effect on eye movement
 Prentice rule
 Instrumental use of prism
 Therapeutic use of prism
 Diagnostic use of prism
 Special cases
 Summary
 References

4. Introduction:
Prism : A portion of a transparent isotropic substance included
between two polished , nonparallel surfaces called refracting surfaces.
A prism causes light to change direction without changing its vergence
but the lens will change the vergence of light rays.
Prism don’t necessarily have flat surface but may have curved surfaces
as in ophthalmic lenses.

5. Some terminologies :
 Apex : The intersection of the refracting surfaces
(extended if necessary) forms the apex of prism
 Refracting angle : The angle between two refracting
surfaces is called as refracting angle. A.k.a Aptical and
denoted by beta (β).
 Base of prism : The thick side of prism which substends
the apical angle is called as base of prism.
 Angle of deviation : the angle between initial direction of
light ray and final direction after refraction is called
angle of deviation. It is denoted by epsilon (ε)
Apex
Base
Refracting
angle
Angle of
deviation

6. Contd..  Base apex line : If any point on the refracting edge is taken and principle
section through that point intersects either refracting surfaces called
base apex line
 Principle section : Any section of prism in a plane perpendicular to the
edge is called as principle section.
Fig : principle section of a prism with base apex line

7. Action of prism :
 A prism refracts the light rays towards base so the object is displaced towards apex.
 Image formed by prism is erect virtual and displaced towards apex.
Object displaced
towards apex

8. Refracting power of thin prism :
Here,
a = Angle of
incidence in 2nd surface
d = Angle of deviation
I2’ =Angle of refraction
n = Refractive index of
prism
n’ = Refractive index
medium

9. Contd. From the previous figure we can say,
Angle of incidence ( i2 ) = a
Angle of refraction (i2’) = a + d
Applying Snellen’s law at 2nd surface
n sina = n’ sin ( a+d )
for small angles (< 10 degree )
n ( a ) = n’ ( a + d )
Finally ,if medium is air ( n’ = 1 ) then, final equation is
with its unit in degree . d◦ = a ( n-1 )
Example ,
If the prism material
is having refractive
index of nearly 1.50
( approx. of CR- 39 )
then d = (a/ 2)

10. Specification of power of ophthalmic prism
 Refracting power of a ophthalmic prism is specified in prism diopter ( ) .
 A prism diopter is defined as deviation of 1 unit at distance a distance of 100 units .
 For convenience amount of deviation is considered in centimeters (cm).
d◦
d▲
x
x
100 units
100 units
Fig 1
Fig 2

11. Contd.
 We know ,
d◦ = a◦ ( n-1 )
From fig-1
tan d◦ = (x/100)
i.e. x= 100 tan d◦
Now, if we can substitute x for prism diopter as we know amount of prism
diopter is equal to the no of units deviation for 100 units of distance.
d▲ = 100 tan d◦
i.e. d▲ = 100 tan a◦(n-1)
Finally tan a◦ = d▲ / 100 (n-1)

12.  Although angle of deviation can be stated in either degree or pd but refracting
angle (d) is always stated in degrees
 Prism diopter differs from degree and radian because prism diopter is unit of
tangent measurement but degree and radian are unit of arc measurment .
 1 radian = (180◦/ π) = 57.5 degree
 1degree = 0.0175 radian
= 1.75 centrad ( because 100 centrad = 1 radian)
 For small angles 1 centrad is equal to 1 prism diopters
so 1 degree = 1.75 prism diopters
1 radian
Arc
equal
To
radius

13. 1 centrad unit
 Less frequently used unit to specify prism power.
 Denoted by (▼) sign
 The difference between the two is that a prism diopter is measured on a flat
plane 1 m away
 whereas the displacement of a centrad is measured on the arc of a circle
having a 1-m radius

14. Ways to specify prism base directions:
 There are many ways used to specify prism orientations . But most
commonly used notation are illustrated in figure.
OD OS
Method 1

15. Method 2

16. Effects of prism on eye movement
Monocular effect
 A person is watching distant object monocularly
 A prism is introduced in front of his open eye at any orientation
 The image of object is displaced towards apex
 So to look the image of object persons eye moves by an amount equal to
angle of deviation.
Orientation Direction of eye movement
Base up Downward
Base down Upward
Base in Outward( abduction)
Base out Inward ( adduction)
nose

17. Binocular effect
 When a person fixates an object with both eye open
 And prism ( suppose equal power ) is introduced infront of both eyes
 If base is in same direction in both eyes ( base in in both eye or base out in
both eyes then it produces vergence movement.
 If base are in opposite direction in both eyes ( base in in one and base out in
next it produce version movement.
Vergence Version

18. Resultant prismatic effects:
 Horizontal prismatic effect:
1. Prism placed before each eye with their bases in same direction for each eye
i.e. both base in or both base out then resultant prismatic effect will be
additive
2. But if bases are in opposite direction resultant effect will be difference
b/w the power of two prism with resultant base towards stronger prism.
Additive effect Subtractive effect

19. Vertical prismatic effect
1. Vertical prisms before each eye in same base direction will give net resultant
effect after subtraction of weaker prism from stronger one and base direction
remains same as overall base direction.
2. If prism before each are indifferent base direction net effect will be additive
and net base direction can be specified with respect to each eye .
Subtractive effect Additive effect

20. Prentice rule:
 An spherical lens can be considered as being made up of infinite number of
prisms stacked together.
 Prism are stacked base to base for plus lens and stacked apex to apex for
minus lens.
 So both positive and negative lens show prismatic properties that gradually
increases towards periphery from the pole of lens.
 Prismatic effect depends not only in the distance from pole also depends on
the power of the the lens.

21. Prentice rule contd..
 Prismatic effect(P)= c F
 Where c is displacement away from pole of lens in (cm) and F is power
of lens in diopters.

22. Prism positioning:
 Prentice position.
 Glass prisms are calibrated for use in this position, so the line of sight makes a right
angle with one of the surfaces( usually back surface).
 Ophthalmic glass prism are calibrated for prentice positioning.
 Minimum deviation position.
 Plastic prisms, including plastic prism bars, are calibrated in this position, so the
line of sight makes an equal angle with each prism surface.
 Difficult to obtain in clinical practice.

23.  Frontal plane positioning.
 Holding plastic prisms in this position, with the back surface flat to
the face of the patient, closely approximates the minimum
deviation position, which would otherwise be difficult to estimate
in clinical practice

24. Instrumental use of
prism

25. Types of prism :
 Dispersive : Abbes , Triangular
 Polarizing : Nicol , Wollaston
 Reflective : Penta , Porro , Dove prism

26. Nicol prism
1. Made from a piece of calcite crystal cut diagonally
in half with the two halves cemented together with
Canada balsam or a low refractive index optical
cement.
2. Incident light is split into ordinary and extra-
ordinary linearly polarized rays in the prism:
- the ordinary rays reaches the interface and is
totally reflected,
- the extra-ordinary ray is transmitted.
3. Best for Haidinger’s Brushes (Nowadays : dichoric
filter)
26

27. Wollaston prism
2 right-angled prisms of equal angle made of a
double refracting surface such as quartz or
calcite cemented together by their hypotenuse
faces to form a rectangular unit.
A beam of unpolarized
light incident on it will
emerge as two diverging
beams which are oppositely
polarized and almost free
of dispersion.
27

28. Porro prism
• Type of reflection prism used to alter the orientation of
image.
• A image travelling through the prism is rotated by 180º
• Exit in the opposite direction offset from its entrance
point.
• In slit lamp double porro prism are used , net effect is
beam parallel to but displaced from its original direction
with image rotated 180º

29. Prism in ophthalmic instruments
Slit lamp
Applanation
tonometry
Indirect
Ophthalmoscope
Keratometer
Phoropter Synaptophore
Exophthalmometer
Pupilometer
Haidinger's
brushes
Koppes Gonio
lens
Operating
microscope

30. Slit lamp
Two Porro prisms
Set of 45-45-90 Porro prism
Overcome the problem of inverted image
produced by compound microscope
Brighter image

31. Ophthalmoscope
 May prism
 Made up of solid glass
 Consists of convex lower end , acting as
condenser and angled upper portion serving as
reflector

32. Applanation tonometry
 GAT uses Bi prism
 Two prism with their bases in opposite direction
are placed in head
 Split the applanated area into 2 and two half
images by 3.06mm

33. Keratometer
1. Wollaston prism
2. Light passing cornea is reflected through obj lens . Diaphragm with 4
aperture
3. Light passing through one of the horizontal aperture also pass
through
Base up prism vertically displaced image
Base out horizontally displaced image

35. Phoropter
 Risley prism (Rotary prism)
 Knob oriented vertically BI /BO prism power introduced
horizontally BU/BD prism power
 Measure phoria , fusional vergence

36. Indirect ophthalmoscope
1. Reflecting prism
2. Erect the inverted image
3. To maintain binocularity reducing IPD

37.  Gonioscope
1. Reflecting prism
2. Overcome the problem of TIR easy viewing of AC angle
 Synaptophore
1. Reflect ray in right angle

38.  Exophthalmometer
 Haidinger's brushes

39. Prism in low vision
A) Half eye prism reading microscope
1. Convex spherical lenses which allow BV
2. Early aid prescribed to reading
3. Prism power = power of microscope + 2
4. Prescribed in power from +5 to +16

40. B) Fresnel prism
1. Plastic sheet of parallel tiny prism of identical
refracting angle
2. Overall prismatic effect is same as of single prism
3. 1 mm plastic polyvinyl chloride material , applied
to base surface of spec
4. Prism placement
side of defect , base towards the defect

41. Condition in which we can prescribe are
 Bitemporal hemianopia
 RP
 Glaucoma
 Brain injury
 Stroke
 Patients may benefit from placement of a Fresnel or ground-in prisms in their
spectacles. It can be used as a definitive treatment for small amounts of
strabismus or as a temporary treatment while waiting for the strabismus to
stabilize.

42.  Advanced glaucoma and RP patients with tubular vision have to
restrict the eye movement to a limited range.
 Prisms help to displace peripheral blind area of visual field toward
the straight ahead position to the seeing area of visual field.
 Small segment of Fresnel prism 5 mm from the center of pupil with
its BI the same direction of restricted visual field will help the
patient.

43. Pros Cons
Lighter – more comfortable VA
Cosmetically superior contrast
Should be chief to manufacture distortion and chromatic
aberration
Convenient Difficult to keep clean
High power possible (0.5 to 30 prism
diopter)
discolor with time inferior optically

44. C) Field expanding channel lenses
1. lens system composed of two12 p.d. lateral prism and one 8pd inferior prism
2. Apex toward central non prismatic channel
3. Distance Rx can be upto +/-8DS and +/-7DC and also bifocals if needed
4. Designed for various degree of peripheral field loss by having pts use specific
size of peripheral field that is remaining
5. Lens channel be aligned with the peripheral edge of functional retina so that
as field ( ) size so too does the width of channel

45.  Designed for improved mobility , pts. viewing object through channel while
having advantage of temporal ,nasal , Inferior awareness
 Recommended for glaucoma, RP
Field loss Recommended size of
channel(mm)
Beyond 20 degree 14
To 20 degree 12
To 15 degree 10
To 10 degree 8
To 6 degree 6

46. Field expanding channel

47. Prism in ARMD
 Used for Image relocation
 Prisms were added to prescription glasses to produce IR to the
presumed preferred retinal locus
 Main outcome measures were best-corrected visual acuity (BCVA)
improved.
 This effect is probably created by facilitation of oculomotor
functions resulting from direct reduction of fixation instability.
2006 Jun;41(3):313-8.
Image relocation with prisms in patients with age-related macular degeneration.
Al-Karmi R1, Markowitz SN.

48. Prism in hemi spatial neglect due to stroke.
 Patients who experience right hemisphere strokes often experience left hemi
spatial neglect.
 Recent studies have shown that yoked prisms which move both visual fields to
the opposite side (to the right) improve function in these patients.

49. Prism in ankylosing spondylitis
 Patients with head or neck positioning problems, such as
patients with severe ankylosing spondylitis, may benefit from
prisms.
 In patients with an orthopedic chin-down posture, for example,
bilateral equal-power, base-up “yoked” prisms can allow for
improvement in straight-ahead vision and thereby facilitate
mobility.

50. Prism in keplerian telescope
 Used to erect the image

51. Prism reading spectacle:
 Bedridden patients are forced to read or watch
television in extreme downgaze.
 If they are provided with 15– 30 Δ , base-down
prism in the form of recumbent spectacle will allow
the patient to read comfortably.
 Prisms in Fresnel form are given temporarily till the
patients recover.

52. Prism in contact lenses
Prism in CL are used in Toric CL for the stabilization of lens ,they are
a) Prism ballast
b) Peri ballast
c) Reverse prism

53. a) Prism ballast
1. 1 to 1.5 pd BD
2. stabilized by prism induced thickness diff

54. b) Peri ballast
 Minus carrier is converted into BD effect
 Uses thickness diff as the stabilizing
component
 To create BD effect , superior carrier is
slabbed – off or chamfered superiorly

55. c) Reverse prism
1. incorporate both prism( BD) and the inferior chamfer (BU)
2. Base-to- base line is located below the geometric center
3. Cyl component limited to optical zone which is prism less

56. Prism in nystagmus
Prismo therapy in nystagmus may be useful as
a) BO prism : stimulate fusional convergence thus improve VA dampening
nystagmus
b) Prism with base opp to preferred dirn of gaze correcting head
posture
eg. If head turn is in left , null position is in dextroversion
BI before RE, BO before LE

57. Prism in nystagmus

58. Therapeutic uses
of prism in
Optometric practice

59. Prism types for therapeutic uses.
Prism
Reliving
prism
Over
corrective
prism
Inverse
prism
Yoked
prism
Rotating
prism
Regional
prism
Corrective
prism

60. Relieving prism:
 Aim : Optically reduces the demand to fusional vergence and stabilizes the sensory motor
fuison
 Amount prescribed: Less than the deviation( phoria or dissociated deviation in strabismus)
 Commonly prescribed for phorias and intermittent deviations.
 Way to prescribe : Type of deviation Prism position
Exo deviation Base in
Eso deviation Base out
Hyper deviation Base down
Hypo deviation Base up

61. Contd…
Mechanism :
1. when base in prism is prescribed the retinal image moves nasally and amount of
convergence needed to obtain bifoveal fixation of real object is optically reduced
for exo deviation.
2. And eye will turn outward with base in prism.
3. When base out prism is prescribed retinal images are moved temporally and
amount of divergence needed to obtain bifoveal fixn object of regard is optically
reduced for esodeviation.
4. And eye will turn inward with base out prism.
5. Similar is for hyper deviations.

62. Relieving prism position
A
A’
A’
A
Exophoria Esophoria

63. Reliving prism prescription criteria's:
Criteria for prescribing reliving prism can be grouped into 3 groups.
1. Minimum prism criteria
3. Associated prism criteria
a. Fusion prism criteria
b. Sheard’s criteria
c. Percival’s criteria
d. Fixation disparity criteria
2. Dissociated prism criteria
a. Percentage prism criteria
b. Residual vergence demand ( RVD )

64. Corrective prisms:
 Aim: Optically eliminates the oculomotor deviation so residual vergence demand is zero
 Amount prescribed: Equal to the size of oculomotor deviation so that retinal images are
centered in fovea.
 Problematic for incomitant deviation
 Ways to prescribe :
Type of deviation Prism position
Exo deviation Base in
Eso deviation Base out
Hyper deviation Base down
Hypo deviation Base up

65. Contd..
 Mechanism : With the use of prism stimulation of corresponding retinal points with
eye remaining in deviated position is possible
 Best for basic esotropia of recent onset and infantile esotropia to establish normal
visual development before surgical procedure.

66. Over corrective prisms:
 Aim: Optically changes the direction of deviation.
 Amount to prescribe: larger than the deviation and commonly
prescribed amount is (deviation +10 pd ).
 Way of prescription: Type of deviation Prism position
Exo deviation Base in
Eso deviation Base out
Hyper deviation Base down
Hypo deviation Base up

67.  Mechanism: Over corrective prism moves the retinal image beyond fovea
in the opposite hemi retina than that was previously stimulated.
 Example : In an esodeviation with application of over corrective prism
esotropic eye changes to optically exo deviated eye as temporal hemi
retina is stimulated
 Uses : 1. To eliminate existing suppression zone in retina.
2. Change ARC to NRC especially in constant esotropes
3. A change in direction of deviation of eye is seen with OCP
during prism cover testing.
 Also called as disruptive prism

68. Figure of over correcting prism
Ray diagram for Esotropic eye Ray diagram after overcorrection

69. Inverse prism:
 Aim : Optically increases the demand to controlling fusional vergence.
 Amount to prescribe: Usually 2-6 pd is prescribed initially with gradually
increment in the strength of prism for constant wear or certain visual
activities.
 Way to prescribe:
Type of deviation Prism position
Exo deviation Base out
Eso deviation Base in
Hyper deviation Base up
Hypo deviation Base down

70.  Mechanism: Optically increases the strength of controlling fusional vergence.
 Uses : 1. Acts as passive therapy method to improve fusional vergence ability when
prescribed for constant wear with spectacles.
2. For cosmesis in small angle constant strabismic patient
(amount usually given : 5 – 10 p.d. )
3. Used to eliminate ARC
4. Used to eliminate eccentric fixation
5. Used as training prism or exercising prism.
6. Also called disruptive prism

71. Yoked prism:
 Aim : To move both eyes synergistically away from primary gaze into
different field of gaze to stabilize binocular vision .
 Amount to prescribe : Differs from patient to patient but usually 5 -10
p.d. are prescribed
 Way to prescribe: Prisms with bases parallel in two eyes with both
prisms bases right or left or up or down

72.  Mechanism : Yoked prism optically moves the retinal images of a fixed
target in parallel direction towards the base of prism the
apparent target towards the apex of the prism.
 Uses : 1. Combined with reliving prism for non comitant deviations , DRS , gaze
palsies
2. In patients with homonymous field loss.
3. To dampen nystagmus by directing eyes to null zone
4. Also applicable for bedridden ( vertical yoked ) and people
with certain physical disabilities
 A.K.A Version prism

73. Yoked prism :
Before After

74. Rotating prism:
 Aim : To change ARC to NRC
 Amount to prescribe: Vary from person to person but 10 pd is most preferred
amount
 Way to prescribe : Base of prism is rotated at certain time interval (1 wk ) from
base out to base in to base up and same sequence is repeated
and prism is given as Fresnel form
 Mechanism: Provides a method of changing sensory input for constant strabismic
patient
 Uses: 1. Monocular viewing rotating prism applicable for amblyopia therapy
2. Binocular viewing rotating prism applicable for treating ARC

75. Regional prism:
 Aim : To stabilize BSV in different gaze or at different distance.
 Way to prescribe : Prescribed as reliving prism often combined with yoked
prism and given in Fresnel form.
 Mechanism : Reduces demand for controlling fusional vergence in more
than one gaze or distance
 Also called as Sector prism

76. Vision therapy in optometry with prisms:
Types of prism used for therapy
Risley prism Prism flipper
Prism bar Loose prism

77. Prism therapy can be given monocularly as well as binocularly.
Monocular therapy are used to improve the patient awareness of his or her
eye movement to enhance fixation and saccade ability.
Binocular prism therapy are used to create vergence demands in free spaces.

78. Types of therapy procedures:
Monocular prism saccade therapy.
Binocular prism jump with loose prism or prism bar
For patient with saccadic deficiency also used
to improve fixation in amblyopic eyes.
For Heterophores and well controlled intermittent deviation
to increase fusional vergence ability and vergence facility.

79. Contd..
Binocular prism jump with prism flipper
 To increase smoothness and speed of fusion.

80. Diagnostic
uses of
prism

81. Diagnostic uses of prism can be explained under
following subheadings:
Assessment of
torsion
Test for retinal
correspondence
Correction of
deviation
Measurement of
angle of
deviation
Investigation of
BSV and its
components
Test for
malingering
Pre operative
diplopia test
Diagnostic
uses of
prism

82. 1. Measurement of angle of deviation .
Prism cover test
1. Objective method
2. Materials required : Prism bar , Occluder , (Near + Distance)
targets
3. Prism placement: Apex towards deviation.
4. Alternate cover test is performed until neutralization.
For large angled deviation
and combined horizontal
& vertical deviation prism
can be divided b/w two
eyes

83. Prism reflex test ( krimsky’s corneal reflex test)
1. Objective method
2. Performed at near with point light as fixing target.
3. Materials required: prism bar , pen light
4. Useful in large squint , deeply amblyopic as well as blind eye

84.  Prism placement: Prism is placed before fixing eye with Apex
towards deviation.
 Prism power is gradually increased till corneal reflex is centered in
squinting eye

85. Simultaneous prism cover test.
1. Objective method and special form of cover test
2. Used to measure tropia without dissociating phorias.
3. Useful for small angled tropia (<10▲) and larger phoria i.e.
monofixation syndromes.
4. Occluder ( in front of fixing eye) and prism ( in front of
deviating eye are placed simultaneously until neutralization
of eye movement
5. Prism placement : Apex towards deviation

86. Maddox rod test:
1. Subjective method to access the angle of deviation.
2. Can detect horizontal , vertical as well as torsional
deviation.
3. It is an dissociating test based on principle of
diplopia.
4. Materials required: Maddox rod , occluder , spot
light target , prism bar.

87. Contd.. 5. Placement of Maddox rod : Axis of rod is
placed parallel to axis of deviation.
6. Then prism of successive increasing power is
placed in front of rod until red streak crosses
the spot .
7. Prism placement : Apex towards deviation .
Limitations of Maddox rod test:
1. Cannot be done in suppression
2. Difficult to perform in large angled deviations.
3. Error may occur in ARC.

88. Double Maddox rod test:
 Used to identify and quantify cyclodeviation
 Materials required : 2 Maddox rods ( red + white) ,
spot light target , and 6 ▲ prism
 Maddox rod are placed with axis in vertical direction
in both eyes.
 Prism placement and use : Base down or up in front of
one eye so that two colored streak can be
differentiated easily
2. Assessment of torsion

89. Maddox double prism test:
 Qualitative test for cyclodeviation.
 Materials required: Two prisms( 4 ▲ ) , Horizontal line target.
 Can be done only at near 33cm.
 Procedure:
1. Prism are mounted base to base and placed before one eye ( say right if left is being tested)
2. Pts asked to view horizontal line target at 33cm.
3 . Patient will see 3 lines middle line by left eye and upper and lower by right.
4 . Based on orientation of central line with respect to two outer line interpretation is made.
RE LE

90. Contd..
Orientation of central line Interpretation
No tilt No cyclodeviation
Tilted outward Left eye incyclodeviation
Tilted inward Left eye excyclodeviation

91. 3. Investigation of BSV and its components
Prism reflex test ( induced tropia test )
1. Used to investigate BSV and find small angled
deviation in infant and children's.
2. Fixation pattern of fixing eye is studied with horizontal
BO prism in front of fellow eye.
3. Same procedure is repeated for next eye too.
4. Commonly used prism is (10▲ BO – 25 ▲ BO)
5. By inducing tropia , fixation preference is studied.

92. Interpretation of prism reflex test.
Result 1 :
If BSV is present with normal motor fusion , after placing prism diplopia will be induced
and both eye will make version movement and fellow eye will show refixation
movement. (Hering’s law obeyed)
Result 2 :
If BSV is not present ,on placing prism over suppressed eye no version movement will be
seen but if placed over non suppressed eye version movement is seen but no refixation
or fusional movement is noted.
Result 3:
In some cases of alternate deviation b/w two eyes alternate fixation preference is seen
b/w two eyes
Vertical prism
superior to
horizontal prism

93. Prism 4▲ test
1. Performed to detect small angled deviation and
presence of central suppression scotoma.
2. Can be performed for both near and far .
3. Materials required : 4 p.d. prism , fixation light
for near and distance.
4. Procedure : 4 p.d. BO prism is place in front of
each eye alternately and necessary
interpretation is made

94. Interpretation of 4 ▲ test
 When BO prism is placed in front of Rt. Eye (say) then :
Result 1 : If normal biphasic corrective movement of
left eye is seen then pts has normal BSV.
Result 2 : Absence of normal biphasic movement
indicates presence of central suppression scotoma
Mechanism :
Prism displaces image towards its base i.e towards a point in temporal half nearly 2
degree . So the relaxation movement of right eye will induce conjugate movement of
left eye ( levoversion) , If right eye has no suppression . This displaces the image in left
towards temporal retina from fovea ,thus left eye now makes fusional movement in
opposite direction, if no foveal suppression is present.
Central suppression scotoma

95. Fusional vergence amplitude:
 Prism bar / Risley prism can be used to measure fusional vergence
amplitude .
 Measured for both near (33cm)and distance(6m).
 Horizontal ( positive and negative ) vergence amplitude as well as vertical
vergence amplitude can be measured.
 Prism bar of increasing strength is placed in front of eye with patient
instructed to look at fixing target at certain distance.
 Three points i.e. Blur point , Break point and Recovery point are noted .
 Fusional vergence amplitude measured by prism bar is step fusional
vergence.

96. Contd..  Normal values:
Category Normal values
Base out ( distance) Blur 9 p.d.
Break 11 p.d.
Recovery 7 p.d.
Base out (near) Blur 17 p.d.
Break 19 p.d.
Recovery 14 p.d.
Base in ( distance ) Blur -
Break 7 p.d.
Recovery 4 p.d.
Base in ( near ) Blur -
Break 13 p.d.
Recovery 10 p.d.
Source :
Clinical management of
Binocular vision 4th
edition by Mitchell
scheiman & Bruce wick

97. Vergence facility test:
 Important test used to detect binocular vision disorders.
 Done at near i.e. 40 cm .
 Purpose : Test ability of vergence system to respond
rapidly and accurately to changing vergence demand.
 Measure as cycle per minutes
 Normal value is 15 cpm .
 Prism flipper with 12 ▲ BO and 3 ▲ BI prism is used.

98. Contd..
Testing procedure:
1. Pts views an vertical target ( 20 / 30 ) at 40 cm .
2. Prism power of 12 ▲ BO and 3 ▲ BI are alternately placed in front of pts
one eye .
3. Pts is instructed to make single and clear view of target and immediately
prism is flipped to next power .
4. And the numbers of complete cycles per minute is noted.
5. Absence of eye movement indicates suppression.

99. Prism are also use to measure AC/A & CA /C ratio

100. 4. Test of retinal correspondence.
Prism bar and red filter test.
 Materials required : prism bar , red filter , spot light for fixation at 6 m .
 Firstly amount of deviation is estimated for distance by PBCT ( objective angle )
 Prism is placed in front of fixing eye .
 Red filter is placed in front of deviating eye and patient is asked to describe
what he sees.

101. Interpretation of result:
 Result 1 : Patient doesn’t sees red light then eye is suppressed.
 Result 2 : Patient sees light as blend of red and white then , NRC.
 Result 3 : Pts sees two lights i.e. white and red light then, ARC
1. Paradoxical diplopia is appreciated.
2. For eso deviation cross diplopia & for exo uncrossed diplopia.
3. To find subjective angle of deviation prism bar is gradually reduced
until diplopia disappears or changes its nature.
4 . By difference b/w SA and OA angle of anomaly is obtained .

102. Prism use in suppression:
Prism for investigation of suppression :
 Area of suppression can be measured with prism.
 Measured by displacing image nasally, temporally, Vertically ( up and down )
 Diplopia will be appreciated when image falls outside scotoma zone .

103. Contd..
Prism as a part of Anti suppression exercise (Diplopia exercise by
prism):
1. Used to make patient aware of physiological diplopia in intermittent
deviations.
2. Vertical prism (BU or BD ) sufficient enough to displace image out of
suppression scotoma is placed in front of one eye
3. Gradually prism power is reduced until finally patient appreciates diplopia
without any vertical prism.
4. In this way patient is taught to appreciate diplopia for both near and distance.

104. 5. Test for malingering
 Prism base down test :
1. Prism of power (4-6) ▲ BD is palced before good eye
2. If patient sees diplopia at the fixation target.
3. It proves malingering.
 Prism base out test .
1. Prism of power (10- 16) ▲ BO is taken
2. Prism placed before alleged blind eye
3. If eye moves inwards i.e. to avoid diplopia
4. It indicates gross vision is present and malingering is proved.

105. 6. Pre-op diplopia test.
 Some portion of adult people suffer from intractable diplopia.
 Prism adaptation test is pre-op diagnostic test to identify potential for BSV and
predict risk of post op diplopia.
 Done before strabismus cosmetic surgery.
 Patient may experience post op diplopia due to poor motor alignment.
 Prism bar is used for this purpose and test should be done for both near &
distance.
 Firstly deviation is neutralized with prism bar by Alt .prism cover test .

106. Contd..
 Then at neutralization point pts is asked whether diplopia is seen or not .
 Then prism bar is removed and again started from zero prism diopter till deviation is
over corrected by 5 - 10 p.d. and pts is instructed to inform If he reports diplopia at
any position .
Interpretation:
 Result 1 : If Pts reports diplopia then prism power is increased and decreased several
times if any position where diplopia is eliminated i.e. if position of fusion is not
obtained ( pts has chance of post-op diplopia)
 Result 2 : If pts doesn’t report diplopia i.e. any position for fusion is obtained then
probably he won’t have post-op diplopia

107. Post operative prism use
 Prism is also used to eliminate post operative intractable diplopia by increasing
the image separation or maintaining BSV .
 Fresnel prism is used for this purpose .
Some examples
Surgically over corrected exotropia ( consecutive esotropia)
1. Mostly pts develop good fusional divergence with in certain day of surgery
2. If stable esotropia persists over 1 months of surgery the BO prism are
prescribed to prevent diplopia
3. For larger overcorrection re-surgery is needed

108. Surgically under corrected exotropia
1. Use of BI prism immediately after surgery is helpful to establish good fusion
2. Usually overcorrecting prism are used ( 10 p.d. greater than deviation)
3. But it has been reported to be unsuccessful for most of the time.
Surgically overcorrected convergence insufficiency
1. May spontaneously develop post op diplopia
2. During this period prism therapy may be useful.

109. Prism prescription in convergence and divergence anomalies
Convergence insufficiency
1. Convergence exercise with prism bar to increase
amplitude of fusional convergence.
2. Base in reliving prism for near work fitted in bifocal.
Convergence paralysis
1. Base in prism are prescribed especially for near work.
2. Plus lens with near add may be required for associated
accommodative anomalies.

110. Contd..
Divergence insufficiency.
1. Base out prism is prescribed to give
comfortable BSV at distant.
2. Not mostly practiced , prism is gradually
reduced and avoided.
Divergence paralysis
1. Same as above.

111. Guidelines for prism prescription to avoid diplopia
 Lowest prism amount that makes pts is comfortable.
 Prism power can be divided into two eyes for good cosmesis and weight.
 Step wise guidelines:
1. First measure amount of deviation vertical as well as horizontal in primary gaze.
2. If both H and V deviation are present resultant oblique prism power is
estimated
3. Starting prism power is equal to = larger deviation + (smaller deviation/2)

112.  Examples:
If pts is 20 p.d. eso and 10 p.d. left hypertropia then starting prism will be:
20 + (10/2) = 25 p.d.
4. So 25 p.d. prism is taken as starting prism.
5. And 25 p.d. will be placed before non-dominating eye always
6. Prism positioning will be b/w base out and slightly base down before left eye.
7. Prism is slightly rotated in clockwise and anticlockwise direction to find oblique prism
position in which diplopia is eliminated at least at primary position .

113. 8. If diplopia is not eliminated with starting prism then prism of slightly small or higher
power are taken and same procedure is repeated as point 7.
9. Finally prism power and its oblique orientation ( angle) is noted and prism is
prescribed In the form of Fresnel prism.
How to Prescribe a Prism for Combined Vertical and Horizontal Strabismus
Written By: J. Scott Kortvelesy, MD
Edited by Ingrid U. Scott, MD, MPH, and Sharon Fekrat, MD
Q) If right eye is 16 p.d. exo and 6 p.d hypo
then what would be necessary prism
amount???

114. Summary :
 Prism has wide application in the field of ophthalmology and optometry .
 And its uses can be categorized into instrumental , therapeutic &
diagnostic use.

115. References:
Squint and orthoptics by A.K. khurana
System of ophthalmic dispensing
Binocular vision by G.K von noorden
Clinical management of Binocular vision 4th edition by Mitchell scheiman
Clinical optics
Internet

116. Hillary
Clinton with
Fresnel
prism in her
spectacle