Retinal Image Height, Anisometropia and Aniseikonia

1
Retinal Image Height, Anisometropia and
Aniseikonia
Page 11.1
2
Terminology

• Isometropia - equal refractive errors OD and OS
• Anisometropia - unequal refractive errors, OD, OS
• Antimetropia - one eye myopic one hyperopic
• Aniseikonia - unequal retinal image heights
• most relevant in corrected anisometropia
• differential blur more important in uncorrected
  anisometrope

3
Effects of Anisometropia

• Spectacle Correction ? unequal prismatic effects
  between right and left lenses. May cause
  diplopia at larger angles of gaze
• Spectacle Correction ? unequal demand on ocular
  accommodation (OD vs. OS chapter 8)
• Correction may ? aniseikonia

Contact lenses eliminate prismatic imbalance and
unequal accommodative demand. Contacts do not
necessarily prevent aniseikonia
4
Symptoms of Aniseikonia

• Symptoms due to unequal corrected retinal image
  heights This is the Classical Theory of
  Aniseikonia
• Will cover Classical Theory first
• Symptoms vague and non-specific asthenopia
  (eye-strain), headaches, photophobia, reading
  problems. Many other conditions produce the same
  symptoms
• ? aniseikonia often misdiagnosed
• Aniseikonia ? iatrogenic (practitioner-induced).
  Can be avoided by appropriate choice of distance
  correction

OBJ
5
Opposing Theories of Aniseikonia

• Theory 1 arises as a direct result of retinal
  image height disparity in corrected anisometropia
  (effectively the Classical Theory)
• Theory 2 arises solely from differential
  prismatic effects in spectacle-corrected
  anisometropes. Counter-arguments
• incidence of ANK after refractive surgery on
  axially anisometropic patients
• aniseikonia symptoms occur in some contact lens
  wearers

Page 11.2
6
Unifying Theory of Aniseikonia?

• Aniseikonia in spectacle-corrected anisometropes
  may be due to both retinal image height disparity
  and differential prismatic effects
• Aniseikonia in contact lens wearers (when it
  occurs) is simply due to retinal image height
  disparity (according to the Classical Theory)

7
Aniseikonia Tolerance

• Small differences in corrected retinal image
  height (e.g. 1) rarely cause symptoms may
  occur in sensitive patients
• Young anisometropic children ? high risk
  patients. Retinal image height disparity may
  impair/prevent development of normal sensory
  fusion? BV anomalies common in anisometropia
• Older anisometropes with normal sensory fusion ?
  tolerate higher degrees of aniseikonia before
  becoming symptomatic
• 5 retinal image height disparity usually causes
  breakdown of binocular vision

8
Astigmatism and Aniseikonia

• Spectacle-corrected astigmats ? meridional
  aniseikonia (real effect according to all
  theories)
• Astigmats most likely to report symptoms of
  spatial distortion (uncommon in spherical
  aniseikonia)
• Meridional aniseikonia increases
• with magnitude of astigmatism
• with axis asymmetry OD vs. OS
• with different magnitude of astigmatism OD vs. OS
• in oblique astigmatism

9
Retinal Image Height for a Distant Object
Emmetropia
Page 11.3
Classical Theory of Aniseikonia
10
Path of the Chief Ray through the Eye (SSE)
Avoid a lot of complicated EnP, ExP, MCR, etc.
steps if we use the Reduced Eye instead of the
SSE. Less accurate, but trends still clear
From chapter 7
11
Chief Ray Path Reduced Eye
P
Fig 11.1, Page 11.3
(EnP ExP)
12
CR Path Retinal Image Height -Distant Object
Fig 11.1, Page 11.3
13
Angular Magnification of Chief Ray
Fig 11.1 Page 11.3
MCR constant for all reduced eyes, because pupil,
EnP and ExP are all at the reduced surface
14
Origin of Ametropia
Page 11.4

• Most cases of significant anisometropia are
  predominantly axial or predominantly refractive
  in origin
• e.g. patient Rx 6 D OD 2 D OS
• ultrasonography ? ? equal axial lengths
• keratometry ? estimated total corneal power ? 3
  to 4 D higher in left eye vs right eye
• This would be a case of predominantly refractive
  anisometropia

15
Importance of Origin of Anisometropia

• The type of correction that will avoid inducing
  aniseikonia depends on the origin of
  anisometropia (primarily axial or primarily
  refractive)
• This is where the wrong choice of correction may
  produce aniseikonia ? iatrogenic condition

16
Axial Ametropia and Anisometropia
Page 11.5
(a) Uncorrected
17
E
Fig 11.2Page 11.5
18
OBJ
This is the reduced eye (easier) version of
Chapter 7 chief ray path and retinal image height

• For a given object angle ? same chief ray path
  works for all axially ametropic eyes
• Chief ray defines a gradually larger uncorrected
  retinal image height with increasing axial length

19

• Remember blur circle diameter increases as the
  ametropic retina moves farther from the
  emmetropic focal plane(? uncorrected vision
  worse)

20
Axial Ametropia and Anisometropia
Page 11.6
(b) Corrected with Spectacles
21
Axial Ametropia corrected with Spectacles
Knapps Law (basis of Classical Theory)
Page 11.6

• When an axially ametropic eye is corrected with a
  spectacle lens placed at the first principal
  focus of the eye ? the corrected retinal image
  height will be the same regardless of the
  magnitude of (axial) ametropia
• Corrected retinal image height will also be the
  same as for the standard emmetropic reduced eye

OBJ
22
Spectacle Magnification

• Spectacle magnification produces a new incident
  chief ray angle at the eye (for a given object)

OBJ

• This change in incident CR angle produces a
  proportional change in refracted CR angle.
• A new refracted CR angle will change RI height

OBJ
23
Axial Hyperopia
Corrected with a Spectacle Lens
Hyperopia ?S gt ? ? ??S gt ??
MR
24
Why is the new incident CR angle (?S) steeper?
Fig 11.3Page 11.6
Plus lens converges parallel rays toward a focus
at the far point
Q
25
Why is the new incident CR angle (?S) steeper?
Plus lens like prisms with bases on axis
26
Why is the new incident CR angle (?S) steeper?
Prisms deviate light toward their base
27
Why is the new incident CR angle (?S) steeper?
Hyperopia ?S gt ? ? ??S gt ??
Q
28
Spectacle Magnification - Hyperope
?S gt ? ? ??S gt ??

• Spectacle magnification in hyperopia increases
  the incident chief ray angle from ? to ?S
• MCR remains unchanged (0.75 for all reduced eyes)
• Greater incident chief ray angle ? proportionally
  greater refracted chief ray angle
• Greater refracted chief ray angle means increased
  retinal image height ? RI height increases with
  spectacle correction of hyperopia

29
Spectacle Correction at Knapps Plane (Fe)
Hyperope

• A positive spectacle lens placed at the first
  principal focus (Knapps Plane) of the axially
  hyperopic eye increases retinal image height to
  become the same as the (standard) emmetropes RI
  height
• This occurs for all magnitudes of axial hyperopia

OBJ
30
Axial Myopia
31
Axial Myopia Corrected with a Spectacle Lens
Page 11.7
32
Axial Myopia Corrected with a Spectacle Lens
Myopia ?S lt ? ? ??S lt ??
33
Spectacle Magnification - Myope
?S lt ? ? ??S lt ??

• Spectacle magnification in myopia decreases the
  incident chief ray angle from ? to ?S
• MCR remains 0.75
• Smaller incident chief ray angle ? proportionally
  smaller refracted chief ray angle
• Smaller refracted chief ray angle means decreased
  retinal image height ? RI height decreases with
  spectacle correction of myopia

34
Spectacle Correction at Knapps Plane (Fe) Myope

• A negative spectacle lens placed at the first
  principal focus (Knapps Plane) of the axially
  myopic eye decreases retinal image height to
  become the same as the (standard) emmetropes RI
  height
• This occurs for all magnitudes of axial myopia

OBJ
35
RI Height - Axial Ametropia corrected at Fe
Page 11.8
CHIEF RAY (u)
CHIEF RAY (u)
All corrected RI heights equal
36
Spectacle Magnification
Page 11.9

• Expressed three different ways (use all three)
• Origin change in incident chief ray path between
  the uncorrected and corrected eye

1. Application change in retinal image height
   between the uncorrected and corrected eye

1. Calculation an equation to determine the value
   for SM in any particular case

?
37
Spectacle Magnification - Hyperope
?S
38
Spectacle Magnification - Hyperope
39
Spectacle Magnification - Hyperope
OBJ
Page 11.10
40
Spectacle Magnification - Myope
41
Spectacle Magnification - Myope
OBJ
42
Axial Ametropia Corrected with a Contact Lens
Page 11.11
43
Axial Ametropia Corrected with a Contact Lens

• Ocular Correction (at the reduced surface) ? no
  effect on incident chief ray path. ? SM 1.0
• Contact lens sits 1.67 mm in front of the reduced
  surface ? f ?CL only differs from ?MR by 1.67 mm.
• ? FCL ? FO

LMR ? FCL
? SM ? 1.0
f?CL ? f?O
? ?CL ? ?
44
Axial Ametropia Corrected with a Contact Lens

• SM for a positive contact lens, very slightly gt
  1.0SM for a negative contact lens, very slightly
  lt 1.0
• This means that corrected retinal image height
  will be virtually the same as uncorrected retinal
  image height
• ? correcting an axial anisometrope with contact
  lenses will induce aniseikonia (uncorrected RI
  heights differ negligible change with contacts ?
  difference remains)

45
Axial Ametropia Corrected with a Contact Lens
h?CL ? h?O ? h?U
Page 11.11
46
Variation on Example 11.1 (p. 11.12)

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane
• Calculate corrected retinal image heights for a
  contact lens correction

47
Test questions will NOT be multi-part e.g. From
Practice Test 3
This is a variant on part (b) of the current
example
48
Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye

O.D. 4 D axial hyperopia ? Fe 60 D A
Femm ? Fe ? Femm A Fe 4 60
64 D
49
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50
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51
Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye

O.S. 9 D axial hyperopia ? Fe 60 D Femm
69 D
52
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53
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54
Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• find uncorrected RI heights for each eye and
  compare with the standard emmetropic reduced eye

h?U (OD) ?0.819 mm h?U (OS) ?0.759
mm h? (SERE) ?0.873 mm
55
Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane

O.D. spectacle correction at Knapps Plane, d
16.67 mm
56
Spectacle Magnification (O.D)
57
O.D.
CHIEF RAY (s)
58
Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane

O.S. spectacle correction at Knapps Plane, d
16.67 mm
59
Spectacle Magnification (O.S)
60
O.S.
61
Variation on Example 11.1

• Axial anisometrope FO 4 D OD 9 D OS
• For a distant object subtending a 3O visual
  angle
• calculate corrected retinal image heights for a
  spectacle correction at Knapps Plane

h?U (OD) ?0.819 mm h?U (OS) ?0.759
mm h? (SERE) ?0.873 mm
h?S (OD) ?0.873 mm h?S (OS) ?0.873
mm h? (SERE) ?0.873 mm ANK 1.0 (no
difference OD vs. OS)
62
Refractive Ametropia
Page 11.15
63
Refractive Ametropia

• (a) Uncorrected

64
RI Height in Uncorrected Refractive Ametropia
Page 11.15
h?E h?U (M) h?U (H)
65
RI Height Correction of Refractive Ametropia

• Uncorrected RI heights the same in all refractive
  ametropes
• Any difference in retinal image height after
  correction will therefore be due to differences
  in spectacle magnification (O.D. vs. O.S.)
• To avoid aniseikonia in refractive anisometropia
  ? want both correcting lenses to have same SM
• ? want to prescribe contact lenses to avoid
  aniseikonia in refractive anisometropia

66
Example 11.2
Pp. 11.16-17

• Demonstrates equality of uncorrected retinal
  image heights in refractive anisometropia
• Shows that a spectacle correction will produce
  ANK that is a direct result of different SM (O.D.
  vs. O.S.) ? more hyperopic eye will have larger
  corrected image due to higher SM
• Shows that contact lenses induce almost zero ANK
  (because a CL ? ocular correction) ? contact
  lenses are correction of choice for refractive
  anisometropia

67
Summary Corrected Retinal Image Height in Axial
and Refractive Anisometropia
Page 11.18
68
Variation in Corrected RI Height Axial Ametropia
This figure is a graph NOT an optical diagram
Page 11.18
69
Variation in Corrected RI Height Refractive
Ametropia
This figure is a graph NOT an optical diagram
Effectively an SM plot for each magnitude of
ametropia
70
Variation in Corrected RI Height with Ametropia
KnappsPlane
ReducedSurface
ReducedSurface
Hy
HIGH
HIGH
My
LOW
LOW
Hy
Em
My
LOW
LOW
Fe
HIGH
HIGH
Hy
My
h?em
h?S (A)
h?S (R)
d
d
BASELINE
AXIAL AMETROPIA
REFRACTIVE AMETROPIA
71
Quantifying Aniseikonia
Page 11.18
Example 11.1 (pp. 11.12-14) Axial anisometrope
corrected (b) with spectacles (page 11.14)
(c) with contacts (page 11.14), put larger image
in numerator
72
Quantifying Aniseikonia
Modified Example 11.1 A 4 D OD, A 9 D OS
Axial anisometrope corrected (b) with spectacles
(c) with contacts, put larger image in numerator
73
Quantifying Aniseikonia
Example 11.3 (pp. 11.16-17 A 5 D OD, A ?5 D
OS refractive anisometrope corrected (b) with
spectacles
(c) with contacts
Reality with contacts (1.67 mm vertex dist) h?OD
slightly gt h?OS Negligible, but not zero,
aniseikonia
74
Relative Spectacle Magnification
Page 11.20
Factors out image height for specific object
angle RSM compares corrected RI height with the
corresponding height for the standard emmetropic
reduced eye ANK then becomes the ratio of RSM
(OD) and RSM (OS) ANK tables list RSM values for
axial and refractive ametropia as a function of
correcting vertex distance
75
Figures are RSM Plots for Axial and Refractive
Ametropia
KnappsPlane
ReducedSurface
ReducedSurface
Hy
HIGH
HIGH
My
LOW
LOW
Hy
Em
My
LOW
LOW
Fe
HIGH
HIGH
Hy
My
h?em
h?S (A)
h?S (R)
d
d
BASELINE
AXIAL AMETROPIA
REFRACTIVE AMETROPIA
76
Relative Spectacle Magnification
Page 11.20
Factors out image height for specific object
angle RSM compares corrected RI height with the
corresponding height for the standard emmetropic
reduced eye ANK then becomes the ratio of RSM
(OD) and RSM (OS) ANK tables list RSM values for
axial and refractive ametropia as a function of
correcting vertex distance
77
Apply RSM Values to Example 11.1
Example 11.1 (pp. 11.12-14) Axial anisometrope
corrected(b) with spectacles (page 11.14)
Get the same answer as Ex. 11.1 (b), but OD and
OS results not tied to a particular object angle
they directly compare h?S to a standard h?
78
RSM Axial Anisometropia
Page 11.18
KnappsPlane
ReducedSurface
OS (Example 11.1)
Fe
My
Hy
Em
My
Hy
OD (Example 11.1)
RSM (OD and OS)
h?em
d
AXIAL AMETROPIA
79
Apply RSM Values to Example 11.1
Example 11.1 (pp. 11.12-14) Axial anisometrope
corrected
(c) with contacts (page 11.14)
Again, same answer as Ex. 11.1 (b), but OD and OS
results not tied to a particular object angle
they directly compare h?S to a standard h?
80
Apply RSM Values to Example 11.1
Example 11.1 (pp. 11.12-14) Axial anisometrope
corrected
(c) with contacts (page 11.14)
81
RSM Axial Anisometropia
Page 11.18
KnappsPlane
ReducedSurface
OS (Example 11.1)
Fe
My
RSM (OS)
Hy
Em
My
h?em
Hy
OD (Example 11.1)
d
RSM (OD)
AXIAL AMETROPIA
82
Aniseikonia Classical Theory vs. Clinical
Findings
Page 11.27
83
ANK Classical Theory vs. Clinical Findings

• With growing popularity of contacts in 1980s
  90s, patients of all types, including axial
  anisometropes, were trying contact lenses.
  According to Knapps Law, these patients should
  have aniseikonia with contacts
• Clinical studies of axial anisomyopes (one eye
  has stretched to become more myopic than the
  other) found some patients to be more tolerant of
  contact lenses than spectacles at Knapps Plane
• Possible explanation stretching of the more
  myopic eye results in lower receptor density ?
  larger cortical receptive fields

84
Optical vs. Cortical Image
Page 11.27
85
Aniseikonia Classical Theory vs. Recent Findings

• Optical versus cortical image
• Do receptive fields stretch due to reduced
  receptor density with increasing axial length?
• does this mean that optical iseikonia is not
  synonymous with cortical iseikonia?

86
OD
Axial anisomyopia Left eye more axially myopic ?
receptors spread further apart? At fovea 11 cone
to ganglion cell correspondence, so cone
separation ? receptive field size
OS
87
Evidence for Differences between Optical and
Cortical Image
Page 11.27

• Winn (1988) studied 18 anisometropes, nearly all
  with axial anisomyopia
• Spectacle correction even with equal retinal
  image sizes, ? found aniseikonia
  present.Aniseikonia increased with increasing
  anisometropia

88
Equal retinal images span UNequal numbers of
receptive fields
89
Evidence for Differences between Optical and
Cortical Image

• Winn (1988) studied 18 anisometropes, nearly all
  with axial anisometropia
• Spectacle correction even with equal retinal
  image sizes, ? found considerable aniseikonia
  present.Aniseikonia increased with increasing
  anisometropia

• Contact lens correction found little or no
  aniseikonia, ? concluded that the unequal
  retinal images covered the same number of
  receptive fields

90
Unequal retinal images span equal numbers of
receptive fields
Cortical images
91
Optical vs. Cortical Image
Fig 11.11Page 11.27
92
Optical vs. Cortical Image
Fig 11.11Page 11.27
Equal retinal images falling on equal numbers of
receptive fields ? Equal cortical images ?
Iseikonia
93
Optical vs. Cortical Image
Fig 11.12Page 11.28
Unequal retinal images falling on different
numbers of receptive fields ? different cortical
image sizes ? Aniseikonia This is what
classical theory predicts to occur e.g. with
contact lens correction of axial anisometropia
94
Optical vs. Cortical Image
Fig 11.13Page 11.28
Equal retinal images falling on different numbers
of receptive fields (RFs stretched over a larger
left eye) ? Different cortical image sizes ?
Aniseikonia This is what clinical findings
suggest may happen with Knapps plane spectacle
correction of axial anisomyopia
95
Optical vs. Cortical Image
Fig 11.14Page 11.29
Unequal retinal images falling on the same number
of receptive fields (RFs stretched over a larger
axially myopic right eye) ? Equal cortical image
sizes ? Iseikonia This is what clinical findings
(from late 80s) suggest will happen with a
contact lens correction of axial anisomyopia
96
Optical vs. Cortical Image
Equal retinal and cortical image heights. No ANK
(both theories)
?
97
Optical vs. Cortical Image
Classical theorys justification of Knapps Law
?
e.g. what would happen if an axial anisometrope
was corrected with contacts OR a refractive
anisometrope was corrected with spectacles
98
Optical vs. Cortical Image
Rationale for axially anisomyopic patients being
intolerant of glasses prescribed according to
Knapps Law
?
99
Optical vs. Cortical Image
Clinical finding that axial anisomyopes seem to
tolerate contact lenses better than glasses
?
100
Differences between Optical and Cortical Image
Page 11.30

• So where does this leave Knapps Law?
• These findings of reduced ANK with contact lens
  correction of axial anisomyopia have not been
  found in all studies
• But, there is ample evidence to show that contact
  lenses do work better in some axial anisomyopes
• Answer measure aniseikonia subjectively ?
  eikonometry

101
Space Eikonometer The Gold Standard
Page 11.30
102
Space Eikonometer The Gold Standard

• Space eikonometer ? provides a quantitative,
  subjective, measure of aniseikonia for the
  patient with their proposed or actual correction
  (e.g. contacts, glasses at d 16.67 mm 14 mm,
  etc.)

103
Space Eikonometer The Gold Standard

• Patient views a group of five targets binocularly
• Appearance and relative positioning of the
  targets gives initial indication of the amount
  and type of aniseikonia

104
Space Eikonometer The Gold Standard
Page 11.30
Afocal (zero equivalent power) size lenses then
added in front of one eye until aniseikonia
reduced to zero ? verified by neutral position
of targets. Lenses can magnify overall (R or L),
horizontally (R or L), vertically (R or L), or
oblique (R or L)
105
The Space Eikonometer
106
The Space Eikonometer

• The magnification change required with the afocal
  size lenses to reduce ANK to zero ? direct
  measure of how retinal image heights must be
  changed in the patients correction to eliminate
  ANK (or reduce it to acceptable levels).
  Choices, e.g.
• change from spectacles to contact lenses
• change from contact lenses to spectacles
• change the vertex distance of the spectacle
  correction
• Eikonic lenses spectacle lenses specifically
  designed to reduce spectacle magnification in one
  eye and/or increase it in the other to bring ANK
  down to an acceptable level
• to increase SM ? can ? lens thickness, ? front
  surface power (compensate with ? back surface
  power), or decrease lens refractive index).
  Opposite changes reduce SM

107
Spectacle Magnification - Thick lenses
SM Shape Factor ? Power
factor
108
Newer Devices to Measure ANK
109
Aniseikonia Inspector (Px wears RG Filters)
110
Aniseikonia Inspector II
111
2.7
112
3.7
113
2.3
114
1.5
115
1.2
116
0.8
117
Renewed interest in Aniseikonia?

• Two reasons unilateral pseudophakia (intraocular
  implant in one eye only) and unequal refractive
  surgery (bilateral emmetropizing correction
  given to previously anisometropic patient)
• Kramer et al. (1999) reported that 40 of all
  previously anisometropic pseudophakes had
  ophthalmic complaints attributable to aniseikonia
  (probably overestimates true optical
  aniseikonia)

118
Renewed interest in Aniseikonia?

• Unequal refractive surgery presents many more
  variables than intraocular implants ? altering
  the refractive component of ametropia differently
  in each eye
• This presents an even stronger argument for
  eikonometry
• In particular, anisometropes who are
  asymptomatic, binocular and fusing with
  spectacles, may be less functional or even
  potentially suffer breakdown of BV after
  refractive surgery (diplopia).
• Example axial hyperopic anisometrope 5.5 D
  O.D., 2 D O.S. wearing spectacles without any
  symptoms of ANK ? would be poor candidate for
  refractive surgery