For residents

1. Refractive Errors
Correction
By Desta Genete(MD, R l)
Advisor Mr. Alemayehu( consultant Optometrist)
July 2018

2. Seminar outline
• Introduction to optical principles
• Correction of myopia
• Correction of hyperopia
• Correction of astigmatism
• Spectacle and accomodation
• Spectacle and binicularity

3. Introduction
• In a nonaccommodating emmetropic eye, a
distant object at infinity is conjugate to the
retina.
• Conversely, if the retina were the object, its
image would be located at an infinite distance
in front of it.

4. Far Point
• The far point denotes the location at
which the far point plane intersects
the optical axis..
• For emmetropia it is at infinity
• Myopia…..finite distance infront of
the eye
• The higher the degree of myopia,
the closer the far point is to the eye
• Hypermetropia ….is virtual ; located
behind the lens.
4

5. • The far point forms the basis
for optical correction of any
ametropia
• Any lens brings light from an
object located at a distance
and images it at that lens'
secondary focal plane.
• Therefore, if that lens is
positioned in front of the eye
so that its
• secondary focal plane is
superimposed on the eye's far
point plane, that lens becomes
a “corrective lens.”

6. Lens Effectivity
• Almost any lens can
become a corrective
lens if its secondary
focal plane is placed to
coincide with the
ametropic eye's far
point.

7. Vertex Distance
• The distance b/n the posterior
surface of spectacle lens and
anterior pole of the eye(cornea)
• Must be given with all
prescriptions over 4-5 diopters
• The position in front of the eye
affects the optical correction of
ametropia.
7

8. • The measurement may be
made with a ruler held
parallel to the arm of the
trial frame.
• Vertexometer
• Add 2mm to correct lid
thickness
8
Back Vertex Distance

9. Brain storm quizes
• An aphakic patient's right eye is refracted at a
vertex distance of 14 mm and found to be
+12.00D.
• What power contact lens would provide the
same optical correction (i.e., the same
effectivity)
• Ans 14.4D

10. • A patient who has been wearing -6.00D
eyeglasses finds that, in order to see clearly,
he must push his
• eyeglasses 1 cm closer to his eyes. What is his
new refractive correction if he wants to use
his current frame?
• -5.66D

11. Myopia
• The incident parallel rays come to focus in front of the retina
when accommodation is at rest.
11

12. Etiology
Etiology Axial
Curvatural
Index
Positional
12

13. Treatmentt
1. Spectacle (concave lens)
2. Contact lens
3. Refractive surgery
– Keratorefractive surgery like radial keratotomy(RK)
– Clear lens extraction for high myopia
– minus phakic IOL
– Excimer laser surgery
13

14. CORRECTION OF MYOPIA
• The far point plane of the myopic eye is
located at a certain distance in front of the eye
and determines the degree of myopia.
• The greater the myopia, the closer the far
point will be.
• Eg. 1D=1m 0.25D= 4m

15. A “minus” lens, a lens that diverges light, is
necessary to accomplish this task.
The closer to the eye the corrective lens is placed,
the less divergence power it needs to allow its
secondary focal plane to be coincident with the
eye's far point plane, therefore necessitating a
weaker minus corrective lens
A contact lens in myopia will not need to be as
strong as the corresponding spectacle

16. • Because of the small difference in power
induced by changes in vertex distance in low
refractive errors,
it is not typically necessary to make
corresponding prescriptive changes unless the
myopia is 4D or greater.

17. Full Versus Partial Correction of
Myopia
• To create clear retinal image, full extent of
myopia must be corrected.
• Refraction without cycloplegia result slight
under correction 0.25D
• Another factor is night myopia, 0.5-1D which
may result in
 blurred vision or
 halos around lights at night

18. • If a significantly myopic patient has never
worn a correction before, prescribing the full
correction may result in significant asthenopia
• Depends on clinical judgment whether or not
to prescribe the full prescription.
• Eg in older patients, partial correction of
myopia recommended.

19. Myopic Progression and Spectacles
• Remains topic of debate and continued
research area.
• The COMET study (Correction of Myopia
Evaluation Trial) was designed to evaluate this
clinical practice and its outcome.

20. • Children were fit with either progressive
lenses or single vision lenses and were
followed for 3 years using cycloplegic
autorefraction.
• Study results revealed that the group wearing
progressives glasses had developed only
about 0.2D less myopia occurring primarily
during the first year of lens wear

21. • There was a corresponding greater increase in
axial length in the single vision lens group
compared to the progressive lens group
• prescribed bifocals showed a slowing of
myopic development over the first two years
(0.1D less development per year for the first 2
years)
• Atropine/bifocal In one study showed lessen
myopic progression of 0.15D/year

22. CORRECTION OF HYPEROPIA
DEFINITION
• It is the refractive state of
eye where in parallel rays
of light coming from
infinity are focused
behind the sensitive layer
of retina with
accommodation being at
rest.
• The posterior focal point
is behind the retina
which receives a blurred
image

23. Etiology
Etiology
Axial hypermetropia
Axial shortning
Commonest
Curvatural
hypermetropia
Flatter curvature of
lens/cornea
Index hypermetropia Aphakia
Positional
hypermetropia
Posterior dislocation
of lens
23

24. Components of hypermetropia
Total
Manifest
Facultative
Absolute
Latent
24

25. • Total hypermetropia is the amount of RE, which is
estimated after complete cycloplegia with atropine
• Manifest hypermetropia -the strongest convex lens
correction accepted for clear distance vision without
cycloplegia.
Components of hypermetropia
25

26. SYMPTOMS
• Principal symptom is blurring of vision for
close work
• Symptoms vary depending upon age of
patient & degree of refractive error
1.ASYMPTOMATIC
• small error produces no symptoms
• Corrected by accommodation of patient

27. 2.ASTHENOPIA
• Refractive error are fully corrected by accommodative
effort Thus vision is normal
• Sustained accommodation produces symptoms
• Asthenopia increases as day progresses
• Increased after prolonged near work
SYMPTOMS
• Tiredness
• Frontal or fronto temporal headache
• Watering
• Mild photophobia

28. BASIS FOR TREATMENT
No Treatment
• Error is small
• Asymptomatic
• Visual acuity normal
• No muscular imbalance

29. Young children(<6 or 7yrs)
• Some degree of hypermetropia is
physiological so no correction
• Treatment is required if error is high or
strabismus is present

30. ADULTS
• If symptoms of eye-strain are marked , correct
as much of the total hypermetropia as
possible , trying as far as we can to relieve the
accommodation.

31. • Some patients with hypermetropia do not
initially tolerate the full correction indicated
by manifest refraction so we undercorrect
them

32. MODE OF TREATMENT
• SPECTACLES
• CONTACT LENS
• SURGICAL

33. SPECTACLES
Basic principle
• Prescribe convex lenses (Plus lenses) so that
rays are brought to focus on the retina
Advantages
• Comfortable
• Easier method
• Less expensive
• Safe

34. CONTACT LENS
ADVANTAGES:
• Cosmetically good
• Increased field of view
• Less magnification
• Elimination of aberrations & prismatic effect

35. REFRACTIVE SURGERY
• Refractive surgery is not
as effective as in myopia
TYPES:
(1)HEXAGONAL
KERATOTOMY(HK)
• Low to moderate degrees
of hypermetropiaIts risk
/benefit ratio is not low
enough to warrant its
continued use.

36. • LASER THERMAL
KERATOPLASTY(LTK)
Procedure done using
Thallium-Holmium-
Chromium
(THC:YAG) laser energy to
heat the cornea
(contraction of collagen)
and increase its curvature

37. • The far point plane (the
image of the retinal
plane) of the hyperopic
eye is located behind
the eye and, so, is
virtual
• A corrective lens for
hyperopia is a “plus”
lens

38. • Many different plus lenses offer equal
effectivity.
• It dependens on vertex distance
• Contact lens is closer to the far point plane so
it must be of higher plus.

39. Full vs partial correction
Prescription of spectacle depends on
 age of patient
Degree of hyperopia
Accomodative and binocular satus

40. • Young patients will be uncomfortable initially if
given full refractive correction due to habitual
accomodative tone all times.
• Manifest refraction may be deceptive.
• As the patient ages , accomodative ability
decreases and latent hyperopia may manifest
.

41. • In the presence of high hyperopia; especially if
AC/A ratio is high may result accomodative
esophoria/tropia
• Prescribing maximum tolerated plus lenses,
often in the form of bifocals, is indicated to
normalize binocularity.

42. • As an under corrected hyperope ages,
accomodative amplitude decreases,
symptoms of early onset presbyopia
• In such cases correcting the hypermetropia
usually alleviates symptoms

43. Astigmatism
• A refractive error in which incident light fail to focus to a
single focal point and may focus at ,pre and post retina.
• The image is formed as a Sturm's conoid
43

44. 44

45. Etiology
Astigmatism
Corneal (common)
Lenticular
Curvatural
Positional
Oblique tilt of lens
Index
Diff index in diff
meridians
Retinal
Oblique placement
of macula
45

46. Classification based on axis of principal meridians
Astigmatism
Regular
With the
rule
Oblique
Against the
rule
Irregular
46

47. Correction of astigmatism
• Unique because it has two far point planes
from the two principal meridians of refractive
errors
• In simple myopic astigmatism, one plane is
located at infinity and another at finite
distance in front of the eye
• In simple hyperopic astigmatism one at
infinity another at the back of the eye.

48. • In compound hyperopic astigmatism, both
planes are located behind the eye at different
distance
• In compound myopic both planes are located
in front of the eye at different distance
• In mixed astigmatism one is located behind
the other in front of the eye.

49. • Cylindrical lenses are
used
• A cylindrical lens has
maximum dioptric
power in one meridian,
while the orthogonal
(perpendicular) meridian
has no dioptric power.
• They create two line
images each with specific
orientations.

50. • A spherocylindrical
lens’s meridians of
maximum power and
minimum power, which
are perpendicular to
each other, are called
the major, or principal
meridians

51. Plus and minus forms of cylinder

52. To transpose a plus-cylinder prescription into
minuscylinder form and vice versa.
• Keeping the signs, add the sphere and cylindrical
powers to arrive at the new sphere value.
• Reverse the cylinder sign to arrive at the new
cylinder power.
• Change the axis by 90 degrees to arrive at the
new axis. (Remember that the axis is never greater
than 180 degrees.)

53. Circle of least confusion
• the circle is dioptrically centered between the
focused images of the two principal meridians

54. circle of least confusion

55. POWER IN AN OBLIQUE MERIDIAN
OF A CYLINDRICAL LENS
Eg. pl −5.00 × 030
Power in the axis meridian(30) is zero and power
meridian(120) is -5D. What about the other
meridians?
Can be calculated by using this formula
Fθ = (Fcyl)sin2θ
• where Fθ is the power in the oblique meridian,
Fcyl is the power in the power meridian, and θ is
the angle between the oblique meridian and the
cylinder axis.

56. • Fθ = (−5.00 D)sin2(30)
• Fθ = −1.25 D
• Oblique meridian
doesnt have focal point
but calculating it is
useful
• In calculations that deal
with lens thickness and
the prism in decentered
cylindrical lenses.

57. • Let’s look at a
spherocylindrical lens.
What’s the power in the
vertical meridian of
+ 2.00 − 3.00 × 160 lens?

58. • In compound or mixed astigmatism , two
simple cylinders oriented 90 degrees apart are
used to create image lines at each of the far
point planes.

59. • Combination of two cylinders result in
spherocylindrical lens
• The spherical equivalent of that lens is the
spherical power that is diopterically half way
between the highest and lowest of the lens
principle meridional powers.

60. • In within the rule astigmatism
The focusing element of the eye require more
convergent power in the 180 degree meridian,
which is corrected by plus cylinder axis 90
degree.

61. • In against the rule astigmatism, the focusing
elements of the eye require more convergent
power in the 90degree meridian, which is
corrected by plus cylinder axis 180 degree
• Or minus cylinder axis 90 degree.
• + 2.00 −3.00 × 90

62. Full versus partial correction of
astigmatism
• The non uniform magnification or minification results
in meredional anisokonia.
This may result in;
• Spatial orientation and asthenopia.
• Common compliant is flat surface appear tilted
• Usually patients adapt rapidly if not reducing
astigmatic component may suffice to alleviate
symptoms.
• Maintaining spherical equivalent ensures that the circle
of least confusion will be maintained at the retina
reducing retinal blur.

63. Effect of spectacle prescribing on
accomodation
• Always keep in mind the effect of the
spectacle on accomodation
• Especially on those elderly or with
accomodative dysfunction.
• Careless refractive exam may result in
prescribing more minus lens than actually
needed.

64. • Overminused prescriptions = excess
accomodaive effort= asthenopia specially at
near task.
• May result in early presbyopia
• ???slightly undercorrecting the myopia may
delay the need for bifocals or progressive.

65. • Hyperopes often benefits from the “the
pushing of plus”
• Minimizes the adverse effects of
accomodation from undercorrecting the
refractive error.

66. Effects of spectacle prescribing on
binocularity
• Accommodation and binocularity are
neurologically linked.
• Important specially in esophoria and esotropia
• All esophoric/ esotropic patients need full
cycloplegic refraction.
• It will uncover any accomodative spasm or
latent hyperopia

67. • Once binocularity is maintained at distance,
• Additional plus power may need to be
prescribed to lessen accomodative demand
and any induced esotropia at near.
• In contrast inadvertant over correction of
myopia or inadequate correction of hyperopia
may result iatrogenic esophoria.

68. • In exophoria/ exotropia, the over correction
with plus lenses or
• Undercorrection with minus lenses will likely
exacerbate the condition because
accomodation is relaxed
• Least plus or over minus may help to restore
proper binocular posture

69. Correction of Anisometropia and
Aniseikonia
• Anisometropia in which patients refractive error
significantly differs between the two eyes.
• The term significant is unclear
• Usually a difference of 2D
• Anisokonia the perception of an image size
difference between the two eyes
• 1% image size difference per diopter
• 1-5% likely to result symptoms and interfere with
fusion.

70. Aniesokonia may result in
• Headach
• Eye ach
• Tearing
• Decreased reading stamina
• vertigo, and spatial distortions

71. The symptoms can be reduced by
• Minimizing the vertex distance
• By fitting contact lenses
• Axial anisomertopia can be minimized by
placing the corrective lenses close to the
anterior focal planes of the eyes(15.7mm in
front of the eye)KnapPs rule

72. • Anisophoria which is differential prism effect
between the two eyes occurs with off axis
viewing through the asymetric spectacle
lenses.
• May result in both asthenopia and diplopia

73. VISUAL HYGIENE
• While reading or doing intensive near work
take a break about every 30 min.
• When reading maintain proper distance
• Sufficient Illumination
• Place a limit spent watching television &
watching videogames
• Sit 5-6 feet away from the television

74. • Duanes ophthalmlogy 2012 edition
• Geometric and visual optics, second edition
• BCSC section 3
• Onlinesources