Understanding Refraction in Ophthalmology

 
Dr Ajai Agrawal
Additional Professor
Department of Ophthalmology
AIIMS, Rishikesh
 
Acknowledgement
 
Photographs in this presentation are courtesy of
    
Kanski’s Clinical Ophthalmology.
 
 
2
 
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At the end of the class, students shall be able to
 
Understand what is refraction.
Have basic knowledge of hypermetropia and astigmatism
and their management.
 
 
 
3
 
Question
 
A patient with a corneal scar is carefully
refracted. Best corrected visual acuity is 6/12.
With a pinhole over his correction, his acuity
   improves to 6/6.  The best explanation for this is
 
a. 
spherical aberration.
b. 
myopic astigmatism.
c. 
cataract.
d
. 
irregular astigmatism.
 
4
 
What is Refraction
 
When rays of light traveling through air enter a
denser transparent medium, the speed of the
light is reduced and the light rays proceed at a
different angle, i.e., they are refracted.
Except when the rays are normal
Refraction in Ophthalmology
Methods for evaluating the optical and refractive
state of the eye
 
 
5
 
Emmetropia
 
Parallel light rays, from an object more than 6 m away,
are focused at the plane of the retina when accomodation
is at rest.
 
Clear image of a distant object formed without any
internal adjustment of the optics of the eye.
 
Absence of emmetropia = Ametropia
 
6
 
Anomalies of the optical state of the eye
Myopia
Hypermetropia
Astigmatism
 
7
 
Hypermetropia
 
8
 
Hypermetropia
 
Refractive or Diopteric state of eye wherein
incident parallel rays of light coming from
infinity are focused behind the retina 
with
accommodation being at rest.
 
9
 
 
Near images can be blurred unless there is
sufficient accommodation, as in a child.
They 
have blurred images for distant objects also
Most children are born about +3 D hyperopic,
but this usually resolves by age 12 years.
 
10
 
Axial is the commonest form.
In this condition the total refractive power of eye is
normal but there is axial shortening of eye ball.
 
11
 
 
Each millimeter of shortening represents approximately
3D of refractive change and thus a hypermetropia of over
6D is uncommon.
 
Physiological
: Infant, child.
Pathological
: Orbital tumour, or inflammatory mass may
indent the posterior pole of the eye and flatten it
 
12
 
 
Curvature Hypermetropia 
: When the radius of
curvature of any of the refracting surfaces is increased,
congenitally (cornea plana) or as a result of trauma
Increase of 1 mm produces a hypermetropia of 6 D.
 
13
 
 
Index Hypermetropia 
: Usually manifests
itself as a decrease in the effective refractivity of
the lens and is responsible for the
hypermetropia which occurs 
physiologically in
old age and pathologically in diabetes.
 
14
 
 
Positional Hypermetropia 
: Posterior placed lens
also produced hypermetropia whether it occurs as a
congenital anomaly or as a result of trauma and disease.
 
Aphakia 
: Surgical, posterior dislocation of lens
 
15
 
Clinical Types:
 
Simple Hypermetropia
 : Commonest form.
It results from normal biological variations in the
development of eye  e.g., axial and curvatural.
Pathological Hypermetropia 
: Either congenital or
acquired  conditions of eyeball which are outside the normal
biological variation of development
Example: index , positional (Aphakia).
Functional Hypermetropia
 : Results from paralysis of
accommodation as seen in patients with third nerve palsy.
 
16
 
Components of hypermetropia
 
Total hypermetropia = Latent+manifest
(facultative + absolute)
Accommodation in Hypermetropia
Contraction of ciliary muscle 
in the act of
accommodation increases the refractive power
of the lens so that it corrects a certain amount of
hypermetropia.
 
17
 
 
Normally there is an appreciable amount corrected by
contraction involved in 
physiological tone 
of ciliary muscle.
Consequently the full degree of hypermetropia is revealed
only when this muscle is paralysed by the use of a drug such
as atropine.
This is called 
latent
 hypermetropia, normally 1D.
 
18
 
Manifest Hypermetropia consists of:
 
Facultative Hypermetropia
: Corrected by effort of
accomodation
Absolute Hypermetropia
: Cannot be overcome by
effort of accomodation
As tone of ciliary muscle decreases with age, some latent
hypermetropia becomes manifest
 
As range of accomodation reduces with age, more
facultative hypermetropia becomes absolute, all of it
after age 60.
 
19
 
Symptoms
 
Vary with degree of hypermetropia and accomodative effort
Blurred vision: near>distant
 
Accomodative asthenopia
 
Convergent squint due to continuous effort of accomodation,
excess of convergence leads to dissociation of muscle balance
Early onset of presbyopia
 
20
 
Signs
 
Small eyeball
Smaller cornea
Shallow anterior chamber predisposes to angle closure
glaucoma since size of lens is normal
Apparent divergent squint
 
21
 
 
Retina :
 Has peculiar sheen : a reflex effect  so
called “shot silk retina” on ophthalmoscopy.
Optic disc : 
Characteristic appearance which
may resemble optic neuritis (Pseudopapillitis).
 
22
 
Treatment
 
In young children below the age of 6-7 years:
some degree of hypermetropia  is physiological
and a correction need be given only if the error is
high or if strabismus is present.
 
In those between 6 and 16 years:
   smaller error may require correction.
 
23
 
 
Refractive correction is required
in middle aged patients
in high hypermetropia
and if patient is symptomatic
 
Optical:
Glasses
Contact lens
 
24
 
 
Convex lenses 
prescribed after full cycloplegic
refraction, particularly in children
Child with convergent squint may need “full
atropine correction”
Contact lens power is a little more than spectacle
power
 
25
 
Surgical Options
 
Conductive keratoplasty
.
Non contact Holmium YAG laser 
thermokeratoplasty
for lower hypermetropia (+1D – 2.5 D).
P
hakic Intraocular lens 
(+6D – +10 D)
 
 
 
26
 
Astigmatism
 
27
 
Astigmatism
|
  
|
 
|
  
Regular
 
Irregular
 
Astigmatism
 
Astigmatism is a type of  refractive error wherein
the 
refraction varies in different meridia
.
Consequently  rays of light entering the eye
cannot converge to a point focus, but form focal
lines.
 
 
28
 
Astigmatism
 
 
Light rays passing through a steep meridian are
deflected more than those passing through a
flatter meridian.
 
29
 
1. Corneal Astigmatism e.g. keratoconus
2. Lenticular Astigmatism
 
(i)
 
Curvatural – e.g. lenticonus
 
(ii) 
 
Positional – subluxation
 
(iii) 
 
Index – cataract
3. Retinal astigmatism – due to oblique placement of macula.
 
30
 
Types of Regular Astigmatism
 
1.
With the rule astigmatism : 
The two principal
meridia are placed at right angles to one another but the
vertical meridian is more curved 
than horizontal meridian
       (most common type)
 
2.
Against the rule astigmatism : 
Horizontal meridian is
more curved than the vertical meridian.
 
3.
 
Oblique astigmatism : 
Is a type of regular astigmatism
where the two principal meridia are not horizontal and
vertical , though they are at right angles to one another
(45 and 135 deg)
 
31
 
Oblique astigmatism :
(i) 
 
Symmetrical  : Cylindrical lens required at same axis
in both eyes.
(ii) 
 
Complementary : Cylindrical lens required at 30
o
 in
one eye and at 150
o
 in the other eye.
4.
 
Bi-oblique astigmatism :
 In this type of regular
astigmatism the two principal meridia are not at
right angles to each other, one eye  at 30
o
 and other
at 100
o
.
 
32
 
Optics of regular astigmatism :
 In regular
astigmatism the parallel rays of light are not focused on a
point but form two focal lines – 
Sturm’s conoid
 
33
 
Refractive types of Regular astigmatism
 
Depending upon the position of
two focal lines in relation to retina,
regular astigmatism is further
classified
Simple : 
Where one focus falls
upon retina, the other focus may
fall in front of or behind, so that
one meridian is emmetropic 
the
other is either hypermetropic or
myopic
.
 
34
 
     Compound : 
 Where neither of two
foci lie upon retina but both are
placed in front or behind it.
      The state of the refraction is then
entirely hypermetropic or entirely
myopic
. The former is known as
compound hypermetropic, the latter
as compound myopic astigmatism.
 
35
 
3. 
Mixed : 
Where one focus is in front of and other behind
retina so that the refraction is 
hypermetropic in one
direction and myopic in the other.
 
36
 
Irregular Astigmatism : 
Refraction in different
meridia are irregular.
Etiological types:
1.
Curvatural irregular astigmatism: irregular healing of
cornea after trauma and inflammation (particularly
ulceration & keratoconus)
2.
Index irregular astigmatism : incipient cataract
 
37
 
Symptoms
 
1.
Defective vision
2.
Blurring of objects
3.
Asthenopic symptoms - eyeache and headache
4.
Running of lines
 
38
 
Treatment
 
Optical
 – Spectacles with cylindrical lenses, Contact lens
(
Toric contact lenses with prism ballast)
 
Surgical
1.
Astigmatic keratotomy: 
Limbal Relaxing Incision,
arcuate keratectomy, removal of sutures
2.
Photo-astigmatic refractive keratotomy (PARK)
3.
La
ser: Excimer laser: LASIK or Femtosecond laser
 
39
 
Guidelines for Optical treatment
 
1.
If the patient does not complain of asthenopic
symptoms small astigmatic errors (0.5 D or less)
generally do not require correction
2.
If asthenopic symptoms 
are present , error should be
corrected by 
cylindrical lenses
.
3.
Undercorrect the error initially
4.
At a later date, full correction may be worn
comfortably.
 
40
 
Question
 
In a patient with astigmatism, all of the following are
true of myopia and hyperopia 
except
 
a. 
In simple myopic astigmatism, one focal line lies in
front of the retina and the other is on the retina.
b. 
In compound myopic astigmatism, both focal lines lie
in front of the retina.
c
. 
In simple hyperopic astigmatism, both focal lines lie
behind the retina.
d. 
In mixed astigmatism, one focal line lies in front of
the retina and one lies behind the retina.
 
41
 
 
 
 
 
                                
Thank you
 
42
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Dr. Ajai Agrawal, an Additional Professor at AIIMS, Rishikesh, presents valuable insights on refraction and common optical anomalies like hypermetropia and astigmatism. The session covers the basics of refractive errors, the concept of emmetropia, and detailed explanations of hypermetropia. Learn about the importance of proper refractive state evaluation and managing refractive errors effectively.


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  1. Dr Ajai Agrawal Additional Professor Department of Ophthalmology AIIMS, Rishikesh

  2. 2 Acknowledgement Photographs in this presentation are courtesy of Kanski s Clinical Ophthalmology.

  3. 3 Learning Objectives At the end of the class, students shall be able to Understand what is refraction. Have basic knowledge of hypermetropia and astigmatism and their management.

  4. 4 Question A patient with a corneal scar is carefully refracted. Best corrected visual acuity is 6/12. With a pinhole over his correction, his acuity improves to 6/6. The best explanation for this is a. spherical aberration. b. myopic astigmatism. c. cataract. d. irregular astigmatism.

  5. 5 What is Refraction When rays of light traveling through air enter a denser transparent medium, the speed of the light is reduced and the light rays proceed at a different angle, i.e., they are refracted. Except when the rays are normal Refraction in Ophthalmology Methods for evaluating the optical and refractive state of the eye

  6. 6 Emmetropia Parallel light rays, from an object more than 6 m away, are focused at the plane of the retina when accomodation is at rest. Clear image of a distant object formed without any internal adjustment of the optics of the eye. Absence of emmetropia = Ametropia

  7. 7 Refractive errors Anomalies of the optical state of the eye Myopia Hypermetropia Astigmatism

  8. 8 Hypermetropia

  9. 9 Hypermetropia Refractive or Diopteric state of eye wherein incident parallel rays of light coming from infinity are focused behind the retina with accommodation being at rest.

  10. 10 Near images can be blurred unless there is sufficient accommodation, as in a child. They have blurred images for distant objects also Most children are born about +3 D hyperopic, but this usually resolves by age 12 years.

  11. 11 Types | | Axial Curvature Index | | | | Positional Absence of lens Axial is the commonest form. In this condition the total refractive power of eye is normal but there is axial shortening of eye ball.

  12. 12 Each millimeter of shortening represents approximately 3D of refractive change and thus a hypermetropia of over 6D is uncommon. Physiological: Infant, child. Pathological: Orbital tumour, or inflammatory mass may indent the posterior pole of the eye and flatten it

  13. 13 Curvature Hypermetropia : When the radius of curvature of any of the refracting surfaces is increased, congenitally (cornea plana) or as a result of trauma Increase of 1 mm produces a hypermetropia of 6 D.

  14. 14 Index Hypermetropia : Usually manifests itself as a decrease in the effective refractivity of the lens and is responsible for the hypermetropia which occurs physiologically in old age and pathologically in diabetes.

  15. 15 Positional Hypermetropia : Posterior placed lens also produced hypermetropia whether it occurs as a congenital anomaly or as a result of trauma and disease. Aphakia : Surgical, posterior dislocation of lens

  16. 16 Clinical Types: Simple Hypermetropia : Commonest form. It results from normal biological variations in the development of eye e.g., axial and curvatural. Pathological Hypermetropia : Either congenital or acquired conditions of eyeball which are outside the normal biological variation of development Example: index , positional (Aphakia). Functional Hypermetropia : Results from paralysis of accommodation as seen in patients with third nerve palsy.

  17. 17 Components of hypermetropia Total hypermetropia = Latent+manifest (facultative + absolute) Accommodation in Hypermetropia Contraction of ciliary muscle in the act of accommodation increases the refractive power of the lens so that it corrects a certain amount of hypermetropia.

  18. 18 Normally there is an appreciable amount corrected by contraction involved in physiological tone of ciliary muscle. Consequently the full degree of hypermetropia is revealed only when this muscle is paralysed by the use of a drug such as atropine. This is called latent hypermetropia, normally 1D.

  19. 19 Manifest Hypermetropia consists of: Facultative Hypermetropia: Corrected by effort of accomodation Absolute Hypermetropia: Cannot be overcome by effort of accomodation As tone of ciliary muscle decreases with age, some latent hypermetropia becomes manifest As range of accomodation reduces with age, more facultative hypermetropia becomes absolute, all of it after age 60.

  20. 20 Symptoms Vary with degree of hypermetropia and accomodative effort Blurred vision: near>distant Accomodative asthenopia Convergent squint due to continuous effort of accomodation, excess of convergence leads to dissociation of muscle balance Early onset of presbyopia

  21. 21 Signs Small eyeball Smaller cornea Shallow anterior chamber predisposes to angle closure glaucoma since size of lens is normal Apparent divergent squint

  22. 22 Retina : Has peculiar sheen : a reflex effect so called shot silk retina on ophthalmoscopy. Optic disc : Characteristic appearance which may resemble optic neuritis (Pseudopapillitis).

  23. 23 Treatment In young children below the age of 6-7 years: some degree of hypermetropia is physiological and a correction need be given only if the error is high or if strabismus is present. In those between 6 and 16 years: smaller error may require correction.

  24. 24 Refractive correction is required in middle aged patients in high hypermetropia and if patient is symptomatic Optical: Glasses Contact lens

  25. 25 Convex lenses prescribed after full cycloplegic refraction, particularly in children Child with convergent squint may need full atropine correction Contact lens power is a little more than spectacle power

  26. 26 Surgical Options Conductive keratoplasty. Non contact Holmium YAG laser thermokeratoplasty for lower hypermetropia (+1D 2.5 D). Phakic Intraocular lens (+6D +10 D)

  27. 27 Astigmatism

  28. 28 Astigmatism Astigmatism is a type of refractive error wherein the refraction varies in different meridia. Consequently rays of light entering the eye cannot converge to a point focus, but form focal lines. Astigmatism | | | Regular Irregular

  29. 29 Astigmatism Light rays passing through a steep meridian are deflected more than those passing through a flatter meridian.

  30. 30 1. Corneal Astigmatism e.g. keratoconus 2. Lenticular Astigmatism (i) Curvatural e.g. lenticonus (ii) Positional subluxation (iii) Index cataract 3. Retinal astigmatism due to oblique placement of macula.

  31. 31 Types of Regular Astigmatism 1. With the rule astigmatism : The two principal meridia are placed at right angles to one another but the vertical meridian is more curved than horizontal meridian (most common type) 2. Against the rule astigmatism : Horizontal meridian is more curved than the vertical meridian. 3. Oblique astigmatism : Is a type of regular astigmatism where the two principal meridia are not horizontal and vertical , though they are at right angles to one another (45 and 135 deg)

  32. 32 Oblique astigmatism : (i) Symmetrical : Cylindrical lens required at same axis in both eyes. (ii) Complementary : Cylindrical lens required at 30o in one eye and at 150o in the other eye. 4. Bi-oblique astigmatism : In this type of regular astigmatism the two principal meridia are not at right angles to each other, one eye at 30o and other at 100o.

  33. 33 Optics of regular astigmatism : In regular astigmatism the parallel rays of light are not focused on a point but form two focal lines Sturm s conoid

  34. 34 Refractive types of Regular astigmatism Depending upon the position of two focal lines in relation to retina, regular astigmatism is further classified Simple : Where one focus falls upon retina, the other focus may fall in front of or behind, so that one meridian is emmetropic the other is either hypermetropic or myopic.

  35. 35 Compound : Where neither of two foci lie upon retina but both are placed in front or behind it. The state of the refraction is then entirely hypermetropic or entirely myopic. The former is known as compound hypermetropic, the latter as compound myopic astigmatism.

  36. 36 3. Mixed : Where one focus is in front of and other behind retina so that the refraction is hypermetropic in one direction and myopic in the other.

  37. 37 Irregular Astigmatism : Refraction in different meridia are irregular. Etiological types: 1. Curvatural irregular astigmatism: irregular healing of cornea after trauma and inflammation (particularly ulceration & keratoconus) 2. Index irregular astigmatism : incipient cataract

  38. 38 Symptoms 1. Defective vision 2. Blurring of objects 3. Asthenopic symptoms - eyeache and headache 4. Running of lines

  39. 39 Treatment Optical Spectacles with cylindrical lenses, Contact lens (Toric contact lenses with prism ballast) Surgical 1. Astigmatic keratotomy: Limbal Relaxing Incision, arcuate keratectomy, removal of sutures 2. Photo-astigmatic refractive keratotomy (PARK) 3. Laser: Excimer laser: LASIK or Femtosecond laser

  40. 40 Guidelines for Optical treatment 1. If the patient does not complain of asthenopic symptoms small astigmatic errors (0.5 D or less) generally do not require correction 2. If asthenopic symptoms are present , error should be corrected by cylindrical lenses. 3. Undercorrect the error initially 4. At a later date, full correction may be worn comfortably.

  41. 41 Question In a patient with astigmatism, all of the following are true of myopia and hyperopia except a. In simple myopic astigmatism, one focal line lies in front of the retina and the other is on the retina. b. In compound myopic astigmatism, both focal lines lie in front of the retina. c. In simple hyperopic astigmatism, both focal lines lie behind the retina. d. In mixed astigmatism, one focal line lies in front of the retina and one lies behind the retina.

  42. 42 Thank you

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