Error of Refraction

1. PRESENTED BY:
PROF.VIJAYREDDY VANDALI
DEPT OF MEDICAL-SURGICAL NURSING
INDIA.
Error of Refraction

3. Emmentropia
Emmentropia can
be defined as a
state of refraction,
when the parallel
rays of light coming
from infinity are
focused at the
sensitive layer of
the retina with the
accommodation
being at rest.

4. Diopter
It is the standard unit to express the refractive power
of optical lenses.
Reciprocal of the distance (in meters) between a
lens and its focus (its focal length).
Thus, if the focal length of a lens is 0.20 m (20 cm),
its power in diopters is 5.0 D (1 / 0.20 = 5.0 D).

5. Accomodation
It is the ability of the eye to change the refraction
of light as it passes through the eye by changing
the curvature of the lens of the eye.
Stimulus: blurred retinal image Contraction of
ciliary muscles  relaxation of zonular fibers 
thickening of the lens  increased refractive
power

8. Refractive errors
(Ametropia)

9. Ametropia
It is defined as a state of refraction, when the parallel
rays of light coming from infinity are focused either
in front or behind the sensitive layer of retina.

10. Symptoms and Signs of Refractive Error
Cardinal sign –
decreased visual acuity
Asthenopia – weakness
or easy fatigability of the
eyes
Headache – discomfort
should be related to the
use of the eyes and
relieved when the eyes
are not in use.

11. Refractive Errors
Hyperopia Farsightedness
Myopia Nearsightedness
Axial Abnormality in the length of the eye
Refractive
Abnormality in the refractive power of cornea
or lens
Astigmatism
Unequal refracting power of the lens in
different meridians
Presbyopia Decreased accomodation with aging
Aphakia Absence of the crystalline lens
Anisometropia
Inequality in the refractive power of the two
eyes

12. hypermetropia

13. hypermetropia
Where in the parallel
rays of the light
coming from infinity
are focused behind
the retina with
accommodation being
at rest. Thus the
posterior focal point is
behind the retina
receives a blurred
image.

15. EFFECT OF POSTERIOR CHAMBER

16. Hyperopia

17. Hyperopia
The refractive condition
in which, with
accomodation
suspended, parallel rays
of light are intercepted by
the retina.
Usually due to:
 eyeball that is too short
 Axial hyperopia
 lens system that is too
weak
 Refractive hyperopia
The image forms behind the
retina

19. ETIOLOGY
 Axial hypermetropia
Curvatural hypermetropia
Index hypermetropia
Positional hypermetropia
 Absence of crystalline lens

20. ABSENCE OF CRYSTALLIN LENS

21. Signs & symptoms of hyperopia
Asymptomatic
Asthenopic symptoms
Defective vision
Size of eyeball – small
Anterior chamber id comparatively
smaller

22. Convex Lens Focuses Light Rays
The outer rays bend
more and more toward
the center, which is
called convergence of
the rays
focal point – single point
through which bent light
passes through

23. Lens System
Composed of four refractive
interfaces:
1. the interface between air and the
anterior surface of the cornea
2. the interface between the
posterior surface of the cornea
and the aqueous humor
3. the interface between the
aqueous humor and the anterior
surface of the lens of the eye
4. the interface between the
posterior surface of the lens
and the vitreous humor.
The anterior surface of
the cornea is the major
refractive surface of
the eye.

25. Aphakia

26. APHAKIA

27. APHAKIA
The crystalline lens
is displaced and
not in line with the
visual axis.
Causes:
Surgical removal
2o
to trauma
Leads to severe
hyperopia and loss
of accommodation

28. Aphakia
Chief symptom –
decreased vision for
both near and far
No symptoms of
ocular discomfort
since accommodation
is not possible.
Diagnosis:
Loss of reflected
image from the
surface of the lens
Excessive
movement of the
iris because of the
loss of support of
the anterior lens
capsule

29. Aphakia
Corrected by means of
intraocular lens
Placed immediately
behind the iris after an
extracapsular cataract
extraction.
An additional convex
lens is required for near
work.

30. Myopia

31. Myopia
Optical condition in
which rays of light
entering the eye parallel
to the visual axis come
to a focus in front of the
retina.
Usually due to:
 eyeball that is too long
 Axial myopia
 A lens system that is
too strong
 Refractive myopia
The image forms in
front of the retina

34. Type Cause
Physiologic
myopia
Axial
Inadequate correlation of the
refractive power of the
cornea and lens with the
length of the globe.
•Onset between 5 – 10 up to
25 years old.
•Gradually increases until 18
y.o when the eye is fully
grown.
•Seldom exceeds 6 diopters.Refractive
Pathologic/
degenerative
myopia
Axial
Axial length of eye is
excessive because of
overgrowth of the posterior
2/3 of the globe
•Commonly begins as
physiologic myopia
•> 6 Diopters
•The eye continues to enlarge
rather than stabilizing when the
globe is adult size.
Lenticular
myopia
Refractive
Due to increasing refractive
power of the crystalline lens
(anterior segment).
•Occurs in uncontrolled DM,
nuclear sclerosis (aging)
•Drugs s.a. hydralazine,
chlorthalidone, phenothiazines.

36. Concave Lens Diverges Light Rays
This is opposite to the
effect in the convex lens,
and it causes the
peripheral light rays to
diverge from the light
rays that pass through
the center of the lens.

37. Radial Keratotomy (RK)
More commonly
performed before the use
of excimer lasers
Surgical procedure used
to decrease myopia by 2
– 6 diopters
Procedure is less
predictable and less
stable than laser vision
correction.

38. PRK - Photorefractive Keratectomy
Original excimer laser
procedure for reshaping
the cornea.
Laser is applied to the
surface of the cornea after
the epithelium is removed
Discomfort is greater and
the visual recovery
prolonged compared to
LASIK
Range of refractive errors
which can be corrected by
PRK is also not as wide as
that of LASIK

39. LASIK(Laser-Assisted In Situ Keratomileusis)
A surgical procedure
intended to reduce a
person's dependency on
glasses or contact lenses.
Most commonly performed
as a cure for myopia), but
can also be used to cure
hyperopia or astigmatism.
Permanently changes the
shape of the cornea using
a special laser and thus
focusing the light rays
exactly on the retina.
Ambulatory procedure

40. Crystalline Lens
Consists of layers of
transparent proteins
having different optical
densities.
Acts as a series of
lenses within the lens
capsule.
Total refractive index is
greater than that of
individual layers.

41. Astigmatism

42. Astigmatism
Optical condition in which
the refracting power of
the lens (or an eye) is
not the same in all
meridians.
This most often results
from too great a
curvature of the cornea
in one plane of the eye.
Parallel rays form an object do
not fall on a single point.

44. Types of Astigmatism
Regular
Meridians of minimal and maximal refraction are at right
angles to each other
Irregular Meridians are not at right angles to each other
Leticular It is rare
Curvatural Abnormalities of curvatural of lens
positional Tilting or oblique placement of lens
Retinal Oblique placement of macula

45. Regular astigmatism
The cornea has two
different radii of
curvature at right angles
to each other.
Causes
 Biologic variant
 Due to weight of upper
eyelid resting on eyeball
 Surgical incisions into the
cornea
 Trauma/ scarring of
cornea
 Tumors of the eyelid/
chalazion
Irregular astigmatism
Cornea has three or
more different curvatures
Causes:
 Corneal scarring
 Keratoconus
 Variations in radii of
curvature of the crystalline
lens  lenticular
astigmatism (minor
degrees of astigmatism)

46. Keratoconus
The cornea
becomes cone –
shaped with the
apex of the cone
below and nasal to
the corneal center.

47. Symptoms of Astigmatism
A distinct retinal image
cannot form
The changes in
accomodation cause
symptoms
Severe astigmatism may
cause the optic disc to
appear oval rather than
nearly circular.

48. Anisometropia

49. Anisometropia
The optical state with equal fraction in
the two eyes is termed isometropia.
When the total refraction of the two
eyes is unequal the condition is caused
anisometropia.

50. Anisometropia
Condition in which the
refractive error of each
eye is different (at least
>2 diopters)
Neutralized by
 Prescribing the
appropriate prism in the
reading segment
 Using separate lenses for
near and far vision

51. Correction of Refractive
Errors by Use of Lenses

52. Concave spherical lensConvex spherical lens

53. Correction of Optical Abnormalities by
Use of Contact Lenses
Held in place by a thin
layer of tear fluid that fills
the space between the
contact lens and the
anterior eye surface.
It nullifies almost entirely
the refraction that
normally occurs at the
anterior surface of the
cornea.

54. Neutralized by cylindrical
lenses which allows a
line of focus to form,
which parallels the axis
of the lens.
Hard contact lenses may
be used to correct
irregular astigmatism.
In keratoconus – corneal
transplant should be
considered
Correction of Astigmatism

56. Correction of Presbyopia
Treated by convex
lenses addded to the
distance correction
 Weakest possible convex
lenses to permit individual
to carry on different tasks.
Trifocal/ Bifocal lenses
 If require lenses for
distance without changing
glasses.

57. The Surgical Correction of
Refractive Errors

58. Surgical procedures:
LASIK (Laser In Situ Keratomileusis)
PRK (Photoreactive Keratectomy)
Radial Keratotomy
INTACS - implants within the corneal stroma
Phakic intraocular lens implants - lens implants in
the eye

59. Advantages
In case of high refractive
errors, the error may be
brought down to a more
manageable level so that
it can be corrected by
other means
No need for glasses,
contact lenses, no
maximum wearing time
Disadvantages
All surgical procedures
have a statistical failure
rate which may range
from improper correction
to loss of the eye
Costly instrumentation
and surgery
Not freely available
Stringent case selection

60. INTACS
 Microthin polymer ring
segments
 Placed under the outer edge
of the cornea.
Advantages:
 reversibility
 tissue is not removed and the
eye is not structurally
weakened
 delivers an excellent quality of
vision
Disadvantages:
 only approved for low levels of
myopia and does not correct
astigmatism

61. Phakic Intraocular Lens Implants
•Lenses made of plastic or silicone
that are implanted into the eye
permanently to reduce a person's
need for glasses or contact lenses
•Phakic refers to the fact that the lens
is implanted into the eye without
removing the eye's natural lens.
•During phakic lens implantation
surgery, a small incision is made in the
front of the eye.
•The phakic lens is inserted through
the incision and placed just in front of
or just behind the iris.

62. Thank You for
Listening!!!