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
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
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.
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
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.
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.
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
32.
33.
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.
35.
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.
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.
43.
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)
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.
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
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
55.
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.
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.
Stimulation of th eshort ciliary branches of the oculomotor nerve (CN III)
A refractive error occurs when parallel rays of light from an object an infinite distance form the eye do not come to focus or form an image exactly at the fovea centralis.
Migraine is not caused by a refractive error.
Headache present on awakening in the morning is not ascribed to excessive use of the eyes the evening before.
A refractive error is that optical condition in which parallel rays of light from an object an infinite distance from the eye do not come to focus (form an image) exactly at the fovea centralis.
In this condition, parallel light rays are not bent sufficiently by the relaxed lens system to come to focus by the time they reach the retina.
Accomodation increases the refractive power of the lens and may compensate for hyperopia.
Pseudopapilledema – hyperopia that exceeds 5 diopters
- distinguished from early papilledema by the normal caliber of veins, their normal pulsation, absence of retinal edema, hemorrhages, and progression
Papillitis – usually unilateral, causes decreased vision, a cental scotoma, and an afferent pupillary defect
The eye is optically equivalent
to the usual photographic camera.
It has a lens system,
a variable aperture system (the pupil), and a retina
that corresponds to the film
Patients with uncorrected myopia do not accommodate to improve vision because accomodation shifts the focus even further anterior to the retina and blurs vision further.
The f
With accomodation in the youthful human eye, the refractive index of the lens increases from about 19 to 33 diopters due to a change in its thickness and curvature.
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. It is also the process by which the refractive power of the anterior lens segment increases so that a near object may be distinctly imaged on the retina.
In this condition, parallel light rays are not bent sufficiently by the relaxed lens system to come to focus by the time they reach the retina. To overcome this abnormality, the ciliary muscle must contract to increase the strength of the lens. By using the mechanism of accommodation, a farsighted person is capable of focusing distant objects on the retina.
Often such patients are asymptomatic until bifocal lenses are prescribed.
When the eyes rotate downward to use the bifocal segment, the difference in power of the two lenses induces a vertical prism so that one ocular image is above the other., causing a vertical diplopia and severe ocular symptoms.
Hyperopia - corrected by convex lenses which are prescribed when visual acuity is decreased, when a convergence excess causes esophoria or esotropia, when hyperopia causes symptoms.
- If visual acuity is good, muscle balance is normal, no symptoms, correction of hyperopia is unnecessary irrespective of its severity.
Myopia – optical correction is achieved with concave lenses for patients dissatisfied with poor visual acuity.
- Wearing corrective lenses has no apparent effect on its progression.
Glass or plastic contact lenses can be inserted that fit snugly against the anterior surface of the cornea. The reason for this is equally in both planes; therefore, in astigmatism, each of the two planes requires a different degree of accommodation. Thus, without the aid of glasses, a person with astigmatism never sees in sharp focus.
most of the procedures are aimed at altering the front surface of the eye (the cornea) so that a state of normal refraction can be achieved