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REFRACTION
Švehlíková G.
Department of Ophthalmology LF UPJS v
Košiciach
Prednosta: prof. MUDr. Juhás T., DrSc
HOW THE EYE SEES
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The process of vision begins
when light rays that reflect
off objects and travel
through the eye's optical
system are refracted and
focused into a point of sharp
focus.
For good vision, this focus
point must be on the retina,
where light-sensitive cells photoreceptors capture
images in much the same
way that film in a camera
does when exposed to light.
These images then are
transmitted through the
eye's optic nerve to the
brain for interpretation.
REFRACTIVE ERROR
An eye that has no refractive error when viewing
a distant object is said to have emmetropia or be
emmetropic.
 An eye that has a refractive error when viewing a
distant object is said to have ametropia or be
ametropic.

Very few people have refraction of exactly 0.0
diopters.
 Approximately 55% of persons between the ages
of 20 and 30 have refraction between + 1 and –1
diopters.
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REFRACTIVE ERROR
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1.
2.
3.
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The total refractive power of an emmetropic eye is
approximately 58 diopters (D), of which 43 D come from the
cornea and the remaining 15 D from the lens, aqueous, and
vitreous.
forms of refractive error:
Myopia (nearsightedness)
Hyperopia (farsightedness)
Astigmatism
refractive error - blurred or distorted vision
REFRACTIVE ERROR
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Causes of Refractive
Errors
The eye's ability to refract
or focus light sharply on
the retina primarily is
based on three eye
anatomy features:
1) the overall length of the
eye
2) the curvature of the
cornea and
3) the curvature of the lens
inside the eye
Eye Length:
 If the eye is too long,
light is focused before
it reaches the retina,
causing myopia.
 If the eye is too short,
light is not focused by
the time it reaches the
retina. This causes
hyperopia.

Curvature of the
Cornea:
 If the cornea is not
perfectly spherical,
then the image is
refracted or focused
irregularly to create
a condition called
astigmatism.
 A person can have
myopia or hyperopia
with or without
astigmatism.

Curvature of the
Lens:
 If the lens is too
steeply curved in
relation to the length
of the eye and the
curvature of the
cornea, this causes
myopia.
 If the lens is too flat,
the result is
hyperopia.

MYOPIA
Rays of light entering
the eye focus in front
of the retina
 The refractive power
of the eye is too strong
for the lenght of the
globe – refractive
myopia
 The eye is too long for
the refractive power –
axial myopia
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Epidemiology:
Approximately 25% of
persons between the
ages of 20 and 30 have
refraction less than –1
diopters.
MYOPIA
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Forms:
Simple myopia: Onset is
at the age of 10–12 years
Usually does not progress
after the age of 20
Refraction rarely exceeds 6
diopters
Pathologic ( progressive,
malignant )myopia:
This disorder is largely
hereditary and progresses
continuously
Overgrowth of the posterior
2/3 of the globe
Degeneration at the retinal
periphery
Special forms:
Lenticular myopia sclerosis of the nucleus of
the lens (cataract) in
advanced age (
Spherophakia (spherically
shaped lens).
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Treatment: minus or
concave lenses
HYPEROPIA
Rays of light entering
the eye focus behind
the retina
 The refractive power
of the eye is too week
for the lenght of the
globe – refractive
hyperopia
 The eye is too short
for the refractive
power – axial
hyperopia
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Epidemiology:
Approximately 20% of
persons between the
ages of 20 and 30
 have refraction
exceeding +1 diopters

HYPEROPIA
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Symptoms:
In young patients,
accommodation can
compensate for slight to
moderate hyperopia.
However, this leads to
chronic overuse of the
ciliary muscle. Reading in
particular can cause
asthenopic symptoms –
eye pain or headache,
blurred vision, ...
As accommodation
decreases with advancing
age, near vision becomes
difficult. For this reason,
hyperopic persons tend to
become presbyopic early.
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Special forms of
hyperopia:
Absence of the lens
(aphakia) due to dislocation.
Postoperative aphakia
following cataract surgery
without placement of an
intraocular lens
Treatment: plus or convex
lenses
Epidemiology:
 42% of all humans
have astigmatism
greater than or equal
to 0.5 diopters.
 In approximately 20%,
this astigmatism is
greater than 1 diopter
and requires optical
correction

ASTIGMATISM
Astigmatism is
derived from the
Greek word stigma
(point) and literally
means lack of a focal
point.
 parallel light rays do
not focus at a point
 The refracting power
in one axis is not the
same than that in an
axis perpendicular to
it
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ASTIGMATISM
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Classification
Simple myopic astigmatism – the
focus of one meridian is on the
retina, other is in front of the retina
Simple hyperopic astigmatismthe focus of one meridian is on the
retina, other is behind the retina
compound myopic astigmatism
– both meridians are focused in
front of the retina
compound hyperopic
astigmatism - both meridians are
focused behind the retina
mixed astigmatism – one is
focused in front and the other
behind the retina
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Regular - only two meridians
approximately perpendicular to
each other
Irregular - there are multiple focal
points
Causes
corneal ulcerations with resulting
scarring of the cornea
penetrating corneal trauma
advanced keratoconus
Cataract
Treatment: Cylinder lenses
Only regular astigmatism can be
corrected with eyeglasses
irregular astigmatism cannot be
corrected with eyeglasses
ACCOMODATION
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Ability of the eye to bring
retinal images of objects in
various distances into
sharp focus
Varying the reftactive
power of the lens
Lens is fixated with elastic
ligaments – the zonules –
at the ciliary muscle.
With contraction of ciliary
muscle, the zonules
relaxes and the lens takes
spherical shape
PRESBYOPIA
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The elasticity of the lens
decreases with increasing
age, and the range of
accommodation decreases
Presbyopia physiologic loss of
accommodation in
advancing age - begins
when the range of
accommodation falls below
3 diopters.
depending on age
presbyopia can be
compensated with
converging lenses of 0.5–3
diopters
ANISOMETROPIA
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difference in refractive
power between the two eyes
Where the difference in
refraction is greater than or
equal to 4 diopters, the size
difference of the two retinal
images becomes too great
for the brain to fuse the two
images into one
aniseikonia
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Epidemiology:
Anisometropia of at least 4
diopters is present in less
than 1% of the population
Symptoms:
Anisometropia is usually
congenital and often
asymptomatic.
binocular functions may
remain underdeveloped
when the correction of the
anisometropia results in
unacceptable aniseikonia,
patients will report
unpleasant visual
sensations of double vision
CORRECTION OF REFRACTIVE ERRORS

the type and degree of refractive error –
computerized automated refractometry
glasses,
 contact lenses,
 refractive surgery
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EYEGLASS LENSES
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Monofocal
Spherical lenses
Toric lenses (cylindrical
lenses) refract light only
along one axis.
Spherical and toric lenses
can be combined where
indicated
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Multifocal
different areas of the lens
have different refractive
powers
Bifocals: The upper
portion of the lens is for the
distance correction; the
lower portion is for the nearfield correction
Patients are able to view
distant objects and read
using one pair of eyeglasses
Progressive addition
lenses: continuously
increasing refractive power
CONTACT LENSES
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quality of the optical
image viewed through
contact lenses is higher
than that viewed
through eyeglasses
Contact lenses have
significantly less
influence on the size of
the retinal image
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The cornea requires
oxygen from the
precorneal tear film.
To ensure this supply,
contact lens materials
must be oxygenpermeable. This becomes
all the more important
the contact lens moves
and permits circulation
of tear fluid.
Contact lenses may be
manufactured from
rigid or flexible
materials.
RIGID CONTACT LENSES
These contact lenses
have a stable, nearly
unchanging shape.
 Patients take some
time to become used to
them
 Today, highly oxygenpermeable materials
such as silicone
copolymers are
available
 correct keratoconus
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SOFT CONTACT LENSES
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The material of the contact
lens, such as hydrogel is soft
Patients find these lenses
significantly more comfortable.
The oxygen permeability of the
material depends on its water
content, which may range from
36% to 85%
Supported by the limbus
The lens is displaced only a few
tenths of a millimeter when the
patient blinks. This greatly
reduces the circulation of tear
film under the lenses.
This requires to be removed at
night to allow regeneration of
the cornea
CONTACT LENSES
Contact lenses may also be
classified by wearing
schedule:
- daily wear contacts must
be removed, cleaned and
stored each night,
- while extended wear
contact lenses are made
from materials which are
safe for overnight wear
- sometimes "continuous
wear" is used for a type of
extended wear lens that
can be worn for up to 30
days
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Contacts can also be
described by
replacement interval
 Daily, weekly, biweekly, monthly or
quarterly
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SPECIAL LENSES
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Therapeutic contact
lenses: In the
presence of corneal
erosion, soft contact
lenses act as a
bandage and thereby
accelerate
reepithelialization of
the cornea. They also
reduce pain.
DISADVANTAGES OF CONTACT LENSES
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Contact lenses exert
mechanical and
metabolic influences on
the cornea.
Therefore, they require
the constant supervision
of an ophthalmologist.
Contact lenses require
careful daily cleaning
and disinfection.
This is more difficult,
time-consuming, and
more expensive than
eyeglass care
CONTACT LENS COMPLICATIONS
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Infectious keratitis (corneal
infiltrations and ulcers)
caused by bacteria,fungi,
and protozoans
Giant papillary
conjunctivitis: This is an
allergic reaction of the
palpebral conjunctiva of the
upper eyelid
Corneal vascularization
may be interpreted as the
result of insufficient supply
of oxygen to the cornea
Severe chronic
conjunctivitis: This usually
makes it impossible to
continue wearing contact
lenses
REFRACTIVE SURGERY
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is surgical (laser or
conventional) configuring of
the curvatures of the eye
(cornea and/or lens) to allow
the rays of light to be focused
on the retina as a point
Categories
Corneal refractive procedures
 Intraocular refractive
procedures
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REFRACTIVE SURGERY
Corneal refractive
procedures
Intraocular refractive
procedures
laser
Phacic intraocular lens
PRK, LASEK,
Epi LASIK
LASIK
other
incisional
keratotomy –
radial,
hexagoanal
intracorneal
ring
Clear lens extraction
REFRACTIVE SURGERY
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lower refractive errors
(in the range +4.00 to 10.00D) are usually
corrected using laser
techniques to reshape
the cornea
higher errors are
corrected using lens
implant based methods
astigmatism and age are
also influential in
determining the most
appropriate technique
LASER REFRACTIVE SURGERY
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an eye drop anesthetic is used
A portion of the cornea is removed, or
creating the flap
The eye is then positioned under an
Excimer laser which has been
programmed to remove microscopic
amounts of corneal tissue.
Removal of the tissue changes the
curvature of the cornea.
If the patient has myopia tissue closer to
the central part of the cornea is removed
to decrease the curvature or flatten the
cornea.
If a patient has hyperopia tissue in the
periphery of the cornea is removed to
increase curvature.
To correct for astigmatism, selected
tissue at certain angles is removed to
insure that the cornea curves equally in
all directions.
After the laser has been used, the flap is
returned to its original position.
LASER REFRACTIVE SURGERY
Profile of the cornea
after fotoablation for
MYOPIA
If the patient has myopia tissue
closer to the central part of the
cornea is removed to decrease
the curvature - flatten the
cornea
Profile of the cornea
after fotoablation for
HYPEROPIA
If a patient has hyperopia
tissue in the periphery of
the cornea is removed to
increase curvature
LASER REFRACTIVE SURGERY
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2.
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divided into
lamellar (LASIK) and
surface ( PRK, LASEK,
and Epi-LASIK)
laser-assisted in situ
keratomileusis (LASIK)
photorefractive
keratectomy (PRK)
laser assisted
subepithelial
keratectomy (LASEK)
PRK, LASEK,
AND
methods of surface
ablation
 these procedures do
not require a partial
thickness cut into the
stroma
 these methods differ
in the way the
epithelial layer is
handled
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EPI-LASIK
PHOTOREFRACTIVE KERATECTOMY
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In photorefractive
keratectomy surgery the
epithelium is removed
The epithelium might be
removed in several ways,
including excimer laser
destruction, mechanical
debridement with a surgical
blade, abrasion with a brush,
or use of alcohol to loosen the
epithelium.
(PRK)
LASER ASSISTED SUBEPITHELIAL
KERATECTOMY (LASEK)
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LASEK – epitelial flap
Epi-LASIK microkeratome
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In LASEK and EpiLASIK surgery the
epithelial flap is folded
back
after the ablation, this
epithelial flap is placed
back into its original
position
the epithelial alignment
is protected from blinks
and eye movements by
the addition of a
bandage contact lens
LASEK
The Advantages
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No stromal flap - much fewer
serious potential
complications ( Lamellar
Keratopathy, Interface
infections.)
Minimal or no induction of
optical aberrations
Quicker recovery of corneal
sensation and the blink
reflex (4 months vs. 1 1/2
years with LASIK)
Higher amounts of refractive
errors can be corrected
The Disadvantages
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More discomfort
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Recovery time is longer
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final results for LASIK and
surface treatments are
similar
LASER-ASSISTED IN SITU KERATOMILEUSIS
(LASIK)
a partial-thickness
corneal flap
 made with a
microkeratome
 depths of 100–200 µm
 femtosecond laser has
been developed,
provides more
accuracy in flap
thickness

LASIK
The Advantages
The Disadvantages
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Rapid visual recovery
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Less postoperative discomfort
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Induces more optical
aberrations
Induces more complications
If there is a complication, it is
usually due to the flap
Only used for mild to
moderate myopia
Much longer time for recovery
of blink reflex when the eye is
dry
COMPLICATIONS
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Corneal scarring/haze (<1-2%)
Although LASEK may carry a decreased rate of corneal haze
relative to PRK, it may still develop secondary to an
inflammatory response to the surgical manipulation of the
corneal surface.
 The inflammation leads to the formation of an opacified
cellular layer that appears as a white haze and restricts light
from transmitting to the back of the eye, thus causing a defect
in vision
 The risk of scar formation increases with increasing ablation
depth, and scars are common when treating more than 8 D of
myopia.
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Keratitis (0.5-1%)
Postoperative infection is more likely when epithelial coverage
is incomplete or when the surgical duration is longer than
average.
 Additionally, contact lenses may serve as a source of infection,
as they may be contaminated with microorganisms. Likely,
because contact lenses are not used postoperatively in LASIK,
LASIK has a lower incidence of keratitis (about 0.2%).
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COMPLICATIONS
Corneal scarring/haze
Keratitis
COMPLICATIONS
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Dry eye syndrome associated with recurrent erosions
 This complication is secondary to decreased corneal
sensation due to corneal denervation. It may last from a
few weeks to 1 year, although, on average, it lasts 1-4
weeks.
 Although this complication occur in LASEK and LASIK, it
is more likely to be associated with a longer duration in
LASIK.
Overcorrection (1%, incidence similar to LASIK and PRK)
Undercorrection (10-15%, incidence similar to LASIK and
PRK)
Macular cyst formation (<0.1%)
Irregular astigmatism (<1%): This complication is
secondary to decentration of the laser optical zone or uneven
healing, leading typically to a wavy corneal surface.
INTRAOCULAR REFRACTIVE PROCEDURES

Clear lens
extraction

range:
myopia/hypermetropia at
any level
Principle = replacement of
the natural lens with an
intraocular lens (IOL)
multifocal IOLs can be
implanted to reduce
spectacle dependence for
near vision
is identical to modern
cataract surgery
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INTRAOCULAR REFRACTIVE PROCEDURES
 Phacic
intraocular lens
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implantation - range: up to 17D myopia; up to +10D
hypermetropia
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Principle = implantation of
a soft flexible artificial lens
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is particularly suited to
younger patients who are
out of range for laser
refractive surgery
INDICATIONS

The major indications for
refractive surgery include
astigmatism, myopia, and
hyperopia, specifically in
patients who are intolerant of
or who desire to be free from
glasses or contact lenses
CONTRAINDICATIONS
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Unstable refractive error
Refractive error outside the
range of correction (The range
varies according to the
surgeon's experience, the
laser used, and the laser
strategy; however, it is
typically approximately 9-14
D of myopia, 4-6 D of
hyperopia, and 2-6 D of
astigmatism.)
Keratoconus
Pellucid marginal
degeneration
Significant dry eye syndrome
Active inflammation of
external eye
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Autoimmune disease
History of or active herpes
simplex keratitis, risc of the
reactivation of the virus
Active collagen vascular
disease
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Uncontrolled diabetes
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Uncontrolled glaucoma
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Pregnancy or breastfeeding
QUESTIONS AND DISCUSSION
THANK YOU FOR YOUR ATTENTION !