New Refractive Errors and management · 2019. 7. 5. · • Spectacle lenses • Monofocal lenses :...
Transcript of New Refractive Errors and management · 2019. 7. 5. · • Spectacle lenses • Monofocal lenses :...
Refractive Errors
and management
Vergence
Parallel Beam Convergence Divergence
VERGENCE
• All naturally occurring sources of light are
divergent
• Light rays traveling parallel have zero
vergence
• Light rays that focus on a point are
convergent
• The unit of measurement of vergence is
the diopter
D= Vergence (Diopters)=___________1_____________
Distance from the source in meters
BASIC OPTICS
• Therefore:
• The closer the light is to its source, the
greater the vergence
• The farther the light is from its source, the
lower the vergence, approaching zero as
distance goes toward infinity.
• Diverging light has negative power (-)
• Converging light has positive power (+)
• Lens power is calculated by : using the
biometry by measuring the axis length and
the corneal convexity.
REFRACTION
• Refraction of light occurs when light
passes from one medium to another of
different refractive index (ie. density)
Refractive Components of the Eye
• Cornea: responsible for the majority of the
refractive power of the eye (40 D) /
constant.
• Lens: 20 D of refractive power, changes
with accommodation.
• Axial length is constant except under
certain conditions
•
• These two refracting elements in the eye
converge the rays of light because:
1-The cornea has a higher refractive index
than air; the lens has a higher refractive
index than the aqueous and vitreous
humours that surround it.
2-The refracting surfaces of the cornea and
lens are spherically convex
Fovea Light rays
Emmetropia (normal vision)
• Adequate correlation between axial length
and refractive power.
• Parallel rays of light from a distant object
are brought to focus on the retina with the
eye at rest (no accommodation) ,such an
individual can see sharply in the distance
without accommodation.
Ametropia (Refractive error)
• Mismatch between axial length and
refractive power.
• Parallel light rays don’t fall on the retina
with the eye at rest (a change in refraction
is required to achieve sharp vision).
• Ametropia may be divided into:
• Nearsightedness (Myopia)
• Farsightedness (Hyperopia)
• Astigmatism
• Presbyopia
Accommodation
• Emmetropic eye
• object closer than 6 M send divergent light
that focus behind retina , adaptative
mechanism of eye is to increase refractive
power by accommodation
• Helm-holtz theory
• contraction of ciliary muscle -->decrease
tension in zonule fibers -->elasticity of lens
capsule mold lens into spherical shape --
>greater dioptic power -->divergent rays are
focused on retina
• contraction of ciliary muscle is supplied by
parasympathetic third nerve.
Myopia
• Parallel rays converge at a focal point
anterior to the retina
• Etiology : not clear , genetic factor
• Causes
• excessive long globe (axial myopia) :
more common
• excessive refractive power (refractive
myopia).
• Increase in the curvature of the cornea
or the surfaces of the crystalline lens.
Uncorrected, light focuses in front of fovea
Corrected by divergent lens, light focuses on fovea
Myopia
The patient is able to focus on objects near but not far away (blurred distance vision).
Typical complaint is difficulty focusing on road signs or the black board.
Usually detected by the young when they discover they cannot see things at a distance as well as their friends do .
The teacher complains that the child makes too many mistakes copying things from the black-board.
The lens is unable to flatten enough to prevent conversion of images before reaching the retina.
Frequently squinting is compensatory mechanism in an attempt to improve uncorrected visual acuity when gazing into the distance.
MYOPIA
• Increases with age roughly until the person stops
growing in height.
• A myopic person can still see some objects clearly,
provided the object is closer than the far point
• Pseudomyopia: accomodative spasm.The patient
cannot relax accomodation when looking in the
distance. For example, an over anxious student
Myopia
• Forms
• Benign myopia (school age myopia): Benign myopia
usually starts in school, develops during adolescence
period and finally stabilizes in the adulthood. It rarely
grows beyond -7.0 D and it is never associated with
any pathologic changes on the eye fundus.
• Progressive and malignant myopia
• interchangeable
• myopia increase rapidly each year
and is associated with vitreous
opacities , fluidity of vitreous and
chorioretinal change.
Myopia
• Congenital myopia
• Myopia > 10 D
• Increase slowly each year
Special forms cataract nuclear sclerosis , keratoconus , spherophakia
Myopia
• Symptoms • Typically do not have “eye-strain”, “watering” of the eyes or
headaches as often as hypermetropes do
• Usually detected by the young when they discover they cannot see things at a distance as well as their friends do
• The teacher complains that the child makes too many mistakes copying things from the black-board
• Blurred distance vision
• Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
• Headache
• Amblyopia – uncorrected myopia > 10 D
Myopia
• Morphologic changes
• deep anterior chamber
• atrophy of ciliary muscle
• vitreous may collapse prematurely -->
opacification
• fundus change : loss of pigment in RPE
,large disc and white crescent-shaped area
on temporal side ,RPE atrophy in macular
area , posterior staphyloma ,retinal
degeneration-->hole-->increase risk of
RD
• Treatment : concave lenses, clear lens
extraction
• *Staphyloma : depression on retina.
PATHOLOGICAL CAUSES OF
MYOPIA
• KERATOCONUS
PATHOLOGICAL CAUSES OF
MYOPIA
* Nuclear Cataract *Diabetes
PATHOLOGICAL CAUSES OF
MYOPIA
Marfan’s Staphyloma
Complication of myopia
1-RD
2-Macular degeneration.
3-Cataract
4-Open angle glaucoma.
Hyperopia
• Parallel rays converge at a focal point
posterior to the retina
• Etiology : not clear , inherited
• Causes: • excessive short globe (axial hyperopia) : more
common
• insufficient refractive power (refractive hyperopia)
• Hyperopia forms a stage in normal development of
the eyes—at birth eyes are hypermetropic (2.5 to 3.0
Diopters).
When persists in adulthood it represents an imperfectly
developed eye.
• Lens changes (cortical cataract).
• Special forms : lens dislocation , postoperative aphakia
HYPEROPIA
• • hyperopic persons must accommodate
when gazing into distance to bring focal
point on to the retina more than normal
people .
• Hyperopia may be partially compensated
for by using the eyes’ accommodative
ability this is possible during the first two to
three decades of life
• • When accommodative ability cannot
keep up with demand, hyperopia is
manifest and images are blurred in the
distance and for near and the need for
reading glasses arises earlier than in the
normal person.
• Typical complaint is difficulty reading.
Hyperopia
• Symptoms
• Typical complaint is difficulty reading
• visual acuity at near tends to blur relatively early
• nature of blur is vary from inability to read fine print to near
vision is clear but suddenly and intermittently blur
• blurred vision is more noticeable if person is tired , printing
or inadequate light .
• Extremely good distant vision (6/4)
• asthenopic symptoms : eyepain, headache in frontal region,
burning sensation in the eyes, blepharoconjunctivitis.
• Young children with significant hypermetropia can also
develop a convergent squint.
• accommodative esotropia : because accommodation is linked
to convergence -->ET
• Amblyopia – uncorrected hyperopia > 5D
Hyperopia
• Fundus in axial hyperopia may reveal
pseudooptic neuritis (indistinct disc margin, no
physiologic cup, maybe elevated disc)
• DDx from optic neuritis by :>=4D, no enlarged
blind spot, no passive congestion of vein
• Treatment : convex lenses, keratorefractive
surgery, refreactive lensectomy with IOL, phakic
IOL
Uncorrected, light focuses behind fovea
Corrected by convergent lens, light focuses on fovea
PATHOLOGICAL CAUSES OF
HYPEROPIA
DISLOCATED LENS
PATHOLOGICAL CAUSES OF HYPEROPIA
RETINAL DETACHMENT
1 mm = 3D
RETINAL FLUID
CHOROIDAL TUMOR
Complication .
They are more susceptible to closed angle glaucoma
because there smaller eyes are more likely to have
shallow A.C and narrow angles.
Astigmatism
• Parallel rays come to focus in 2 focal lines rather than a
single focal point
• Etiology : hereditary
• Cause : Astigmatism present when the cornea
curvature is irregular (Refractive power of the cornea in
different planes is not equal).
In short, astigmatism occurs whenever the vertical
curvature of the cornea is different from the
horizontal.
Astigmatism
• Classification
• Regular astigmatism : power and orientation of
principle meridians are constant
• With the rule astigmatism , Against the rule
astigmatism , Oblique astigmatism
• Simple or Compound myopic astigmatism ,
Simple or Compound hyperopic astigmatism ,
Mixed astigmatism
• Irregular astigmatism : power and orientation of
principle meridians change across the pupil.
• Keratoconus is pathological cause of
irregular astigmatism
• Simple astigmatisum : one focal point on
retina other in front
• Compound astigmatism : 2 points in front
or 2 behind
• Hypermetropic : 2 points behind retina.
• Mixed : 1 point in front other behind.
• With rule astigmatism : when the steeper
meridian is vertical.
• Against rule : when steeper meridian is
horizontal .
Astigmatism
• Symptoms
• asthenopic symptoms ( headache ,
eyepain)
• blurred vision
• distortion of vision
• head tilting and turning
• Amblyopia – uncorrected astigmatism >
1.5 D
• Treatment
• Regular astigmatism :cylinder lenses with
or without spherical lenses(convex or
concave), Sx
• Irregular astigmatism : rigid CL , surgery
Treatment of astigmatism
• 1. Laser, Intraocular toric lenses, Limbal
relaxing incision
• Contraindicated to do LASIK as it needs
good thickness of cornea and regular
curvature .
Pathologic Causes of
Astigmatism
• Corneal: post surgical, traumatic,
infectious
• External pressure on cornea: lid masses
• Lens: pressure on lens from tumors
Presbyopia
• Physiologic loss of accommodation in advancing age due to age related loss of elasticity of the capsule.
• deposit of insoluble proteins in lens in advancing age-->elasticity of lens progressively decrease-->decrease accommodation.
• Around 45 years of age , accommodation become less than 3 D-->reading is possible at 40-50 cm-->difficultly reading fine print , headache , visual fatigue.
• This occurs earlier in hypermetropes than myopes.
Presbyopia
• Treatment
• convex lenses in near vision
• Reading glasses
• Bifocal glasses
• Trifocal glasses
• Progressive power glasses
Anisometropia
• Difference in refractive power between 2 eyes
• refractive correction often leads to different
image sizes on the 2 retinas( aniseikonia)
• aniseikonia depend on degree of refractive
anomaly and type of correction
• closer to the site of refraction deficit the
correction is made-->less retinal image changes
in size ??
Anisometropia
• Glasses : magnified or minified 2% per 1 D
• Contact lens : change in size less than glasses
• Tolerate aniseikonia ~ 5-8%
• Symptoms : usually congenital and often
asymptomatic
• Treatment
• anisometropia > 4D-->contact lens
• unilateral aphakia-->contact lens or
intraocular lens
Diagnosis OF refractive errors :
not required
•Diagnosis of refractive errors is made by an
optician or ophthalmologist.
•Instruments used to diagnose refractive errors
include:
- pinhole glasses
- Autorefractor (measures how light changes as
it enters the eye).
-Retinoscope (measures the refractive
condition of the eye).
- Phoropter.
Pinhole glasses Autorefractor
View through an autorefractor Phoropter
Pinhole Test
• Light rays that are
perpendicular to the lens
do not refract , but go
straight forward.
• If visual acuity improves
with the pinhole test, then
this is a refractive error; if
it worsens, then this is a
central field defect.
Types of optical correction • Spectacle lenses
• Monofocal lenses : spherical lenses ,
cylindrical lenses
• Multifocal lenses
• Contact lenses:
• higher quality of optical image and less
influence on the size of retinal image than
spectacle lenses
• indication : cosmetic , athletic activities ,
occupational , irregular corneal
astigmatism , high anisometropia ,
corneal disease.
• Can be: soft, hard, gas-permeable,toric
• Contact lenses
• disadvantages : careful daily cleaning and
disinfection , expense
• complication : infectious keratitis , giant
papillary conjunctivitis , corneal
vascularization , severe chronic
conjunctivitis
• Intraocular lenses
• replacement of cataract crystalline lens
• give best optical correction for aphakia , avoid
significant magnification and distortion caused
by spectacle lenses
Hard lenses:
• Plastic polymer.
• Most durable.
• Rigid, therefore may scratch the cornea.
• Not gas permeable corneal hypoxia corneal
ulcers.
• Cannot be worn continuously.
• Difficult to get used to (because they are very rigid).
• Less susceptible to infection and allergy.
• Best for treating astigmatism (smoothes out the
uneven curvature).
Hard contact lenses are used in orthokeratology: the
fitting of a series of rigid contact lenses to reshape the
cornea. It is an alternative to glasses and surgery
Soft lenses:
* High water content.
* Less durable.
* Permeable for both gases + liquids.
* Could be worn for long periods.
* Tolerated much better.
* They do correct astigmatism.(MILD ONE )
* Are the most comfortable lenses.
* Are the least durable lenses (must be replaced
more often).
•Susceptible to accumulation of deposits (because
they absorb more water, which binds proteins). This
accumulation of protein deposits leads to allergic
conjunctivitis and other allergies.
Gas permeable lenses
(Semi-rigid):
•They are permeable only to gases.
•They are more comfortable than hard
lenses, but less than soft ones.
•They are more durable than soft lenses, but
less than hard lenses.
•Allow oxygen to pass, but also allow
proteins to deposit.
Toric lenses:
Are similar to soft contact lenses, but
have a couple of extra characteristics:
•They have 2 powers in them: 1 for
spherical correction + 1 for astigmatism.
•They are designed to keep the lens in a
stable position even on movement.
They offer the comfort of soft lenses ,and at
the same time they correct astigmatism, but
their disadvantage is that the lens will
sometimes rotate, and this creates a very
irritating change in vision as the lens rotates.
Contraindications to using
contact lenses:
* History of atopy.
* Dry eyes.
* Previous glaucoma filtration
surgery.
* Inability to handle and cope
with the lenses.
Refractive Surgery Techniques
• Radial Keratotomy (RK) not used anymore.
• Freeze keratomileusis not used anymore.
• Photorefractive Keratectomy (PRK)
• Laser Epithelial Keratomileusis (LASEK)
• Laser-assisted in-situ Keratomileusis (LASIK)
• Others: • Astigmatic Keratotomy (AK)
• Intracorneal ring segments (Intacs)
• Phakic Intraocular Lens Implants
• Refractive lensectomy
Surgery classified into :
• Flab : LASIK > and doing a flap needs
good thickness of cornea and regular
surface {so not made for keratoconus nor
astigmatism} and being not susceptible to
trauma after surgery unlike boxer or
solider.
• Surface ablation : SPRK (LASER) ,
LASEK > here we remove part of the
regenerative epithelium .
• LASIK has faster(2-3 days) post op
recovery than Surface ablation(5-7days) .
Laser technology
• Excimer laser: EXCited dIMER
• AKA “cool laser beam” because little thermal damage to adjacent tissues.
• 193nm wavelength ultraviolet laser with sufficient energy to disrupt intermolecular bonds within the corneal stromal tissue (photoablative decomposition).
• First excimer lasers FDA approved in 1995, with beam width 4-5mm, now available less than 100 microns.
• Each laser pulse removes a given volume of stroma
• Three types of laser application: wide-area ablation, scanning slit, and flying spot lasers.
Laser technology
• In myopia, laser flattens central cornea to decrease its focusing power to bring secondary focal point back to retina.
• In hyperopia, the laser removes peripheral corneal tissue thereby secondarily steepening the central cornea, increasing the focusing power of the cornea.
• Astigmatism treated with elliptical or cylindrical beams that flatten the steepest corneal meridian.
• To minimize glare and halos, optical zone should be larger than the dilated pupil.
Myopic photorefractive keratectomy
PRK is recommended for some patients
who cannot undergo LASIK. These
include:
Patients whose corneas are too thin to
have LASIK &Patients whose corneas are
scarred (from infection or trauma).
• PRK can effectively treat low to mod myopia or hyperopia +/- astigmatism.
• Performed as outpt with topical anesthesia.
• First, the corneal epithelium in the area to be ablated is removed to expose Bowman’s layer and the underlying corneal stroma (spatula, laser).
• Excimer laser then applied as directed by the corneal topography-driven computer program.
• Topical antibiotics, steroids, and NSAIDs applied, along with a bandage contact lens (BCTL)
PRK • In the post-op period, pt may experience tearing,
photophobia, blurred vision, and discomfort due to abrasion of central epithelium.
• This can be controlled with topical steroids and NSAIDs.
• Pts occ. require systemic analgesia for severe pain
• BCTL removed once epithelial defect healed (avg 3-4 days).
• Abx continued several more days, and steroids for up to 3 months post-op.
• Visual acuity improves once the epithelial defect heals, but fluctuates for a few months and finally stabilizes at ~3 months.
• Glare, halos, and dry eye symptoms common in the first month post-op, usually diminishing/disappearing by 3-6 months.
Laser Sub-Epithelial
Keratomileusis
• LASEK can treat mild to moderate myopia and hyperopia +/- astigmatism.
• Can be performed as an outpt with topical anesthesia
• The corneal epithelium is incompletely incised using a microkeratome with a 70 micron deep blade.
• A hinge is left at the 12 o’clock position.
• Dilute alcohol (20%) drops are applied to the exposed tissue and left for ~30 seconds. The area is then washed with water and allowed to dry. The excimer laser is applied as in PRK to the sub-epithelial stroma.
• The epithelial flap is repositioned afterward.
LASIK
• The use of the suction ring helps hold the cornea steady and provides for a uniform cut by the microkeratome.
• Flaps can be formed by an automated process involving a blade guide on the suction ring to guide a turbine-driven microkeratome, producing a very smooth, regular cut
• Patients usually sent home on topical antibiotics, steroids, and NSAID drops
• Benefits include little pain, quick recovery of vision, and potential to treat higher levels of myopia. LASIK enhancements are also easily performed.
LASIK
LASIK Complications
• Potential complications: • Intra-operative flap complications: microkeratome
complication with a higher rate with surgeon inexperience
• Post-operative flap complications
• Flap-bed interface epithelialization: that epithelial growth at the interface could significantly be reduced by irrigating the stromal surfaces and using a BCTL for one day.
• Irregular astigmatism
• Infection:
• Diffuse lamellar keratitis (DLK): (AKA Sands of Sahara syndrome) Wavy inflammatory reaction at LASIK flap interface 1-3 days post-op of unknown cause. Treatment involved high-dose topical steroids or lifting the flap to irrigating the interface.
• Progressive corneal ectasia: progressive corneal thinning and steepening with worsening irreg. astigmatism thought to result from too thin a stromal bed after LASIK. Most believe stromal bed thickness should be at least 250 microns.
Implantable Contact Lenses:
-Between iris + lens.
-Preserves accommodation.
-Complications include:
* Over correction.
* Under correction.
* Infection.
* Increased intraocular
pressure.
Clear lens extraction + IOL:
•IOL: Intra ocular lens.
•Same as cataract extraction.
•Implantation of artificial lens.
•Lose accommodation (patient will need reading
glasses).
2 contraindications of refractive eye
surgery
Keratoconus. Thin cornea.
Pregnant. SLE, RA. Autoimmune
disorders. Glaucoma. Diabetes.
• Glasses:
• Advg > comfortable , no risk for infection, or
dryness or allergy.
• Disadvg > cosmatic , poor quality of vision
esp in high degree , not convenient for
some jobs.
• Contact lens :
• Advg : cosmatic , good quality of vision ,
more convenient .
• Disadvg : infection , allergy , dryness.