Refractive Errorsand management

Ahmed AlMumtin, MD

Basic Optics

• Optics is the properties of light as it is acted on by optical systems

• Geometric optics: uses line diagrams to depict the behavior of light

• Wavefront optics: uses principles of waves to describe the behavior of light

Vergence

Prallel 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 (+)

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)• Lens: 20 D of refractive power, changes

with accommodation• Axial length

REFRACTIVE PHYSIOLOGY

• Light rays are focused on the retina because they are refracted by passing through the cornea and lens (Snell’s Law)

• Corneal refractive power is constant• Lens refractive power is modifiable with

accommodation• Axial length of the eye is constant except

under certain conditions

FoveaLight rays

Emmetropia

• Adequate correlation between axial length and refractive power

• Parallel light rays fall on the retina (no accommodation)

Ametropia (Refractive error)

• Mismatch between axial length and refractive power.

• Parallel light rays don’t fall on the retina (no accommodation)• 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 increase refractive power by accommodation

• Helm-holtz theory• contraction of ciliary muscle -->decrease tensio

n 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

• Forms• Benign myopia (school age myopia)

• onset 10-12 years , myopia increase until the child stops growing in height

• Progressive and malignant myopia• interchangeable • myopia increase rapidly each year and i

s associated with vitreous opacities , fluidity of vitreous and chorioretinal change

• rate of increase in amount of myopia generally tapers off at about 20 years of age

Myopia

• Congenital myopia• Myopia > 10 D• Increase slowly each year

Myopia• Special forms : nuclear sclerosis , keratoconus , s

pherophakia• 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

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• For a -2 D myope, the far point is 0.5 meters

(D=1/distance in meters), so any objects inside 0.5 m are clear as long as they are not too close at which point clarity may be limited by accommodation.

• Pseudomyopia: accomodative spasm.The patient cannot relax accomodation when looking in the distance. For example, an over anxious student

PATHOLOGICAL CAUSES OF MYOPIA

• KERATOCONUS

PATHOLOGICAL CAUSES OF MYOPIA

* Cataract * Diabetes

PATHOLOGICAL CAUSES OF MYOPIA

Marfan’s Staphyloma

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)

• The length of the eyeball is shorter than it should be

• 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 (cataract).

Hyperopia

• Special forms : lens dislocation , postoperative aphakia

• hyperopic persons must accommodate when gazing into distance to bring focal point on to the retina

• Symptoms• distance vision is impaired in high refractive error( > 3 D) a

nd in older patient• “Eye-strain”(ciliary muscle is straining to maintain

accommodation) / “watering” / “redness”• Headaches in later part of the day • time• Young children with significant hypermetropia can also

develop a convergent squint

HYPEROPIA

• Hyperopia may be partially compensated for by using the eyes’ accommodative ability

• When accommodative ability cannot keep up with demand hyperopia is manifest and images are blurred in the distance and for near

Uncorrected, light focuses behind fovea

Corrected by convergent lens, light focuses on fovea

Hyperopia

• Symptoms• 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 is weak or light inadequate

• asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis

• 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, may elevate disc) • DDx from optic neuritis by > 4 D , no enlarged

blind spot, no passive congestion of vein• Treatment : convex lenses, keratorefractive

surgery, refreactive lensectomy with IOL, phakic IOL

PATHOLOGICAL CAUSES OF HYPEROPIA

DISLOCATED LENS

PATHOLOGICAL CAUSES OF HYPEROPIA

RETINAL DETACHMENT

1 mm = 3D RETINAL FLUID

CHOROIDAL TUMOR

Astigmatism

• Parallel rays come to focus in 2 focal lines rather than a single focal point

• Etiology : heredity • Cause : refractive media is not spherical-->refract

differently along one meridian than along meridian perpendicular to it-->2 focal points ( punctiform object is represent as 2 sharply defined lines)

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

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 sp

herical lenses(convex or concave), Sx• Irregular astigmatism : rigid CL , surgery

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 advanci

ng age• deposit of insoluble proteins in lens in advanci

ng 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

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 imag

e sizes on the 2 retinas( aniseikonia)• aniseikonia depend on degree of refractive anom

aly and type of correction• closer to the site of refraction deficit the correctio

n is made-->less retinal image changes in size

Anisometropia

• Glasses : magnified or minified 2% per 1 D• Contact lens : change less than glasses• Tolerate aniseikonia ~ 5-8%• Symptoms : usually congenital and often asympt

omatic• Treatment

• anisometropia > 4 D-->contact lens• unilateral aphakia-->contact lens or intraocula

r lens

Correction of refractive errors

• Far point• point on the visual axis conjugate to the re

tina when accommodation is completely relaxed

• placing an object or imaging an object at far point will cause a clear image of that object to be relayed to the retina

• use correcting lenses to form an image of infinity at the far point , correcting the eye for distance

What is the Far Point

• The farthest an eye can see

• An emmetrope it is infinity or 20 feet

• 4.00D myope without glasses it is 25cm in front of the eye

• 1.00D myope without glasses it is 1meter in front of the eye

• 3.00D hyperope without glasses 33.3 cm behind the eye

What is the Near Point

• The closest the eye can see

• The near point is the sum of the power of the eye and accommodation

• An emmetrope with 10.00D of accommodation and a -4.00D error without glasses has a near point of 14.00D or 7.15cm in front of the eye

What is the Near Point

• A 50 year old with 2.00D of accommodation and a -1.00D error without glasses has a far point of 1 meter and a near point of 33.3 cm

• With glasses on the far point is infinity or 20 feet and the near point is 50cm in front of the eye

What is the Near Point

• A +3.00D without glasses hyperope with 6.00D of accommodation has a far point of 33.3 cm behind the eye and a far point of 33.3 cm in front of the eye.

• With glasses on the far point is infinity and the near point 16.6 cm.

Remember!!

• A myope has too much plus in their eyes

• Whereas a hyperope has too much minus

Diagnosis:

•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 autorefractorPhoropter

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

• Contact lenses• disadvantages : careful daily cleaning and disi

nfection , expense• complication : infectious keratitis , giant papilla

ry 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 + allergy.• Best for treating astigmatism (smoothes out the uneven

curvature).

Soft lenses:

* High water content.* Less durable.* Permeable for both gases + liquids.* Could be worn for long periods.* Tolerated much better.* They do not correct astigmatism.* 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. •More durable than soft lenses, but less durable than hard ones.

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.

Refractive Surgery Techniques

• Radial Keratotomy (RK)• Freeze keratomileusis• 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

• First used in U.S in 1978.• Treats low to mod myopia in outpt

setting using topical anesthetics.• RK reduces myopia by steepening

the cornea peripherally, which secondarily flattens the cornea centrally.

• The surgeon makes deep radial incisions with a diamond blade in a spoke-like pattern, leaving a clear optical zone in the center.

• Refractive effect determined by the number, length, and depth of the incisions, as well as the size of the spared central optical zone.

• The smaller the optical zone, the greater the central corneal flattening (reduction in myopia), but greater risk of side-effects.

Radial Keratotomy

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 kertectomy

• 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 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

• LASIK can treat mild, moderate, and high myopia and hyperopia +/- astigmatism.

• Can also be performed as an outpt with topical anesthesia• LASIK is now the most commonly performed refractive

surgery in the world.• A suction ring is applied to the anesthetized cornea and a

microkeratome is used to raise a corneal flap of ~160microns thickness (25-30% of the corneal thickness), hinged at the 12 o’clock position.

• The suction is turned off and the flap is lifted aside, exposing stromal tissue

• The excimer laser is applied as with PRK and LASEK, controlled by the topography-driven computer software, to reshape the cornea.

• The flap is replaced on the stromal bed without sutures or a BCTL, as the endothelial pumps create a driving force to keep the flap in position.

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. Pt is usually seen ~POD 1, and 7, then at 1, 3 and 6 months.

• 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.