Ophtha - Eor And Lens

ERROR OF REFRACTION

Outline I. Introduction A. Optics B. Refraction C. Emmetropia and Ametropia II. General Symptomatology III. Errors of Refraction A. Hyperopia B. Myopia C. Astigmatism D. EOR secondary to Aging E. Congenital EOR F. Iatrogenic EOR IV. Measurement of EOR V. Management

OPTICS

LENS - a device which causes light to either converge and concentrate or to diverge, usually formed from a piece of shaped glass

OPTICS

POSITIVE OR CONVERGING LENS

If the lens is biconvex or plano-convex, a collimated or parallel beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens.

NEGATIVE OR DIVERGING LENS If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread) The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens

Diopter - is the standard unit to express the refractive power of optical lenses. It is the reciprocal of the distance, expressed in meters, between a lens and its focus (its focal length)

D =

1 f

D: diopter meters)

f: focal length (in

OPTICS D = 1/f

Example:

Focal length = 1 meterFocal length = 0.5 meter Focal length = 0.25 meter D = 1/1 D = 1/0.5 D = 1/0.25 D = 1 diopter D = 2 diopters D = 4 diopters

OPTICS MAIN OPTICAL COMPONENTS OF THE EYE: •

Cornea 43 D

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Lens

17 D

The anterior surface of the cornea is the major refractive surface of the eye

DEFINITION OF TERMS:

Emmetropia •

optical condition in which an eye does not have an error of refraction

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Light from a distant object is focused on the retina.

AMETROPIA optical condition in which parallel rays of light from an object an infinite distance from the eye does not come to focus (form an image) exactly at the retina Three general types: HYPEROPIA, MYOPIA AND ASTIGMATISM

Ammetropia Axial ametropia - Abnormal length of the globe Curvature Ametropia - Abnormal curvature of the cornea or the lens Index Ametropia - Abnormal refractive indices of the media Abnormal position of the lens

GENERAL SYMPTOMATOLOGY  Decreased visual acuity Pin hole test  Ocular discomfort  increased sensitivity to light, decreased efficiency, various aches and fatigue  Headache

HYPEROPIA •

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Farsightedness The refractive power of the cornea and lens are weak, or a relatively short eyeball light from distant objects focus behind the retina Axial Hyperopia - the eyeball is shorter than average

Hyperopia •

Accomodation increases the refractive power of the lens and may compensate for hyperopia Headache – due to excessive sustained accomodation required for clear vision

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Corrected by: Convex lenses

Hyperopia

MYOPIA •

Nearsightedness

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The refractive power of the cornea and lens are too strong, or a relatively long eyeball light from distant objects focus in front of the retina Axial myopia - The eyeball is longer than average

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Curvature Myopia – due to abnormal curvature of the cornea (Keratoconus)

MYOPIA

Corrected by: Concave lenses

Astigmatism 

Abnormalities in corneal shape A condition wherein the light rays entering the eye focus on two or more separate lines instead of one point This happens when the refractive power is not uniform in all meridians Principal symptom: inability to have clear image Usually accompanied by myopia or hyperopia

TREATMENT: Cylindrical Lens

TYPES OF REGULAR ASTIGMATISM 1. Compound myopic Both anterior and posterior focal lines are in front of the retina 2. Simple Myopic Anterior focal line is in front of the retina while posterior focal line is on the retina 3. Mixed Anterior focal line is in front of retina while posterior focal line is at the back of the retina 4. Simple Hyperopic Anterior focal line is on the retina while posterior focal line is behind the retina 5. Compound hyperopic Both focal lines are behind the retina

1. Compound myopic

4. Simple Hyperopic

2. Simple Myopic 5. Compound hyperopic

3. Mixed

ERRORS OF REFRACTION 2O TO AGING Accomodation -

As object comes closer to the eye, lens increases its power by altering its shape to become more convex

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Accommodation = oculomotor nerve stimulation (parasympathetic fibers) Presbyopia - manifest in 5th decade - overcome by convex lens (Reading glasses)

PRESBYOPIA •

loss of accommodation due to aging

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Inability to read small print Due to a reduction in the deformability of the lens Corrected by: convex lenses (reading glasses), bifocals or progressive lenses

ERRORS OF REFRACTION 2O TO AGING •

“Second Sight” – Myopic shift ↑ density of lens nucleus (nuclear sclerosis) •

↑ refractive power → myopia

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distant vision → blurred and out of focus near vision→ improves

Anisometropia •

difference in refractive power between the two eyes major cause of amblyopia due to failure of both eyes to accommodate independently

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cause complications in refractive correction due to: differences in size of retinal images (aniseikonia)

CONGENITAL ERRORS OF REFRACTION 

Hyperopic at birth (+1.50 D) Corneal curvature is much steeper (6.59 mm radius) at birth and flattens to adult curvature (7.71 mm) by about 1 y/o Cornea and lens grow and eyes elongate with child growth thus, HYPEROPIA --> EMMETROPIA OR MYOPIA

IATROGENIC ERRORS OF REFRACTION 

Cataract Surgery: a. Tension in suture astigmatism •

Reduced by suture removal

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Induced astigmatism may persist

b. Removal of lens •

Artificial lens replacement

MEASUREMENT OF EOR 1. Subjective method - utilizes the ability of the individual to choose the lens that gives him the best image - a set of trial lenses is usually used

2. Objective method Retinoscopy • makes use of an instrument (retinoscope) that catches the rays of light reflected at the patient’s retina whose source comes from a mirror near the examiner’s eye • useful in children, the low intelligent, semiconscious or unconscious

MEASUREMENT OF EOR 3. Cyclopegic Refraction  drugs that paralyze accommodation are used to measure accurately the refractive error the individual has by the objective method  usually performed in children

4. Keratometry  measurement of the corneal astigmatism  the curvature of the cornea is measured in various meridians by reflected light on the cornea coming from plates with various transparent configurations

CORRECTION OF EOR •

Spectacle lenses - safest method of refractive correction - correct low degrees of ametropia - bifocals: lens for both near and distant vision in single frame - progressive: graduated lens for objects at any distance - may be cosmetically unacceptable for high degrees of ametropia

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Limitation of Most spectacles are worn Spectacles: successfully. Problems occur with increasing refractive power.

Cosmesis : convex magnify,concave minify •

Distortion of objects in peripheral vision

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Magnification of objects by highly convex lenses Weight

Contact Lenses  worn beneath the eyelids anterior to the cornea

DISADVANTAGES: not appropriate for children

 May be hard or soft Maintenance  Used to neutralize ametropia  To protect the healing cornea  To conceal unsightly damaged eyes

Size Irritation infection

TYPES OF CONTACT LENSES: 1. Hard contact lenses

 Offer clear, crisp vision and are durable  Correct major errors of refraction  Indicated for correction of irregular astigmatism  Correct by changing the curvature of the anterior surface of the eye

2. Soft contact lenses

 More comfortable  Require little adjustment to their use  Easier to insert and remove than hard lenses little correction of astigmatism Provide poorer corrected vision than do hard lenses

Contact Lens Care •

• • •

Do not exceed the recommended wearing time Observe meticulous hygiene Do not clean with tap water or saliva Remove if the eye becomes sore or inflammed Remove soft lenses while administering preservative containing drops

Complications of Contact Lens use  Giant papillary conjunctivitis – a condition in which wearers of (usually) soft contact lenses develop increasing ocular discomfort and itching  Corneal vascularization - more common in soft lens due to larger surface area covering the part of the sclera and limbus  Corneal ulceration  Corneal infection

CORRECTION FOR EOR •

Keratorefractive Surgery Radial Keratotomy •

Photorefractive Keratectomy (PRK) Laser Assisted Intrastromal Keratomileusis (LASIK)

Keratorefractive surgery Radial keratotomy •

incisional technique to alter corneal curvature

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Cornea becomes flatter as incision heals

Excimer laser: shaves off ultra-thin discs of corneal tissue to reduce refractive power in myopia

PRK (photorefractive keratectomy) LASIK (laser assisted in-situ keratomileusis)

Photorefractive Keratectomy (Excimer laser) •

Uses an excimer laser to reshape the outer surface of the cornea Benefits: -myopia reliable(-2 correction of low to moderate to -6 diopters) - improvement in unaided visual acuity

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Complications: - corneal haze - post op pain - loss of best-corrected visual acuity - regression of effect

LASIK •

corneal flap is created --- Excimer laser reshapes the cornea reliable correction of low to high myopia (-2 to -12 diopters)

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Benefits •

Clear vision in 24 hours Minimal post-op pain

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Limitation Additional surgical step

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CORRECTION OF EOR •

Intraocular lenses- phakic IOLs

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Clear lens extraction for myopia - extraction of non-cataractous lenses - eyes must be highly myopic in order for the operation to be a success.

Lens

Lens •

transparent, avascular, biconvex

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Held in position by zonules

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Surrounded by the lens capsule

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1 of the main refracting surfaces of the eye

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Has inherent elasticity

Lens Cross section

Causes of Cataract •

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Age related •

50 % in 65 – 74 yo

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70 % in > 75 yo

3 main types •

Subcapsular Cataract

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Nuclear Cataract

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Cortical Cataract

Cataract •

Subcapsular cataract •

May be anterior or posterior

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Anterior type is associated with fibrous metaplasia of the anterior epithelium of the lens

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Posterior type is associated with posterior migration of the epithelial cells of the lens

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Cataract

Nuclear Cataract •

Sclerosis and yellowing of the lens nucleus

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Myopic shift “second sight”

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Advanced cases, nucleus becomes opaque and brown (brunescent nuclear cataract)

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Cataract Cortical cataract •

Hydropic swelling of the lens fibers

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Due to ionic changes in the lens fibers

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Formation of radial cortical spokes.

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Mature Cataract – Entire cortex becomes opaque

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Hypermature Cataract – Leakage of degenerated cortical material through the lens capsule leaving the capsule wrinkled and shrunken.

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Morgagnian Cataract – further liquefaction of the cortex allows free movement of the nucleus

Cataract Mature Cortical Cataract

Cataract Hypermature Cataract

Cataract Morgagnian Cataract

Traumatic Cataract •

Direct Penetrating Injury - lens capsule ruptured with hydration of lens fibers

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Blunt injury •

stellate or rosette shaped opacification

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Vossius ring – imprinting of iris pigment onto the anterior lens capsule

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Electric shock – causes protein coagulation and cataract formation

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Ionizing radiation - damage to actively growing lens cells, younger patients more susceptible

Traumatic Cataract Severe penetrating trauma

Traumatic cataract Vossius ring

Metabolic Cataract Diabetes Mellitus Inc. glucose in aqueous humor

“Snow flake” Cataract

Glucose enters lens by diffusion Influx of water in the lens

Glucose converted to sorbitol

Metabolic Cataract •

Galactosemia •

Autosomal recessive

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Inability to convert galactose to glucose

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Symptoms of Malnutririon,hepatomegaly, jaundice and mental deficiency

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75 % develops cataract few weeks after birth

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Inc. galactose and glactiol in the lens cells

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Nucleus and deep cortex opacified

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“Oil droplet” cataract

Metabolic Cataract •

Wilson’s disease •

Autosomal recessive

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Disorder in copper metabolism

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Cuprous oxide (reddish brown pigment) is deposited in the anterior lens capsule

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“sunflower cataract”

Toxic Cataract

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Steroid induced cataract •

•

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Phenothiazines •

Yellow brown granules on the anterior capsule

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Visually insignificant

Miotics - cholinesterase inhibitors •

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Posterior subcapsular cataract

Anterior subcapsular vacuoles

Amiodarone • •

anterior subcapsular lens opacities Visually Insignificant

Secondary Cataract •

Chronic Anterior Uveitis Most common cause of secondary cataract •

Formation of posterior synechiae

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Thickening of anterior lens capsule

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Formation of fibrovascular membrane

Chronic Anterior Uveitis

Secondary Cataract •

Acute angle closure glaucoma •

glaukomflecken

Congenital Cataract •

lens opacity present at birth Rubella •

Maternal infection with rubella virus

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Cardiac defects, deafness, mental retardation

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Cataracts are characterized by pearly white nuclear opacification

Abnormalities of Lens Shape and Position •

Coloboma-congenital absence of an eye structure Lenticonus-cone formation in the anterior or posterior pole of the lens

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Microphakia-lens with a smaller than normal diameter Microsherophakia-not only small but spherical

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Ectopia Lentis Marfan Syndrome’s Widespread abnormality of connective tissue •

Autosomal Dominant •

Cardiac anomalies - aneurysm of the ascending aorta. Skeletal anomalies – limbs are inappropriately long. •

• •

Arachnodactyly - excessively long fingers

Muscular underdevelopment

Zonular attachment of the lens usually are intact but become stretched and elongated

Marfan’s Syndrome

Marfan’s Syndrome •

Ocular feature •

Bilateral lens subluxation – 80 % of cases

Ectopia Lentis •

Homocystinuria Autosomal recessive •

Error of methionine metabolism

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Lenses are usually subluxated inferiorly

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Zoules have high levels of cysteine and tend to be brittle

Management of Cataract •

Cataract Surgery Intracapsular Cataract Surgery •

Extracapsular Cataract Surgery Phacoemulsification

Extracapsular Cataract Extraction

Phacoemulsification

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After Cataract

YAG laser capsulotomy

Complications of Cataract Surgery •

Early Inc intraocular pressure •

Iris prolapse

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Wound leak

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Acute Bacterial Endophthalmitis

Pupil Block

Iris Prolapse

Wound Leak

Bacterial Endophthalmitis

Complications of Cataract Surgery •

Late Toxic suture syndrome •

Corneal Decompensation Irvine-Gass Syndrome

Toxic Suture Syndrome

Corneal Decompensation

Irvine-Gass Syndrome

Cystoid macular edema

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