R EFRACTION Švehlíková G. Department of Ophthalmology LF UPJS v Košiciach Prednosta: prof. MUDr. Juhás T., DrSc.

REFRACTION

Švehlíková G.

Department of Ophthalmology LF UPJS v Košiciach

Prednosta: prof. MUDr. Juhás T., DrSc

HOW THE EYE SEES

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.

REFRACTIVE ERROR 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:

1. Myopia (nearsightedness)2. Hyperopia (farsightedness)3. Astigmatism

refractive error - blurred or distorted vision

REFRACTIVE ERROR 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

Epidemiology: Approximately 25% of persons between the ages of 20 and 30 have refraction less than –1 diopters.

MYOPIA 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).

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

Epidemiology: Approximately 20% of persons between the ages of 20 and 30

have refraction exceeding +1 diopters

HYPEROPIA 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.

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

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

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 Classification Simple myopic astigmatism

– the focus of one meridian is on the retina, other is in front of the retina

Simple hyperopic astigmatism- the 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

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 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 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

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

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

EYEGLASS LENSES

Monofocal

Spherical lenses Toric lenses

(cylindrical lenses) refract light only along one axis.

Spherical and toric lenses can be combined where indicated

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 near-field correction

Patients are able to view distant objects and read using one pair of eyeglasses

Progressive addition lenses: continuously increasing refractive power

CONTACT LENSES

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

The cornea requires oxygen from the precorneal tear film.

To ensure this supply, contact lens materials must be oxygen-permeable. 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 oxygen-permeable materials such as silicone copolymers are available

correct keratoconus

SOFT CONTACT LENSES

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

Contacts can also be described by replacement interval

Daily, weekly, bi-weekly, monthly or quarterly

SPECIAL LENSES

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

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 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

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

REFRACTIVE SURGERY

laser PRK, LASEK, Epi LASIK

LASIK

other incisional keratotomy – radial, hexagoanal

intracorneal ring

Phacic intraocular lens

Clear lens extraction

Corneal refractive procedures

Intraocular refractive procedures

REFRACTIVE SURGERY 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

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

Profile of the cornea after fotoablation for

HYPEROPIA

If the patient has myopia tissue closer to the central part of the cornea is removed to decrease the curvature - flatten the cornea

If a patient has hyperopia tissue in the periphery of the cornea is removed to increase curvature

LASER REFRACTIVE SURGERY

divided into1. lamellar (LASIK) and 2. surface ( PRK, LASEK,

and Epi-LASIK)

laser-assisted in situ keratomileusis (LASIK)

photorefractive keratectomy (PRK)

laser assisted subepithelial keratectomy (LASEK)

PRK, LASEK, AND EPI-LASIK

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

PHOTOREFRACTIVE KERATECTOMY (PRK)

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.

LASER ASSISTED SUBEPITHELIAL KERATECTOMY (LASEK)

LASEK – epitelial flap

Epi-LASIK - microkeratome

In LASEK and Epi-LASIK 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

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

More discomfort Recovery time is longer final results for LASIK and

surface treatments are similar

The Advantages The Disadvantages

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

Rapid visual recovery Less postoperative

discomfort

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

The Advantages The Disadvantages

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.

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%).

COMPLICATIONS

COMPLICATIONS

Corneal scarring/haze Keratitis

COMPLICATIONS

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

INTRAOCULAR REFRACTIVE PROCEDURES

Phacic intraocular lens

implantation - range: up to -17D myopia; up to +10D hypermetropia

Principle = implantation of a soft flexible artificial lens

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

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

Autoimmune disease History of or active herpes

simplex keratitis, risc of the reactivation of the virus

Active collagen vascular disease

Uncontrolled diabetes Uncontrolled glaucoma Pregnancy or

breastfeeding

QUESTIONS AND DISCUSSION

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