Refractive Errorsand management

Asma’ Al-husamia

REFRACTIVE ERRORS OBJECTIVE

Refraction Refractive power of the eye Accommodation Types of refractive errors Detailed description if myopia , hyperopia ,

astigmatism Management of refractive errors

REFRACTION Light moves in the air at speed ( 300,000

km/sec) When light moves from one media to another

media of different density (ie. Different refraction index ) >> it’s speed changes

And it will be refracted

• A refractive error simply means that the shape of your eye does not bend light correctly, resulting in a blurred image.

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

What’s responsible for the refractive power

The refractive power of any lens is expressed by

If we consider the eye as a single lens then the refractive power is 59 D ≈ 60 D (Total refractive power of the eye )

The maximum total refractive power of an emmetropic eye is

63 diopterswith an axial length of the globe measuring 23.5mm. The cornea

accounts for43 diopters and the lens for 10–20

diopters, depending on accommodation.

(Ophthalmology A Short Textbook)

“Guyton and Hall Textbook of Medical Physiology”

Cornea account for (43 D) of the refractive power

responsible for the majority of the refractive power of the eye (2/3)(Corneal refractive power is constant)

Lens account for ( 17 D) of refractive power (1/3)

(Lens refractive power is modifiable with accommodation)

Axial length(Axial length of the eye is constant except under certain conditions)

Light rays

43 D

17 D

Accommodation

Adjustment of the refractive power of the lens inside the eye for seeing objects at various distances. It is done by changing the curvature of the lens

The refractive power of the lens = 17 D By accommodation it can be increased in children up to 34 D “Guyton and Hall Textbook of Medical Physiology”

negative accommodation adjustment of the eye for long distances by relaxation of the ciliary muscles.

positive accommodation adjustment of the eye for short distances by contraction of the ciliary muscles.

Helm-holtz theorycontraction 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 retinacontraction of ciliary muscle is supplied by parasympathetic third nerve

Changes during accommodation: (A), contraction of ciliary muscles; (B), approximation of ciliary muscles to lens; (C), relaxation of suspensory ligament; (D), increased curvature of anterior surface of lens.

EMMETROPIA (GK, EMMETROS, PROPORTIONED/

FITTING , OPSIS, VISION)

Image of distant objects focus exactly on the retina

No refractive error

AMETROPIA (REFRACTIVE ERROR)GK, AMETROS, IRREGULAR, OPSIS, SIGHT

Mismatch between axial length and refractive power.

Parallel light rays don’t fall on the retina.

Nearsightedness (Myopia) Farsightedness (Hyperopia) Astigmatism Presbyopia Anisometropia

Types of Refractive errors

Myopia

Hyperopia

Astigmatism

Presbyopia

Anisometropia

DefinitionA discrepancy between the refractive power and axial length of the eye such that parallel light rays converge at a focal point anterior to the retina

Etiology : not clear , genetic factor 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

In myopia, the far point (distance from the eye = A) can be calculated using the formula: A (m) = 1/D, where D is myopia in diopters.

For ex. a -2 D myopia, the far point is 0.5 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

Myopia

CLASSIFICATION OF MYOPIA

Simple

Pathologic

By cause

axial myopia

refractive myopia

by their clinical

appearance

Simple myopia

Degenerative myopia

Nocturnal myopia

psedumyopia 

Induced myopia, a

by age of onset

Congenital myopia,

Young onset myopia

School myopia

Early adult: onset

myopia

Late adult: onset

myopia

Forms Simple myopia (school age myopia)

Onset is at the age of 10–12 years. Usually the myopia does not progress after the age

of 20. Refraction rarely exceeds 6 diopters. However, a benign progressive myopia also exists,

which stabilizes only after the age of 30. Pathologic myopia: This disorder is largely hereditary and progresses continuously independently of external influences.

FORMS OF MYOPIA (BY THE CAUSE)

Axial myopia: (more common) increase in the eye's axial length( it’s too long for its optical power)

Refractive myopia: excessive refractive power

(eye is optically too powerful for its axial length)- Curvature myopia is attributed to excessive, or increased, curvature of one or more of the refractive surfaces of the eye, especially the cornea , or the lens

- Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.

FORMS OF MYOPIA (by clinical classification )

Simple myopia, more common than other types of myopia,

characterized by an eye that is too long for its optical power  or optically too powerful for its axial length Both genetic and environmental factors, contribute to the development of simple myopia. Degenerative myopia, also known as malignant,

pathological, or progressive myopia, is characterized by marked fundus changes, such as posterior staphyloma

Nocturnal myopia, also known as night or twilight myopia,

Pseudomyopia is the blurring of distance vision brought about by spasm of the ciliary muscle >> accommodative spasm.. For example, an over anxious student

Induced myopia, also known as acquired myopia, results from exposure to various pharmaceuticals, increases in glucose levels, nuclear sclerosis, oxygen toxicity , etc..

FORMS OF MYOPIA ( by age of onset )

Congenital myopia, also known as infantile myopia, is present at birth .

Myopia > 10 D ,Increase slowly each year Young onset myopia occurs prior to age 20 . School myopia appears during childhood,

particularly the school-age years Early adult: onset myopia occurs between

ages 20 and 40

Late adult: onset myopia occurs after age 40

SPECIAL FORMS OF REFRACTIVE MYOPIA:

Myopic sclerosis of the nucleus of the lens (cataract) in advanced age . This causes a secondary focal point to develop, which can lead to monocular diplopia (double vision).

Keratoconus (increase in the refractive power of the cornea).

Spherophakia (spherically shaped lens).

PATHOLOGICAL CAUSES OF MYOPIA

KERATOCONUS

PATHOLOGICAL CAUSES OF MYOPIA

* Cataract * Diabetes

PATHOLOGICAL CAUSES OF MYOPIA

Marfan syndrome(Spherophakia )

Posterior Staphyloma

SYMPTOMS• Usually detected when they discover that 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 Frequent squinting of eyes (The term

“myopia” comes from this squinting; the Greek word “myein” means to squint or close the eyes. In attemps to improve uncorrected visual acuity when gazing into the distance)

Eye strain or headaches from trying to focus

Morphologic changesProgressive myopia in particular is characterized by :

deep anterior chamber atrophy of ciliary muscle The volume of the vitreous body is too small for

the large eye, and it may collapse prematurely. This results in vitreous opacifications that the patient perceives as floaters.

fundus change : loss of pigment in “Retinal pigment epithelium”, large disc and white crescent-shaped area on temporal side , RPE atrophy in macular area , posterior staphyloma , retinal degeneration-->hole-->increase risk of Rretinal detachment

Treatment :-concave lenses (minus lenses)

-Contact lens- In certain special cases removal of the crystalline

lens (Χ retinal detachment)

- implanting an anterior chamber intraocular lens (diverging lens) anterior to the natural lens to reduce refractive power

Types of Refractive errors

Myopia

Hyperopia

Astigmatism

Presbyopia

Anisometropia

Definition:

there is a discrepancy between the refractive power and axial length of the eye such that parallel light rays converge at a focal point posterior to the retina

To bring the focal point on to the retina, a farsighted person must accommodate even when gazing into the distance .Close objects remain blurred because the eye is unable to accommodate any further in near vision. As accommodation is linked to convergence, this process can result in esotropia (accommodative esotropia or accommodative convergent strabismus).

Hyperopia

Causes:o excessive short globe (axial hyperopia) :

more common .The length of the eyeball is shorter than it should beAxial hyperopia is usually congenital and is characterized by a shallow anterior chamber with a thick sclera and well developed ciliary muscle.

Hyperopic eyes are predisposed to acute angle closure glaucoma because of their shallow anterior chamber.

insufficient refractive power (refractive hyperopia)

Lens changes (cataract). 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.

Special forms of refractive hyperopia:

Absence of the lens (aphakia) due to dislocation.

Postoperative aphakia following cataract surgery without placement of an intraocular lens

PATHOLOGICAL CAUSES OF HYPEROPIA

DISLOCATED LENS

PATHOLOGICAL CAUSES OF HYPEROPIA

Retinal pigment epithelial detachment

RETINAL FLUID

Circumscribed choroidal

haemangioma

SYMPTOMS In young patients, accommodation can compensate for slight

to moderate hyperopia. However, this leads to chronic overuse of the ciliary muscle. As accommodation decreases with advancing age, near vision becomes increasingly difficult. For this reason, hyperopic persons tend to become presbyopic early. accommodative esotropia : because accommodation

is linked to convergence . 30s-40s : blurring of near vision due to decreased

accommodation > 50s : blurring of distance vision due to severely

decreased accommodation

Reading in particular can cause asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, , blepharoconjunctivitis, blurred vision, and rapid fatigue.

Fundus in axial hyperopia may reveal Pseudo-papilloedema (a slightly blurred optic disk that may be elevated)

DDx from optic neuritis by > 4 D , no enlarged blind spot, no passive congestion of vein

Uncorrected, light focuses behind fovea

Corrected by convergent lens, light focuses on fovea

Treatment: convex lenses, keratorefractive

surgery, refractive

lensectomy

Types of Refractive errors

Myopia

Hyperopia

AstigmatismPresbyopi

a

Anisometropia

AstigmatismDefinition : (GK stigma means (point) so Astigmatism literally means lack of a focal point.) The disorder is characterized by a curvature anomaly of the refractive media such that parallel light rays do not converge at a point but are drawn apart to form a line.

Etiology : heredity Affects approximately 30% of population with prevalence increasing with age

Pathophysiology: The refractive media of the astigmatic eye are not spherical but

refract differently along one meridian than along the meridian perpendicular to it). This produces two focal points.

Therefore, a punctiform object is represented as a sharply defined line segment at the focal point of the first meridian but also appears as a sharply defined line segment rotated 90 degrees at the focal point of the second meridian.

Midway between these two focal points is the “circle of least confusion.” (the location with the least loss of image definition.) The aggregate system lacks a focal point. The combined astigmatic components of all of the refractive media

comprise the total astigmatism of the eye. These media include: Anterior surface of the cornea. Posterior surface of the cornea. Anterior surface of the lens. Posterior surface of the lens. Rarely, nonspherical curvature of the retina may also contribute to

astigmatism.

TYPES Regular _ curvature uniformly different in meridians at right

angels to each other involving only two meridians approximately perpendicular to each other . This is presumably caused by excessive eyelid tension that leads to astigmatic changes in the surface of the cornea. Irregular –curvature and the refractive power of the

refractive media are completely irregular .There are multiple focal points, which produces a completely blurred image on the retina

may be caused by the following diseases: Corneal ulcerations with resulting scarring of the cornea. Penetrating corneal trauma. Advanced keratoconus. Cataract. Lenticonus.

Astigmatism can be classified as follows: External astigmatism: astigmatism of the

anterior surface of the cornea. Internal astigmatism: the sum of the

astigmatic components of the other media.

Symptoms asthenopic symptoms ( headache , eyepain) distortion of vision

Treatment Regular astigmatism :cylindrical lenses with or without spherical lenses(convex or concave)to shift this focal point on to the retina if necessary Irregular astigmatism : This form cannot be corrected with eyeglasses. - External astigmatism may be managed with a rigid

contact lens ,keratoplasty, or surgical correction of the refractive error.

-internal astigmatism is usually lens-related. In this case, removal of the lens with implantation of an intraocular lens is indicated Refractive surgery

PATHOLOGIC CAUSES OF ASTIGMATISM

Corneal: post surgical, traumatic, infectious External pressure on cornea or lens : lid

masses , tumors..

Types of Refractive errors

Myopia

Hyperopia

Astigmatism

Presbyopia

Anisometropia

Presbyopia Physiologic loss of accommodation in

advancing age (Normal aging process (>40) ) Near image cannot be focused onto the

retina ( focus behind the retina) Accommodative power is 14D at age of 10

year diminishes to 3.5Dby 40 y deposit of insoluble proteins in lens in

advancing age-->elasticity of lens progressively decrease-->decrease accommodation

Hardening /reduced deformability of the lens

CLINICAL FEATURE

hold reading material farther away but distance vision remains unaffected

myopic person removes distance glasses to read

symptoms of presbyopia occur earlier In hyperopic persons.

PRESBYOPIA

Treatment convex lenses in near vision

Reading glasses Bifocal glasses Trifocal glasses Progressive power glasses

Types of Refractive errors

Myopia

Hyperopia

Astigmatism

Presbyopia

Anisometropia

AnisometropiaDefinition: In anisometropia, there is a difference in refractive power between the two eyes.

Etiology: The reason for the varying development of the two eyes is not clear. This is primarily a congenital disease.

Second most common cause of amblyopia in childern

Pathophysiology:

- This refractive difference can be corrected separately for each eye with different lenses as long as it lies below 4 diopters If the difference in refraction is >= 4 diopters, the size

difference of the two retinal images becomes too great for the brain to fuse the two images into one (Known as aniseikonia)

aniseikonia depend on degree of refractive anomaly and type of correction

The closer to the site of refraction deficit the correction is made-the less retinal image changes in size. Correction with intraocular lenses results in almost no difference in image

size. Contact lenses produce a slight and usually irrelevant

difference in image size. However, eyeglass correction resulting in a difference of

more than 4 diopters leads to intolerable aniseikonia

ANISOMETROPIA

Symptoms: Anisometropia is usually congenital and often

asymptomatic. Children are not aware that their vision is abnormal.

However, as binocular functions may remain underdeveloped ;there is a tendency toward strabismus

Where the correction of the anisometropia results in unacceptable aniseikonia, patients will report unpleasant visual sensations of double vision.

Treatment If Anisometropia exceeding 4 diopters -> Contact

lenses are used and in rare cases, surgical treatment . Patients with unilateral aphakia or who do not tolerate

contact lenses will require implantation of an intraocular lens.

Thank you