Doctor Lecture4

8/14/2019 Doctor Lecture4

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MYOPIA


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Myopia , also called near- orshort-

sightedness, is a refractive defect of the

eye in which collimated light produces

image focus in front of the retina when

accommodation is relaxed.
http://en.wikipedia.org/wiki/Refractive_errorhttp://en.wikipedia.org/wiki/Eyehttp://en.wikipedia.org/wiki/Collimated_lighthttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Retinahttp://en.wikipedia.org/wiki/Accommodation_(eye)http://en.wikipedia.org/wiki/Accommodation_(eye)http://en.wikipedia.org/wiki/Retinahttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Collimated_lighthttp://en.wikipedia.org/wiki/Eyehttp://en.wikipedia.org/wiki/Refractive_error


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Those with myopia see nearby objects

clearly but distant objects appearblurred.

With myopia, the eyeball is too long, or the

cornea is too steep, so images are

focused in the vitreous inside the eye

rather than on the retina at the back of the

eye.
http://en.wikipedia.org/wiki/Visual_perceptionhttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Corneahttp://en.wikipedia.org/wiki/Retinahttp://en.wikipedia.org/wiki/Retinahttp://en.wikipedia.org/wiki/Corneahttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Visual_perception


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Classification

Axial myopia is attributed to an increase

in the eye's axial length

Refractive myopia is attributed to thecondition of the refractive elements of the

eye


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Refractive myopia

further sub classified :

Curvature myopia is attributed toexcessive, or increased, curvature of oneor more of the refractive surfaces of theeye, especially the cornea

Index myopia is attributed to variation in theindex of refraction of one or more of theocular media
http://en.wikipedia.org/wiki/Index_of_refractionhttp://en.wikipedia.org/wiki/Index_of_refraction


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Clinical entity

Various forms of myopia have been described

by their clinical appearance

Simple myopia is more common than other

types of myopia and is characterized by an eyethat is too long for its optical power(which is

determined by the cornea and crystalline lens) or

optically too powerful for its axial length. Both

genetic and environmental factors, particularlysignificant amounts of near work, are thought to

contribute to the development of simple myopia.
http://en.wikipedia.org/wiki/Optical_powerhttp://en.wikipedia.org/wiki/Corneahttp://en.wikipedia.org/wiki/Lens_(anatomy)http://en.wikipedia.org/wiki/Lens_(anatomy)http://en.wikipedia.org/wiki/Corneahttp://en.wikipedia.org/wiki/Optical_power


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Degenerative myopia, also known as

malignant, pathological, or progressive

myopia, is characterized by marked fundus

changes, such as posteriorstaphyloma, andassociated with a high refractive error and

subnormal visual acuity after correction. This

form of myopia gets progressively worse over

time. Degenerative myopia has been reported asone of the main causes ofvisual impairment
http://en.wikipedia.org/wiki/Staphylomahttp://en.wikipedia.org/wiki/Visual_impairmenthttp://en.wikipedia.org/wiki/Visual_impairmenthttp://en.wikipedia.org/wiki/Staphyloma


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Nocturnal myopia, also known as night myopia

ortwilight myopia, is a condition in which the eyehas a greater difficulty seeing in low illuminationareas, even though its daytime vision is normal.Essentially, the eye's far point of an individual's

focus varies with the level of light. Night myopiais believed to be caused by pupils dilating to letmore light in, which adds aberrations resulting inbecoming more nearsighted. A stronger

prescription for myopic night drivers is oftenneeded. Younger people are more likely to beaffected by night myopia than the elderly.


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Pseudomyopia is the blurring of distance

vision brought about by spasm of the

ciliary muscle.
http://en.wikipedia.org/wiki/Ciliary_musclehttp://en.wikipedia.org/wiki/Ciliary_musclehttp://en.wikipedia.org/wiki/Ciliary_muscle


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Induced myopia, also known as

acquired myopia, results from exposure

to various pharmaceuticals, increases in

glucose levels, nuclear sclerosis, or otheranomalous conditions. The encircling

bands used in the repair of

retinal detachments may induce myopiaby increasing the axial length of the eye.
http://en.wikipedia.org/wiki/Pharmaceuticalhttp://en.wikipedia.org/wiki/Glucosehttp://en.wikipedia.org/wiki/Nuclear_sclerosishttp://en.wikipedia.org/wiki/Retinal_separationhttp://en.wikipedia.org/wiki/Retinal_separationhttp://en.wikipedia.org/wiki/Nuclear_sclerosishttp://en.wikipedia.org/wiki/Glucosehttp://en.wikipedia.org/wiki/Pharmaceutical


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Index myopia is attributed to variation in

the index of refraction of one or more of

the ocular media. Cataracts may lead to

index myopia.


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deprivation myopia is a type of myopiathat occurs when the eyesight is deprived

by limited illumination and vision range, or

the eye is modified with artificial lenses or

deprived of clear form vision. In lower

vertebrates this kind of myopia seems to

be reversible within short periods of time.

Myopia is often induced this way invarious animal models to study the

pathogenesis and mechanism of myopia

development.


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Degree

Myopia, which is measured in diopters by

the strength oroptical powerof a

corrective lens that focuses distant images

on the retina, has also been classified bydegree or severity
http://en.wikipedia.org/wiki/Dioptershttp://en.wikipedia.org/wiki/Optical_powerhttp://en.wikipedia.org/wiki/Optical_powerhttp://en.wikipedia.org/wiki/Diopters


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Low myopia usually describes myopia of

3.00 diopters or less


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Medium myopia usually describes

myopia between 3.00 and 6.00 diopters

. Those with moderate amounts of myopia

are more likely to havepigment dispersion syndrome or

pigmentary glaucoma
http://en.wikipedia.org/wiki/Dioptershttp://en.wikipedia.org/wiki/Pigment_dispersion_syndromehttp://en.wikipedia.org/wiki/Glaucomahttp://en.wikipedia.org/wiki/Glaucomahttp://en.wikipedia.org/wiki/Glaucomahttp://en.wikipedia.org/wiki/Pigment_dispersion_syndromehttp://en.wikipedia.org/wiki/Diopters


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High myopia usually describes myopia of

6.00 or more. People with high myopia

are more likely to have

retinal detachments andprimary open angle glaucoma. They are

also more likely to experience floaters,

shadow-like shapes which appear singlyor in clusters in the field of vision. Roughly

30% of myopes have high myopia
http://en.wikipedia.org/wiki/Retinal_detachmenthttp://en.wikipedia.org/wiki/Glaucomahttp://en.wikipedia.org/wiki/Floaterhttp://en.wikipedia.org/wiki/Visual_fieldhttp://en.wikipedia.org/wiki/Visual_fieldhttp://en.wikipedia.org/wiki/Floaterhttp://en.wikipedia.org/wiki/Glaucomahttp://en.wikipedia.org/wiki/Retinal_detachment


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Age of onset

Congenital myopia, also known as

infantile myopia, is present at birth and

persists through infancy


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Youth onset myopia occurs prior to age

20.

appears during childhood, particularly the

school-age years. This form of myopia isattributed to the use of the eyes for close

work during the school years.


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Adult onset myopia

Early adult onset myopia occurs

between ages 20 and 40.

Late adult onset myopia occurs after

age 40


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Epidemiology

The global prevalence of refractive errors

has been estimated from 800 million to 2.3

billion. The incidence of myopia within

sampled population often varies with age,country, sex, race, ethnicity, occupation,

environment, and other factors. Variability

in testing and data collection methodsmakes comparisons of prevalence and

progression difficult.
http://en.wikipedia.org/wiki/Agehttp://en.wikipedia.org/wiki/Sexhttp://en.wikipedia.org/wiki/Race_(classification_of_human_beings)http://en.wikipedia.org/wiki/Ethnicityhttp://en.wikipedia.org/wiki/Ethnicityhttp://en.wikipedia.org/wiki/Race_(classification_of_human_beings)http://en.wikipedia.org/wiki/Sexhttp://en.wikipedia.org/wiki/Age


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In some areas, such as China, India and

Malaysia, up to 41% of the adult

population is myopic to -1dpt

A recent study involving first-year

undergraduate students in the United

Kingdom found that 50% ofBritish whites

and 53.4% of British Asians were myopic
http://en.wikipedia.org/wiki/Chinahttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/Malaysiahttp://en.wikipedia.org/wiki/United_Kingdomhttp://en.wikipedia.org/wiki/United_Kingdomhttp://en.wikipedia.org/wiki/Malaysiahttp://en.wikipedia.org/wiki/Indiahttp://en.wikipedia.org/wiki/China


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In Greece, the prevalence of myopia among 15

to 18 year old students was found to be 36.8%.

In the United States, the prevalence of myopia

has been estimated at 20%. Nearly 1 in 10(9.2%) American children between the ages of 5

and 17 have myopia. Approximately 25% of

Americans between the ages of 12 and 54 have

the condition
http://en.wikipedia.org/wiki/Greecehttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/Greece


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Ethnicity and race

The prevalence of myopia has been

reported as high as 70-90% in some Asian

countries, 30-40% in Europe and the

United States, and 10-20% in Africa
http://en.wikipedia.org/wiki/Asiahttp://en.wikipedia.org/wiki/Asia


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Education, intelligence, and IQ

A number of studies have shown that the

prevalence of myopia increases with level

of education and many studies have

shown a relationship between myopia andIQ.

According to Arthur Jensen, myopes

average 7-8 IQ points higher than non-myopes
http://en.wikipedia.org/wiki/IQhttp://en.wikipedia.org/wiki/Arthur_Jensenhttp://en.wikipedia.org/wiki/Arthur_Jensenhttp://en.wikipedia.org/wiki/IQ


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The relationship also holds within families,

and siblings with a higher degree of

refraction erroraverage higher IQs than

siblings with less refraction error. Jensenbelieves that this indicates myopia and IQ

are pleiotropically related as they are

caused or influenced by the same genes.
http://en.wikipedia.org/wiki/Refraction_errorhttp://en.wikipedia.org/wiki/Pleiotropichttp://en.wikipedia.org/wiki/Pleiotropichttp://en.wikipedia.org/wiki/Refraction_error


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No specific mechanism that could cause a

relationship between myopia and IQ has

yet been identified. Another possible

explanation is that high levels of reading isa common cause for both intelligence and

myopia, as it supposed that large amounts

of close work is a causal factor of myopia


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Etiology and pathogenesis

Because in the most common, "simple"

myopia, the eye length is too long, any

etiologic explanation must account for

such axial elongation. To date, no singletheory has been able to satisfactorily

explain this elongation
http://en.wikipedia.org/wiki/Etiologichttp://en.wikipedia.org/wiki/Theoryhttp://en.wikipedia.org/wiki/Theoryhttp://en.wikipedia.org/wiki/Etiologic


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Among mainstream researchers and

eye care professionals, myopia is now

thought to be a combination of genetic

and environmental factors
http://en.wikipedia.org/wiki/Eye_care_professionalhttp://en.wikipedia.org/wiki/Eye_care_professional


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There are currently two basic mechanisms

believed to cause myopia: also known as

pattern deprivation and optical defocus.
http://en.wikipedia.org/wiki/Defocus_aberrationhttp://en.wikipedia.org/wiki/Defocus_aberration


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Form deprivation occurs when the image quality

on the retina is reduced; optical defocus occurs

when light focuses in front of or behind the

retina. Numerous experiments with animals have

shown that myopia can be artificially generatedby inducing either of these conditions. In animal

models wearing negative spectacle lenses, axial

myopia has been shown to occur as the eye

elongates to compensate for optical defocus.
http://en.wikipedia.org/wiki/Retinahttp://en.wikipedia.org/wiki/Retina


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Theories

Combination of genetic and

environmental factors: In China,myopia is more common in those with

higher education background and some

studies suggest that near work may

exacerbate a genetic predisposition to

develop myopia


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myopia determined by heritage some

people are at a higher risk to developmyopia when exposed to modern

environmental conditions with a lot of

extensive near work like reading. In otherwords, it is often not the myopia itself

which is inherited, but the reaction to

specific environmental conditions and

this reaction can be the onset and the

progression of myopia.


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Genetic factors The wide variability of

the prevalence of myopia in different

ethnic groups has been reported as

additional evidence supporting the role ofgenetics in the development of myopia


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Genetic factors include an inherited,

increased susceptibility for environmental

influences like excessive near work, and

the fact that some people do not developmyopia in spite of very adverse conditions

is a clear indication that heredity is

involved somehow in any case


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Environmental factors It has beensuggested that a genetic susceptibility to

myopia does not exist. A high heritability

of myopia (as for any other condition)does not mean that environmental factors

and lifestyle have no effect on the

development of the condition


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Other theories suggest that the eyes

become strained by the constant extra

work involved in "nearwork" and get stuck

in the near position, and eye exercisescan help loosen the muscles up thereby

freeing it for far vision.


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These primarily mechanical models

appear to be in contrast to research

results, which show that the myopic

elongation of the eye can be caused by

the image quality, with biochemicalprocesses as the actuator


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Common to both views is, however, that

extensive near work and corresponding

accommodation can be essential for the

onset and the progression of myopia


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Near work

Diet and nutrition - One 2002 articlesuggested that myopia may be caused by

over-consumption of bread in childhood,

or in general by diets too rich in

carbohydrates, which can lead to chronic

hyperinsulinemia.

Stress has been postulated as a factor in

the development of myopia
http://news.bbc.co.uk/1/hi/health/1909075.stmhttp://en.wikipedia.org/wiki/Carbohydrateshttp://en.wikipedia.org/wiki/Hyperinsulinemiahttp://en.wikipedia.org/wiki/Hyperinsulinemiahttp://en.wikipedia.org/wiki/Carbohydrateshttp://news.bbc.co.uk/1/hi/health/1909075.stm


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Diagnosis

A diagnosis of myopia is typically

confirmed during an eye examination by

an ophthalmologist or an optometrist.

Frequently an autorefractororretinoscopeis used to give an initial objective

assessment of the refractive status of

each eye, then a phoropteris used tosubjectively refine the patient's

eyeglass prescription.
http://en.wikipedia.org/wiki/Eye_examinationhttp://en.wikipedia.org/wiki/Ophthalmologisthttp://en.wikipedia.org/wiki/Optometristhttp://en.wikipedia.org/wiki/Autorefractorhttp://en.wikipedia.org/wiki/Retinoscopehttp://en.wikipedia.org/wiki/Phoropterhttp://en.wikipedia.org/wiki/Eyeglass_prescriptionhttp://en.wikipedia.org/wiki/Eyeglass_prescriptionhttp://en.wikipedia.org/wiki/Phoropterhttp://en.wikipedia.org/wiki/Retinoscopehttp://en.wikipedia.org/wiki/Autorefractorhttp://en.wikipedia.org/wiki/Optometristhttp://en.wikipedia.org/wiki/Ophthalmologisthttp://en.wikipedia.org/wiki/Eye_examination


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Treatment, management, and

prevention

Eyeglasses, contact lenses, and

refractive surgery are the primary options

to treat the visual symptoms of those with

myopia. Orthokeratology is the practice ofusing special rigid contact lenses to flatten

the cornea to reduce myopia
http://en.wikipedia.org/wiki/Eyeglasseshttp://en.wikipedia.org/wiki/Contact_lenseshttp://en.wikipedia.org/wiki/Refractive_surgeryhttp://en.wikipedia.org/wiki/Orthokeratologyhttp://en.wikipedia.org/wiki/Orthokeratologyhttp://en.wikipedia.org/wiki/Refractive_surgeryhttp://en.wikipedia.org/wiki/Contact_lenseshttp://en.wikipedia.org/wiki/Eyeglasses


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Eye-exercises and biofeedback

Practitioners and advocates of

alternative therapies often recommend

eye exercises and relaxation techniques.

However, the efficacy of these practices isdisputed by scientists and eye care

practitioners. A 2005 review of scientific

papers on the subject concluded thatthere was "no clear scientific evidence"

that eye exercises were effective in

treating myopia.
http://en.wikipedia.org/wiki/Alternative_medicinehttp://en.wikipedia.org/wiki/Alternative_medicine


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Prevention

There is no universally accepted method

of preventing myopia. Commonly

attempted preventative methods include

wearing reading glasses, eye drops andparticipating in more outdoor activities


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Some clinicians and researchers

recommend plus power (convex) lenses in

the form of reading glasses when engaged

in close work or reading instead of usingsingle focal concave lens glasses

commonly prescribed
http://en.wikipedia.org/wiki/Concave_lenshttp://en.wikipedia.org/wiki/Concave_lens


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Pirenzepine eyedrops had a limited effect

on retarding myopic progression in a

recent, placebo-control, double-blinded

prospective controlled study
http://en.wikipedia.org/wiki/Pirenzepinehttp://en.wikipedia.org/wiki/Prospective_studyhttp://en.wikipedia.org/wiki/Prospective_studyhttp://en.wikipedia.org/wiki/Pirenzepine


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Daylight may prevent myopia. Australian

researchers had concluded that exposure to

daylight appeared to play a critical role in

restricting the growth of the eyeball, which is

responsible for myopia or short-sightedness.They compared children from other developed

countries such as Singapore and Australian

children spent about 2-3 hours a day outdoors

which could increased dopamine in the eyes thatrestrict distorted shaping of the eye


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Myopia control

Various methods have been employed in an

attempt to decrease the progression of myopia.

The use of reading glasses when doing close

work may provide success by reducing oreliminating the need to accommodate. Altering

the use of eyeglasses between full-time, part-

time, and not at all does not appear to alter

myopia progression. Bifocal andprogressive lenses have not shown significant

differences in altering the progression of myopia.
http://en.wikipedia.org/wiki/Progressive_lenseshttp://en.wikipedia.org/wiki/Progressive_lenses


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Hyperopia


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Hyperopia, also known as far-sightedness,

long-sightedness orhypermetropia, is adefect ofvision caused by an imperfection in theeye (often when the eyeball is too short or whenthe lens cannot become round enough), causinginability to focus on near objects, and in extremecases causing a sufferer to be unable to focuson objects at any distance. As an object movestoward the eye, the eye must increase its powerto keep the image in focus on the retina. If the

power of the cornea and lens is insufficient, as inhyperopia, the image will appear blurred.
http://en.wikipedia.org/wiki/Defecthttp://en.wikipedia.org/wiki/Visual_systemhttp://en.wikipedia.org/wiki/Eyehttp://en.wikipedia.org/wiki/Lens_(anatomy)http://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Distancehttp://en.wikipedia.org/wiki/Distancehttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Lens_(anatomy)http://en.wikipedia.org/wiki/Eyehttp://en.wikipedia.org/wiki/Visual_systemhttp://en.wikipedia.org/wiki/Defect


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Clinical entity

People with hyperopia can experience

blurred vision, asthenopia,

accommodative dysfunction,

binocular dysfunction, amblyopia, andstrabismus.
http://en.wikipedia.org/wiki/Defocus_aberrationhttp://en.wikipedia.org/wiki/Asthenopiahttp://en.wikipedia.org/wiki/Accommodation_(eye)http://en.wikipedia.org/wiki/Binocular_visionhttp://en.wikipedia.org/wiki/Amblyopiahttp://en.wikipedia.org/wiki/Strabismushttp://en.wikipedia.org/wiki/Strabismushttp://en.wikipedia.org/wiki/Amblyopiahttp://en.wikipedia.org/wiki/Binocular_visionhttp://en.wikipedia.org/wiki/Accommodation_(eye)http://en.wikipedia.org/wiki/Asthenopiahttp://en.wikipedia.org/wiki/Defocus_aberration


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Hyperopia is often confused withpresbyopia, another condition that

frequently causes blurry near vision.

Presbyopes who report good far vision

typically experience blurry near vision

because of a reduced

accommodative amplitude brought about

by natural aging changes with thecrystalline lens.
http://en.wikipedia.org/wiki/Presbyopiahttp://en.wikipedia.org/wiki/Accommodative_amplitudehttp://en.wikipedia.org/wiki/Crystalline_lenshttp://en.wikipedia.org/wiki/Crystalline_lenshttp://en.wikipedia.org/wiki/Accommodative_amplitudehttp://en.wikipedia.org/wiki/Presbyopia


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Diagnosis

Visual acuity is affected according to the

amount of hyperopia, as well as the

patient's age, visual demands, and

accommodative ability

T t t
http://en.wikipedia.org/wiki/Visual_acuityhttp://en.wikipedia.org/wiki/Visual_acuity


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Treatment

Various eye care professionals, including

ophthalmologists, optometrists,

orthoptists, and opticians, are involved in

the treatment and management ofhyperopia. At the conclusion of an

eye examination, an eye doctor may

provide the patient with aneyeglass prescription forcorrective lenses
http://en.wikipedia.org/wiki/Eye_care_professionalhttp://en.wikipedia.org/wiki/Ophthalmologisthttp://en.wikipedia.org/wiki/Optometristhttp://en.wikipedia.org/wiki/Opticianhttp://en.wikipedia.org/wiki/Eye_examinationhttp://en.wikipedia.org/wiki/Eyeglass_prescriptionhttp://en.wikipedia.org/wiki/Corrective_lenshttp://en.wikipedia.org/wiki/Corrective_lenshttp://en.wikipedia.org/wiki/Eyeglass_prescriptionhttp://en.wikipedia.org/wiki/Eye_examinationhttp://en.wikipedia.org/wiki/Opticianhttp://en.wikipedia.org/wiki/Optometristhttp://en.wikipedia.org/wiki/Ophthalmologisthttp://en.wikipedia.org/wiki/Eye_care_professional


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Minor amounts of hyperopia are sometimes leftuncorrected, however, larger amounts may be

corrected with convex lenses in eyeglasses or

contact lenses. Convex lenses have a positive

dioptric value, which causes the light to focuscloser than its normal range.

Hyperopia is sometimes correctable with various

refractive surgery procedures (LASIK). It can

also be corrected with special concave lenses.
http://en.wikipedia.org/wiki/Lens_(optics)http://en.wikipedia.org/wiki/Eyeglasseshttp://en.wikipedia.org/wiki/Contact_lenseshttp://en.wikipedia.org/wiki/Dioptrehttp://en.wikipedia.org/wiki/Refractive_surgeryhttp://en.wikipedia.org/wiki/Refractive_surgeryhttp://en.wikipedia.org/wiki/Dioptrehttp://en.wikipedia.org/wiki/Dioptrehttp://en.wikipedia.org/wiki/Contact_lenseshttp://en.wikipedia.org/wiki/Eyeglasseshttp://en.wikipedia.org/wiki/Lens_(optics)


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Astigmatism


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Astigmatism is an optical defect in which

vision is blurred due to the inability of the

optics of the eye to focus a point object

into a sharp focused image on the retina.This may be due to an irregular or toric

curvature of the cornea orlens
http://en.wikipedia.org/wiki/Corneahttp://en.wikipedia.org/wiki/Lens_(anatomy)http://en.wikipedia.org/wiki/Lens_(anatomy)http://en.wikipedia.org/wiki/Cornea


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An optical system with astigmatism is

one where rays that propagate in two

perpendicularplanes have different foci. If

an optical system with astigmatism is usedto form an image of a cross, the vertical

and horizontal lines will be in sharp focus

at two different distances
http://en.wikipedia.org/wiki/Optical_systemhttp://en.wikipedia.org/wiki/Ray_(optics)http://en.wikipedia.org/wiki/Plane_(mathematics)http://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Crosshttp://en.wikipedia.org/wiki/Crosshttp://en.wikipedia.org/wiki/Focus_(optics)http://en.wikipedia.org/wiki/Plane_(mathematics)http://en.wikipedia.org/wiki/Ray_(optics)http://en.wikipedia.org/wiki/Optical_system


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http://www.google.cn/imgres?imgurl=http://astro.berkeley.edu/~jrg/Aberrations/astigmatism8.jpg&imgrefurl=http://astro.berkeley.edu/~jrg/Aberrations/node8.html&h=309&w=375&sz=25&tbnid=Cp3mqGxOK9yqpM::&tbnh=101&tbnw=122&prev=/images%3Fq%3Dastigmatism%2Bpicture&hl=zh-CN&usg=__U3fztATiOp2r6Ic_YvFNAYbYEFI=&ei=tT2ySfXZOca-kAXpo7i0BA&sa=X&oi=image_result&resnum=2&ct=image&cd=1


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http://www.google.cn/imgres?imgurl=http://www.optometrists.asn.au/freestyler/files/Astigmatism.gif&imgrefurl=http://www.optometrists.asn.au/eyevision/disorders/referrors/astigmatism/details&h=700&w=300&sz=123&tbnid=F8bgf8k3idK2XM::&tbnh=140&tbnw=60&prev=/images%3Fq%3Dastigmatism%2Bpicture&hl=zh-CN&usg=__Nf04lVJXEcRWdEk9G0CqK9H_hkI=&ei=tT2ySfXZOca-kAXpo7i0BA&sa=X&oi=image_result&resnum=4&ct=image&cd=1


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http://www.medicinenet.com/astigmatism/article.htm


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Types

Based on focus of the principal meridians

Simple astigmatism

Simple hyperopic astigmatism first focal line

coincides with the retina while the second islocated behind the retina

Simple myopic astigmatism first focal line is

located in front of the retina while the secondfocal line is located on the retina


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Compound astigmatism

Compound hyperopic astigmatism both

focal lines are located behind the retina

Compound myopic astigmatism both

focal lines are located in front of the retina


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Mixed astigmatism focal lines are on

both sides of the retina (straddling the

retina)


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Prevalence

According to an American study published in

Archives of Ophthalmology, nearly 3 in 10

children between the ages of 5 and 17 have

astigmatism A recent Brazilian study found that 34% of the

students in one city were astigmatic. Regarding

the prevalence in adults, a recent study in

Bangladesh found that nearly 1 in 3 (32.4%) ofthose over the age of 30 had astigmatism


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A number of studies have found that the

prevalence of astigmatism increases with

age


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Diagnosis

1. Symptoms

Although mild astigmatism may be

asymptomatic, higher amounts of

astigmatism may cause symptoms such

as blurry vision, squinting, asthenopia,

fatigue, or headaches
http://en.wikipedia.org/wiki/Asthenopiahttp://en.wikipedia.org/wiki/Asthenopia


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2.Signs and tests

A keratometermay be used to measure

the curvature of the steepest and flattest

meridians in the cornea's front surface. A

may also be used to obtain a more

accurate representation of the cornea's

shape.
http://en.wikipedia.org/wiki/Keratometerhttp://en.wikipedia.org/wiki/Keratometer


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Treatment

Astigmatism may be corrected with

eyeglasses, contact lenses, or refractive

surgery. Various considerations involving

ocular health, refractive status, andlifestyle frequently determine whether one

option may be better than another


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In those with keratoconus, rigid gas permeablecontact lenses often enable patients to achieve

bettervisual acuities than eyeglasses. If the

astigmatism is caused by a problem such as

deformation of the eyeball due to a chalazion,treating the underlying cause will resolve the

astigmatism. Should the person suffer from

severe astigmatism, they will be advised to wear

hard rigid gas permeable lenses rather than themore comfortable soft lenses

Presbyopia
http://en.wikipedia.org/wiki/Keratoconushttp://en.wikipedia.org/wiki/Visual_acuityhttp://en.wikipedia.org/wiki/Chalazionhttp://en.wikipedia.org/wiki/Chalazionhttp://en.wikipedia.org/wiki/Visual_acuityhttp://en.wikipedia.org/wiki/Keratoconus


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Presbyopia

Presbyopia is a vision condition in which

the crystalline lens of your eye loses its

flexibility, which makes it difficult for you to

focus on close objects


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Presbyopia may seem to occur suddenly,

but the actual loss of flexibility takes place

over a number of years. Presbyopia

usually becomes noticeable in the early tomid-40s. Presbyopia is a natural part of

the aging process of the eye. It is not a

disease, and it cannot be prevented.


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Similar to grey hair and wrinkles,

presbyopia is a symptom caused by thenatural course of aging


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The ability to focus on near objects

declines throughout life, from an

accommodation of about 20 dioptres

(ability to focus at 50 mm away) in a childto 10 dioptres at 25 (100 mm) and leveling

off at 0.5 to 1 dioptre at age 60 (ability to

focus down to 1-2 meters only).
http://en.wikipedia.org/wiki/Accommodation_(eye)http://en.wikipedia.org/wiki/Dioptrehttp://en.wikipedia.org/wiki/Dioptrehttp://en.wikipedia.org/wiki/Accommodation_(eye)


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Symptoms

The first symptoms most people notice are

difficulty reading fine print, particularly in

low light conditions, eyestrain when

reading for long periods, blur at near ormomentarily blurred vision when

transitioning between viewing distances.

Many advanced presbyopes complain thattheir arms have become "too short" to hold

reading material at a comfortable distance
http://en.wikipedia.org/wiki/Asthenopiahttp://en.wikipedia.org/wiki/Asthenopia


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A delayed onset of seeking correction for

presbyopia has been found among those

with certain professions and those with

mioticpupils.In particular, farmers andhomemakers seek correction later,

whereas service workers and construction

workers seek eyesight correction earlier.
http://en.wikipedia.org/wiki/Miotichttp://en.wikipedia.org/wiki/Pupilhttp://en.wikipedia.org/wiki/Pupilhttp://en.wikipedia.org/wiki/Miotic


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In optics, the closest point at which an

object can be brought into focus by the

eye is called the eye's near point. A

standard near point distance of 25 cm istypically assumed in the design of optical

instruments, and in characterizing optical

devices such as magnifying glasses.
http://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Magnifying_glasshttp://en.wikipedia.org/wiki/Magnifying_glasshttp://en.wikipedia.org/wiki/Optics


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In the classic book, 'Eye and Brain' by Gregory,

for example, the lens is said to be suspended bya membrane, the 'zonula', which holds it under

tension. The tension is released, by contraction

of the ciliary muscle, to allow the lens to fatten,

for close vision. This would seem to imply thatthe ciliary muscle, which is outside the zonula

must be circumferential, contracting like a

sphincter, to slacken the tension of the zonula

pulling outwards on the lens

T


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Treatment

Presbyopia is not routinely curable - theloss of focusing ability can becompensated for by corrective lenses

including eyeglasses orcontact lenses. Insubjects with other refractory problems,convex lenses are used. In some cases,the addition ofbifocals to an existing lens

prescription is sufficient. As the ability tochange focus worsens, the prescriptionneeds to be changed accordingly.
http://en.wikipedia.org/wiki/Corrective_lenseshttp://en.wikipedia.org/wiki/Eyeglasseshttp://en.wikipedia.org/wiki/Contact_lenseshttp://en.wikipedia.org/wiki/Convexhttp://en.wikipedia.org/wiki/Bifocalshttp://en.wikipedia.org/wiki/Bifocalshttp://en.wikipedia.org/wiki/Convexhttp://en.wikipedia.org/wiki/Contact_lenseshttp://en.wikipedia.org/wiki/Eyeglasseshttp://en.wikipedia.org/wiki/Corrective_lenses


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In order to reduce the need for bifocals orreading glasses, some people choose

contact lenses to correct one eye for near

and one eye for far with a method called"monovision


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Surgery

New surgical procedures may also providesolutions for those who do not want to wearglasses or contacts, including theimplantation of accommodativeintraocular lenses (IOLs). , which increasethe space between the ciliary body andlens, have not been found to provide

predictable or consistent results in thetreatment of presbyopia
http://en.wikipedia.org/wiki/Intraocular_lenshttp://en.wikipedia.org/wiki/Intraocular_lens


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Documents

Doctor Lecture4