clinical optics and refractive errors 09

$: Clinical Optics and Refractive Errors 09$
8/8/2019 Clinical Optics and Refractive Errors 09

http://slidepdf.com/reader/full/clinical-optics-and-refractive-errors-09 1/33

CLINICAL OPTICS AND

REFRACTIVE ERRORS



Lens

A lens is a transparent refractingmedium, bounded by two surfaceswhich form a part of a sphere (sphericallens) or a cylinder (cylindrical or toriclens)

Power of a lens is defined as ability of the lens to converge a beam of light

falling on the lens. For a converging(convex) lens the power is taken aspositive and for a diverging (concave)lens power is taken as negative.



Lens

The change in direction of rays of light in the optic system is called refraction.It is measured as reciprocal of the focal

length in meters, i.e. D=1/F. Lenses aremeasured in diopters (D). One diopteris the power of a lens of focal lengthone meter.



Spherical lenses are bounded by

two spherical surfaces and aremainly of two types: convex andconcave. Convex lens (or plus lens)is a converging lens. It may be of

biconvex, plano-convex orconcavo-convex type. Plus lensesare used for the correction of

hypermetropia, presbyopia andaphakia.



Concav e lens (or min us lens ) is a div ergin g lens . It is of further three

types : biconcav e, plano-concav ean d conv exo-concav e. Min us lens es are us ed to correc t myopia.

A c ylin drical l ens ac ts onl y in on e

axis , i.e., power is incorporated in on e axis , the other axis hav in gzero power. A c yl in drical l ens maybe conv ex (pl us ) or concav e(min us ). C yl in drical (or toric )l ens es are presc ribed to correc tas tigmatis m.



The eye as the opticalinstrument

The total dioptric powerof the eye is about +60,OD. The cornea hasan index of refraction of 1.376 and contributesabout +40-42D to theeye. The crystalline lenshas an idex of refractionabout 1.41 with a powerof+16-18D.



Clinical refraction

It is a position of the principalfocus concerning the sensitivelayer of retina

Emmetropia (clinical refraction of optically normal eye). The eyeis considered to be emmetropicif the parallel light rays coming

from infinity are focused at theplane of the retina with the

accomodation being at rest.



Clinical refraction

Hypermetropia (hyperopia)or farsightedness (long-sightedness) is the refractivestate of the eye wherein

parallel rays of light comingfrom infinity are focused behindthe retina (with accomodationbeing rest) for refractive power

is the same in all meridians. Sothe focused image is formedbehind the plane of the retina.This is weak refraction.



Hypermetropia Clinical pictur e:1. A small a mount of r ef racti ve err or in youn g

pati ent s i s u suall y c orr ect ed by mil dacc omodati ve effort wit hout pr oducin g an y

sympt om.2. Ast hen opia i s t he r esult of pr ol on gedacc omodati ve effort s. Ast hen opic sympt omsinclu de tir edn ess of eyes, f r ontal hea dac hes,wat erin g. These a st hen opic sympt oms ar e

especiall y a ssociat ed wit h n ear wor k an dincr ea se t owar ds evenin g.3. The pati ent s c omplain of defecti ve vi si on

mor e for n ear t han di stanc e.



Hypermetropia

Th e signs of h ig h hyp erm etro p ia:

1. Sma ll size of t h e ey eba ll

2. Ant erior ch am ber is com p arati vely s h a llo w

3. Fun dus examination r evea ls a sma ll

o p ti c dis c wh i ch s eems som etim es "s wo llen " an d sim ulat e p a p i llitis



HypermetropiaComplications of uncorrected hypermetropia:

Reccurent styes, blepharitis or chalazia Accomodative convergent squint (usually in

children by age of 2-3 years) Amblyopia Spasm of accomodationIn Russia the cl inic al cl assific ation of

hypermetropia is used:1) Small Hm- (+) 0.25D - (+) 2,OD

2) Average Hm - (+) 2,25D - (+) 5,OD3) High Hm - (+) 5,25D and more.

T r eatm ent o hyp erm etro p ia: to p r escri becon vex (+) len ses, conta ct len ses, r ef ra cti vesur ger y



Myopia

Myopia ornearsightedness (short-sightedness) is therefractive error in which

parallel rays of light coming from infinity arefocused in front of theretina (with

accomodation is at rest).Myopia is strongrefraction



Etiological types of myopia

Axial myopia - the anteroposteriordiameter of the eye is longer thannormal, although the corneal and lenscurvatures are normal.

C urvature myopia - occurs due toincreased curvature of the cornea(congenitally or in keratoconus), lens(as in moderate to severe

hyperglycemia) or both Index myopia - the sclerotic change in

the lens increases the index ofrefraction



Etiological types of myopia

Position al myopia - produced byan terior placemen t of crystallin e len s in the eye

C on gen ital myopia. It is usually presen tsin ce birth, however, it is usuallydiagn osed by the age of 2-3 years

Simple myopia (developmen tal) is the

common est variety. It is con sidered as aphysiological error n ot associated withan y diseases of the eye. Its cause islon g work at a close distan ce.



Clinical picture Simplemyopia

Poor vision for distance is the mainsymptom of myopia. Asthenopicsymptoms may occur in patientswith small degree of myopia.

The myopic eyes typically are largeand somewhat prominent; anterior

chamber is slightly deeper thannormal; fundus is normal. Rarelytemporal myopic crescent may beseen.



Clinical picture Simplemyopia

Pathological (degenerativ e,progress iv e) myopia is a rapidlyprogress iv e error res ulting in highmyopia during early adult life which is us ually ass ociated with degenerativ echanges in the eye.

The etiological hypothes is for

pathological myopia can be s ummarizedas follow: 1) genetic factors (play majorrole); 2) general growth process (plays minor role).



C

linical classification of myopia

1) Small myopia - (-) 0.25D - (-) 3,OD

2) Average myopia - (-) 2.25D - (-)6,OD

3) High myopia - (-) 6,25D and more



Clinical picture High myopia

Blurred distance vision. Due toprogressive degenerative changes anuncorrectable loss of vision may occur;

Night blindness may be complained byvery high myopes having markeddegenerative changes;

Floating black opacities in front of the

eyes as the result of degeneration of thevitreous body.



Clinical picture High myopia The eyes are often prominent. Anterior

chamber is deep. Fundus examination(ophthalmoscopy):

1) Large and pale optic disc 2) Temporal myopic crescent,

sometimes peripapillary crescentencircling the disc may be seen. 3) Degenerative changes in retina and

choroid (white atrophic patches,particulary in the central area); Foster-

Fuch's spot (dark red circular patch dueto subretinal haemorrhage) may bepresent at the macula. Cystoiddegeneration may be seen at theperiphery.



High myopia

Complications

Retinal

detachment Vitreous and

choroidhaemorrhage

Complicatedcataract

Treatment Prescription of concave (-)

lenses (in form of spectaclesand contact lenses)

Surgical treatment

a) strengthening of the posteriorpole (scleroplastics)

b) refractive coneal surgery (itsbasic principle is to decrease

the curvature of cornea), radialkeratotomy, photo-refractivekeratectomy (is perfomed withexcimer laser)



Astigmatism

Astigmatism is a type of refractive errorwherein the refraction varies in the

different meridians. It is a combinationof different types of refraction ordifferent degrees of one kind of refraction in one eye. There are twoprincipal meridians apart 90°



Classification of astigmatism

1. Regular astigmatism - It i nclu des horizont o-verti cal astigmatism , when t wopri nci pal m eri dia ns ar e pla ced i n t hehorizontal a nd verti cal pla nes a nd r ef ra cti ve

power is t he sam e i n one m eri dia n, a ndobli qu e astigmatism , wher e t he t wo pri nci pal m eri dia ns ar e not t he horizontal a nd verti cal .

2. Irr

egular

astigmatism

- It is

chara

ct e

rizedby a n irr egular cha ng e of r ef ra cti ve power i n

one m eri dia n (it is usuall y t he r esult of dis or ders of cor nea ).



Refractive types of regularastigmatism

1. Sim p l e ast . - it is a com bi nati on of E mm et rop ia i n one m eri dia n a ndm yop ia (or hyp erm et rop ia ) i n t he ot her(sim p l e m yop i c or hyp erm et rop i c ast )

2. C om p ou nd ast - it is a com bi nati on of di fferent degrees of t he sam e ki nd of refra cti on (com p ou nd m yop i c ast ,

com p ou nd hyp erm et rop i c ast ) 3. M i xed ast - it is a com bi nati on of

m yop ia i n one m eri dia n a ndhyp erm et rop ia i n t he ot her m eri dia n



Astigmatism

cl in ic al symptoms

defective vision blurring of objects

sometimes -asthenopicsymptoms

Tr eatment

Optical correction -cylindrical lenses in theform of spectacles,contact lenses

Surgical treatment (astigmatic corneal

surgery)



Anisometropia

it is a state in which there is adifference in the refractive errors of thetwo eyes. This condition may becongenital or aquired due to

asymmetric age changes or disease. Inanisometropia the patient may be madevisually uncomfortable by

Visual acuity differences between the

two eyes Aniseikonia: difference in size of the

ocular image in each eye



Anisometropia

Anisophoria: varying heterophoria (muscleimbalance) in different fields of gazedepending on the eye used for fixation

Ambliopia, or strabismus (which may develop

in young anisometropes) T reatmen for ametropiat:

1. T he corrective glasses can be tolerated upto a maximum difference of 2,OD. After that

there occurs diplopia. 2. Contact lenses

3. Surgical treatment (refractive surgery)



Accomodation

The cornea is a static or fixed surface. Thecrystalline lens, however, is capable of increasingits plus power. This is reffered to as focusing, oraccomodation (ability of the eye to see at different distances). The lens is suspended'-in

the eye by thousands of chemical strands, whichform Zinns ligaments, that are attached to theciliary body at one end and the lens capsule at the opposite end. When the ciliary muscle isrelaxed, the strands maintain a slight tension on

the capsule. Constriction results in a slight relaxation of Zinns ligament. The lens becomesmore convex (because of its elasticity), therebyincreasing the power of the lens.



Accomodation

Accomodation is ability of the eye tosee at different distances. Anemmetrope who wants to view a nearby

object contracts the ciliary muscle,which results in an increase of power of accomodation to focus the image backto the plane of the retina.



Accomodation

The nearest point at which small objects canbe seen clearly is called near- point (orpunctum proximum) and the distant (farthest) point is called far- point (or

punctum remotum). The distance betweenthe near point and the far point is called therang e of accomodation. The differencebetween the dioptric power needed to focus

at near point (P) and far point (R) is calledamplitude of accomodation (A).

Thus A = P (±R).



Anomalies of accomodation

paralysis of accomodation, spasm of accomodation

presbyopia.

Paralysis of accomodation . Drugs (atropine, homatropine or other

parasympatholytic drugs)

2. Syphilis, alcoholism, cerebral and

meningeal diseases 3. Complete paralysis of parasympathetic part

of N.oculomatorius (intracranial or orbitalcauses)



Spasm of accomodation

Drugs (strong miotics)

Spontaneous spasm. It usually occurswhen the eyes are used for excessivenear work in unfavourablecircumstances such as bad illumination,

bad reading position, mental stress.



Presbyopia

physiologic decrease in the amplitude of accomodation associated with aging (because

of the decreasing in the elasticity andplasticity of the crystalline lens)

Symptoms: 1) difficulty in near vision; 2)asthenopic symptoms

T reatment : is prescription of convex glassesfor near work.



The average eyeglass for variousage levels (for the emmetrope)

AGE(YEARS) POWER OF GLASSES

40 +1.0D

45 +1.5D

50 +2.0D

55 +2,5D

60 +3.0D

65 and more +3,5D

clinical optics and refractive errors 09

Documents