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Refraction Dr.Ani, SpM

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Refraction

Dr.Ani, SpM

Refraction

• Consists of :

– General Optics

– The optical system of the eye

– Clinical anomalies : refractive errors

Optic

• Dioptri (D) : Lens power unit, is an inverse of focal distance in meters

D = 1/f

• 1 D lens, parallel light will be directed into focal spot in 1 meter distance

2 D = 1/f ----> f = ?

If f = 25 cm , ----> D = ?

• Parallel rays will be converged to the focus ---> Plus lens (+)

• or will be diverged as if it comes from the focus ----> Minus Lens (-)

• Rays coming from distance > 5 m

parallel rays

• Rays coming from distance < 5m

divergent rays

Principles

• Spherical lens– Is a lens with the same curvature diameter in

all meridians

Spherical Convex (+) Spherical Concave (-)

• A convex lens may be regarded as a series of prisms bases toward the middle of the lens

• A concave lens may be regarded as a series of prisms apex toward the middle of the lens

• Prismatic Effect that occur on eye glasses explain :– Against motion with (+) Lens – With motion, with (-) Lens

• Spherical Lens :– Plus sphere : Convex

• characteristic : makes larger and nearer images

Biconvex Plano K

+2 +2 0 +4

Concave K

+5 -1

• Minus sphere : Concave– Characteristic : makes smaller and farther images

Bi Concave Plano K Convex K

• Parallel rays will be centered or diverged from the focus

-2 -2 -40 +1 -5

Cylindrical Lens• Is a kind of lens that have two

meridians that are perpendicular to each other

• The meridian that has no power is called the axis

• The other meridian, has the power

• Spherocylindrical Lens– Is a combination between spherical lens and

cylindrical lens– Example :

• S + 2.00 D C + 1.00 D X 90 0

+

+ 2.00

+ 2.00

0.00

+ 1.00

+ 2.000.00

+ 2.00+ 1.00

+ 2.00

+ 3.00

• Transposition– Methods :

• Sphere : Sum with algebra ways SPH + CYL• Cylinder : replace power marks (Neg Pos),

axis change 90 degrees• Example : S + 2.00 C + 1.00 X 90

S + 3.00 C - 1.00 X 180

0

0

Eye as an Optical Instrument

• Refraction media :– Cornea n = 1.33– Humour Aqueous n = 1.33– Lens n = 1,41– Vitreous body n = 1.33

• Haziness on refraction media --> disturbances of vision

• Power of refraction of the eye ball– Totally : 60 dioptri– Cornea : 40 dioptri– Lens : 20 dioptri

• Accommodation Process– Capability of adding the refraction power of the

eye, by increasing the convexity of the lens

– normal : rays that come from > 5 m - distance

object regarded as parallel light; the eyes are in

relax position, the images are focused right on

the retina (fovea centralis)

• For object at less than 5 meters distance, the rays do not come parallel but divergent. If the eyes are still in relax position, the images will be focused behind the retina. So the object will be seen blurred. These images must be moved forward so it will be focused on the retina by increasing the convexity of the lens. This process is called accommodation

process.

• This accommodation

process happens as a

result from the

contraction of M. ciliaris

in the ciliary body

• These reflexes also happen during the accommodation process :– Accommodation– Miosis– Convergents

Near Reflex

Refraction Anomalies

• Normal : Emetropia

• Anomalies : (ametropia)• Myopia

• Hypermetropia

• Astigmatism

• Presbiopia

• Emmetropia– Is the condition when the parallel rays focused

exactly on the retina of the eye in relax condition ---> the visual acuity is maximum

• Ametropia– Is the condition when the parallel rays are not

focused exactly on the retina of the eye in relax condition.

– The focal point may be behind or in front of the retina

Hal 47, 4.2 Duke Elder

• Myopia– Refractive condition in which, with

accommodation completely relaxed, parallel rays are brought to a focus in front of the retina.

– Myopic eye : refractive state over plus power

• Factors that causing myopia :– Axial : The antero-posterior axis of the eye ball > normal

• in this case, the refraction power of the cornea, lens and the lens position are normal. The eye usually looks like proptosis

– Curvature : • The size of the eye ball ---> normal, but there is a increasing of the

cornea/lens curvature

• The change of the lens e.g. : intumescens cataract

– Increasing of the refraction index• could occur on Diabetic patient

– Changes of the lens location• changes of the lens position to the anterior after glaucoma surgery

• lens subluxation

• Clinical findings :– Farsightedness are blurred, nearsightedness are normal

– Asthenopia

– On high myopia : hemeralopia occurred caused by periphery retinal degeneration

– Floating spots visualization caused by vitreous degeneration

– screw up the eye lids together, in order to get a better vision

• On high myopia ----> proptosis simulation, deep Anterior Chamber

• Funduscopy : Tigroid fundus ---> thin retina and the choroid, myopic crescent arround the papilla area, sthaphyloma posterior

• Complication :– Commonly occurred on high myopia

1. Degenarated and liquefied vitreous

2. Retinal detachment

3. Pigmentation changes + Macular bleeding

4. Strabismus

• Myopia classification :– < 3.00 D = low myopia– 3.00 - 6.00 D = moderate myopia– > 6.00D = high myopia/gravis

• Treatment :– Low and moderate myopia : full correction with

weakest spherical lens that give the best visual acuity• Example :

VOD = 5/60 S -2.50 D = 6/7

S -2.75 D = 6/6

S -3.00 D = 6/6

S -3.25 D = 6/7

The glasses are S - 2.75 D

– On high myopia, usually full correction are not given due to headache that may occurred. If necessary, reading glasses can be given ---> bifocal glasses

• Prognosis :

– Simplex/stationer, after puberty will be

constant

– Progressive myopia, the myopia will be

continuously higher and complication may

occurred

Hypermetropia• Is a refraction anomaly that without accommodation

parallel rays will be focused behind the retina

• Divergent rays from near object, will be focused farther behind the retina

• Etiology :

– Axial ---> eye ball diameter < N

– Deminished convexity of cornea/lens curvature

– Decreasing Refractive index

– Changed lens position

• Clinical manifestation :– H. Manifest ---> is detected without

paralazing accommodation and is represented by the strongest convex glass needed , the patient sees most distinctly. It correspons to the amount of accommodation which he relaxes when a convex lens is placed before the eye. Devided into two types :

• Facultative : Can be overcome by an effort of accommodation

• Absolute : Can not be overcome

– Total Hipermetrop : detected after the

accommodation has been paralyzed with

cylcopegic agents

– Latent Hypermetrop : is the diference of the

total hypermetrop with the manifest

hypermetrop

Latent Hypermetrop

Hypermetrop manifest

Hypermetrop

• Clinical finding :

– Nearsightness are blurred

– High hypermetropia at old age : farsightedness

also blurred

– Astenophia accommodative (eye strain)

– Children : high hypermetropia usually

occurring convergent strabismus (convergent

squint)

• Treatment :

– If foria/tropia not present, apply strongest

positive spherical lens that give the best visual

acuity

– If foria/tropia present, total hypermetrop

correction. If necessary : bifocal eye glasses

astigmatism• Refractive condition of the eye in which there is a

difference in degree of refraction in diferent meridian, each will focused parallel rays at a different point. The shape of the images :– Line, oval, circle, never a point

• Manifestation :– Regular astigmatism

• Difference in the degree of refraction in every meredian.

• Two principles meridian :– Maximmum refraction

– Minimum refraction

– Irregular astigmatism• Difference in refraction not only in different

meridians, but also in different parts of the same meridian.

Right angle to each other

• Etiology of astigmatism :– Corneal curvature disturbances ---> 90%

– Lens curvature disturbances ---> 10%

• Type of Astigmatism :– Ast. M. Simplex C-2.00 X 90

– Ast. H. Simplex C+2.00 X 45

– Ast. M Compositium S-1.50 C-1.00 X 60

– Ast. H Compositium S+3.00 C+2.00 X 30

– Ast. Mixtus S+2.00 C-5.00 X 180

0

0

0

0

0

Ast. M. Simplex Ast. H. Simplex

Ast. M Compositium Ast. H Compositium

Ast. Mixtus

Presbiopia

• Physiological changes because accommodation capability is lowering at old age

Accommodation

Age

16

10

6

2

10 20 40 50 60

• Presbiopia correction :– 40 years old S + 1.00 D– 45 years old S + 1.50 D– 50 years old S + 2.00 D– 55 years old S + 2.50 D– 60 years old S + 3.00 D

• Consider the type of previous/history work– Tailor– Architect– Weld engineer

Refraction Examination Technique

• Subjective :– Snellen chart/projector, alphabet , inverse E, picture,

Landolt ring– Trial lens– Trial frame

• Objective :– Children, incooperative, difficult correction, strabismus :

• Ophthlamoscopy• Retinoscopy• Refractometer

• Subjective– Check firstly just one eye : OD– Distance : 5 or 6 meters– VOD : …...(basic right eye visus)

a. Trial and error• apply S + 0.50, better visus , add S+ until visus = 6/6

• S +0.50, lower visus, change to S -, increase S - until visus = 6/6

• S +/- not working ----> cylindrical

• With astigmatism dial, stenoplic slit, cross cylinder

• astigmatism dial :– Blurred line ----> C negative lens axis

• One by one fogging

– S + sp. Lens --> blurred vision, step by step distracting

---> best sp.

– Both eyes at one time at required distance : use jaeger

chart

– Example :I. AVOD 2/60 S - 3.50 = 6/6

AVOS 3/60 S - 3.00 = 6/6

II.AVOD 2/60 S - 3.00 = 6/7

AVOS 3/60 S - 2.75 = 6/7

Give Eye Glasses according to II

ODS 6/6headache, eye strain

ODS 6/6w/o headache, eye strain

• Objective– Use cyclopegic

1. Ophthlamoscopy : papilla clearly seen with which lens

2. Retinoscopy : • Ordinary ---> light source outside

• streak -----> light source inside

3. Refactometer• Computerized

• Lensmeter principal

• Ideally :– Subjective– Objective with cyclopegic– Subjective once more without cyclopegic

• Lens meter– Measuring lens power– Measuring focus distance

• Measuring Pupillary Distance– drop the flashlight light onto both eyes, the

light is coming from in front of the patient, The patient look at the observer forehead or the light ----> measure the distance of light spot between OD and OS ----> as near pupil distance

– Far distance : • add 2 mm ---> for pupil distance less than 60

mm. • Add 3 mm ---> for pupil distance more than

60 mm

• Eye Glasses• Monofocal• Bifocal• Progressive

• Eye Glasses Prescription, the components are :– Which eye (OD or OS)

– Power of the lens ( + or - , Power, axis)

– Pupil distance far/near

– Name of the patient

Binocular Optical Defects

• Anisometropia :– Condition wherein the refractions of the two eyes are

an equal– variation : Myopia M

M. E.

H. E.

H. H.

M. H

Antimetropia

• Vision in Anisometrop– difference < 2.50 D : still get fusion + single

binocular vision– difference > 2.50 D : fusion difficulties ---->

weak eye suppression ---> amblyopic– alternans vision : left and right alternate

• Aniseikonia :– The difference of shape and size of the images

between right and left eye

• Limitation of the eye glasses– cannot applied for anisometropia more than 2.50

Dioptri– anisometropia causing aniseikonia

• Contact lens : Hard ---> rigid lensSoft

– Indication :• High anisometropia• irregular astigmatism• Front asymmetry, orbit• Aniridia• Descemetocele• Sports • Cosmetics