OPTICS OF AMETROPIA

Presenter: Dr.ShamikaModerator: Dr.Amit C.

28/1/2016

Ametropia is defined as a state of refraction wherein the parallel rays of light coming from infinity are focussed either in front or behind the retina which therefore receives a blurred image.It includes- Myopia Hyperopia Astigmatism Aphakia

HYP

ERM

ETRO

PIA

Long sightedness. Images are focussed behind retina. ie. Posterior focal point is behind retina.

Mechanisms-

1. AXIAL: 1mm shortening of AP diameter of eye causes 3D of hypermetropia.

2. CURVATURAL: curvature of cornea or lens or both is flatter than normal. 1mm decrease in radius of curvature causes 6D of hypermetropia.

3. INDEX: due to change in refractive index of lens in old age and in diabetics.

4. POSITIONAL: posteriorly placed lens.5. ABSENCE OF CRYSTALLINE LENS.

CLINICAL TYPES:1)SIMPLE HYPERMETROPIA: due to biological

variation in size and shape of eyeball. Could be axial or curvatural.

2)PATHOLOGICAL: A)CONGENITAL: Associated with

microphthalmos, micro cornea congenial posterior subluxation of lens or congenital aphakia.B)ACQUIRED:i. SENILE: could be curvatural or index (due

to cortical sclerosis)ii. POSITIONAL: subluxation of lensiii. APHAKIA.iv. CONSECUTIVE:v. ORBITAL MASS: tumors or edema may push

the retina forward.C) FUNCTIONAL: due to paralysis of accommodation as in 3rd nerve palsy.

COMPONENTS OF HYPERMETROPIA:

TOTAL

MANIFEST

FACULTATIVE

ABSOLUTE

LATENT

Total- after complete cycloplegia with atropine.Latent- about 1D of hyperopia that is corrected by inherent tone of ciliary muscles.Manifest- remainder of hyperopia.Facultative- can be corrected by patient’s accommodative effort.Absolute- residual of manifest hyperopia.

MYO

PIA

Short sightedness. Parallel rays coming from infinity focus in front of retina with accommodation at rest.

MECHANISMS:1. AXIAL: increased axial length of the eye

ball. Commonest form.2. CURVATURAL: increased curvature of

cornea or lens or both.3. INDEX: increased refractive index of lens

with nuclear sclerosis.4. EXCESSIVE ACCOMMODATION: in spasm of

accommodation.

In myopia the image of a distant object is formed of the divergent beam.

Far point of the myopic eye is at a finite point in front of the eye.

Nodal point of the eye is further away from the retina. Hence the image of the object formed is larger than that of the emmetropic eye or spectacle corrected eye. This compensates for visual acuity to some extent.

They do not need to accommodate. Hence it is not well developed and they may suffer from convergence insufficiency, exophoria or early presbyopia.

ASTI

GM

ATIS

MAstigmatism is a refractive error in which therefraction varies in different meridia.ETIOLOGY: CORNEAL: due to abnormalities in the

curvature of cornea. Maybe congenital or acquired (often irregular).

LENTICULAR: CURVATURAL: due to abnormal curvature of the

lens. eg- lenticonus. POSITIONAL: due to oblique placement or tilting of

the lens eg-in subluxation. INDEX: due to difference of refractive index of the in

different meridia. RETINAL: due to oblique placement of macula.

ASTI

GM

ATIS

M Regular astigmatism – principal

meridians are perpendicular. With-the-rule astigmatism – the vertical

meridian is steepest (a rugby ball or American football lying on its side).

Against-the-rule astigmatism – the horizontal meridian is steepest (a rugby ball or American football standing on its end).

Oblique astigmatism – the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.

Irregular astigmatism – principal meridians are not perpendicular.

ASTI

GM

ATIS

M Simple astigmatism

Simple myopic astigmatism – first focal line is in front of the retina, while the second is on the retina.

Simple hyperopic astigmatism – first focal line is on retina, while the second is located behind the retina.

Compound astigmatism Compound myopic astigmatism – both

focal lines are located in front of the retina.

Compound hyperopic astigmatism – both focal lines are located behind the retina.

Mixed astigmatism – focal lines are on both sides of the retina (straddling the retina)

FAR

POIN

TFar point of the eye is the position of an object such that its image falls on the retina of the relaxed eye (ie. With accommodation relaxed). For emmetropia it is at

infinity. For myopia it lies at a

finite distance in front of the eye.

In hypermetropia it is virtual (as only converging light can focus on the retina in hyperope).

OPT

ICAL

CO

RREC

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N O

F AM

ETRO

PIA

The purpose of optical correction is to deviate the rays of light so that they appear to come from the far point.

EFFE

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OF

LEN

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In uncorrected hyperopia the image of an object falls behind the retina.The purpose of convex lens is to bring the image forward. If the correcting lens is itself moved forward the image will move still forward.ie- the effectivity of the lens is increased. Thus a weaker lens is required to project the image onto the retina.

Similarly in uncorrected myopia the image falls in front of the retina. The purpose of the concave lens is to bring the image behind. If the correcting lens is itself moved forward the image moves still forward.ie- the effectivity of the lens is reduced.Thus a stronger lens is required to project the image onto the retina

EFFE

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Thus the convex lens in hypermetropia has to be made weaker and the concave lens in myopia has to be made stronger when the lens is moved further away from the eye

Hence aphakics or high hyperopes pull their glasses down their nose to read.While myopes do not like their glasses slipping down.

EFFE

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Formula to calculate the new focal length of lens at the new distance-

F2= 1/ f1- d or F2= F1/ 1- dF1Where, F1= power of the original lens in diopters

F2= power of lens in diopters at new positionf1= focal length in meters of original lensd= distance moved in meters. It is taken positive if

moved toward the eye and negative if moved away from the eye.

BACK

VER

TEX

DIS

TAN

CEFor any prescription greater than 5D especially in aphakics the refractionist must state how far the trial frame was placed, to adjust the power of contact lens is used or if the glasses are to be worn at a different distance.The distance between the back of the lens and the cornea must be measured.Measurement can be made with a ruler held parallel to the arm of the trial frame or slipped through a steanopic slit till it touches thee closed lid. 2mm should be added to correct for the thickness of the lid.

BACK

VER

TEX

DIS

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CEExample 1: A patient has been prescribed glasses with +16.00D sphere at a BVD of 14mm. He selects a frame that fits him at a BVD of 16mm. What is the power of the new lens?

Ans: +15.50D

BACK

VER

TEX

DIS

TAN

CEExample 2: A aphakic patient requires a +10.00D lens at BVD 15mm. He now wants a contact lens. What should be the power of the contact lens?

Ans- +11.75D

BACK

VER

TEX

DIS

TAN

CEExample 3: A patient was given a prescription of -16.00D at a BVD of 14mm. He selects a spectacle frame of BVD 16mm. What will be the power of the new lens?

Ans- -16.50D

BACK

VER

TEX

DIS

TAN

CEExample 4: A high myope whose spectacle correction is -10.00D at BVD 14mm requires a contact lens. What is the power of the contact lens?

Ans- -8.75 D

SPEC

TACL

E M

AGN

IFIC

ATIO

NThe optical correction of ametropia is associated with in a change in the retinal image size.

Spectacle magnification = corrected image size

uncorrected image size

Relative sp. magnification = corrected image size

emmetropic image size

SPEC

TACL

E M

AGN

IFIC

ATIO

NIn axial ametropia, if the correcting lens is placed at the anterior focal point of the eye then the image size is same as that of emetropia.But in refractive ametropia the image size differs even if lens is placed at the anterior focal point.

In refractive hypermetropia the image size is increased. RSM>1While in refractive myopia the image size is reduced.RSM<1

As the distance of the lens approaches the eye the image size approaches the emmetropic size

SPEC

TACL

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AGN

IFIC

ATIO

NRSM= 1.36 for aphakia with lens at anterior focal point ie 23.2mm

RSM= 1.33 for aphakia with lens placed at 12-15mm

RSM= 1.1 for contact lenses.

normalcontact lens

spectacles

OPT

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PRO

BLEM

S IN

APH

AKIA

1. SPECTACLE MAGNIFICATION:

The spectacle magnification produced by aphakic glasses is 1.33. thus the image is one third times larger than emmetropes.The patient thus tends to misjudge distances.Objects appear closer to the eye than they are.Leads to enhanced performance in visual acuity tests.

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AKIA

2. DISTORTION OF IMAGES DUE TO ABERRATIONS:

Straight lines appear curved except through a small central portion of the lens. At the periphery of the lens the lines appear to be more curved- pincushion effect.Thus the environment appears as curves as the patient moves his eyes across different parts of the lens. Patients adapt to this by moving their head rather than eyes.

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3) PRISMATIC EFFECT OF LENS:

The prismatic effect increases towards the periphery of the lens.It produces a troublesome ring scotoma at the edge of the lens. Hence they can trip over unseen objects.The direction of the ring scotoma changes and objects disappear into the scotoma and appear to reappear out of it- jack in the box phenomenon.

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4) DUE TO WEIGHT OF THE GLASSES:

Aphakic glasses are very heavy and tend to slip down the nose. Plastic glasses are lighter but less scratch resistant.Lenticular form of lenses reduce weight but also reduce field of vision.

OPT

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BLEM

S IN

APH

AKIA

5) UNILATERAL APHAKIA WITH NORMAL FELLOW EYE:The image in aphakic eye is one third larger hence causes aniseikonia. Patient is unable to fuse these images and hence suffers from diplopia.The use of contact lenses and intra ocular implants reduce this effect.Aniseikonic glasses though available are very heavy and costly.

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