Download MBBS Ophthalmology PPT 50 Refraction II Lecture Notes

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Acknowledgement

? Photographs in this presentation are courtesy of

Kanski's Clinical Ophthalmology.

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Learning Objectives

At the end of the class, students shall be able to

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? Understand what is refraction.
? Have basic knowledge of hypermetropia and astigmatism

and their management.

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Question

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? A patient with a corneal scar is carefully

refracted. Best corrected visual acuity is 6/12.

With a pinhole over his correction, his acuity

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improves to 6/6. The best explanation for this is

? a. spherical aberration.
? b. myopic astigmatism.

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? c. cataract.
? d. irregular astigmatism.
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What is Refraction

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? When rays of light traveling through air enter a

denser transparent medium, the speed of the
light is reduced and the light rays proceed at a

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different angle, i.e., they are refracted.

? Except when the rays are normal
Refraction in Ophthalmology
? Methods for evaluating the optical and refractive

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state of the eye

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Emmetropia

? Parallel light rays, from an object more than 6 m away, are

focused at the plane of the retina when accomodation is at

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rest.

? Clear image of a distant object formed without any

internal adjustment of the optics of the eye.

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? Absence of emmetropia = Ametropia


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Refractive errors

Anomalies of the optical state of the eye

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? Myopia

? Hypermetropia

? Astigmatism

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Hypermetropia
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Hypermetropia

? Refractive or Diopteric state of eye wherein

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incident parallel rays of light coming from

infinity are focused behind the retina with

accommodation being at rest.

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? Near images can be blurred unless there is

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sufficient accommodation, as in a child.

? They have blurred images for distant objects also
? Most children are born about +3 D hyperopic,

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but this usually resolves by age 12 years.

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Types

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Axial Curvature Index Positional Absence of lens

? Axial is the commonest form.

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? In this condition the total refractive power of eye is

normal but there is axial shortening of eye ball.

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? Each millimeter of shortening represents approximately

3D of refractive change and thus a hypermetropia of over
6D is uncommon.

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? Physiological: Infant, child.

? Pathological: Orbital tumour, or inflammatory mass may

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indent the posterior pole of the eye and flatten it
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? Curvature Hypermetropia : When the radius of

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curvature of any of the refracting surfaces is increased,

? congenitally (cornea plana) or as a result of trauma

? Increase of 1 mm produces a hypermetropia of 6 D.

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? Index Hypermetropia : Usually manifests

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itself as a decrease in the effective refractivity of

the lens and is responsible for the hypermetropia

which occurs physiologically in old age and

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pathologically in diabetes.
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? Positional Hypermetropia : Posterior placed lens

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also produced hypermetropia whether it occurs as a

congenital anomaly or as a result of trauma and disease.

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? Aphakia : Surgical, posterior dislocation of lens

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Clinical Types:

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? Simple Hypermetropia : Commonest form.

? It results from normal biological variations in the

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development of eye e.g., axial and curvatural.

? Pathological Hypermetropia : Either congenital or

acquired conditions of eyeball which are outside the normal

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biological variation of development

? Example: index , positional (Aphakia).

? Functional Hypermetropia : Results from paralysis of

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accommodation as seen in patients with third nerve palsy.
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Components of hypermetropia

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Total hypermetropia = Latent+manifest
(facultative + absolute)
Accommodation in Hypermetropia
? Contraction of ciliary muscle in the act of

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accommodation increases the refractive power of
the lens so that it corrects a certain amount of
hypermetropia.

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? Normally there is an appreciable amount corrected by

contraction involved in physiological tone of ciliary muscle.

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? Consequently the full degree of hypermetropia is revealed

only when this muscle is paralysed by the use of a drug such

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as atropine.

? This is called latent hypermetropia, normally 1D.
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Manifest Hypermetropia consists of:

? Facultative Hypermetropia: Corrected by effort of

accomodation

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? Absolute Hypermetropia: Cannot be overcome by

effort of accomodation

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? As tone of ciliary muscle decreases with age, some latent

hypermetropia becomes manifest

? As range of accomodation reduces with age, more

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facultative hypermetropia becomes absolute, all of it

after age 60.

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Symptoms

? Vary with degree of hypermetropia and accomodative effort

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? Blurred vision: near>distant

? Accomodative asthenopia

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? Convergent squint due to continuous effort of accomodation,

excess of convergence leads to dissociation of muscle balance

? Early onset of presbyopia

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Signs

? Small eyeball

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? Smaller cornea

? Shallow anterior chamber predisposes to angle closure

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glaucoma since size of lens is normal

? Apparent divergent squint

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? Retina : Has peculiar sheen : a reflex effect so

called "shot silk retina" on ophthalmoscopy.

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? Optic disc : Characteristic appearance which

may resemble optic neuritis (Pseudopapillitis).
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Treatment

? In young children below the age of 6-7 years:

some degree of hypermetropia is physiological

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and a correction need be given only if the error is
high or if strabismus is present.

? In those between 6 and 16 years:

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smaller error may require correction.

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? Refractive correction is required

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? in middle aged patients
? in high hypermetropia
? and if patient is symptomatic

? Optical:

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Glasses
Contact lens
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? Convex lenses prescribed after full cycloplegic

refraction, particularly in children

? Child with convergent squint may need "full

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atropine correction"

? Contact lens power is a little more than spectacle

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power

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Surgical Options

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? Conductive keratoplasty.

? Non contact Holmium YAG laser thermokeratoplasty

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for lower hypermetropia (+1D ? 2.5 D).

? Phakic Intraocular lens (+6D ? +10 D)

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Astigmatism

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Astigmatism

? Astigmatism is a type of refractive error wherein

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the refraction varies in different meridia.

? Consequently rays of light entering the eye

cannot converge to a point focus, but form focal

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lines.

Astigmatism

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Regular

Irregular
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Astigmatism

? Light rays passing through a steep meridian are

deflected more than those passing through a

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flatter meridian.

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1. Corneal Astigmatism e.g. keratoconus

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2. Lenticular Astigmatism

(i) Curvatural ? e.g. lenticonus

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(ii) Positional ? subluxation

(iii) Index ? cataract

3. Retinal astigmatism ? due to oblique placement of macula.

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Types of Regular Astigmatism

1. With the rule astigmatism : The two principal meridia

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are placed at right angles to one another but the vertical

meridian is more curved than horizontal meridian

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(most common type)

2. Against the rule astigmatism : Horizontal meridian is

more curved than the vertical meridian.

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3. Oblique astigmatism : Is a type of regular astigmatism

where the two principal meridia are not horizontal and

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vertical , though they are at right angles to one another

(45 and 135 deg)

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Oblique astigmatism :

aCy

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sa

xi lr

s in

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m l

eqa

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e S:

t

du y

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ri

enir(i

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s )

mm

cal

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ed et

rical

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in both eyes.

in

30o

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a r

t equiCy

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rl l

ed Com

i

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enn

s d

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rip:

ca(i

l l i)

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ementary

one eye and at 150o in the other eye.

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4. Bi-oblique astigmatism : In this type of regular

astigmatism the two principal meridia are not at
right angles to each other, one eye at 30o and other
at 100o.

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? Optics of regular astigmatism : In regular

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astigmatism the parallel rays of light are not focused on a

point but form two focal lines ? Sturm's conoid

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Refractive types of Regular astigmatism

? Depending upon the position of

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two focal lines in relation to retina,

regular astigmatism is further

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classified

? Simple : Where one focus falls

upon retina, the other focus may

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fall in front of or behind, so that

one meridian is emmetropic the

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other is either hypermetropic or

myopic.

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Compound : Where neither of two

foci lie upon retina but both are

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placed in front or behind it.

The state of the refraction is then

entirely hypermetropic or entirely

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myopic. The former is known as
compound hypermetropic, the latter
as compound myopic astigmatism.

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3. Mixed : Where one focus is in front of and other behind

retina so that the refraction is hypermetropic in one

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direction and myopic in the other.
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? Irregular Astigmatism : Refraction in different

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meridia are irregular.

Etiological types:

1. Curvatural irregular astigmatism: irregular healing of

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cornea after trauma and inflammation (particularly
ulceration & keratoconus)

2. Index irregular astigmatism : incipient cataract

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Symptoms

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1. Defective vision
2. Blurring of objects
3. Asthenopic symptoms - eyeache and headache
4. Running of lines
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Treatment

? Optical ? Spectacles with cylindrical lenses, Contact lens

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(Toric contact lenses with prism ballast)

? Surgical

1. Astigmatic keratotomy: Limbal Relaxing Incision,

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arcuate keratectomy, removal of sutures

2. Photo-astigmatic refractive keratotomy (PARK)

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3. Laser: Excimer laser: LASIK or Femtosecond laser

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Guidelines for Optical treatment

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1. If the patient does not complain of asthenopic

symptoms small astigmatic errors (0.5 D or less)
generally do not require correction

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2. If asthenopic symptoms are present , error should be

corrected by cylindrical lenses.

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3. Undercorrect the error initially

4. At a later date, full correction may be worn comfortably.
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Question

? In a patient with astigmatism, all of the following are

true of myopia and hyperopia except

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? a. In simple myopic astigmatism, one focal line lies in

front of the retina and the other is on the retina.

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? b. In compound myopic astigmatism, both focal lines lie

in front of the retina.

? c. In simple hyperopic astigmatism, both focal lines lie

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behind the retina.

? d. In mixed astigmatism, one focal line lies in front of the

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retina and one lies behind the retina.

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Thank you

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