Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Ophthalmology PPT 10 Refraction II Lecture Notes
Department of Ophthalmology
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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
? Understand what is refraction.
? Have basic knowledge of hypermetropia and astigmatism
and their management.
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What is Refraction
? 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
different angle, i.e., they are refracted.
? Except when the rays are normal
Refraction in Ophthalmology
? Methods for evaluating the optical and refractive
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
rest.
? Clear image of a distant object formed without any
internal adjustment of the optics of the eye.
? Absence of emmetropia = Ametropia
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Refractive errors
Anomalies of the optical state of the eye
? Myopia
? Hypermetropia
? Astigmatism
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Hypermetropia
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Hypermetropia
? Refractive or Diopteric state of eye wherein
incident parallel rays of light coming from
infinity are focused behind the retina with
accommodation being at rest.
? Near images can be blurred unless there is
sufficient accommodation, as in a child.
? They have blurred images for distant objects also
? Most children are born about +3 D hyperopic,
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.
? In this condition the total refractive power of eye is
normal but there is axial shortening of eye wall.
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? Each millimeter of shortening represents approximately
3D of refractive change and thus a hypermetropia of over
6D is uncommon.
? Physiological: Infant, child.
? Pathological: Orbital tumour, or inflammatory mass may
indent the posterior pole of the eye and flatten it
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? Curvature Hypermetropia : When the radius of
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.
? Index Hypermetropia : Usually manifests itself as a
decrease in the effective refractivity of the lens and is
responsible for the hypermetropia which occurs
physiologically in old age and pathologically in diabetes.
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? Positional Hypermetropia : Posterior placed lens
also produced hypermetropia whether it occurs as a
congenital anomaly or as a result of trauma and disease.
? Aphakia : Surgical, posterior dislocation of lens
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Clinical Types:
? Simple Hypermetropia : Commonest form.
? It results from normal biological variations in the
development of eye e.g., axial and curvatural.
? Pathological Hypermetropia : Either congenital or
acquired conditions of eyeball which are outside the
normal biological variation of development e.g.
? index , positional (Aphakia).
? Functional Hypermetropia : Results from paralysis
of accommodation as seen in patients with third nerve
palsy.
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Components of hypermetropia
Total hypermetropia = Latent+manifest (facultative +
absolute
Accommodation in Hypermetropia
? Contraction of the ciliary muscle in the act of
accommodation increases the refractive power of the
lens so that it corrects a certain amount of
hypermetropia.
? Normally there is an appreciable amount corrected by
the contraction involved in the physiological tone of this
muscle.
? Consequently the full degree of hypermetropia is
revealed only when this muscle is paralysed by the use of
a drug such as atropine.
? This is called latent hypermetropia, normally 1D.
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Manifest Hypermetropia consists of:
? Facultative Hypermetropia: Corrected by
the effort of accomodation
? Absolute Hypermetropia: Cannot be
overcome by effort of accomodation
? As tone of ciliary muscle decreases with age,
some latent hypermetropia becomes manifest
? As range of accomodation reduces with age,
more facultative hypermetropia becomes
absolute, all of it after age 60.
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Symptoms
? Vary with degree of hypermetropia and accomodative
effort
? Blurred vision: near>distant
? Accomodative asthenopia
? 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
? Smaller cornea
? Shallow anterior chamber predisposes to angle closure
glaucoma since size of lens is normal
? Apparent divergent squint
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? Retina : Have peculiar sheen : a reflex effect so
called "shot silk retina" on ophthalmoscopic
finding.
? Optic disc : Characteristic appearance which
may resemble an optic neuritis (Pseudopapillitis).
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Treatment
? In young children below the age of 6-7 years, some
degree of hypermetropia is physiological and a
correction need be given only if the error is high or if
strabismus is present.
? In those between 6 and 16 years especially when they
are working strenuously at school smaller error may
require correction.
? Required in middle aged patient, in high
hypermetropia and if patient is having symptoms
? Optical:
Glasses
Contact lens
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? Convex lenses prescribed after full cycloplegic
refraction, particularly in children
? Child with convergent squint may need "full
atropine correction"
? Contact lens power is a little more than spectacle
power
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Surgical
? Conductive keratoplasty.
? Non contact Holmium YAG laser
thermokeratoplasty 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 where
in the refraction varies in the different meridia.
? Consequently the ray of light entering in the eye
cannot converge to a point focus but form focal
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 flatter meridian.
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1. Corneal Astigmatism e.g. keratoconus
2. Lenticular Astigmatism
(i) Curvatural ? e.g. lenticonus
(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
are placed at right angles to one another but the vertical
meridian is more curved then horizontal- more common.
2. Against the rule astigmatism : Horizontal meridian is
more curved than the vertical meridian.
3. Oblique astigmatism : Is a type of regular astigmatism
where the two principal meridia are not horizontal and
vertical though they are at right angles to one another
(45 and 135 deg)
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Oblique astigmatism :
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one eye and at 150o in the other eye.
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
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
two focal lines in relation to retina,
regular astigmatism is further
classified
? Simple : Where one of the foci
falls upon retina, the other focus
may fall in front of or behind so
that one meridian is emmetropic
the other is either hypermetropic
or myopic.
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2. Compound : Where neither of
two foci lie upon the retina but
both are placed in front or
behind it.
The state of the refraction is
then entirely hypermetropic or
entirely 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 the
retina so that the refraction is hypermetropic in one
direction and myopic in the other.
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? Irregular Astigmatism : Refraction in different
meridia are irregular.
Etiological types:
1. Curvatural irregular astigmatism: irregular healing of
cornea after trauma and inflammation (particularly
ulceration & keratoconus)
2. Index irregular astigmatism : incipient cataract
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Symptoms
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
(Toric contact lenses with prism ballast)
? Surgical
1. Astigmatic keratotomy: Limbal Relaxing Incision,
arcuate keratectomy, removal of sutures
2. Photo-astigmatic refractive keratotomy (PARK)
3. Laser: Excimer laser: LASIK or Femtosecond laser
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Guidelines for Optical treatment
1. If the patient does not complain of asthenopic
symptoms small astigmatic errors (0.5 D or less)
generally do not require correction
2. If asthenopic symptoms are there, error should be
corrected by cylindrical lenses.
3. Undercorrect the error initially
4. At a later date, full correction may be worn comfortably.
This post was last modified on 07 April 2022