Download MBBS (Bachelor of Medicine, Bachelor of Surgery) 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
? Understand what is refraction.
? Have basic knowledge of hypermetropia and astigmatism
and their management.
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Question
? A patient with a corneal scar is carefully
refracted. Best corrected visual acuity is 6/12.
With a pinhole over his correction, his acuity
improves to 6/6. The best explanation for this is
? a. spherical aberration.
? b. myopic astigmatism.
? c. cataract.
? d. irregular astigmatism.
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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.
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? 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 ball.
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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.
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? 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
? Example: 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 ciliary muscle in the act of
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.
? 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 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 : Has peculiar sheen : a reflex effect so
called "shot silk retina" on ophthalmoscopy.
? 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
and a correction need be given only if the error is
high or if strabismus is present.
? In those between 6 and 16 years:
smaller error may require correction.
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? Refractive correction is required
? in middle aged patients
? in high hypermetropia
? and if patient is symptomatic
? 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 Options
? 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 wherein
the refraction varies in different meridia.
? Consequently rays of light entering 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 than horizontal meridian
(most common type)
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 focus 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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Compound : Where neither of two
foci lie upon 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
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 present , error should be
corrected by cylindrical lenses.
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
? a. In simple myopic astigmatism, one focal line lies in
front of the retina and the other is on the retina.
? b. In compound myopic astigmatism, both focal lines lie
in front of the retina.
? c. In simple hyperopic astigmatism, both focal lines lie
behind the retina.
? d. In mixed astigmatism, one focal line lies in front of the
retina and one lies behind the retina.
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Thank you
This post was last modified on 07 April 2022