Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Ophthalmology PPT 9 Refraction I 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 myopia and its 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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Progress of refractive state of eye

? Birth : +2 to +3 D

? 90% of children at age 5 yrs are Hypermetropic

? 50% of children at age 16 yrs are Hypermetropic

? After the period of growth has passed the refractive state

tends to remain stationary, until in old age a further
tendency of hypermetropia is evident.

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? Normal axial length 24 mm
? Change in axial length by 1mm = ?3D

? Refraction at corneal surface= +40 to 45(+43)D
? Change in Corneal Curvature by 1mm = ?6D

? Refraction by unaccomodated lens= +16 to

20(+17)D

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Angle kappa ()

? M = Macula

? D= Centre of pupil, on cornea

? N = Nodal point

M

Optic axis

D

N

(

B

FD = Pupillary line

F

FNM = Visual axis

= "Between the visual axis and pupillary line, hence roughly corresponds to

angle ".

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Anisometropia

? Anisometropia is a state in which there is a difference in

the refractive errors of the two eyes, i.e., one eye is
myopic and the other hyperopic, or both are hyperopic or
myopic but to different degrees.

? This condition may be congenital or acquired due to

asymmetric age changes or disease.

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Refractive errors

Anomalies of the optical state of the eye

? Myopia

? Hypermetropia

? Astigmatism

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What is Myopia ?

? Diopteric condition of the eye

where parallel incident rays from

optical infinity

focus anterior to light sensitive layers

of retina

when accomodation is at rest

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Myopia ? Optics

Emmetropia

Diverging lens
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Optics of Myopic eye

? Far point is at a finite distance inversely proportional to

the degree of myopia

? Weakest concave lens that diverges rays just sufficiently

to focus them at the retina is to be used

? Poor visual acuity is compensated to some extent by

enlarged image size due to the nodal point being further

from the retina

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Causes of Myopia

? The causes of myopia are not known.

? Epidemiological correlation suggest...

lengthy periods of close work are probably a

contributory factor

there is some genetic predisposition to myopia and its

severity

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Types of myopia

? Axial

? Curvature

? Index

? Positional

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Axial Myopia

? AP diameter increased to 25.5 to 32.5 mm

? 90-95% cases

? There may be...

pseudoproptosis resulting from the abnormally large

anterior segment,

a peripapillary myopic crescent from an exaggerated

scleral ring,

posterior staphyloma
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Curvature Myopia

? Corneal curvature steeper than average, e.g.,

keratoconus,

? Radius <7-8.5 mm (normal); 1 mm=6 D

? Lens curvature is increased
? moderate to severe hyperglycemia (intumescence)

lenticonus (ant/post)

spasm of accomodation
spherophakia

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Index Myopia

? Increased index of refraction in early to moderate

nuclear sclerotic cataracts in the elderly.

? Many people find themselves ultimately able to read

without glasses or having gained "second sight."

? Decrease in refractive index of cortex ? diabetic

myopia

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Positional Myopia

? Anterior movement of the lens is often seen after

glaucoma surgery and will increase the myopic error in
the eye.

? Axial myopia of buphthalmos is countered to a large

extent due to posterior displacement of lens-iris
diaphragm and flattening of the cornea

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Clinical course

?

Simple

?

Pathological
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Simple Myopia

? Rarely present at birth, but often begins to

develop as the child grows.

? Usually detected by age 9 or 10 years in the

school vision tests

? May increase during the years of growth,

stabilizing around the mid-teens, usually at

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Pathological Myopia

? 2-3% population

? Increases by as much as 4 D/yr

? Usually stabilizes at about age 20 years and frequently

results in myopia ? 10 to 20 D.

? If progress is rapid from age 15-20, likely to reach 20-30

dioptres

? Commoner in women, Jews and Japanese
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Pathological Myopia-Etiology

? Developmental defect affecting posterior segment

? Retina grows extensively stretching sclera

? Adjuvants- growth influences during puberty and

physical debility

? Excessive convergence- stretching

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Pathological Myopia

? Associated vitreous floaters, liquefaction, posterior

staphyloma and chorioretinal changes.

? Degeneration is not necessarily comparable with degree

of myopia

? Genetic predisposition in offspring as per laws of

recessive Mendelian inheritance ? if both parents
affected, close supervision needed

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School/ Physiologic/Pseudo-Myopia

? 2D
? Excessive near work causing accomodative

spasm

? Inherited predisposition-more in Orientals and

Jews

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Clinical features of Myopia
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Symptoms

1. Blurred distance vision.

2. Squinting to sharpen distance vision by attempting a

pinhole effect through narrowing of the palpebral

fissures.

3. Eye strain seen in patients with uncorrected low

myopic errors

4. Closer working distance at near that typically gets

closer and closer as the person sustains working at

near.

6. Floaters, photopsiae

7. Visual deterioration

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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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Clinical Signs ?

Apparent convergent squint

? The problem begins at near and spreads to distance

? Results usually in apparent convergent squint due

to excess convergence

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Clinical Signs ?

True divergent squint

? Excess convergence for near work disorients

accommodation which may increase causing ciliary

spasm or

? more frequently, attempt at convergence is given up, its

latent insufficiency causing muscular imbalance till

? advantage of binocular vision is given up, one eye is

relied upon for vision while the other deviates outwards

causing true divergent squint

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Pathology

? Eye appears large and prominent ? pseudoproptosis

? Deep anterior chamber

? Large, sluggish pupil

? Post segment sclera is thinned up to 25% of normal

? Post vitreous detachment ? Weiss ring

? Liquefaction ? muscae volitantes, large floaters

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Fundus

? Atrophy of retina and choroid ?

depigmentation

? Tigroid fundus with prominent

choroidal vessels

? Patches of choroidal atrophy

surrounded by pigment associated

with haemorrhages

? Atrophic patch at macula

associated with loss of central

vision

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Fundus

? Appearance of dark pigmented area at macula-Foster-

Fuch's fleck ? rare, sudden, proliferation of pigmentary

epithelium with intra-choroidal haemorrhage or

thrombosis

? Macular bunches of dilated capillaries or aneurysms

? Myopic crescent ? temporal or annular

? Nasal supertraction crescent

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Macular haemorrhage

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Posterior staphyloma

? Herniation of posterior pole

? Crescentric shadow 2-3 DD temporal to disc,

? Sudden kinking of retinal vessels as they dip over the edges,

? Gross atrophy

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Peripheral

Degenerations

Not requiring

prophylaxis:

Paving stone

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Predisposing Degenerations

Lattice, snailtrack, retinoschisis, white without

pressure

Snailtrack

Retinoschisis

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Lattice degeneration

Figure:

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Complications

? Atrophy ? scotomata

? macular most incapacitating

Horseshoe Tear

? Vitreous degeneration + floaters
? Tears + haemorrhages
? Detachment ? post traumatic or spontaneous

associated with peripheral degenerations due to

? Lenticular opacities, esp. posterior cortical
? Open angle glaucoma

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Night myopia

?

Manifest in reduced illumination

?

~ 0.5 D

?

Cone-rod shift in retina, pupillary dilatation,

ciliary muscle activity

?

If night vision appears seriously impaired,

appropriate correction may be given

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Treatment

1. Optical correction after subjective and objective

refraction

Spectacles

Contact lens (including Orthokeratology)

2. Visual hygiene
3. Refractive surgery

LASIK

o

LASEK

Wavefront Lasik

o

Clear lens Extraction

Phakic IOL

o

ICRS

4. Pharmacological intervention

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Optical correction

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Myopia ? Optics

Diverging lens

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Cycloplegic Refraction

? Cycloplegia is the employment of pharmaceutical

agents to paralyze the ciliary muscle temporarily

to stabilize the accommodative reflex of the eye

so that a definitive end point may be measured.

? Benefit of relaxing the accommodative tone is

especially important in young individuals.

? Cycloplegic + Mydriatic = Relaxes accomodation

+ dilates pupil for better reflex

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Cycloplegic Refraction

Drug

Actions

Onset

Duration

Remarks

Atropine

Strong

6 ? 24 hr

10 ? 15 days

Slow,

Prolonged

Homatropine

Weak

1 hr

1 ? 2 days

Weak,

Prolonged

Phenylephrine

Mydriatic

Tropicamide

Weak

20 ? 30 min

4 ? 10 hr

Fast, Short

Cyclopentolate

Weak

10 ? 30 min

12 ? 24 hr

Fast,

intermediate

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Visual Hygiene

? Proper illumination
? Proper posture
? Clear print
? Better contrast
? Avoid ocular fatigue
? Proper occupation in case of degenerative

myopia

? May need special institutions if low vision

dictates
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Summary

? Refraction is a method for evaluating the optical

and refractive state of the eye.

? Myopia is a diopteric condition of the eye

where parallel incident rays from optical infinity
focus anterior to light sensitive layers of retina
when accomodation is at rest.

? Myopia is corrected by concave lenses prescribed

after cycloplegic refraction.