Acknowledgement
? Photographs in this presentation are courtesy of
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Kanski's Clinical Ophthalmology.
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Learning Objectives
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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
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? When rays of light traveling through air enter adenser 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
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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
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focused at the plane of the retina when accomodation is atrest.
? Clear image of a distant object formed without any
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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
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? 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
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tends to remain stationary, until in old age a further
tendency of hypermetropia is evident.
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Refractive data in adult
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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
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? Change in Corneal Curvature by 1mm = ?6D? Refraction by unaccomodated lens= +16 to
20(+17)D
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Angle kappa ()
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? M = Macula? D= Centre of pupil, on cornea
? N = Nodal point
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M
Optic axis
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DN
(
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B
FD = Pupillary line
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FFNM = Visual axis
= "Between the visual axis and pupillary line, hence roughly corresponds to
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angle ".
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Anisometropia
? Anisometropia is a state in which there is a difference in
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the refractive errors of the two eyes, i.e., one eye ismyopic and the other hyperopic, or both are hyperopic or
myopic but to different degrees.
? This condition may be congenital or acquired due to
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asymmetric age changes or disease.
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Refractive errorsAnomalies of the optical state of the eye
? Myopia
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? Hypermetropia
? Astigmatism
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What is Myopia ?
? Diopteric condition of the eye
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where parallel incident rays fromoptical infinity
focus anterior to light sensitive layers
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of retina
when accomodation is at rest
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12Myopia ? Optics
Emmetropia
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Diverging lens
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Optics of Myopic eye
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? Far point is at a finite distance inversely proportional to
the degree of myopia
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? Weakest concave lens that diverges rays just sufficientlyto focus them at the retina is to be used
? Poor visual acuity is compensated to some extent by
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enlarged image size due to the nodal point being further
from the retina
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14Causes of Myopia
? The causes of myopia are not known.
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? Epidemiological correlation suggest...
lengthy periods of close work are probably a
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contributory factorthere is some genetic predisposition to myopia and its
severity
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Types of myopia
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? Axial
? Curvature
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? Index? Positional
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Axial Myopia
? AP diameter increased to 25.5 to 32.5 mm
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? 90-95% cases? There may be...
pseudoproptosis resulting from the abnormally large
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anterior segment,
a peripapillary myopic crescent from an exaggerated
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scleral ring,posterior staphyloma
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Curvature Myopia? Corneal curvature steeper than average, e.g.,
keratoconus,
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? Radius <7-8.5 mm (normal); 1 mm=6 D
? Lens curvature is increased
? moderate to severe hyperglycemia (intumescence)
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lenticonus (ant/post)
spasm of accomodation
spherophakia
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Index Myopia
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? Increased index of refraction in early to moderatenuclear sclerotic cataracts in the elderly.
? Many people find themselves ultimately able to read
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without glasses or having gained "second sight."
? Decrease in refractive index of cortex ? diabetic
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myopia19
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Positional Myopia? Anterior movement of the lens is often seen after
glaucoma surgery and will increase the myopic error in
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the eye.? Axial myopia of buphthalmos is countered to a large
extent due to posterior displacement of lens-iris
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diaphragm and flattening of the cornea20
Clinical course
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?
Simple
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?Pathological
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Simple Myopia? Rarely present at birth, but often begins to
develop as the child grows.
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? Usually detected by age 9 or 10 years in the
school vision tests
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? May increase during the years of growth,stabilizing around the mid-teens, usually at
about 5 D or less.
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Pathological Myopia
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? 2-3% population? Increases by as much as 4 D/yr
? Usually stabilizes at about age 20 years and frequently
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results in myopia ? 10 to 20 D.
? If progress is rapid from age 15-20, likely to reach 20-30
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dioptres? Commoner in women, Jews and Japanese
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Pathological Myopia-Etiology? Developmental defect affecting posterior segment
? Retina grows extensively stretching sclera
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? Adjuvants- growth influences during puberty and
physical debility
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? Excessive convergence- stretching24
Pathological Myopia
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? Associated vitreous floaters, liquefaction, posterior
staphyloma and chorioretinal changes.
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? Degeneration is not necessarily comparable with degreeof myopia
? Genetic predisposition in offspring as per laws of
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recessive Mendelian inheritance ? if both parents
affected, close supervision needed
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25School/ Physiologic/Pseudo-Myopia
? 2D
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? Excessive near work causing accomodativespasm
? Inherited predisposition-more in Orientals and
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Jews
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Clinical features of Myopia27
Symptoms
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1. Blurred distance vision.2. Squinting to sharpen distance vision by attempting a
pinhole effect through narrowing of the palpebral
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fissures.
3. Eye strain seen in patients with uncorrected low
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myopic errors4. Closer working distance at near that typically gets
closer and closer as the person sustains working at
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near.
5. Delayed dark adaptation
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6. Floaters, photopsiae7. Visual deterioration
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Signs
? Small eyeball
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? Smaller cornea? Shallow anterior chamber predisposes to angle closure
glaucoma since size of lens is normal
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? Apparent divergent squint
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Clinical Signs ?
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Apparent convergent squint
? The problem begins at near and spreads to distance
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leading to a cascade of changes in the findings over time? Results usually in apparent convergent squint due
to excess convergence
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Clinical Signs ?
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True divergent squint? Excess convergence for near work disorients
accommodation which may increase causing ciliary
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spasm or
? more frequently, attempt at convergence is given up, its
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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
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causing true divergent squint
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Pathology
? Eye appears large and prominent ? pseudoproptosis
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? Deep anterior chamber? Large, sluggish pupil
? Post segment sclera is thinned up to 25% of normal
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? Post vitreous detachment ? Weiss ring
? Liquefaction ? muscae volitantes, large floaters
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32Fundus
? Atrophy of retina and choroid ?
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depigmentation
? Tigroid fundus with prominent
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choroidal vessels? Patches of choroidal atrophy
surrounded by pigment associated
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with haemorrhages
? Atrophic patch at macula
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associated with loss of centralvision
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33Fundus
? Appearance of dark pigmented area at macula-Foster-
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Fuch's fleck ? rare, sudden, proliferation of pigmentary
epithelium with intra-choroidal haemorrhage or
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thrombosis? Macular bunches of dilated capillaries or aneurysms
? Myopic crescent ? temporal or annular
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? Nasal supertraction crescent
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35Posterior staphyloma
? Herniation of posterior pole
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? Crescentric shadow 2-3 DD temporal to disc,
? Sudden kinking of retinal vessels as they dip over the edges,
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? Gross atrophy36
Peripheral
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Degenerations
Not requiring
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prophylaxis:Paving stone
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Predisposing Degenerations
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Lattice, snailtrack, retinoschisis, white without
pressure
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SnailtrackRetinoschisis
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Lattice degeneration
Figure:
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Complications
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? Atrophy ? scotomata? macular most incapacitating
Horseshoe Tear
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? Vitreous degeneration + floaters
? Tears + haemorrhages
? Detachment ? post traumatic or spontaneous
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associated with peripheral degenerations due tovitreous adhesion
? Lenticular opacities, esp. posterior cortical
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? Open angle glaucoma40
Night myopia
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?
Manifest in reduced illumination
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?~ 0.5 D
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Cone-rod shift in retina, pupillary dilatation,
ciliary muscle activity
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?If night vision appears seriously impaired,
appropriate correction may be given
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Treatment
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1. Optical correction after subjective and objective
refraction
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SpectaclesContact lens (including Orthokeratology)
2. Visual hygiene
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3. Refractive surgeryLASIK
o
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LASEK
Wavefront Lasik
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oClear lens Extraction
Phakic IOL
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o
ICRS
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4. Pharmacological intervention42
Optical correction
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43Myopia ? Optics
Diverging lens
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Cycloplegic Refraction
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? Cycloplegia is the employment of pharmaceuticalagents to paralyze the ciliary muscle temporarily
to stabilize the accommodative reflex of the eye
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so that a definitive end point may be measured.
? Benefit of relaxing the accommodative tone is
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especially important in young individuals.? Cycloplegic + Mydriatic = Relaxes accomodation
+ dilates pupil for better reflex
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Cycloplegic Refraction
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Drug
Actions
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OnsetDuration
Remarks
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Atropine
Strong
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6 ? 24 hr10 ? 15 days
Slow,
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Prolonged
Homatropine
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Weak1 hr
1 ? 2 days
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Weak,
Prolonged
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PhenylephrineMydriatic
Tropicamide
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Weak
20 ? 30 min
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4 ? 10 hrFast, Short
Cyclopentolate
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Weak
10 ? 30 min
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12 ? 24 hrFast,
intermediate
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Visual Hygiene
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? Proper illumination? Proper posture
? Clear print
? Better contrast
? Avoid ocular fatigue
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? Proper occupation in case of degenerativemyopia
? May need special institutions if low vision
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dictates
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Summary
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? Refraction is a method for evaluating the optical
and refractive state of the eye.
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? Myopia is a diopteric condition of the eyewhere parallel incident rays from optical infinity
focus anterior to light sensitive layers of retina
when accomodation is at rest.
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? Myopia is corrected by concave lenses prescribed
after cycloplegic refraction.
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