Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Ophthalmology PPT 49 Refraction I Lecture Notes
2
Acknowledgement
? Photographs : Courtesy of
Kanski's Clinical Ophthalmology.
3
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.
4
Question
? You have gone fishing and see a fish in the water.
You do not have a fishing rod. The only
equipment that you have is a spear to catch the
fish. Where do you throw the spear?
a. in front of the fish.
b. behind the fish.
c. directly at the fish.
d. It is not possible to hit the fish as it is a virtual
image.
5
What is Refraction
? When rays of light traveling through air enter a
denser transparent medium, the speed of 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 optical and refractive
state of the eye
6
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
7
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 , refractive state
tends to remain stationary, until in old age a further
tendency of hypermetropia is evident.
8
Refractive data in adult
? 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
9
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 ".
10
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.
11
Refractive errors
Anomalies of the optical state of the eye
? Myopia
? Hypermetropia
? Astigmatism
12
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.
13
Myopia ? Optics
Emmetropia
Diverging lens
14
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
15
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
16
Types of myopia
? Axial
? Curvature
? Index
? Positional
17
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
18
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 (anterior/posterior)
spasm of accomodation
spherophakia
19
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
20
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
21
Clinical course
?
Simple
?
Pathological
22
Simple Myopia
? Rarely present at birth, but often begins to
develop as the child grows.
? Usually detected by age 9 or 10 years in school
vision tests
? May increase during years of growth, stabilizing
around the mid-teens, usually at about 5 D or
less.
23
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
24
Pathological Myopia-Etiology
? Developmental defect affecting posterior segment
? Retina grows extensively stretching sclera
? Adjuvants- growth influences during puberty and
physical debility
? Excessive convergence- stretching
25
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
26
School/ Physiologic/Pseudo-Myopia
? 2D
? Excessive near work causing accomodative
spasm
? Inherited predisposition-more in Orientals and
Jews
27
Clinical features of Myopia
28
Symptoms
1. Blurred distance vision.
2. Squinting to sharpen distance vision by
attempting a pinhole effect through narrowing of
palpebral fissures.
3. Eye strain seen in patients with uncorrected low
myopic errors
29
Symptoms
4. Closer working distance at near that typically
gets closer and closer as the person sustains
working at near.
5. Delayed dark adaptation
6. Floaters, photopsiae
7. Visual deterioration
30
Clinical Signs ?
Apparent convergent squint
? The problem begins at near and spreads to distance
leading to a cascade of changes in the findings over time
? Results usually in apparent convergent squint due
to excess convergence
31
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
32
Clinical Signs
? 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
33
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
34
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
35
Macular haemorrhage
36
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
37
Peripheral
Degenerations
Not requiring
prophylaxis:
Paving stone
38
Predisposing Degenerations
Lattice, snailtrack, retinoschisis, white without
pressure
Snailtrack
Retinoschisis
39
Lattice degeneration
Figure:
40
Complications
? Atrophy ? scotomata
? macular most incapacitating
Horseshoe Tear
? Vitreous degeneration + floaters
? Tears + haemorrhages
? Detachment ? post traumatic or spontaneous
associated with peripheral degenerations due to
vitreous adhesion
? Lenticular opacities, esp. posterior cortical
? Open angle glaucoma
41
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
42
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
43
Optical correction
44
Myopia ? Optics
Diverging lens
45
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
46
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
47
Visual Hygiene
? Proper illumination
? Proper posture
? Clear print
? Better contrast
48
Visual Hygiene
? Avoid ocular fatigue
? Proper occupation in case of degenerative
myopia
? May need special institutions if low vision
dictates
49
Summary
? Refraction is a method for evaluating 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.
50
Question
? Which of the following drugs can cause acquired
myopia?
? a. chloroquine.
? b. sulfonamides.
? c. phenothiazines.
? d. benzodiazepines.
51
Question
? Which of the following is a cause of acquired
myopia?
? a. orbital tumor.
? b. central serous chorioretinopathy.
? c. intravitreal silicone oil.
? d. childhood glaucoma.
52
Thank you
This post was last modified on 07 April 2022