Download MBBS Ophthalmology PPT 49 Refraction I Lecture Notes

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

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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


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

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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
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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

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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
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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

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




















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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

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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

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