This document discusses the optical system of the eye and various types of refractive errors. It begins by explaining how the cornea and lens are responsible for most of the eye's refractive power. It then discusses schematic eyes and different types of ametropia including myopia, hypermetropia, and astigmatism. The document provides details on causes, symptoms, signs, and treatment options for each refractive error.

1. Dr. Mohd Najmussadiq Khan
M S (Ophth) DiSSO(ESASO)

3.  the main refraction by the eye takes place at
 Anterior corneal surface about 40-45D due to
more curvature and high difference in the
refractive indices of air (1) &cornea(1.37)
 Lens is resposible for 16-20D due to high
nuclear density and curved surfaces

4. Schematic eye--- bye above discussion it is clear that the
optical system of eye reduces itself into a simplified
schematic eye which has only three pairs of cardinal
poits
 Two principal points(P&P1)
 Two nodal points(N&N1)
 Two focal points(F&F1)
Reduced eye---in schematic eye,the two principal and two
nodal points are very close so can be consider as one
thus the optical system of the eye is being treated as a
single refracting surface.This is known as reduced eye
of Donder.It has
 only two media of refractive indices 1 and 1.336
 one principal and one nodal point(acting as optical
centre)
 two principal foci , anterior 15.7 mm and posterior
24.13mm from cornea

6.  differaction of light in which a blurred disc of light
forms instead of a point
 chromatic aberration in which the white light splits
into its color components after refraction
 spherical aberration because the periphery bof the lens
has a high refractive power than the central part
 decentration because the centrs of curvature of the
separate lens systems of the eye are never exactly
placed on the optic axis.the optic axis is a line passing
through the centres of the various media of the eye
while visual axis is the line passing through the macula
and nodal point.the angle between optic and visual
axis is known as angle alpha.
 Peripheral aberration due to impaired image clarity at
retinal periphery.

7.  Abnormal length of the globe(axial ametropia)
 Abnormal curvature of the refractive surfaces
of the cornea or lens(curvature ametropia)
 Abnormal refractive indices of the media(index
ametropia)
 Abnormal lens position (backwards or
forwards)
 Obliquity of the media like subluxated lens or
oblique IOL
 Absence of the lens like aphakia

9.  The parallel rays of light are brought to a focus
posterior to the retina when accomodation is at
rest.

10.  Axial hypermetropia due to less axial length
,common in all new
borns(physiological).pathological causes
include retinal detachment,retinal and orbital
tumours.it is the commonest type.
 Curvature hypermetropia in cornea plana,lens
plana
 Index hypermetropia due to increased
refractive index of the lens cortex in old age
 In aphakia

11. Latent hypermetropia which is corrected by tone of the ciliary
muscle.(about 1D).It is revealed after atropine cycloplegia.
Menifest hypermetropia having two components and
measured by the strongest convex lens
 Facultative type which is corrected by the effort of
accomodation
 Absolute type which can not be corrected by accomodation
and measured by the weakest convex lens
 Th=lh+mh(fh+ah)
 With age due to decrease in ciliary muscle tone some lh
becomes mh and due to lack of accomodation with age some
of the fh becomes ah.After the age of 60 years all of
hypermetropia becomes absolute type.

12.  Blurred vision more for near
 Eye strain(accomodative asthenopia)
 Artificial myopia due to excessive use of
accomodation
 Converengent squint due to continuous effort
of accomodation
 Early onset of presbyopia

13.  Small eye ball with small cornea
 Shallow anterior chamber
 Apparent divergent squint due to large
positive angle alpha as macula is far away from
the disc
 Ophthalmoscopy shows optic disc small
hyperaemic with ill defined edges(pseudo-
neuritis),shot silk retina,retinal vessels with
undue tortusity and abnormal branching

14.  Amblyopia
 Accomodative convergent squint
 Angle closure glaucoma

15.  The mild hypermetropia does not require any
treatment especially for young individual.The
treatment is required in middle aged patient
 Glasses—the convex lenses are prescribed after full
cycloplegic correction.
 Contact lens—useful for high hypermetropia and
cosmetic reason.The power is slightly more than
spectacle power.
 Surgical—
Keratophakia,Epikeratophakia,Keratomileusis and
IOL implantation for aphakic hypermetropia
 Laser—photorefractive keratoplasty with eximer
laser(PRK)
Aetiology—Aetiology—

16.  The parallel rays come to a focus anterior to the
retina when accomodation is at rest.

17.  Axial myopia is the commonest (more axial
length)
 Curvature myopia-
keratoconus,megalocornea,lenticonus
 Index myopia-nuclear sclerosis and diabeties
 Forward displacement of lens

18. Congenital or developmental myopia-
 The child is born with elongated eyes
 The refraction up to -10 D
 Typical fundus changes are seen but progression is rare
Simple myopia-
 Commonest type upto -6 D
 Does not progress much after adolescence
 No degenerative fundus changes
Pathological myopia-
 It appears in childhood and increase with age upto -15
to -25D
 Typical degenerative fundus changes present
 Strongly hereditary and more common in
female,japanese,chinese,jews.
 Prognosis is usually poor
 If the myopia is more than -6D it is called high myopia.

19.  Impaired distant vision
 Small degree eye strain
 Divergent squint due to lack of accomodation
use
 Black floaters due to vitreous degeneration and
flashes of light
 Delayed dark adaptation
 Sudden loss of vision due to retinal detachment
especially in pathological myopia

20.  Prominenet eyye ball (pseudo-proptosis)
 Large cornea,deep anterior chamber,a large sluggish
pupil
 Apparent convergent squint
 Posterior cortical changes ,vitreous degeneration
 Ophthalmoscopically
 in simple myopia no changes except large disc and
temporal crescent .
 in pathological myopia the disc is large and pale with
large physiological cup,temporal or annular
crescent,posterior staphyloma,macular degeneration
with foster-fuchs spots,pale and tesselated
fundus,choroidal sclerosis and atrophy,cystoid
degeneration of peripheral retina.

21.  Open angle glaucoma is more common
 Retinal tears & vitreous haemrrhage
 Retinal detachment & vitreous degeneration
 Posterior cortical cataract

22.  Glasses-myopia never be over corrected.in case of low
myopia upto-6D full correction is given but in high
myopia of more than -6D slight under correction is
given.Problem with high power negative glasses
include minification,image distortion,reduced
peripheral field of vision,not good cosmetically.
 Contact lenses-less minification,no image
distortion,normal field of vision,cosmetically good.
 Keratorefractive surgeries like redial
keratotomy,keratophakia,keratomileusis
 Fukalas operation for high myopia in which crystalline
lens is removed to make patient aphakic
 Negative anterior chamber IOL
 Scleroplasty for posterior staphyloma
 Laser surgery include photorefractive
keratoplasty(PRK)
 Genetic counselling

23. Indications and pre-requisites:
 When residual bed after LASIK is likely to be
less than 250µ
 When the initial corneal thickness is less than
480µ
 Refractive error between the ages of 21-45
 ACD greater than 2.8 mm
 Stable refraction (<0.5D change in previous 12
months)
 No ocular pathology (NSC, glaucoma, lid
pathology, etc)
 Mesopic pupil <6.0mm

24.  Implantable contact lens is indicated for placement
in the posterior chamber of the phakic eye for
correction of moderate to high myopia ranging –3.0 D
to –20.0 D. Toric ICL (TICL) can correct upto -3 to -
23 D of sphere and + 1.0 to + 6.0 D of cyl. The toric
ICL has the same overall design as the spherical ICL
with the addition of a toric optic. The is made from a
combination of copolymer and collagen called
Collamer. This Collamer implantable contact lens
reduces reflections and glare, and the collagen makes
it extremely biocompatible. It is made-up of 60%
poly-HEMA, Water (36%), Benzophenone (3.8%) and
Collagen (0.2%), it attracts the deposition of
fibronectin on the lens surface, inhibits aqueous
protein binding and makes the lens invisible to the
immune system.

27.  Corneal curvature astimatism which may be with
the rule (direct astigmatism) when vertical meridian
is more curved and against the rule(indirect
astigmatism)when horizontal meridian is more
curved.
 Curvature lenticular astigmatism in lenticonus.
 Corneal diseases like
pterygium,keratoconus,corneal scar
 Index astigmatism in early cataract which may
cause polyopia
 Decentration of the lens in subluxation and tilted
IOL

28.  the two principal meridians of greatest and least
curvature are at right angle to each other.In
bioblique astigmatism the axises are not right right
angled but crossed obliquely and in oblique
astigmatism the two meridians not in principal
planes but right angled to each other.The regular
astigmatism may be
 Simple myopic or simple hypermetropic
astigmatism
 Compound myopic or compound hypermetropic
astigmatism
 Mixed astigmatism

29.  here no geometrical analysis is
 possible and it can not be treated by glasses.It may
be due to corneal
scarring,keratoconus,lenticonus,incipient
cataract,after penetrating keratoplasty etc.
 The rays after refraction will have two separate
foci for horizontal and vertical planes.the whole
bundle of rays is called STRUMS CONOID and the
distance between two foci is called focal interval of
strum.The point at which two opposite tendencies
are equal is known as the circle of least confusion.

30.  Decreased visual acuity
 Asthenopia or eye strain more in
hypermetropic astigmatism
 Eyeache or headache
 Running letters together while reading

31.  Head tilt in children especially in oblique
astigmatism
 Half closure of the lids
 Signs of causative factors like
scarring,decentration
 Optic disc oval or tilted in high astigmatism

32.  Retinoscopy
 keratometry
 Jacksons cross cylinder
 Astigmatic fan and astigmatic dial
 Placido disc
 Photokeratoscope
 Computarised corneal topography

33.  Spectacles with cylinderical lenses used for
distant and near vision
 Rigid contact lenses for high astigmatism and
soft contact lenses for little astigmatism
 Surgery by giving cuts in the direction of more
curved axis and removal of sutures
 Eximer lasers

34.  Best treated by hard contact lenses
 Phototherapeutic keratectomy for superficial
scarring

35.  useful for correcting astigmatism.
 AK is a simple procedure where the surgeon
places incisions in the cornea to change its
curvature in a controlled fashion.
 It is often a useful enhancement procedure
following previous LASIK or PRK. While the
incisions usually go 90% of the total corneal
thickness in depth, a perforation can occur if the
blade cuts too deeply.
 AK probably reduces the strength of the globe so
that any direct trauma, like a fist or air bag to the
eye, may cause the globe to rupture more easily.

36.  It means absence of the crystalline lens from its normal
anatomical position in the papillary area(patellar fossa).
Aetiology—
 Congenital rare
 Post operative commonest(needling,ICCE,ECCE)
 Post traumatic
 Post inflammatory after cornea perforation
 Couching by quacks
Optics of aphakia
 Acquired high hypermetropia +10D
 Against the rule of astigmatism since the cornea is less
curved in vertical meridian due to suturing and fibrosis
 Absence of accommodation
Change of colour vision due to more entry of ultraviolet and
infra red rays

37.  Blurred vision for distance and near
 h/o cataract operation
 h/o injury,perforation

38.  unaided vision finger counting 2-3 ft. patient may
have thick convex glasses
 sutures or scar may present at upper limbus
 deep anterior chamber ,iridodonesis,peripheral
iridectomy, jet black pupil
 absence of 3rd &4th purkinje images in ICCE and
only absence of 3rd image in ECCE
 Retinoscopy shows high hypermetropia and
astigmatism
 Ophthalmoscopically hypermetropic fundus with
a small optic disc

39.  Spectacle correction—glasses are given after 6
weeks of surgery.
 Distant vision +10D sph. +2Dcyl. At 1800
 Near vision +13Dsph. +2Dcyl. At 1800

40.  25-30% image magnification
 Spherical aberration like pin cushion effect
 Lack of fine movements
 Roving ring scotoma (50-65 0 from central fixation) or
jack in box phenomenon due to prismatic aberration at
the edge of the lens
 Restriction of the visual field
 In case monocular aphakic correction diplopia due to
aniseikonia
 Coloured vision due to absence of crystalline lens filter
 Thick and heavy glasses which are not good
cosmetically

41.  Less image magnification 7-8 %
 All types of aberrations are less
 In monocular cases no diplopia
 In cases where IOL can not be put inside the
eye
 Cosmetically well accepted

42.  Not good for old patients
 Lens spoilage and foreign body sensation
 Corneal erosion,ulcer,edema,vascularisation
 Spectacles are required for reading

43.  Image magnification only 0-2%
 No spherical and prismatic aberration
 Minimum or no aniseikonia
 Normal visual field and good hand eye co-
ordination
 Cosmetically very good

44.  Complications like lens displacement, corneal
decompensation, chronic iridocyclitis, posterior
capsule opacification
 More cost,trained surgeon,microscope required

45.  Epikeratophakia –a plus donor lenticule is
sutured over anterior corneal surface
 Keratophakia—a plus donor lenticule is
inserted intra lamellarly in the cornea
 Eximer laser PRK—tried recently to correct
aphakia

46.  In this the refraction of two eyes are unequal.It may be
congenital or due to corneal diseases or
cataract.surgical or non surgical trauma can also cause
it.
Vision in anisometropia—
 Binocular vision ;each 0.25D difference between two
eyes causes 0.5%difference in the size between two
retinal images and upto 5% difference can be tolerated
by retina
 Alternating vision in which each of the two eyes is
used at a time. One eye hypermetropic or emmetropic
and other eye is myopic
 Exclusively uniocular where one eye is better and other
eye is amblyopic
 Convergent squint in childreb and divergent squint in
adults
 Diplopia due to unequal image size

47.  Iseikonic lenses
 Treatment for amblyopia
 Eximer laser PRK

48.  It is the ability to see the near object clearly by
increasing the converging power of the eye with
increase in the curvature of the anterior lens
surface.The ciliary muscle contraction causes the
lens to become more spherical.
 Range of accomodation is the difference between
far point and near point and amplitude of
accomodation is the difference in the refractive
power of the eye between these two points.the
amplitude of accomodation decreases with age
from 14D at 10 years to 4 D at45 years to 1D at 60
years of age.

49.  Presbyopia affects everybody around age 40.
Presbyopia refers to the eyes’ diminishing
ability to focus up close with age.
 Usually in the early 40’s, this point of near
focus is at arm length, and it becomes difficult
to keep things clear. This is when most people
get reading glasses.
 Every few years, the reading prescription will
change because the focusing ability continues
to decline until around age 60 when there is
almost no more to lose.

50.  Symtoms include gradual difficulty in
reading& near work,headache with near work
and arms are not enough is a common
complain.
 Emmetropes usually notice presbyopia around
age 40. However, patients with hyperopia tend
to notice presbyopia earlier than emmetropes.
patients with a small amount of myopia tend to
notice presbyopia later than age 40 .

51.  Lens matrix is harder and less elastic
 Lens capsule is less elastic
 Progressive increase in the lens size
 Weaking of the ciliary muscles

52. Glasses-
 Convex lenses for reading +1D at 40 years and then increase
by +0.5D for every 5 years upto 60 years.It is better to under
correct.Glasses can be unifocal or bifocals or trifocals.
surgery
 The Surgical Reversal of Presbyopia (SRP)-- This technique
assumes that presbyopia is due to slackening of fibers
attached to the lens. By using four “scleral expansion
bands”, this procedure supposedly makes these fibers taut
again, restoring the ability to change focus.
 Another procedure, called Anterior Ciliary Sclerotomy or
ACS, also supposedly makes the fibers attaching to the lens
taut. ACS involves placing several partial thickness
incisions on the sclera or white part of the eye in a radial
pattern, somewhat reminiscent of radial keratotomy. Some
surgeons are experimenting by placing silicone implants
inside the radial incisions, trying to prevent the regression.

53.  Method to estimate the condition of refraction
with accomodation at rest.The doctor watches
illuminated retinal image with retinoscope.
 The cycloplegia is required in children and
young adults.atropine 1% ete oint. Is used for 3
days in patients less than 5 years.Between 5-15
years 1% cyclopentolate or 2% homatropine
e.d. is used three times 1 hr before.
 The refraction under cycloplegia is always
pathological because the shape of the lens has
been altered so post mydriatic test(PMT) is
advisable.
 Doctor sits 1 meter away from patient in a dark
roomwith patient looking distance

54.  The retinoscope is moved slowly from one to
other direction
 In hypermetropia,emmetropia and myopia <-
1D the reflex moves in the same direction then
add increasing convex lens till no movement of
shadow
 In myopia of -1D there is no movement of
shadow
 In myopia of >-1D the shadow moves in the
opposite direction then add increasing concave
lens till no movement of shadow
 The retinoscopy is done in two principal
merdians

55. Calculations-
 If the end point is with + 3D lens then
refraction
 -1D +3D=2D
 If end point is with +1D lens then refraction
 -1D +1D=0

56.  in it a clear retinal image of a test object is
formed by an optical system and with the help
of a computer gives a measure of ametropia.It
is excellenrt for quick screening of refractive
errors.