Refractive errors occur when there is a mismatch between the eye's optical power and its axial length, causing light rays to focus in front or behind the retina. The most common refractive errors are myopia, hyperopia, and astigmatism. Diagnosis involves using instruments like autorefractors and retinoscopes to measure how light enters the eye. Optical corrections include spectacle lenses, contact lenses, and intraocular lenses, with the type chosen based on factors like comfort, durability, and amount of correction needed.

1. Refractive Errors
and management
Ahmed AlMumtin, MD

2. Basic Optics
• Optics is the properties of light as it is
acted on by optical systems
• Geometric optics: uses line diagrams to
depict the behavior of light
• Wavefront optics: uses principles of waves
to describe the behavior of light

3. Vergence
Prallel Beam Convergence Divergence

4. VERGENCE
• All naturally occurring sources of light are
divergent
• Light rays traveling parallel have zero
vergence
• Light rays that focus on a point are
convergent
• The unit of measurement of vergence is
the diopter
D= Vergence (Diopters)=___________1_____________
Distance from the source in meters

5. BASIC OPTICS
• Therefore:
• The closer the light is to its source, the
greater the vergence
• The farther the light is from its source, the
lower the vergence, approaching zero as
distance goes toward infinity.
• Diverging light has negative power (-)
• Converging light has positive power (+)

6. REFRACTION
• Refraction of light occurs when light
passes from one medium to another of
different refractive index (ie. density)

7. Refractive Components of the Eye
• Cornea: responsible for the majority of the
refractive power of the eye (40 D)
• Lens: 20 D of refractive power, changes
with accommodation
• Axial length

8. REFRACTIVE PHYSIOLOGY
• Light rays are focused on the retina
because they are refracted by passing
through the cornea and lens (Snell’s Law)
• Corneal refractive power is constant
• Lens refractive power is modifiable with
accommodation
• Axial length of the eye is constant except
under certain conditions

9. Fovea
Light rays

10. Emmetropia
• Adequate correlation between axial length
and refractive power
• Parallel light rays fall on the retina (no
accommodation)

11. Ametropia (Refractive error)
• Mismatch between axial length and
refractive power.
• Parallel light rays don’t fall on the retina
(no accommodation)
• Nearsightedness (Myopia)
• Farsightedness (Hyperopia)
• Astigmatism
• Presbyopia

12. Accommodation
• Emmetropic eye
• object closer than 6 M send divergent light that
focus behind retina , adaptative mechanism of
eye is increase refractive power by accommoda
tion
• Helm-holtz theory
• contraction of ciliary muscle -->decrease
tension in zonule fibers -->elasticity of lens caps
ule mold lens into spherical shape -->greater di
optic power -->divergent rays are focused on re
tina
• contraction of ciliary muscle is supplied by
parasympathetic third nerve

13. Myopia
• Parallel rays converge at a focal point
anterior to the retina
• Etiology : not clear , genetic factor
• Causes
• excessive long globe (axial myopia) :
more common
• excessive refractive power (refractive
myopia).
• Increase in the curvature of the cornea or
the surfaces of the crystalline lens

16. Uncorrected, light focuses in front of fovea
Corrected by divergent lens, light focuses on fovea

17. Myopia
• Forms
• Benign myopia (school age myopia)
• onset 10-12 years , myopia increase
until the child stops growing in height
• Progressive and malignant myopia
• interchangeable
• myopia increase rapidly each year
and is associated with vitreous opaciti
es , fluidity of vitreous and chorioretin
al change
• rate of increase in amount of myopia
generally tapers off at about 20 years
of age

18. Myopia
• Congenital myopia
• Myopia > 10 D
• Increase slowly each year

19. Myopia
• Special forms : nuclear sclerosis , keratoconus ,
spherophakia
• Symptoms
• Typically do not have “eye-strain”, “watering” of the eyes or
headaches as often as hypermetropes do
• Usually detected by the young when they discover they
cannot see things at a distance as well as their friends do
• The teacher complains that the child makes too many
mistakes copying things from the black-board
• Blurred distance vision
• Squint in an attempt to improve uncorrected visual
acuity when gazing into the distance
• Headache
• Amblyopia – uncorrected myopia > 10 D

20. Myopia
• Morphologic changes
• deep anterior chamber
• atrophy of ciliary muscle
• vitreous may collapse prematurely -->
opacification
• fundus change : loss of pigment in RPE ,large
disc and white crescent-shaped area on temporal side ,
atrophy in macular area , posterior staphyloma ,retinal
eneration-->hole-->
increase risk of RD
• Treatment : concave lenses, clear lens extraction

21. MYOPIA
• Increases with age roughly until the person stops
growing in height.
• A myopic person can still see some objects clearly,
provided the object is closer than the far point
• For a -2 D myope, the far point is 0.5 meters
(D=1/distance in meters), so any objects inside 0.5
m are clear as long as they are not too close at
which point clarity may be limited by
accommodation.
• Pseudomyopia: accomodative spasm.The patient
cannot relax accomodation when looking in the
distance. For example, an over anxious student

22. PATHOLOGICAL CAUSES OF
MYOPIA
• KERATOCONUS

23. PATHOLOGICAL CAUSES OF
MYOPIA
* Cataract * Diabetes

24. PATHOLOGICAL CAUSES OF
MYOPIA
Marfan’s Staphyloma

25. Hyperopia
• Parallel rays converge at a focal point posterior to
the retina
• Etiology : not clear , inherited
• Causes:
• excessive short globe (axial hyperopia) : more common
• insufficient refractive power (refractive hyperopia)
• The length of the eyeball is shorter than it should be
• Hyperopia forms a stage in normal development of the eyes—at
birth eyes are hypermetropic (2.5 to 3.0 Diopters).
• When persists in adulthood it represents an imperfectly developed
eye.
• Lens changes (cataract).

26. Hyperopia
• Special forms : lens dislocation , postoperative
aphakia
• hyperopic persons must accommodate when gazing
into distance to bring focal point on to the retina
• Symptoms
• distance vision is impaired in high refractive error( > 3 D)
and in older patient
• “Eye-strain”(ciliary muscle is straining to maintain
accommodation) / “watering” / “redness”
• Headaches in later part of the day
• time
• Young children with significant hypermetropia can also
develop a convergent squint

27. HYPEROPIA
• Hyperopia may be partially compensated
for by using the eyes’ accommodative
ability
• When accommodative ability cannot keep
up with demand hyperopia is manifest and
images are blurred in the distance and for
near

29. Uncorrected, light focuses behind fovea
Corrected by convergent lens, light focuses on fovea

31. Hyperopia
• Symptoms
• visual acuity at near tends to blur relatively early
• nature of blur is vary from inability to read fine print to near
vision is clear but suddenly and intermittently blur
• blurred vision is more noticeable if person is tired , printing
is weak or light inadequate
• asthenopic symptoms : eyepain, headache in frontal region,
burning sensation in the eyes, blepharoconjunctivitis
• accommodative esotropia : because accommodation is linked
to convergence -->ET
• Amblyopia – uncorrected hyperopia > 5D

32. Hyperopia
• Fundus in axial hyperopia may reveal
pseudooptic neuritis (indistinct disc margin, no ph
ysiologic cup, may elevate disc)
• DDx from optic neuritis by > 4 D , no enlarged
blind spot, no passive congestion of vein
• Treatment : convex lenses, keratorefractive
surgery, refreactive lensectomy with IOL, phakic
IOL

33. PATHOLOGICAL CAUSES OF
HYPEROPIA
DISLOCATED LENS

34. PATHOLOGICAL CAUSES OF HYPEROPIA
RETINAL DETACHMENT CHOROIDAL TUMOR
1 mm = 3D RETINAL FLUID

35. Astigmatism
• Parallel rays come to focus in 2 focal lines rather
than a single focal point
• Etiology : heredity
• Cause : refractive media is not spherical-->refract
differently along one meridian than along meridian
perpendicular to it-->2 focal points ( punctiform ob
ject is represent as 2 sharply defined lines)

37. Astigmatism
• Classification
• Regular astigmatism : power and orientation of
principle meridians are constant
• With the rule astigmatism , Against the rule
astigmatism , Oblique astigmatism
• Simple or Compound myopic astigmatism ,
Simple or Compound hyperopic astigmatism ,
Mixed astigmatism
• Irregular astigmatism : power and orientation of
principle meridians change across the pupil

39. Astigmatism
• Symptoms
• asthenopic symptoms ( headache , eyepain)
• blurred vision
• distortion of vision
• head tilting and turning
• Amblyopia – uncorrected astigmatism > 1.5 D
• Treatment
• Regular astigmatism :cylinder lenses with or without
spherical lenses(convex or concave), Sx
• Irregular astigmatism : rigid CL , surgery

40. Pathologic Causes of
Astigmatism
• Corneal: post surgical, traumatic,
infectious
• External pressure on cornea: lid masses
• Lens: pressure on lens from tumors

41. Presbyopia
• Physiologic loss of accommodation in
advancing age
• deposit of insoluble proteins in lens in
advancing age-->elasticity of lens progressivel
y decrease-->decrease accommodation
• around 45 years of age , accommodation
become less than 3 D-->reading is possible at
40-50 cm-->difficultly reading fine print , heada
che , visual fatigue

42. Presbyopia
• Treatment
• convex lenses in near vision
• Reading glasses
• Bifocal glasses
• Trifocal glasses
• Progressive power glasses

43. Anisometropia
• Difference in refractive power between 2 eyes
• refractive correction often leads to different
image sizes on the 2 retinas( aniseikonia)
• aniseikonia depend on degree of refractive
anomaly and type of correction
• closer to the site of refraction deficit the
correction is made-->less retinal image changes i
n size

44. Anisometropia
• Glasses : magnified or minified 2% per 1 D
• Contact lens : change less than glasses
• Tolerate aniseikonia ~ 5-8%
• Symptoms : usually congenital and often
asymptomatic
• Treatment
• anisometropia > 4 D-->contact lens
• unilateral aphakia-->contact lens or
intraocular lens

45. Correction of refractive errors
• Far point
• point on the visual axis conjugate to the
retina when accommodation is completely
relaxed
• placing an object or imaging an object at far
point will cause a clear image of that object t
o be relayed to the retina
• use correcting lenses to form an image of
infinity at the far point , correcting the eye for
distance

46. What is the Far Point
• The farthest an eye can see
• An emmetrope it is infinity or 20 feet
• 4.00D myope without glasses it is 25cm in
front of the eye
• 1.00D myope without glasses it is 1meter in
front of the eye
• 3.00D hyperope without glasses 33.3 cm
behind the eye

47. What is the Near Point
• The closest the eye can see
• The near point is the sum of the power of
the eye and accommodation
• An emmetrope with 10.00D of
accommodation and a -4.00D error without
glasses has a near point of 14.00D or
7.15cm in front of the eye

48. What is the Near Point
• A 50 year old with 2.00D of accommodation
and a -1.00D error without glasses has a
far point of 1 meter and a near point of
33.3 cm
• With glasses on the far point is infinity or 20
feet and the near point is 50cm in front of
the eye

49. What is the Near Point
• A +3.00D without glasses hyperope with
6.00D of accommodation has a far point of
33.3 cm behind the eye and a far point of
33.3 cm in front of the eye.
• With glasses on the far point is infinity and
the near point 16.6 cm.

50. Remember!!
• A myope has too much plus in their eyes
• Whereas a hyperope has too much minus

51. Diagnosis:
•Diagnosis of refractive errors is made by an optician
or ophthalmologist.
•Instruments used to diagnose refractive errors
include:
- pinhole glasses
- Autorefractor (measures how light changes as it
enters the eye).
-Retinoscope (measures the refractive condition of
the eye).
- Phoropter.

52. Pinhole glasses Autorefractor
Phoropter View through an autorefractor

53. Types of optical correction
• Spectacle lenses
• Monofocal lenses : spherical lenses ,
cylindrical lenses
• Multifocal lenses
• Contact lenses:
• higher quality of optical image and less
influence on the size of retinal image than spe
ctacle lenses
• indication : cosmetic , athletic activities ,
occupational , irregular corneal astigmatism , h
igh anisometropia , corneal disease.
• Can be: soft, hard, gas-permeable

54. • Contact lenses
• disadvantages : careful daily cleaning and
disinfection , expense
• complication : infectious keratitis , giant
papillary conjunctivitis , corneal vascularization
, severe chronic conjunctivitis
• Intraocular lenses
• replacement of cataract crystalline lens
• give best optical correction for aphakia , avoid
significant magnification and distortion caused
by spectacle lenses

55. Hard lenses:
• Plastic polymer.
• Most durable.
• Rigid, therefore may scratch the cornea.
• Not gas permeable  corneal hypoxia  corneal ulcers.
• Cannot be worn continuously.
• Difficult to get used to (because they are very rigid).
• Less susceptible to infection + allergy.
• Best for treating astigmatism (smoothes out the uneven
curvature).

56. Soft lenses:
* High water content.
* Less durable.
* Permeable for both gases + liquids.
* Could be worn for long periods.
* Tolerated much better.
* They do not correct astigmatism.
* Are the most comfortable lenses.
* Are the least durable lenses (must be replaced more often).
•Susceptible to accumulation of deposits (because they
absorb more water, which binds proteins). This accumulation
of protein deposits leads to allergic conjunctivitis and other
allergies.

57. Gas permeable lenses (Semi-
rigid):
•They are permeable only to gases.
•They are more comfortable than hard lenses, but
less than soft ones.
•They are more durable than soft lenses, but less
than hard lenses.
•Allow oxygen to pass, but also allow proteins to
deposit.
•More durable than soft lenses, but less durable
than hard ones.

58. Toric lenses:
Are similar to soft contact lenses, but have a
couple of extra characteristics:
•They have 2 powers in them: 1 for spherical
correction + 1 for astigmatism.
•They are designed to keep the lens in a stable
position even on movement.
They offer the comfort of soft lenses ,and at the
same time they correct astigmatism, but their
disadvantage is that the lens will sometimes rotate,
and this creates a very irritating change in vision as
the lens rotates.

59. Refractive Surgery Techniques
• Radial Keratotomy (RK)
• Freeze keratomileusis
• Photorefractive Keratectomy (PRK)
• Laser Epithelial Keratomileusis
(LASEK)
• Laser-assisted in-situ Keratomileusis
(LASIK)
• Others:
• Astigmatic Keratotomy (AK)
• Intracorneal ring segments (Intacs)
• Phakic Intraocular Lens Implants
• Refractive lensectomy

60. Radial Keratotomy
• First used in U.S in 1978.
• Treats low to mod myopia in outpt
setting using topical anesthetics.
• RK reduces myopia by steepening
the cornea peripherally, which
secondarily flattens the cornea
centrally.
• The surgeon makes deep radial
incisions with a diamond blade in a
spoke-like pattern, leaving a clear
optical zone in the center.
• Refractive effect determined by the
number, length, and depth of the
incisions, as well as the size of the
spared central optical zone.
• The smaller the optical zone, the
greater the central corneal
flattening (reduction in myopia), but
greater risk of side-effects.

61. Laser technology
• Excimer laser: EXCited dIMER
• AKA “cool laser beam” because little thermal
damage to adjacent tissues.
• 193nm wavelength ultraviolet laser with
sufficient energy to disrupt intermolecular
bonds within the corneal stromal tissue
(photoablative decomposition).
• First excimer lasers FDA approved in 1995,
with beam width 4-5mm, now available less
than 100 microns.
• Each laser pulse removes a given volume of
stroma
• Three types of laser application: wide-area
ablation, scanning slit, and flying spot lasers.

62. Laser technology
• In myopia, laser flattens central cornea to
decrease its focusing power to bring
secondary focal point back to retina.
• In hyperopia, the laser removes peripheral
corneal tissue thereby secondarily
steepening the central cornea, increasing
the focusing power of the cornea.
• Astigmatism treated with elliptical or
cylindrical beams that flatten the steepest
corneal meridian.
• To minimize glare and halos, optical zone
should be larger than the dilated pupil.

63. Myopic photorefractive kertectomy
• PRK can effectively treat low to
mod myopia or hyperopia +/-
astigmatism.
• Performed as outpt with topical
anesthesia.
• First, the corneal epithelium in the
area to be ablated is removed to
expose Bowman’s layer and the
underlying corneal stroma
(spatula, laser).
• Excimer laser then applied as
directed by the corneal
topography-driven computer
program.
• Topical antibiotics, steroids, and
NSAIDs applied, along with a
bandage contact lens (BCTL)

64. PRK
• In the post-op period, pt may experience tearing,
photophobia, blurred vision, and discomfort due to
abrasion of central epithelium.
• This can be controlled with topical steroids and
NSAIDs.
• Pts occ. require systemic analgesia for severe pain
• BCTL removed once epithelial defect healed (avg 3-
4 days).
• Abx continued several more days, and steroids for
up to 3 months post-op.
• Visual acuity improves once the epithelial defect
heals, but fluctuates for a few months and finally
stabilizes at ~3 months.
• Glare, halos, and dry eye symptoms common the
first month post-op, usually diminishing/disappearing
by 3-6 months.

65. Laser Sub-Epithelial
Keratomileusis
• LASEK can treat mild to moderate myopia and
hyperopia +/- astigmatism.
• Can be performed as an outpt with topical
anesthesia
• The corneal epithelium is incompletely incised
using a microkeratome with a 70 micron deep
blade.
• A hinge is left at the 12 o’clock position.
• Dilute alcohol (20%) drops are applied to the
exposed tissue and left for ~30 seconds. The
area is then washed with water and allowed to
dry. The excimer laser is applied as in PRK to
the sub-epithelial stroma.
• The epithelial flap is repositioned afterward.

66. LASIK
• LASIK can treat mild, moderate, and high myopia and
hyperopia +/- astigmatism.
• Can also be performed as an outpt with topical anesthesia
• LASIK is now the most commonly performed refractive
surgery in the world.
• A suction ring is applied to the anesthetized cornea and a
microkeratome is used to raise a corneal flap of
~160microns thickness (25-30% of the corneal thickness),
hinged at the 12 o’clock position.
• The suction is turned off and the flap is lifted aside,
exposing stromal tissue
• The excimer laser is applied as with PRK and LASEK,
controlled by the topography-driven computer software, to
reshape the cornea.
• The flap is replaced on the stromal bed without sutures or
a BCTL, as the endothelial pumps create a driving force to
keep the flap in position.

67. LASIK
• The use of the suction ring helps hold the
cornea steady and provides for a uniform
cut by the microkeratome.
• Flaps can be formed by an automated
process involving a blade guide on the
suction ring to guide a turbine-driven
microkeratome, producing a very smooth,
regular cut
• Patients usually sent home on topical
antibiotics, steroids, and NSAID drops. Pt
is usually seen ~POD 1, and 7, then at 1,
3 and 6 months.
• Benefits include little pain, quick recovery
of vision, and potential to treat higher
levels of myopia. LASIK enhancements
are also easily performed.

68. LASIK

69. LASIK Complications
• Potential complications:
• Intra-operative flap complications: microkeratome
complication with a higher rate with surgeon inexperience
• Post-operative flap complications
• Flap-bed interface epithelialization: that epithelial growth at
the interface could significantly be reduced by irrigating the
stromal surfaces and using a BCTL for one day.
• Irregular astigmatism
• Infection:
• Diffuse lamellar keratitis (DLK): (AKA Sands of Sahara
syndrome) Wavy inflammatory reaction at LASIK flap
interface 1-3 days post-op of unknown cause. Treatment
involved high-dose topical steroids or lifting the flap to
irrigating the interface.
• Progressive corneal ectasia: progressive corneal thinning
and steepening with worsening irreg. astigmatism thought to
result from too thin a stromal bed after LASIK. Most believe
stromal bed thickness should be at least 250 microns.