REFRACTIVE ERRORS

Dr. LEOW THYE YNG BSc (Hons), MB BCh BAO (NUI), Dip.Opt., MCOptom, MRCOphth, FRCS (Glasg).

Lenses

POSITIVE or CONVERGINGNEGATIVE or DIVERGING

positive negative

The POWER of a lens , P

The “ DIOPTRE (D ) “A positive lens of one dioptre

(+1.00D) converge parallel light rays to a ‘real’ focal point one metre from (after) the lens.

A negative lens of one dioptre (-1.00D) diverge parallel light rays as if they are coming from a ‘virtual’ point one metre infront of the lens.

ONE DIOPTRE LENS

P = +1.00 D f = +1.0 m or

+100 cm

P = - 1.00 D f = -1.0 m or

-100 cm

The DIOPTRE (D)

1 Power, P (D) = --------------------------- Focal Length, f (metre)

eg. If P = +2.00 D, f = +0.5 m or +50 cmeg. If P = +4.00 D, f = +0.25m or +25 cmeg. If P = -2.00 D, f = -0.5 m or -50 cmeg. If P = -4.00 D, f = -0.25m or -25 cm

Refractive Index (n) Velocity of light in vacuum

n = --------------------------------------- Velocity of light in the medium

eg. Air n = 1 eg. Water n = 1.33 eg. Cornea n = 1.376 eg. Crystalline lens n = 1.38 to 1.42 eg. Crown glass n = 1.52 eg. Flint glass n = 1.6

Refractive Power of a curved surface ( P )

n2 - n1 P (dioptre)= ---------------------

r (metre)where r =

radius of curvature of the refractive surface in metres

Refractive Power of the anterior corneal surface

n1 = 1.0 (air) , n2 = 1.376 (cornea) , r = 8mm or 0.008m (radius of curvature

of cornea)P = (1.376 - 1) / 0.008 = 47 D

Refractive Power of the Cornea

The total refractive power of the cornea is approx. +40 D (ie. less than +47D for the anterior surface as this is reduced by the negative power of the posterior surface)

Refractive Power of the Eye and its axial length

Power of cornea ~ 40 DPower of the crystalline lens ~ 20DRefractive Power of the ave. eye ~ 60DAssuming n = 1.33 for the eye,

ave. length = n / power = 22.22mmThe axial length of most eyes fall between

22 to 24mm (ultrasound scan).

Key WordsEMMETROPIAAMETROPIA

- Myopia or ‘Short-sightedness’ - Hypermetropia (Hyperopia)

or ‘Long-sightedness’ - Astigmatism

ACCOMMODATIONPRESBYOPIAAnisometropia , Amblyopia

EMMETROPIA

Light rays from distant objects (parallel rays) are focused onto the retina in a fully relaxed eye

MYOPIALight rays from distant objects are

focused infront of the retina in a fully relaxed eye

Usually too long eyeball length, or sometimes too high refractive power

Myopia - Far Point

A myopic person can see objects placed at the far point or nearer.

HYPERMETROPIA (HYPEROPIA)

Light rays from distant objects are focused behind the retina in a fully relaxed eye

Eye too short, or refractive power is too low (eg. Aphakia where there is no crystalline lens)

ASTIGMATISM

In many people, the corneal surface is not perfectly spherical (radius of curvature the same in all meridians) like a soccer ball surface.

Many corneas have different curvature (hence different power) in different meridian, like a rugby ball surface.

Astigmatic eyeAny combination of positions of focal

points in relation to the retina is possible - myopic, hyperopic or mixed astigmatism

Astigmatism - circle of least confusion

ACCOMMODATION

Contraction of the ciliary muscles in the eye allow the crystalline lens to increase its power. This increases the power of the eye so that it can focus at near objects.

It also allows young hyperopes to overcome the hypermetropia if this degree is not too high.

AMPLITUDE OF ACCOMMODATION

The range of accommodation decreases with age as the crystalline lens and, to a lesser extent, the ciliary muscles become less elastic.

Amplitude of accommodation with age

PRESBYOPIA

By 40 to 45 years of age onwards, the amplitude of accommodation may not be sufficient to allow a person to read at near.

This is PRESBYOPIA.Additional plus lens power is usually

required.

Anisometropia

Difference in the refractive errors of the two eyes.

If sufficiently different in both eyes, amblyopia (“lazy eye”) will occur in the eye with the more blurred image.

Importance of early detection in children as correction before 8 to 9 years of age can prevent amblyopia.

Correction of refractive errors

Spectacle lensesContact lensesIntraocular lens implants esp. after

cataract removalREFRACTIVE SURGERY -

Excimer Laser (“LASIK” or “PRK”)- Intracorneal ring implants- Intraocular ‘contact lens’ (“ICL”) or

Phakic Intraocular lens

Myopia - correction with a minus lens

Hypermetropia - correction with a positive lens

Astigmatism requires a spherocylindrical lens

Spherocylindrical lenses have different powers in different meridians

Presbyopia - spectacle correction

Two pairs of glasses - one distant, one near

Bifocals lenses

Multifocals or Progressive lenses

EXCIMER LASEREXCIMER = “ Excited Dimer “193 nm (ultraviolet)Breaks the intramolecular bonds of the

corneal tissue (photoablation )PRK - “Photorefractive Keratectomy”LASIK - “Laser in-situ keratomileusis”Flatten the corneal curvature ie.reduce the

refractive power of the cornea in myopiamay also correct hyperopia and astigmatism

PRK – Photorefractive Keratectomy

LASIK – Laser in-situ keratomileusis

Lasik 1

Lasik 2

Lasik 3

Lasik 4

Lasik 5

Lasik 6

Lasik - Complications

Corneal stromal flap complicationsInfectionsCorneal melting, corneal haze, corneal

ectasiaDry eyesGlare - esp. night drivingLoss of visual acuity or constrast sensitivityRetinal detachment

Epi-LASIK

Epithelial Flap instead of Corneal Stromal Flap i.e. more superficial cut

Excimer laser as in PRKSaid to be safer than LASIK

Intraocular Contact Lens / Phakic Intraocular Lens

Phakic IOL

Phakic IOL

CONDUCTIVE KERATOPLASTY (CK) for presbyopiaRadiowaves

applied to corneal periphery to alter the shape of the cornea i.e. steepen the corneal curvature

Reduce hypermetropia / increase myopia

Visual Acuity

Minimum angle of resolution of the eye ~ 1 min. of arc (60 sec)

The normal eye can discriminate two points as separate if they subtend at least an angle of 1 min. at the eye

Snellen Charts

The Snellen “ E “

D ( D - distance m. this letter subtend 5 min. eg. 60, 36, 24, 18, 12, 9, 6, 5 metres)

Snellen Acuity

Recording Visual Acuity (Snellen Acuity)

Test Distance (m.) Snellen Acuity = ------------------------------

Distance (m.) at which the smallest visible letter subtend 5 min. of arc

Test Distance is usually at 6 m.eg. 6/5, 6/6, 6/9, 6/12, 6/18, 6/24, 6/36,

6/60 ; 5/60, 4/60, 3/60, 2/60, 1/60 ; CF (Counting fingers), HM (Hand movements), PL (Perception of light), NPL (No PL)

Determination of Refractive Errors

OBJECTIVE - does not require a response 1) Infants and young children requires retinoscopy under cycloplegia (Cyclopentolate 1% or rarely atropine 1% eyedrops are used to immobilise the ciliary

muscles and hence block accommodation) 2) AUTOREFRACTORS (computerised)

SUBJECTIVE - patient asked to choose between lenses

Importance of vision checks on young children

In addition to manifest squints, high degrees of anisometropia, astigmatism, hyperopia and myopia can cause amblyopia (lazy vision) due to blurred image on the fovea of one or both eyes.

A sharp retinal image is essential for development of a normal visual acuity

Importance of early detection of visual problems for early treatment

Treatment of Amblyopia

Optical correction of refractive errors (with or without patching of the better eye) before 8 to 9 years of age is crucial.

The younger the age at commencement of treatment, the better the results.

Results are generally disappointing after 9 to 10 years old.

Change of refractive errors with age

Low grade hyperopia (ave.~ 2D) at birthSlight increase in hyperopia during first 7

yearsGradual decrease in hyperopia

throughout primary schoolTrend to drift into myopia by end of

primary/early secondary, and increase in myopia throughout secondary school

Change in refractive errorsIf hyperopia of about +2.50D at 6 years,

tend to be emmetropic at 14 years; if > +2.50D at 6 yrs., some hyperopia will remain at 14 yrs.

Myopia tend to increase through secondary school till early 20’s, then level off

Some drift towards hyperopia esp. after 40 yrs., but hardening of the lens nucleus cause a shift into myopia esp. in the older age.

Factors in development of myopia

Genetic - family, uniovular twins, race - Japanese, Chinese, Jews, Germans

Environment - close work - indoors?Pre-existing astigmatism?Lack of exercise, ?food?Role of parasympathetic system - ?Use of

parasympathetic blocker like atropine eyedrops