Myopia

MYOPIA

SHORT SIGHTEDNESS

DIOPTERIC CONDITION IN WHICH INCIDENT PARALLEL RAYS COME TO A FOCUS ANTERIOR TO THE LIGHT SENSITIVE LAYER OF RETINA WITH ACCOMODATION AT REST.

MYOPIA

1. AXIAL MYOPIA COMMONEST FORM INCREASE IN ANTERO-POSTERIOR LENGTH OF THE

EYEBALL

2. CURVATURAL MYOPIA INCREASED CURVATURE OF CORNEA, LENS OR BOTH

3. POSITIONAL MYOPIA PRODUCED BY ANTERIOR PLACEMENT OF CRYSTALLINE

LENS IN EYE

4. INDEX MYOPIA INCREASE IN THE REFRACTIVE INDEX OF CRYSTALLINE

LENS ASSOCIATED WITH NUCLEAR SCLEROSIS

5. MYOPIA DUE TO EXCESSIVE ACCOMODATION SPASM OF ACCOMODATION

ETIOLOGICAL CLASSIFICATION

1. Congenital myopia2. Simple or developmental myopia3. Pathological or degenerative myopia4. Acquired myopia which may be

Post traumatic Post keratitic Drug induced Pseudomyopia Space myopia Night myopia Consecutive myopia

CLINICAL VARIETIES

Since birthDiagnosed by 2-3 yearsMostly unilateralManifests as anisometropiaChild may develop convergent squint in order to

preferentially see clear at its far point (10-12cms)

CONGENITAL MYOPIA

Associated with cataract, micropthalmos, aniridia, megalocornea, congenital separation of retina.

Developmental myopia- commonest varietySchool myopia (school going age 8-12 years)Etiology

Axial type: physiological variation in length of eye ball precocious neurological growth during childhood

SIMPLE MYOPIA

Curvatural type Underdevelopment of eye ball

Role of diet in early childhoodRole of genetics

Prevalence in children both parents myopic(20%) One parent myopic(10%) No parent myopic(5%)

Symptoms Poor vision for distance(short sightedness) Asthenopic symptoms Half shutting of eyes

CLINICAL PICTURE

Signs Prominent eyeballs Anterior chamber - deeper than normal Pupils- Large, sluggishly reacting Fundus- normal; rarely temporal myopic crescent may be

seen Magnitude of refractive error

Increasing at rate -0.5+- 0.30/ year. Does not exceed 6 to 8

DiagnosisConfirmed by performing retinoscopy

Degenerative/ progressive myopiaRapidly progressive error which starts in

childhood at 5-10 years of ageHigh myopia in early adult life with

degenerative changes

PATHOLOGICAL MYOPIA

Role of heredity Heredity linked growth of retina is the

determinant in developmental myopia Sclera due its distensibility follows retinal

growth but choroid undergoes degeneration due to stretching, which in turn causes degeneration of retina Progressive myopia is

Familial More common in chinese,japanese,arabs and

jews Uncommon among negroes,nubians and

sudanese

ETIOLOGY

Role of general growth processLengthening of the posterior segment of globe commences only during the period of active growth and ends with termination of active growth

Genetic factors (play major role)General growth process(minor)

More growth of retina

Stretching of sclera

Increase axial length

Degeneration of choroid

Degeneration of retina

Degeneration of vitreous

Defective visionMuscae volitantes

Floating black opacities in front of eyes Degenerated liquified vitreous

Night blindness

SYMPTOMS

Prominent eye balls Elongation of eye ball mainly affects posterior

pole and surrounding areaCornea-largeAnterior chamber -deepPupils-slightly large ,react sluggishly to light

SIGNS

Fundus examination:Optic disc

large and pale Temporal edge presents a characteristic myopic crescent Peripapillary crescent encircling the disc may be present, where

choroid and retina is distracted away from disc margin Super traction crescent may be present on nasal side (retina

pulled over disc margin)

Degenerative changes in retina and choroid

Common in progressive myopia Characterized by white atrophic patches at macula

with a little heaping of pigment around them

• FOSTER-FUCH’S SPOT:• Dark red circular

patch due to sub-retinal neo vascularization and choroidal haemorrhage• Present at macula

• CYSTOID DEGENERATION – at periphery• Advanced cases:

Total retinal atrophy in central area

Posterior staphyloma Due to ectasia of sclera at posterior pole It may be apparent as an excavation with vessels bending

backward over margins

Degenerative changes in vitreous include: Liquefaction Vitreous opacities Posterior vitreous detachment(PVD)- Weiss’ reflex

Visual fields Contraction Ring scotoma may be

seenERG reveals subnormal

electroretinogram due to chorioretinal atrophy

Retinal detachmentComplicated cataractVitreous haemorrhageChoroidal haemorrhageStrabismus fi xus convergence

COMPLICATIONS

Optical treatment of myopia Concave lenses Basic rule – minimum acceptance providing

maximum visionModes of prescribing concave lens-1. Spectacles2. Contact lens

TREATMENT OF MYPOIA

Contact lenses are used in case of high myopia as they avoid peripheral distortion and minifi cation produced by strong concave spectacle lens

Radial keratotomy Making deep radial incisions in peripheral part

of cornea leaving the central a 4mm optical zone

These incisions on healing ; flatten the central cornea thereby reducing its refractive power

Correct low to moderate myopia(2-6D)DISADVANTAGES:

Cornea is weakened – globe rupture in sports persons

Uneven healing – irregular astigmatism Patient may feel glare at night

SURGICAL TREATMENT OF MYOPIA

Photo refractive keratectomy (PRK)

A central optical zone of anterior corneal stroma is photoablated using excimer laser (193nm uv flash) to cause flattening of central cornea

Correction for -2 to -6D of myopia

Disadvantages:Post operative recovery is slowPain and discomfortResidual corneal haze in centre affecting visionExpensive

Refractory surgery of choice for myopia of upto -12D

LASER ASSISTED IN-SITU KERATOMILEUSIS(LASIK)

Flap of 130-160 micron thickness of anterior corneal tissue is raised

Midstromal tissue is ablated directly with an excimer laser beam

ultimately flattening the cornea

1. Patients >20 years2. Stable refraction for at least 12 months3. Motivated patient4. Absence of corneal pathology

Absolute contraindication for LASIK Presence of ectasia Corneal thickness <450mm

PATIENT SELECTION CRITERIA

Customised(C)-LASIK:

Based on wave front technology

Corrects spherical, cylindrical and other aberations present in eye

Gives vision beyond 6/6 i.e.,6/5 or 6/4

ADVANCES IN LASIK

Epi-(E) LASIK: Only epithelial

sheet is separated with Epiedge Epikeratome

Devoid of complications related to corneal stromal flap

Minimal or no postoperative painRecovery of vision is very early as compared to

PRKNo risk of perforation during surgery and

rupture of globe due to trauma like RKNo residual haze unlike PRK where subepithelial

scarring may occurLASIK is eff ective in correcting myopia of -12D

ADVANTAGES OF LASIK

Expensive Requires greater surgical skill than RK and PRK Flap related complications

Intraoperative flap amputation Wrinkling of flap on repositioning Postoperative flap dislocation/subluxation Epithelization of flap – bed interface Irregular astigmatism

DISADVANTAGES

Fucala’s operationMyopia of -16 to -18D in unilateral casesClear lens extraction with intraocular lens

implantation of appropriate power is the refractive surgery for myopia of >12D

EXTRACTION OF CLEAR CRYSTALLINE LENS

Intraocular contact lens implantation for correction of myopia of >12D

Special type of IOL is implanted in anterior chamber or posterior chamber anterior to natural crystalline lens

PHAKIC INTRAOCULAR LENS

Into the peripheral cornea at approximately 2/3rd stromal depth

Flattening of central cornea, decreasing myopiaAdvantage: reversible procedure

INTRACORNEAL RING (ICR) IMPLANTATION

A non-surgical reversible method of molding the cornea with overnight wear unique rigid gas permeable contact lenses

Myopia correction upto -5DUsed in patients below 18 years of age

ORTHOKERATOLOGY

General measures : Balanced diet rich in vitamins and proteins Early management of associated debilitating disease

Low vision aids indicated in patients with progressive myopia with

advanced degenerative changesProphylaxis Genetic counselling