AMBLYOPIA:cLAssIfIcAtIOn, etIOLOgY And

dIAgnOsIs

BY:RAJU KAItIOPtOMetRIst

AMBLYOPIA

• Defn

– “A unilateral or bilateral decrease of visual acuity caused by pattern vision deprivation or abnormal binocular interaction for which no obvious causes can be detected by physical examination of the eye and cannot be corrected by optical or surgical means but in appropriate cases is reversible by therapeutic measures.’

• Researches have pointed to the fact that amblyopia was not simply a reduction in the visual acuity in an eye, but a complex visual processing disorder that involved the decrement virtually in all areas of visual functions including-

– Accommodative accuracy and facility– Fixation stability– Pursuit and saccadic accuracy– Localization in space – Contrast sensitivity

• Amblyopia originated from Greek word:

Amblyos - dullness / blunt

Ops – vision

• Condition in which the observer sees nothing & patient very little

• Significance difference in acuity between eyes may be sign of amblyopia

• Mainly result due to visual stimulus deprivation / suppression usually associated with strabismus or anisometropia

• conflicting inputs from the two eyes to the visual cortex that result in active suppression and development of amblyopia in non dominant eye

• usually unilateral

• Bilateral amblyopia can occur when high hypermetropic (+4D) or myopic (-8 D) or astigmatic refractive error not corrected during visual immaturity

• Defective VA even after correction

• may be due to organic causes

AMBLYOPIA

• Prevalence

– Variable

– 2.0 -2.5 % of general population

– Preschool/school age children : 4-5.3%

sensItIve PeRIOd Developmental time frame early in life during which

there is robust plasticity within the visual system, particularly the visual cortex.

– Retinocortical connection not firmly established

– Period – sensitive /critical / susceptible period

– Strabismic amblyopia – 7 years

– Sensitive period for recovery - ? 8 years

• Critical period - 2 months of age

RIsK fActORs

• 4 times more prevalent in– LBW & Premature baby

• 6 times more in– delayed milestones & CNS disorders

• parent with amblyopia• Maternal smoking

• Criteria for Excellent VA

-early retinal stimulation of each eye

-proper ocular alignment

-binocularity

- stereopsis

• Any blurred retinal image• Difference in VA b/w two eyes

• Leads to abnormal fixation • Inhibition of visual cortex

Amblyopia

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cLAssIfIcAtIOn• Can be divided in to two groups;• Functional amblyopia

Stimulus deprivation Strabismic

RefractiveAnisometropicPsychogenic

• Organic amblyopiaDue to retinal diseasesNutritionalToxicIdiopathic

stIMULAtIOn dePRIvAtIOn AMBLYOPIA

• primary cause is due to disuse/under stimulation of the retina – i.e. opacities or occlusion • cong cataract, Ptosis, corneal opacities, surgical lid

closure ,Vitreous haemorrhage, may be due to occlusion amblyopia

• unilateral or bilateral– unilateral more severe and often associated with

secondary ET or XT & anisometropia

• eccentric fixation may develop.

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• Pattern deprivation amblyopia

- dense congenital cataract -corneal opacity (Peters' anomaly ) -corneal opacity ( Trauma, Infections, HSV) - large uncorrected refractive errors -Nystagmus - poor visual outcome

stIMULUs dePRIvAtIOn AMBLYOPIA

Aphakia

Corneal scar Congenital ptosis

Congenital cataract

Unilateral ocular abnormalities are much more likely to lead amblyopia than binocular ones. If one eye has a

competitive advantage over the other, its afferent connections become stronger and more numerous

while those of other eye atrophied and retract.

• WHICH IS MORE LIKELY TO PRODUCE AMBLYOPIA-UNILATERAL OR BILATERAL PTOSIOS.WHY??????

StrabiSmic amblyopia• occurs as a result of neural changes in the deviated eye

• pt having one eye for fixation in unilateral rather than alternating fixation more likely to develop Strabismic amblyopia

• often seen in ET than XT– the fovea of the deviated eye has to compete with a strong

temporal hemi field of the fellow eye

• always unilateral & caused by active inhibition

• aetiology similar to that of suppression.– thus called suppression amblyopia.

• turning and consequent disuse of one eye will arrest the development of VA –amb. of arrest.– if amblyopia of arrest is allowed to persist suppression

amblyopia develops – amblyopia of extinction.

• constant untreated acquired ET under 3 years will dev strabismus amb in100% of cases

• doesn’t occur in X(T)

aniSometropic amblyopiao abnormal binocular interaction caused by unequal fovea

images in the two eyes causes dev of the Anisometropic amb.

o always unilateral.

o active inhibition of the fovea as the Strabismic amb.

o 30% of the cases are associated with strabismus.

o with reduction in central VA , overall reduction of the contrast sensitivity

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• it occurs when dioptric power differs over +1D in hyperopes , more than –3D in myopes &more than 1.5 D in astigmatism.

• when corrected optically resulting aneisokonia may be amblypiogenic factor since retinal images of dif sizes present an obstacle fusion

• more common in anisohyperopia than anisomyopia

refractive amblyopia• caused by uncorrected ref error where there is blur image

at all distance

• may be unilateral or bilateral

• if unilateral then Anisometropic amblyopia

• meridional amb. occurs in principal meridian of high uncorrected astigmatism.

refractive : iSoametropic • Hyperopia : > + 5.00 Ds

• Myopia : > -8.00 Ds

• Astigmatism : > +/- 2.50 Ds

• Cause – Equal pattern deprivation

• Other term – Meridional amblyopia • Selective visual deprivation for visual stimuli of a certain spatial

orientation

pSychogenic amblyopia

• also called hysterical amblyopia.

• occurs as a visual conversion reaction.

organic amblyopia Irreversible type which results from some pathological or anatomical abnormalities of the retina

Retinal eye diseaseseg.-neonatal macular hemorrhage receptor dystrophy pathogenic lesion affecting the fovea & surrounding retinal area such as

toxoplasmosis chorioretinitis ,a retinoblastoma, traumatic retinal lesion

May be associated with abnormality of visual pathway

• Nutritional amblyopia-Occurs from nutrition deficiencies

• Toxic amblyopia -visual loss due to damage to the optic nerve fibrosis due to effect of exogenous or endogenous poisons

• -Its types are as follows:

– tobacco amblyopia– ethyl alcohol amblyopia– methyl alcohol amblyopia– quinine amblyopia– ethambutol amblyopia

Tobacco amblyopiaTypically occurs in men in pipe smokers, heavy drinkers ,diet deficiencies in protein & vit.B complex deficiencies

Pathogenesis:Toxic agent – cyanide found in tobaccoExcessive tobacco smoking - Excessive cyanide in blood – degeneration of ganglion cells

particularly in macular lesion – degeneration of papillomacular bundle in the nerve - toxic amblyopia

• Characterized by gradually progressive impairment in the central vision

• Patient complains of fogginess & difficulty in doing near works

• V.F.-B/L centrocaecal scotoma with diffuse margins,defects more for red than white

• Fundus: normal/slight temporal pallor

• ethyl alcohol amblyopiaUsually in association with tobacco amblyopia

May occur in nonsmoker but heavy drinkers suffering from chronic gastritis

• Clinical picture same as tobacco amblyopia

MEthyl alcohol amblyopiaIt is typically acute usually resulting in optic atrophy &

permanent blindness

Etiology : usually occurs due to intake of wood alcohol or

methylated spirit in cheap adulterated /fortified beverages

- sometimes may be due to inhalation of fumes in industries

• Pathogenesis; Metabolized very slowly thus stays in body for longer period of time – oxidized in to formic acid & formaldehyde in the tissues -toxic agents cause edema followed by degeneration of the ganglion cells of the retina resulting in complete blindness due to optic atrophy

• Clinical features: • Symptoms:• In acute poisoning - headache , vomiting ,nausea ,dizziness,

abdominal pain • Presence of characteristic odor • Patient usually brought with complete blindness noticed

after 2-3 days

• Fundus: mild disc edema , markly narrowed blood vesselsFINALLY: B/L primary optic atrophy

• Quinine amblyopia

o May occur even with small doses of the drugs in susceptible individuals.

• Near total blindness ,deafness & tinnitus

• Pupil –fixed & dilated

• Fundus - retinal edema , marked pallor of the disc , extreme attenuation of retinal vessels

• V.F.-markly contracted

• Ethambutol amblyopia

• Caused due to anti -tubercular drugs

• Used in doses of 15mg/kg per day

• usually occurs in patient who have associated alcoholism & diabetes

• Fundus: sign of papillitis

• Idiopathic amblyopia

• -unknown etiology • -may be cortical or sub cortical

• Is color vision affected in amblyopia??

– Generally not affected– Mild abnormalities reported in severe amblyopia,

particularly those with loss of foveal fixation

• Does amblyopia cause a relative afferent pupillary defect??

– Generally not affected– Pathologic changes located in posterior visual

pathway, not in retina or optic nerve

??? Necessity for testiNg AmblyopiA

• Differential Diagnosis and prognosis .

• Differentiating the functional from the organic amblyopia.

• The decision , type and extent of the amblyopia therapy depends on the test results.

• Guides the therapy.

DetectioN & iNvestigAtioN

• Full routine examination• History and symptoms• age of onset • onset of strabismus• previous treatment; with glasses ,occlusion

• V.A measurementfirst amblyopic eye then non-amblyopic eye

-line acuity

-Single letter acuity

• Line acuityCalled morphoscopic acuityAsk the patient to read until the real limit of

acuity is reachedWhere there is eccentric fixation , small foveal scotoma may result in patient missing out letters

• Single letter acuitycalled angular acuityE cube , S.G charts- measure minimum recognizable acuity in children

Usually higher V.A with angular acuity than morphoscopic acuity called crowding phenomenon due to contour interaction

spAtiAl iNterActioN (crowDiNg pheNomeNoN)

• Persons with amblyopia have increased difficulty identifying test letters when they are presented in a linear or two-dimensional array rather than as isolated characters. - "separation difficulty/ crowding phenomenon"

– when figures near the limit of resolution are surrounded by other closely spaced forms

• A similar effect can be produce by placing interactive bars around a single letter

• In the normal fovea, contour interaction – when forms are separated by a distance of 1 to 3

minutes of arc (0.4 to 0.6 times the overall size of 6 meter Snellen letter)

– In the normal periphery its extent is much greater.

• In the amblyopic fovea,– contour interaction typically extends over an

increased distance, to a degree that is roughly proportional to the reduction in acuity.

• Crowding phenomenon more enhanced in Strabismic

amblyopia

• Contrast sensitivityStrabismic & Anisometropic amblyopia -Have

poorer C.S than normal eye

• Electrodiognostic testsHelps in detecting the presence of organic amblyopia in which there is no response to treatment

• PinholeHelps to confirms the presence of amblyopia if

VA is not improved with its use

• Bruckner test

• Fixation pattern & amblyopia in strabismus• Presence of free alteration indicates equal V.A

• No alternate fixation ,likely to suppress so develops amblyopia

• Eccentric fixation may be present

ecceNtric fixAtioN• Unfavorable prognostic factor for therapy.

• Objectively determined by Visuoscopy

• Subjectively determined when the patient fixating the centre of the rotating field that creates the Haidingers brush effect , and the brush is seen eccentric to the central fixation spot rather than superimposed on it.

• Eccentric fixation tests

• Visuoscope / fixation graticule present in ophthalmoscope

• Area used for fixation is notedParafoveal fixation- 1-3 degreesparamacular fixation- 3-5 degrees peripheral fixation- more than 5 degrees

• Neutral density filter • difference between organic & strabismus amblyopia

• In strabismus amblyopia, there is eccentric fixationamblyopic eye is not affected by the filter as the slightly peripheral retina adapts better since it contains rod & cones

• In organic amblyopia –usually central fixation, likely to be reduction of several lines

• After image transfer method

• asked to indicate the position of the after Image in relation to the fixation point in eccentric fixation ,image will appear slightly to the side of the fixation pattern

telescope test• Chart is viewed with amblyopic eye with

2.5xtelescope.

• If organic amblyopia acuity increase by a factor of 2.5

• (20\100 improve to 20\40)

• If further improvement in visual acuity it is functional amblyopia.

• (20\100 to 20\25)

coNtrAst seNsitivity fuNctioN

• Useful in predicating the degree and the rate of improvement before amblyopia therapy.

• Mild , middle spatial frequency (6to 12 cycles\degree) support a better prognosis for recovery.

Visually EVokEd PotEntial tEsting

• Stimulus subtense is increased in size(lower spatial frequency)until a minute of arc size is found where the monocular wave form looks similar with regard to latency and amplitude.

• The higher the spatial frequency at which this occurs , the better the prognosis for improvement.

• Below 14 minutes of arc the prognosis is excellent below 55 minutes it is poor.

• Entopic phenomenonHaidingers brushes and Maxwell's spot

• These phenomenon are centered on the fovea, eccentrically fixing patient will not see them at the point of fixation

-will be slightly to the side of fixationSome cannot see at all

• Perimetry method

• Amsler chartsUsed to show early signs of organic amb where

there is typically a small dense central scotoma

• Past pointing test

• Gives an indication if the localization of objects in space has been disturbed with an amblyopic eye

TypeType PrognosisPrognosis TreatmentTreatment

OrganicOrganic

NutritionalNutritional

TobaccoTobacco

Toxic Toxic congenitalcongenital

Good Good

Good Good

Poor –fair Poor –fair PoorPoor

Diet Diet

Abstinence Abstinence

Medical attention Medical attention Functional vision Functional vision therapytherapy

FunctionalFunctional

Hysterical Hysterical Light Light deprivation deprivation Refractive Refractive

StrabismicStrabismic

Good Good

Poor Poor

GoodGood

GoodGood

Psychotherapy Psychotherapy Remove obstacleRemove obstacle

Ophthalmic lensesOphthalmic lenses

Functional vision Functional vision therapytherapy

Functional amblyopia Organic amblyopia

Normal decrease in VA with neutral density filter

Marked decrease in VA

Disproportionate increase in VA with 2.5x telescope

Expected improvement in VA

Normal color vision Abnormal color vision

Interigity of Haidingers brush Diminution of Haidingers brush

Normal electroretinogram Abnormal electroretinogram

Normal VEP Attenuated VEP

Prognostic Factors in amblyoPiaPositive factor Negative factor

functional organic

Central fixation Eccentric fixation

Random dot stereopsis No random dot stereopsis

Short duration Long duration

Young patient ,motivated Older patient, un -motivated

diagnosis

• Amblyopia – Diagnosis of exclusion

U/L amblyopia

- fixation behavior differs in two eyes

-difference –not eliminated by corrective lenses

• Not attributable to a structural abnormalities

diagnosis

B/L amblyopia

- diagnosed when sig. Refractive errors present

- fixation behaviors falls below the N range

- acuity is not normalized by corrective lenses - reduced VA is not attributable to the ocular

findings

PrEVEntion & Early dEtEction• Paramount important

• Infants & children- susceptible to permanent central loss

• Screening program

- Red reflex test ( media opacity )

-Penlight corneal reflex (Hirschberg test)

-Cover test

-Binocular red reflex test (Buckner test)

• Others :

• Photographic Screening

• Auto refractive devices

• Refractive errors & Amblyopia – can be detected by VA screening of normal children > 3 years

• Unfortunately cases – missed • Children with risk factors for Amblyopia• Referral system: • -Premature baby

-Low birth weight 6 mo - Prenatal complications

-CNS & delayed milestones 18 mo -Genetic syndromes with others

diagnosing thE risk oF amblyoPia

MEAC approach

• Monitor qualitative fixation:– VA evaluation-measure gross fixation abilities

• Evaluate red reflex:– Bruckner test-whiter/brighter reflex represents a

Strabismic eye

• Alignment evaluation:– Hirschberg with infant or CT for toddler

• Compare refractive error:– Cycloplegic refraction

thumb occludEr (Fixation PrEFErEncE)

Glasses wear Patching good eye