I. Binocular Vision and Retinal CorrespondenceII. Amblyopia

Alvina Pauline D. Santiago, MDPediatric Ophthalmology & Adult Strabismus

Basic Course Lectures in OphthalmologySentro Oftalmologico Jose RizalPhilippine General Hospital September 2014

Binocular Vision & Retinal Correspondence

Theories of Binocular VisionCorrespondence & disparity

sensory binocular cooperation based on correspondence & disparity

single visual impression, no depth binocular rivalry: diplopia horizontal disparity: depth within Panum’s

fusional space

TermsHoropter

locus of all object pts imaged on corresponding retinal elements at a given fixation distance

Panum’s fusional space or area of SBV region in front of and at the back of the

horopter that still allows SBV allows depth perception

Horopter Elliptical line that

stimulate corresponding points on the retina

Panum’s fusional space Area in front and behind

horopter where objects stimulate non correspondings points and yet are still fusible as one retinal image

Physiologic diplopia

Uncrossed diplopia

Crossed diplopia

Theories of Binocular VisionNeurophysiologic theory

visual stimuli from retina to visual cortex modified and coded

only 25% binocularly driven cells are stimulated equally; 75% graded influence from R & L eye

Macaque monkey: Ar 18; Rhesus: Ar 17-18 Lost binocular neurons do not recover

Binocular Vision Reduced panorama of visionUpright positionFrontal eye positionVisuomotor tasks improvedDepth perceptionOrientation of body to environment

Visual Pathway

Normal Visual Development

Development of Visual Acuity

Macula Development Fine visual

discrimination characteristic of high VA require sharply focused small objects as appropriate stimuli

Amblyoscope or haploscope

Amblyoscope 2 mirrors at elbow

that reflect images from picture slides

arms may be moved to align targets on each fovea

AmblyoscopeSubjective angle

amount in degrees the examiner must move amblyoscope arms for patient to see 2 pictures as superimposed

during binocular viewing conditionsObjective angle

AmblyoscopeSubjective angleObjective angle

deviation measured by alternate prism cover test

during monocular viewing conditions

AmblyoscopeAngle of anomaly

displacement of the true fovea from pseudofovea

zero if NRC; subjective = objective angle (+) if alternate cover test (objective angle)

is not equal to subjective angle

Worth’s level of fusionFirst degree (simultaneous perception)

Dissimilar targets presented Perceived at the same time in the same

visual directionSecond degreeThird degree

Worth’s level of fusionFirst degree (simultaneous perception)Second degree (flat fusion)

similar targets with dissimilar components (monocular suppression checks)

Third degree

Worth’s level of fusionFirst degree (simultaneous perception)Second degree (flat fusion)Third degree (stereopsis)

Same targets as 2nd degree + disparity

First degree simultaneous

perception

2nd degree flat fusion

3rd degree Stereopsis

Normal retinal correspondence RE: circle LE: black dot

Orthotropic Black dot in circle

Normal retinal correspondenceOrganization of visual space is such

that the visual direction of each fovea is the same

Angle of anomaly = 0Patients with NRC always use fovea as

center of reference

NRC, with strabismus

To see black dot in circle, need to move arms of amblyoscope total of 40 PD

Difficulties in TestingNRC and strabismus

If with dense large regional suppression, no subjective angle, difficult to superimpose images

If monofixator, use targets for peripheral retina

Harmonious ARC subjective angle = 0

measured under binocular conditions

Pseudofovea compensates for angle of deviation

No subjective misalignment

Unharmonious ARC

pseudofovea does not compensate fully for angle of deviation

Angle of anomaly = objective angle - subjective angle

AmblyoscopeMeasures fusional vergenge amplitudesAngle of deviationArea of suppressionRetinal correspondenceTorsion Instrument convergence

Terms

Retinal correspondence single vision: hallmark

ARC vs NRCRetinal disparity & rivalry:

diplopia and visual confusion

Abnormalities of binocular visionDiplopiaVisual confusionSuppressionHorror fusionisAnomalous retinal correspondence

Diplopia vs Visual ConfusionDiplopia

Object of interest seen by fovea of one eye and peripheral retina of the other eye

Visual confusion Fovea of both eyes (or corresponding

retinal points) sees 2 objects of interest interpreted as coming from the same point in space

Diplopia vs Visual Confusion

SuppressionSuppression: alteration of visual

sensation that results in inhibition or prevention of one eye’s image from reaching consciousness

Physiologic suppression prevents physiologic diplopia from reaching consciousness

Central vs peripheral suppressionCentral:

pathologic extension of retinal rivalry prevents foveal image of deviating eye

from being perceived

Peripheral: Prevents awareness of the image on

peripheral retina (2nd image)

Suppression ScotomaEsotropia Exotropia

Monocular vs Alternating suppressionMonocular: unidirectional

Dominant eye always predominate over image from deviating eye

Alternating: bidirectional Suppression switches between the two

eyes

Facultative vs Obligatory SuppressionFacultative

Present only when eyes are deviated

Obligatory Present at all times, whether eyes are

deviated or aligned

Amblyopia

Definition: Amblyopia

Etymology: dullness of vision Greek:

amblyos = dull Stem: ops = vision

Observer (MD) sees nothing, and the patient sees very little

von Graefe

DefinitionUnilateral or bilateral decrease in VA

caused by pattern vision deprivation or abnormal binocular interaction for which no cause can be detected by physical examination, and which in appropriate cases, reversible by therapeutic measures

von Noorden

PrevalenceApproximately 2-2.5% of general

population has amblyopia

Military Recruits 1-3.2% Preschool children 0.5-3.5% Ophthalmic patients 4-5.3%

Normal Visual Development

Basic Mechanisms Abnormal binocular interactionFoveal pattern vision deprivationCombination of both

Susceptible Period Most sensitive first 2-3

years Decreases until age 6-

7 years complete visual

maturation retinocortical pathways

and visual centers resistant to abnormal visual input

Classification Reversible (functional)

Strabismic amblyopia Anisometropic / ametropic amblyopia Visual deprivation amblyopia Idiopathic amblyopia Toxic / Nutritional amblyopia

Irreversible (organic) Amblyopia in nystagmus Other ocular pathologies

Strabismic Amblyopia Higher prevalence in ET than XT because of

nasotemporal asymmetry of retinocortical projections

ET: fovea of deviating eye competes with strong temporal field

XT: fovea competes with weaker nasal field

Strabismic Amblyopia

always unilateral, caused by active inhibition of retinocortical

pathway originating from fovea of deviating eye due to visual confusion

lack of adequate stimulation of the foveaDuration of strabismus rather than age

at time strabismus develops correlates more closely with amblyopia

Sensory AdaptationsDouble vision: diplopia

obj of interest seen by fovea of one eye and peripheral retina of other eye

Visual confusion fovea of each eye sees 2 different obj

interpreted by brain to be in the same visual direction

Diplopia vs Visual Confusion

Suppression ScotomaEsotropia Exotropia

Lateral Geniculate Nucleus in Strabismic Amblyopia

NORMAL

Normal PET Scan

Relative cerebral glucose metabolism most active in tip of visual cortex

equal activity with monocular viewing of either eye

Right eye viewing

Left eye viewing

Strabismic Amblyopia Normal right eye

active glucose metabolism in tip of visual occipital cortex extending to Area 19 and 7

Amblyopic left eye decreased glucose

metabolism at tip activation of Areas 19

and 7

Right eye viewing

Left eye viewing

Strabismic Amblyopia Amblyopic left eye

minimal glucose metabolism in visual cortex

Normal right eye active glucose

metabolism left temporal lobe

more active than right

Left eye viewing

Right eye viewing

Other AssociationsEccentric FixationAnomalous CorrespondenceLoss of fusionRetinal disparity and rivalryCrowding phenomenonSmooth pursuit asymmetry

Visual Acuity Prediction

Amblyopia vs SuppressionBoth occur to eliminate visual confusion

from dissimilar retinal imagesSuppression occurs only under

binocular conditionsAmblyopia persists after closure of

fixating eye

Anisometropic AmblyopiaActive inhibition of fovea Eliminate sensory interference caused

by superimposition of a focused and defocused image (abnormal binocular interaction)

Aniseikonia may be a factorFoveal pattern vision deprivation

Normal PET Scan

Relative cerebral glucose metabolism most active in tip of visual cortex

equal activity with monocular viewing of either eye

Right eye viewing

Left eye viewing

Anisometropic Amblyopia Greater glucose

metabolism when sound eye (left) is viewing

Note contralateral hemisphere activation with either eye viewing

Left eye viewing

Right eye viewing

Visual Deprivation Amblyopia

Disuse or understimulation of retinaUnilateral more severe than bilateralBoth pattern vision deprivation and

abnormal binocular interaction are factors in monocular

Only pattern vision deprivation in binocular

Mechanism of Ambyopia

Causes AbN binoc Forminteraction Deprivation

Strabismus + -Anisometropia + +Visual Deprivation

Unilateral + +Bilateral - +

Organic AmblyopiaSuperimposed functional component

can be improved with amblyopia therapy

Wrong orientation of retinal receptors?Changes in lateral geniculate nucleus

and striate cortex

DiagnosisFixation preference

but many normal patients have fixation preference

Crossed fixation does not always imply absence of

amblyopia

10 PD Base Up Test

DiagnosisVisual acuity

difference in acuity in 2 eyes but certain small difference exist in normal

population difference of 2 lines for most clinical

purposes

Amblyopia TherapyOcclusion or patching even in

nystagmus1 week per year of life then reevaluate

does not signify end of therapy evaluate occlusion amblyopia

Full time vs part timeRole of tapering

Amblyopia Therapy3-6 weeks in older child because of

schoolRationale:

force to use the bad eye allows neural circuitry to be reestablished

Occlusive Patch under Eyeglasses

Alternative to PatchingPenalization

Atropine (Hunter, Johns Hopkins protocol) Optical penalization (over plus: fogging) Neutral density filters Blurring better eye by any means

Factors Affecting RecoveryStage of maturity of visual connections

at which abnormal visual experience began

Type of abnormal visual experience amblyopia more common in ET than XT may be related to nasotemporal

asymmetry (stronger temporal hemifield)

Factors Affecting RecoveryDuration of deprivationAge at which treatment was startedComplianceNeurologic statusPsychological and psychosocial

considerations

Problems with Amblyopia TherapyPatient compliance & acceptanceParental understanding of rationaleParent-child dynamicsPoor follow-upSmart children memorize charts!Differing methods of assessmentWhen to stop patching?

Results of Treatment

Single optotype improves more rapidly than line acuity

Crowding has prognostic valueSingle E acuity represents true

potentialDifferent charts, different results

Normalization of fixation

Timing of Surgery In general, maximize amblyopia

treatment prior to straightening eyes Enhances fusion-lockMost of the time, more practical to start

patching, continue even after surgery

References

Rosenbaum & Santiago: Clinical Strabismus Management

Von Noorden: Binocular Vision and Ocular Motility

Wright: Pediatric Ophthalmology and Strabismus Taylor:Pediatric Ophthalmology Nelson:Pediatric Ophthalmology Moses & Hart:Adler’s Physiology of the Eye PEDIG Amblyopia Studies

Thank you