Sensory Adaptations in Strabismus

Sensory Adaptations in Strabismus

To avoid confusion and diplopia, the visual system can use the mechanisms of suppressionand ARC .It is important to realize that pathologic suppression and ARC develop only in the immature visual system.

Suppression• Suppression is the alteration of visual sensation that

occurs when the images from 1 eye• are inhibited or prevented from reaching

consciousness during binocular visual activity.• Pathologic suppression results from strabismic

misalignment of the visual axes, Such• suppression can be seen as an adaptation of a visually

immature brain to avoid diplopia,• Physiologic suppression is the mechanism that

prevents physiologic diplopia (diplopia• elicited by objects outside Panum's area) from

reaching consciousness,

Suppression classification• Central versus peripheral. Central suppression is the term used to describe the

mechanism• that keeps the foveal image of the deviating eye from reaching consciousness,• thereby preventing confusion, Peripheral suppression is the mechanism that

eliminates• diplopia by preventing awareness of the image that falls on the peripheral• retina in the deviating eye, the image that resembles the image falling on the

fovea• of the fixating eye. This form of suppression is clearly pathologic, developing as a• cortical adaptation only within an immature visual system, Adults may be unable• to develop peripheral suppression and therefore may be unable to eliminate the

peripheral• second image of the object viewed by the fixating eye (the object of regard)• without closing or occluding the deviating eye,

Suppression classification

Nonalternating versus alternating. If suppression is unidirectional or always causes

the image from the dominant eye to predominate over the image from the deviatingeye, the suppression is non alternating. This type of mechanism may lead to the establishment

of strabismic amblyopia. If the process is bidirectional or switches over

time between the images of the 2 eyes, the suppression is described as alternating .

Suppression classificationFacultative versus obligatory. Suppression may be considered facultative ifPresent only when the eyes are in the deviated state and absent in all otherstates. Patients with intermittent exotropia, for instance, often experiencesuppression when the eyes are divergent but may enjoy high-grade

stereopsiswhen the eyes are straight.In contrast, obligatory suppression is present at all times, whether the eyesare deviated or aligned. The suppression scotoma in the deviating eye may beeither relative (in the sense of permitting some visual sensation) or absolute(permitting no perception oflight).

Tests of suppression

• If a patient with strabismus and NRC does not have diplopia, suppression is present provided

• the sensory pathways are intact. In less clear-cut situations, several simple tests are

• available for clinical diagnosis of suppression.

Management of suppressionTherapy for suppression often involves the treatment of the strabismus itself: • proper refractive correction • occlusion or pharmacologic penalization, to permit equal and alternate use of

each eye and to overcome any amblyopia that may be present • alignment of the visual axes, to permit simultaneous stimulation of correspondingretinal elements by the same objectOrthoptic exercises may be attempted to overcome the tendency of the image fromone eye to suppress the image from the other eye when both eyes are open. TheseExercises are designed to make the patient aware of diplopia first, then attempt tofuse the imagesboth on an instrument and in free space. The role of orthoptics in thetherapy of suppression is controversial.

Anomalous Retinal CorrespondenceAnomalous retinal correspondence (ARC) can be described as a condition whereinthe fovea of the fixating eye has acquired an anomalous common visual direction witha peripheral retinal element in the deviated eye; that is, the 2 foveas have differentvisual directions. ARC is an adaptation that restores some sense of binocularcooperation.Anomalous binocular vision is a functional state superior to that prevailing in thepresence of total suppression. In the development of ARC, the normal sensorydevelopment is replaced only gradually and not always completely. The more longstanding the deviation, the more deeply rooted the ARC may become. The periodduring which ARC may develop probably extends through the first decade of life.

Anomalous Retinal Correspondence

Paradoxical diplopia can occur when ARC persists aftersurgery. When esotropic patients whose eyes have beenset straight or nearly straight report, postoperatively, acrossed diplopic localization of foveal or parafovealstimuli, they are experiencing paradoxical diplopia.Clinically, paradoxical diplopia is a fleeting postoperativephenomenon, seldom lasting longer than a few days toweeks. However, in rare cases, this condition haspersisted for much longer.

Testing for ARCTesting in patients with ARC is performed to determine howpatients use their eyes in normal life and to seek out anyvestiges of normal correspondence. ARC is a sensoryadaptation to abnormal ocular alignment. Because the depthof the sensory rearrangement can vary widely, an individualcan test positive for both NRC and ARC. Tests that closely simulate everyday use of the eyes are morelikely to give evidence of ARC. The more dissociative the test,the more likely the test will produce an NRC response unlessthe ARC is deeply rooted.

Testing for ARC

Some of the more common tests, in order ofmost dissociating to least dissociating, are the

afterimage test, Worth 4-dot test, red-glasstest (dissociation increases with the density of

the red filter), amblyoscope, and Bagolinistriated glasses. If the patient gives an

anomalous localization response in the more dissociative

tests, then the depth of ARC is greater.

Testing for ARCThe tests for ARC can basically be divided into 2 groups: those that stimulate thefovea of 1 eye and an extrafoveal area of the other eye, and those that stimulate the

fovealarea in each eye. Note that ARC is a binocular phenomenon, tested for and

documentedin both eyes simultaneously, Eccentric fixation is a monocular phenomenon found ontesting 1 eye alone; it is not necessarily related to ARC. In eccentric fixation, patientsdo not fixate with the fovea when the fellow eye is covered. On cover testing, the eyeremains more or less deviated, depending on how far the nonfoveolar area of fixationis from the fovea. Because some tests for ARC depend on separate stimulation of eachfovea, the presence of eccentric fixation can significantly affect the test results.

Subjective Testing for Sensory Adaptations

All tests are tainted by the inability of the testingconditions to reproduce the patient's conditionof casual seeing, The more dissociative the test,the less the test simulates everyday use of theeyes. These tests should always be performed inconjunction with a cover test to decide whethera fusion response is due to orthophoria or ARC.

Red-glass testIn a patient with strabismus, the red-glass (diplopia) test involves stimulation of bothThe fovea of the fixating eye and an extrafoveal area of the other eye. First, thepatient's deviation is measured objectively. Then a red glass is placed before theNon deviating eye while the patient fixates on a white light. This test can be performedboth at distance and at near.If the patient sees only 1 light (either red or white), suppression is present .A 5L1 orlOL1 prism base-up in front of the deviated eye can be used to move the image out ofthe suppression scotoma, causing the patient to experience diplopia. With NRC, thewhite image will be localized correctly: the white image is seen below and to the rightof the left image .With ARC, the white image will be localized incorrectly: it is seendirectly below the image.

The following responses are possible with the red-glass test:

The patient may see a red light and a white light. If the patient has esotropia,The images appear uncrossed (eg, the red light is to the left of the white lightwith the red glass over the left eye). This response is known as homonymous,or uncrossed, diplopia. This can easily be remembered because the esotropicpatient sees the red light on the same side as the red glass .If the patient hasexotropia, the images appear crossed (eg, the red light is to the right of thewhite light with the red glass over the left eye). This response is known asheteronymous, or crossed, diplopia. If the measured separation between the2 images equals the previously determined deviation, the patient has NRC.

Red-glass testIf the patient sees the 2 lights superimposed so that they appear pinkishdespite a measurable esotropia or exotropia, an abnormal localization ofretinal points is present. This condition is known as harmonious anomalousretinal correspondence .• If the patient sees 2 lights (with uncrossed diplopia in esotropia and withcrossed diplopia in exotropia), but the separation between the 2 images isfound to be less than the previously determined deviation, the patient hasunharmonious anomalous retinal correspondence. Some investigatorsconsider unharmonious ARC to be an artifact of the testing situation.

Worth 4-dot testIn the Worth 4-dot test, a red glass is worn in front of 1 eye and a green glassin front of the other .The eye behind the red glass can see red light but notgreen light because the red glass blocks this wavelength. Similarly, the eyebehind the green glass can see green light but not red light. A polarized Worth 4-dot test is available; it is administered and interpreted much like thetraditional test except that polarized glasses are worn rather than red andgreen ones. As with the red-glass test, the Worth 4-dot test can produce adiplopic response in nonsuppression heterotropic NRC and either a diplopicor a fusion response in ARC, depending on the depth of the ARC adaptation.As mentioned earlier, this test must be performed in conjunction with covertesting.

Worth 4-dot testWhen testing a patient for monofixation syndrome (see the sectionMonofixation Syndrome later in this chapter), the Worth 4-dot test can beused to demonstrate both the presence of peripheral fusion and the absenceof bifixation. The standard Worth 4-dot flashlight projects onto a centralretinal area of 10 or less when viewed at 10 ft, well within the 10 _40scotoma characteristic of monofixation syndrome. Therefore, patients withmonofixation syndrome will report 2 or 3 lights when viewing at 10 ft,depending on their ocular fixation preference. As the Worth 4-dot flashlight isbrought closer to the patient, the dots begin to project onto peripheral retinaoutside the central monofixation scotoma until a fusion response (4 lights) isobtained. This usually occurs between 2 and 3 ft.

Bagolini glassesBagolini striated glasses are glasses of no dioptric power thathave many narrow striations running parallel in one meridian.These glasses cause the fixation light to appear as an elongatedstreak, like micro-Maddox cylinders. The glasses are usuallyplaced at 135° in front of the right eye and at 45° in front of theleft eye. The advantages of the Bagolini glasses are that theyafford the most lifelike testing conditions and permit theexaminer to perform cover testing during the examination.

4 Δ base-out prism testThe 4 Δ base-out prism test is a diagnostic maneuver performed primarily todocument the presence of a small facultative scotoma in a patient withmonofixation syndrome and no manifest small deviation .In this test, a 4 Δ base-out prism is quickly placed before 1 eye and then theother during binocular viewing, and motor responses are observed .Patientswith bifixation usually show a version (bilateral) movement away from theeye covered by the prism followed by a unilateral fusional convergencemovement of the eye not behind the prism. A similar response occursregardless of which eye the prism is placed over. Often, no movementis seen in patients with monofixation syndrome when the prism is placedbefore the nonfixating eye. A refixation version movement is seen when theprism is placed before the fixating eye, but the expected fusional convergencedoes not occur.

4 Δ base-out prism test

The 4 Δ base-out prism test is the least reliable methodused to document the presence of a macular scotoma.An occasional patient with bifixation recognizesdiplopia when the prism is placed before an eye butmakes no convergence movement to correct for it.Patients with monofixation syndrome may switchfixation each time the prism is inserted and show nomovement, regardless of which eye is tested.

Afterimage test

The test can be performed by covering a cameraflash with black paper and then exposing only anarrow slit, the center of which is covered withblack tape to serve as a fixation point, as well asto protect the fovea from exposure. This testinvolves the stimulation, or labeling, of themacula of each eye with a different linearafterimage, 1 horizontal and 1 vertical.

Afterimage testBecause suppression scotomata extend along thehorizontal retinal meridian and may obscure most of ahorizontal afterimage, the vertical afterimage is placedon the deviating eye and the horizontal afterimage onthe fixating eye simply by having each eye fixate thelinear light filament separately.The central zone of the linear light is occluded to allowthe fovea to fixate and remain unlabeled. The patient isthen asked to draw the relative positions of theperceived afterimages.

Amblyoscope testingAlthough its use has declined in recent years, the majoramblyoscope in various forms (eg, Clement Clarke synoptophore, American Optical troposcope), was for decades a mainstay in thefield of strabismus. The amblyoscope can be used in themeasurement of horizontal, vertical, and torsional deviations; inthe diagnosis of suppression and retinal correspondence; and inthe determination of fusional amplitudes and the degree ofstereopsis, with testing usually performed by an orthoptist. Themajor amblyoscope can also be used in exercises designed toovercome suppression and expand fusional amplitudes.

Monofixation Syndrome

The term monofixation syndrome is used to describe a particular presentation of a

sensory state in strabismus. The essential feature of this syndrome is the presence of

peripheral fusion with the absence of bimacular fusion due to a physiologic macular

scotoma.


A patient with mono fixation syndrome may have nomanifest deviation but usually has a small heterotropia(less than 86), most commonly esotropia. Stereoacuityis present but reduced. Amblyopia is a common finding.The original description of this entity states that retinalcorrespondence is normal regardless of whether thereis a manifest deviation; this has been questioned byother authors.


Monofixation may be a primary condition. It is afavorable outcome of infantile strabismussurgery. This syndrome can also result fromanisometropia or macular lesions. It can be thecause of unilaterally reduced vision when noobvious strabismus is present. If amblyopia isclinically significant, occlusion therapy isindicated.

Monofixation Syndrome Diagnosis

To make the diagnosis of monofixation syndrome, theclinician must demonstrate the absence of bimacularfusion by documenting a macular scotoma in thenonfixating eye under binocular conditions; and thepresence of peripheral binocular vision (peripheralfusion). Several binocular perimetric techniques havebeen described to plot the monoftxation scotoma.However, they are rarely used clinically.

Monofixation Syndrome Diagnosis

Vectographic projections of Snellen letters can be usedclinically to document the facultative scotoma of themonoftxation syndrome. Snellen letters are viewedthrough polarized analyzers or goggles equipped withliquid crystal shutters in such a way that some lettersare seen with only the right eye, some with only theleft eye, and some with both eyes. Patients withmonofixation syndrome delete letters that are imagedonly in the nonftxating eye.

Monofixation Syndrome DiagnosisTesting stereoacuity is an important part of the monofixation syndromeevaluation. Any amount of gross stereopsis conftrms the presence ofperipheral fusion. Most patients with monoftxation syndrome demonstrate200- 3000 sec of arc stereopsis. However, because some patients with thissyndrome have no demonstrable stereopsis, other tests for peripheral fusion,such as the Worth 4-dot test and Bagolini glasses, must be used inconjunction with stereoacuity measurement. Fine stereopsis (better than 67sec of arc) is present only in patients with bifixation.

Amblyopia

AmblyopiaAmblyopia is a unilateral or, less commonly, bilateralreduction of best-corrected VA that cannot beattributed directly to the effect of any structuralabnormality of the eye or the posterior visual pathways.Amblyopia is caused by abnormal visual experienceearly in life resulting from one of the following:• strabismus• anisometropia or high bilateral refractive errors(isometropia)• stimulus deprivation

Amblyopia

Amblyopia is responsible for more unilaterallyreduced vision of childhood onset than all othercauses combined, with a prevalence of 2%- 4%in the North American population.This fact is particularly distressing because, inprinciple, most amblyopic vision loss ispreventable or reversible with timely detectionand appropriate intervention.

AmblyopiaChildren with amblyopia or at risk for amblyopia should be identified at ayoung age, when the prognosis for successful treatment is best. Screeningplays an important role in detecting amblyopia and other vision problems atan early age and can be performed in the primary care practitioner's office,allowing the primary care physician to help coordinate the care of thesepatients, or in community-based vision screening programs. Repeated screening is important for continuing to check for the developmentof vision problems and is also helpful in detecting false-positive results. Aconsensus about the best method and the appropriate age to screen has notyet emerged.

Amblyopia

Amblyopia is primarily a defect of central vision; theperipheral visual field is usually normal.Experimental studies on animals and clinical studies ofinfants and young children support the concept ofcritical periods for sensitivity in developing amblyopia.These critical periods correspond to the period whenthe child's developing visual system is sensitive toabnormal input caused by stimulus deprivation,strabismus, or significant refractive errors.

Amblyopia

In general, the critical period for stimulusdeprivation amblyopia occurs earlier than thatfor ocular misalignment or anisometropia.Furthermore, the time necessary for amblyopiato occur during the critical period is shorter forstimulus deprivation than for strabismus oranisometropia.

Amblyopia

Although the neurophysiologic mechanisms thatunderlie amblyopia are far from clear, the study ofexperimental modification of visual experience inanimals and laboratory testing of humans withamblyopia have provided some insights. Animal modelshave revealed that a variety of profound disturbancesof visual system neuron function may result fromabnormal early visual experience.

Amblyopia

Cells of the primary visual cortex can completely losetheir innate ability to respond to stimulation of 1 orboth eyes, and cells that remain responsive may showsignificant functional deficiencies. Abnormalities alsooccur in neurons in the lateral geniculate body.Evidence concerning involvement at the retinal levelremains inconclusive; if present, changes in the retinamake at most a minor contribution to the overall visualdefect.

AmblyopiaSeveral findings from both animals and humans, such asincreased spatial summation and lateral inhibition when lightdetection thresholds are measured using different-sized spots, suggest that the receptive fields of neurons in the amblyopicvisual system are abnormally large. This disturbance mayaccount for the crowding phenomenon (also known as contourinteraction), whereby Snellen letters or equivalent symbols of agiven size become more difficult to recognize if they are closelysurrounded by similar forms, such as a full line or field of letters.

Amblyopia Classification

Amblyopia has traditionally been subdivided interms of the major disorders that may beresponsible for its occurrence.

Strabismic AmblyopiaThe most common form of amblyopia develops in theconsistently deviating eye of a child with strabismus.Constant, nonalternating heterotropias (typicallyesodeviations) are most likely to cause significantamblyopia. Strabismic amblyopia is thought to resultfrom competitive or inhibitory interaction betweenneurons carrying the non fusible inputs from the 2eyes, which leads to domination of cortical visioncenters by the fixating eye and chronically reducedresponsiveness to input by the nonfixating eye.

Strabismic Amblyopia

Amblyopia itself does not as a rule prevent diplopia.Older patients with long-standing deviations mightdevelop double vision after strabismus surgery despitethe presence of substantially reduced visual acuity fromamblyopia.


Several features of typical strabismic amblyopiaare uncommon in other forms of amblyopia. Instrabismic amblyopia, grating acuity, the abilityto detect patterns composed of uniformlyspaced stripes, is often reduced considerablyless than Snellen acuity.


Apparently, the affected eye sees forms in a twisted ordistorted manner that interferes more with letterrecognition than with the simpler task of determiningwhether a grating pattern is present. This discrepancymust be considered when the results of tests based ongrating detection, such as Teller card preferentiallooking (a method of estimating acuity in infants andtoddlers), are interpreted


• When visual acuity is checked with the use of a neutral-density filter, the acuity of an

• eye with amblyopia tends to decline less sharply than that of a normal eye. This phenomenon

• is called the neutral-density filter effect.

Ecccentric fixationEccentric fixation refers to the consistent use of a nonfoveal regionof the retina for monocular viewing by an amblyopic eye, Minordegrees of eccentric fixation, detectable only with special testssuch as visuscopy, are seen in many patients with strabismicAmblyopia and relatively mild acuity loss. A visuscope projects atarget with an open center surrounded by 2 concentric circlesonto the retina, and the patient is asked to fixate on the target. Ifthe target is not directed at the fovea, the degree of eccentricfixation can be measured using the concentric circles as a guide,Many ophthalmoscopes are eq uipped with a visuscope.

Ecccentric fixationClinically evident eccentric fixation, detectable by observing thenoncentral position of the corneal reflection from the amblyopiceye while it fixates a light with the dominant eye covered,generally implies visual acuity of 20/200 or worse. use of thenonfoveal retina for fixation cannot, in general, be regarded asthe primary cause of reduced acuity in affected eyes.The mechanism of this interesting phenomenon, long a source ofspeculation, remains unknown.

Anisometropic AmblyopiaSecond in frequency to strabismic amblyopia, anisometropicamblyopia develops when unequal refractive errors in the 2 eyescauses the image on 1 retina to be chronically defocused.This condition is thought to result partly from the direct effect ofimage blur on visual acuity development in the involved eye andpartly from interocular competition or inhibition similar (but notnecessarily identical) to that responsible for strabismic amblyopiaRelatively mild degrees of hyperopic or astigmatic anisometropia(1-2 D) can induce mild amblyopia.

Anisometropic Amblyopia

Mild myopic anisometropia (less than -3 D)usually does not cause amblyopia, but unilateralhigh myopia (-6 D or greater) often results insevere amblyopic vision loss, Unless strabismus ispresent, the eyes of a child with anisometropicAmblyopia look normal to the family andprimary care physiCian, typically causing a delayin detection and treatment.

Ametropic AmblyopiaAmetropic amblyopia, a bilateral reduction in acuity that is usually relativelymild, results from large, approximately equal, uncorrected refractive errors inboth eyes of a young child. Its mechanism involves the effect of blurredretinal images alone. Hyperopia exceeding about 5 D and myopia in excess of6 D carry a risk of inducing bilateral amblyopia.Uncorrected bilateral astigmatism in early childhood may result in loss ofresolving ability limited to the chronically blurred meridians (meridionalamblyopia). The degree of cylindrical ametropia necessary to producemeridional amblyopia is not known, but most ophthalmologists recommendcorrection of greater than 2 D of cylinder.

Stimulus Deprivation AmblyopiaDeprivation amblyopia may occur when the visual axis is obstructed. Themost common cause is a congenital or early acquired cataract, but cornealopacities and vitreous hemorrhage may also be implicated. Deprivationamblyopia is the least common but most damaging and difficult to treat ofthe various forms of amblyopia. Amblyopic vision loss resulting from aunilateral occlusion of the visual axis tends to be worse than that producedby bilateral deprivation of similar degree because interocular effects add tothe direct developmental impact of severe image degradation. Even inbilateral cases, however, acuity can be 20/200 or worse.

Stimulus Deprivation AmblyopiaIn children younger than 6 years, dense congenital cataracts that occupy thecentral 3 mm or more of the lens must be considered capable of causingsevere amblyopia. Similar lens opacities acquired after age 6 years aregenerally less harmful. Small polar cataracts, around which retinoscopy canbe readily performed, and lamellar cataracts, through which a reasonablygood view of the fundus can be obtained, may cause mild to moderateamblyopia or may have no effect on visual development. Occlusion amblyopiais a form of deprivation amblyopia that may be seen after therapeuticpatching.

Diagnosis

Amblyopia is diagnosed when a patient is foundto have a condition known to increase the risk ofamblyopia and when reduced visual acuitycannot be explained entirely on the basis ofphysical abnormalities of the eye. Characteristicsof vision alone cannot be used to reliablydifferentiate amblyopia from other forms ofvision loss.

DiagnosisThe crowding phenomenon‘ for example, is typical of amblyopia but is notpathognomonic or uniformly demonstrable.Afferent pupillary defects rarely occur in amblyopia, and then, only in severecases. Amblyopia sometimes coexists with vision loss directly caused by an uncorrectable structural abnormality of the eye such as optic nervehypoplasia or coloboma. When the clinician encounters doubtful orborderline cases of this type ("organiC amblyopia") in a young child, it isappropriate to undertake a trial of occlusion therapy. Improvement invision confirms that amblyopia was indeed present.

Diagnosis

Multiple assessments using a variety of tests orperformed on different occasions are sometimesrequired to make a final judgment concerningthe presence and severity of amblyopia. Tryingto determine the degree of amblyopic vision lossin a young patient should keep certain specialconsiderations in mind.

Diagnosis

The fixation pattern, which indicates the strength ofpreference for 1 eye or the other under binocularviewing conditions, is a test for estimating the relativelevel of vision in the 2 eyes for preverbal children withstrabismus. This test is quite sensitive for detectingamblyopia, but results can be falsely positive, showinga strong preference when vision is equal or nearlyequal in the 2 eyes, particularly with small-anglestrabismus.

DiagnosisA variety of optotypes can be used to directly measure acuity in children 3-6years old. When possible, it is best to use linear symbols to measure visualacuity. Often, however, only isolated symbols can be used, which may lead tounderestimated amblyopic vision loss due to the crowding phenomenon.Crowding bars help alleviate this problem. In addition, the young child's briefattention span frequently results in measurements that fall short of the truelimits of acuity; these results can mimic bilateral amblyopia or obscure orfalsely suggest a significant interocular difference.

Treatment

Treatment of amblyopia involves the followingsteps: 1. Eliminate (if needed) any obstacle to vision,

such as a cataract. 2. Correct any significant refractive error. 3. Force use of the poorer eye by limiting use of the better eye.

Treatment - Cataract RemovalCataracts capable of producing amblyopia require surgery withoutunnecessary delay. In young children, amblyopia may develop as quickly as 1week per age of life. Removal of visually significant congenital lens opacitiesduring the first 4-6 weeks of life is necessary for optimal recovery of vision. Insymmetric bilateral cases, the interval between operations on the first andsecond eyes should be no more than 1-2 weeks. Acutely developing severetraumatic cataracts in children younger than 6 years should be removedwithin a few weeks of injury, if possible. Significant cataracts with uncertaintime of onset also deserve prompt and aggreSSive treatment duringchildhood if recent development is at least a possibility

Treatment - Refractive Correction

In general, optical prescription for amblyopic eyesshould be based on the refractive error as determinedwith cycloplegia. Because an amblyopic eye's ability tocontrol accommodation tends to be impaired, this eyecannot be relied on to compensate for uncorrectedhyperopia as would a normal child's eye. Sometimes,however, symmetric decreases in plus lens power maybe required to foster acceptance of spectacle wear by achild.

Treatment - Refractive CorrectionRefractive correction for aphakia following cataract surgery in childhood

mustbe provided promptly to avoid compounding the visual deprivation effect ofthe lens opacity with that of a severe optical deficit. Both anisometropic andametropic amblyopia may improve or resolve with refractive correction aloneover several months. Given this, many ophthalmologists wait to initiatepatching or penalization. in order to see whether the vision improves withspectacle correction alone. The role of refractive surgery in those patientswho fail conventional treatment with spectacles and/ or contact lenses isunder investigation.

Occlusion and Optical Degradation

Full-time occlusion of the sound eye is defined asocclusion during all waking hours. This treatment isusually performed using commercially availableadhesive patches. Spectaclemounted occluders orspecial opaque contact lenses can be used as analternative to fulltime patching if skin irritation orinadequate adhesion is a significant problem, providedthat close supervision ensures that the spectaclesremain in place consistently.

Occlusion and Optical DegradationRarely, strabismus may result during full-time patching; it is notknown whether strabismus would have occurred with otherforms of amblyopia treatment. Therefore, the child whose eyesare consistently or intermittently straight may benefit by beinggiven some opportunity to see binocularly. Modest reductions inpatching are employed by many ophthalmologists (removing thepatch for an hour or two a day) to reduce the likelihood ofocclusion amblyopia or of inducing strabismus.

Occlusion and Optical DegradationPart-time occlusion, defined as occlusion for 1-6 hours per day, has beenshown to achieve the same results as the prescription of full-time occlusion.The relative duration of patch-on and patch-off intervals should reflect thedegree of amblyopia; for moderate to severe deficits, at least 6 hours per dayis preferred.Compliance with occlusion therapy for amblyopia declines with increasingage. The effectiveness of more acceptable part-time patching regimens inolder children is being actively investigated. Furthermore, studies in olderchildren with amblyopia have shownthat treatment can still be beneficialbeyond the first decade of life. This is especially true in children who have notpreviously undergone treatment.

Other methods of amblyopia treatment

involve optical degradation of the better eye's image to the point

that it becomes inferior to the amblyopic eye's, an approachoften called penalization. Use of the amblyopic eye is thuspromoted within the context of binocular seeing. Studies havedemonstrated that pharmacologic penalization can be used tosuccessfully treat moderate levels of amblyopia. The improvement in vision has been shown to be similar to thatobtained with the prescription of patching.


A cycloplegic agent (usually atropine 1% drops or homatropine 5% drops) isadministered to the better-seeing eye so that it is unable to accommodate. As a result, the better eye experiences blur with near viewing and, ifuncorrected hyperopia is present, with distance viewing. This form oftreatment has been demonstrated to be as effective as patching for mild tomoderate amblyopia (visual acuity of 20/100 or better in the amblyopic eye).Depending on the depth of amblyopia and the response to prior treatment,the hyperopic correction of the dominant eye can be reduced to enhance theeffect. Regular follow-up of patients whose amblyopia is being treated withcycloplegia is important to avoid reverse amblyopia in the previouslypreferred eye.


Pharmacologic penalization offers the particular advantage of being difficultto thwart even if the child objects. Alternative methods of treatment basedon the same principle involve prescribing excessive plus-power lenses(fogging) or diffusing filters. These methods avoid potential pharmacologicside effects and may be capable of inducing greater blur. If the child iswearing glasses, application of translucent tape or a Bangerter foil (aneutral-density filter) to the spectacle lens can be tried. Proper utilization (nopeeking!) of spectacle-borne devices must be closely monitored.Another benefit of pharmacologic penalization and other nonoccludingmethods in patients with straight eyes is that the eyes can work together, agreat practical advantage in children with latent nystagmus.

Complications of TherapyAny form of amblyopia therapy introduces the possibility ofovertreatment leading to amblyopia in the originally better eye.Full-time occlusion carries the greatest risk of this complicationand requires close monitoring, especially in the younger child.The first follow-up visit after initiation of treatment should occurwithin 1 week for an infant and after an interval correspondingto 1 week per year of age for the older child (eg, 4 weeks for a 4year-old). Subsequent visits can be scheduled at longer intervalsbased on early response.

Complications of TherapyPart-time occlusion and optical degradation methods allow forless frequent observation, but regular follow-up is still critical.The parents of a strabismic child should be instructed to watchfor a switch in fixation preference and to report its occurrencepromptly. Iatrogenic amblyopia can usually be treated successfully withjudicious patchingof the better-seeing eye or by alternatingocclusion. Sometimes, simply stopping treatment altogether fora few weeks leads to equalization of vision.

Complications of TherapyThe desired endpoint of therapy for unilateral amblyopia is freealternation of fixation (although 1 eye may still be usedsomewhat more frequently than the other), linear Snellen acuitythat differs by no more than 1 line between the 2 eyes, or both.The time required for completion of treatment depends on thefollowing:• degree of amblyopia• choice of therapeutic approach• compliance with the prescribed regimen• age of the patient

Complications of Therapy

More severe amblyopia, less complete obstruction ofthe dominant eye's vision, and older age are allassociated with a need for more prolonged treatment.Full-time occlusion during infancy may reversesubstantial strabismic amblyopia in 1 week or less. In contrast, an older child who wears a patch only afterschool and on weekends may require a year or more oftreatment to overcome a moderate deficit.

Compliance issuesLack of compliance with the therapeutic regimen is a common problem thatcan prolong the period of treatment or lead to outright failure. If difficultiesderive from a particular treatment method, a suitable alternative should besought. Families who appear to lack sufficient motivation should becounseled concerning the importance of the project and the need forfirmness in carrying it out. They can be reassured that once an appropriateroutine is established and maintained for a short time, the daily effortrequired is likely to diminish, especially if the amblyopia improves.

Compliance issuesThe problems associated with an unusually resistant child vary according toage. In infancy, restraining the child through physical methods such as armsplints or mittens or merely making the patch more adhesive with tincture ofbenzoin may be useful. For children older than 3 years, creating goals and offering rewards tends towork well, as does linking patching to play activities (eg, decorating the patcheach day or patching while the child plays a video game). Authoritative wordsdirected specifically toward the child by the doctor may also help. The

toddlerperiod (1-3 years) is particularly challenging.

UnresponsivenessIn some cases, even conscientious application of an appropriatetherapeutic program fails to improve vision at all or beyond acertain level. Complete or partial unresponsiveness to treatmentoccasionally affects younger children but most often occurs inpatients older than 5 years. The decision of whether to initiate orcontinue treatment in a prognostically unfavorable situationshould take into account the wishes of the patient and family.Primary therapy should generally be terminated if there is a lackof demonstrable progress over 3-6 months with goodcompliance.

UnresponsivenessBefore it is concluded that intractable amblyopia is present,refraction should be rechecked, the pupils carefully reevaluated,and the macula and optic nerve critically inspected for subtleevidence of hypoplasia or other malformation that might havebeenpreviously overlooked. Neuroimaging might be consideredin cases that inexplicably fail to respond to treatment. Amblyopiaassociated with unilateral high myopia and extensive myelinationof retinal nerve fibers is a specific syndrome in which treatmentfailure is particularly common.

Recurrence

When amblyopia treatment is discontinued after fullyor partially successful completion, approximately 25%of patients show some degree of recurrence, which canusually be reversed with renewed therapeutic effort.Institution of a maintenance regimen such as patchingfor 1-3 hours per day, optical penalization withspectacles, or pharmacologic penalization with atropine1 or 2 days per week can prevent backsliding.

RecurrenceIf the need for maintenance treatment is established, treatmentmust be continued until stability of visual acuity is demonstratedwith no treatment other than regular spectacles. This mayrequire periodic monitoring until age 8-10 years. As long asvision remains stable, intervals of up to 6 months betweenfollow-up visits are acceptable. The improvement in visual acuitythat is obtained in most children treated between 7 and 12 yearsof age is sustained following cessation of treatment.

Diagnostic Techniques forStrabismus and

Amblyopia

Documents

Sensory Adaptations in Strabismus