Sensory Adaptations in Strabismus

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

2. 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,

3. 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,

4. 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 strabismicamblyopia. If the process is bidirectional or switches over time between the images of the 2 eyes, the suppression is described as alternating .

5. Suppression classification Facultative versus obligatory. Suppression may be considered facultative if Present only when the eyes are in the deviated state and absent in all other states. Patients with intermittent exotropia, for instance, often experience suppression when the eyes are divergent but may enjoy high-grade stereopsis when the eyes are straight. In contrast, obligatory suppression is present at all times, whether the eyes are deviated or aligned. The suppression scotoma in the deviating eye may be either relative (in the sense of permitting some visual sensation) or absolute (permitting no perception oflight).

6. 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.

7. Management of suppression Therapy 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 corresponding retinal elements by the same object Orthoptic exercises may be attempted to overcome the tendency of the image from one eye to suppress the image from the other eye when both eyes are open. These Exercises are designed to make the patient aware of diplopia first, then attempt to fuse the imagesboth on an instrument and in free space. The role of orthoptics in the therapy of suppression is controversial.

8. Anomalous Retinal Correspondence Anomalous retinal correspondence (ARC) can be described as a condition wherein the fovea of the fixating eye has acquired an anomalous common visual direction with a peripheral retinal element in the deviated eye; that is, the 2 foveas have different visual directions. ARC is an adaptation that restores some sense of binocular cooperation. Anomalous binocular vision is a functional state superior to that prevailing in the presence of total suppression. In the development of ARC, the normal sensory development is replaced only gradually and not always completely. The more long standing the deviation, the more deeply rooted the ARC may become. The period during which ARC may develop probably extends through the first decade of life.

9. Anomalous Retinal Correspondence Paradoxical diplopia can occur when ARC persists after surgery. When esotropic patients whose eyes have been set straight or nearly straight report, postoperatively, a crossed diplopic localization of foveal or parafoveal stimuli, they are experiencing paradoxical diplopia. Clinically, paradoxical diplopia is a fleeting postoperative phenomenon, seldom lasting longer than a few days to weeks. However, in rare cases, this condition has persisted for much longer.

10. Testing for ARC Testing in patients with ARC is performed to determine how patients use their eyes in normal life and to seek out any vestiges of normal correspondence. ARC is a sensory adaptation to abnormal ocular alignment. Because the depth of the sensory rearrangement can vary widely, an individual can test positive for both NRC and ARC. Tests that closely simulate everyday use of the eyes are more likely to give evidence of ARC. The more dissociative the test, the more likely the test will produce an NRC response unless the ARC is deeply rooted.

11. Testing for ARC Some of the more common tests, in order of most dissociating to least dissociating, are the afterimage test, Worth 4-dot test, red-glass test (dissociation increases with the density of the red filter), amblyoscope, and Bagolini striated glasses. If the patient gives an anomalous localization response in the more dissociative tests, then the depth of ARC is greater.

12. Testing for ARC The tests for ARC can basically be divided into 2 groups: those that stimulate the fovea of 1 eye and an extrafoveal area of the other eye, and those that stimulate the foveal area in each eye. Note that ARC is a binocular phenomenon, tested for and documented in both eyes simultaneously, Eccentric fixation is a monocular phenomenon found on testing 1 eye alone; it is not necessarily related to ARC. In eccentric fixation, patients do not fixate with the fovea when the fellow eye is covered. On cover testing, the eye remains more or less deviated, depending on how far the nonfoveolar area of fixation is from the fovea. Because some tests for ARC depend on separate stimulation of each fovea, the presence of eccentric fixation can significantly affect the test results.

13. Subjective Testing for Sensory Adaptations All tests are tainted by the inability of the testing conditions to reproduce the patient's condition of casual seeing, The more dissociative the test, the less the test simulates everyday use of the eyes. These tests should always be performed in conjunction with a cover test to decide whether a fusion response is due to orthophoria or ARC.

14. Red-glass test In a patient with strabismus, the red-glass (diplopia) test involves stimulation of both The fovea of the fixating eye and an extrafoveal area of the other eye. First, the patient's deviation is measured objectively. Then a red glass is placed before the Non deviating eye while the patient fixates on a white light. This test can be performed both at distance and at near. If the patient sees only 1 light (either red or white), suppression is present .A 5L1 or lOL1 prism base-up in front of the deviated eye can be used to move the image out of the suppression scotoma, causing the patient to experience diplopia. With NRC, the white image will be localized correctly: the white image is seen below and to the right of the left image .With ARC, the white image will be localized incorrectly: it is seen directly below the image.

15. 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 light with the red glass over the left eye). This response is known as homonymous, or uncrossed, diplopia. This can easily be remembered because the esotropic patient sees the red light on the same side as the red glass .If the patient has exotropia, the images appear crossed (eg, the red light is to the right of the white light with the red glass over the left eye). This response is known as heteronymous, or crossed, diplopia. If the measured separation between the 2 images equals the previously determined deviation, the patient has NRC.

16. Red-glass test If the patient sees the 2 lights superimposed so that they appear pinkish despite a measurable esotropia or exotropia, an abnormal localization of retinal points is present. This condition is known as harmonious anomalous retinal correspondence . • If the patient sees 2 lights (with uncrossed diplopia in esotropia and with crossed diplopia in exotropia), but the separation between the 2 images is found to be less than the previously determined deviation, the patient has unharmonious anomalous retinal correspondence. Some investigators consider unharmonious ARC to be an artifact of the testing situation.

17. Worth 4-dot test In the Worth 4-dot test, a red glass is worn in front of 1 eye and a green glass in front of the other .The eye behind the red glass can see red light but not green light because the red glass blocks this wavelength. Similarly, the eye behind 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 the traditional test except that polarized glasses are worn rather than red and green ones. As with the red-glass test, the Worth 4-dot test can produce a diplopic response in nonsuppression heterotropic NRC and either a diplopic or a fusion response in ARC, depending on the depth of the ARC adaptation. As mentioned earlier, this test must be performed in conjunction with cover testing.

18. Worth 4-dot test When testing a patient for monofixation syndrome (see the section Monofixation Syndrome later in this chapter), the Worth 4-dot test can be used to demonstrate both the presence of peripheral fusion and the absence of bifixation. The standard Worth 4-dot flashlight projects onto a central retinal area of 10 or less when viewed at 10 ft, well within the 10 _40 scotoma characteristic of monofixation syndrome. Therefore, patients with monofixation syndrome will report 2 or 3 lights when viewing at 10 ft, depending on their ocular fixation preference. As the Worth 4-dot flashlight is brought closer to the patient, the dots begin to project onto peripheral retina outside the central monofixation scotoma until a fusion response (4 lights) is obtained. This usually occurs between 2 and 3 ft.

19. Bagolini glasses Bagolini striated glasses are glasses of no dioptric power that have many narrow striations running parallel in one meridian. These glasses cause the fixation light to appear as an elongated streak, like micro-Maddox cylinders. The glasses are usually placed at 135° in front of the right eye and at 45° in front of the left eye. The advantages of the Bagolini glasses are that they afford the most lifelike testing conditions and permit the examiner to perform cover testing during the examination.

20. 4 Δ base-out prism test The 4 Δ base-out prism test is a diagnostic maneuver performed primarily to document the presence of a small facultative scotoma in a patient with monofixation syndrome and no manifest small deviation . In this test, a 4 Δ base-out prism is quickly placed before 1 eye and then the other during binocular viewing, and motor responses are observed .Patients with bifixation usually show a version (bilateral) movement away from the eye covered by the prism followed by a unilateral fusional convergence movement of the eye not behind the prism. A similar response occurs regardless of which eye the prism is placed over. Often, no movement is seen in patients with monofixation syndrome when the prism is placed before the nonfixating eye. A refixation version movement is seen when the prism is placed before the fixating eye, but the expected fusional convergence does not occur.

21. 4 Δ base-out prism test The 4 Δ base-out prism test is the least reliable method used to document the presence of a macular scotoma. An occasional patient with bifixation recognizes diplopia when the prism is placed before an eye but makes no convergence movement to correct for it. Patients with monofixation syndrome may switch fixation each time the prism is inserted and show no movement, regardless of which eye is tested.

22. Afterimage test The test can be performed by covering a camera flash with black paper and then exposing only a narrow slit, the center of which is covered with black tape to serve as a fixation point, as well as to protect the fovea from exposure. This test involves the stimulation, or labeling, of the macula of each eye with a different linear afterimage, 1 horizontal and 1 vertical.

23. Afterimage test Because suppression scotomata extend along the horizontal retinal meridian and may obscure most of a horizontal afterimage, the vertical afterimage is placed on the deviating eye and the horizontal afterimage on the fixating eye simply by having each eye fixate the linear light filament separately. The central zone of the linear light is occluded to allow the fovea to fixate and remain unlabeled. The patient is then asked to draw the relative positions of the perceived afterimages.

24. Amblyoscope testing Although its use has declined in recent years, the major amblyoscope in various forms (eg, Clement Clarke synoptophore, American Optical troposcope), was for decades a mainstay in the field of strabismus. The amblyoscope can be used in the measurement of horizontal, vertical, and torsional deviations; in the diagnosis of suppression and retinal correspondence; and in the determination of fusional amplitudes and the degree of stereopsis, with testing usually performed by an orthoptist. The major amblyoscope can also be used in exercises designed to overcome suppression and expand fusional amplitudes.

25. 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.

26. Monofixation Syndrome A patient with mono fixation syndrome may have no manifest deviation but usually has a small heterotropia (less than 86), most commonly esotropia. Stereoacuity is present but reduced. Amblyopia is a common finding. The original description of this entity states that retinal correspondence is normal regardless of whether there is a manifest deviation; this has been questioned by other authors.

27. Monofixation Syndrome Monofixation may be a primary condition. It is a favorable outcome of infantile strabismus surgery. This syndrome can also result from anisometropia or macular lesions. It can be the cause of unilaterally reduced vision when no obvious strabismus is present. If amblyopia is clinically significant, occlusion therapy is indicated.

28. Monofixation Syndrome Diagnosis To make the diagnosis of monofixation syndrome, the clinician must demonstrate the absence of bimacular fusion by documenting a macular scotoma in the nonfixating eye under binocular conditions; and the presence of peripheral binocular vision (peripheral fusion). Several binocular perimetric techniques have been described to plot the monoftxation scotoma. However, they are rarely used clinically.

29. Monofixation Syndrome Diagnosis Vectographic projections of Snellen letters can be used clinically to document the facultative scotoma of the monoftxation syndrome. Snellen letters are viewed through polarized analyzers or goggles equipped with liquid crystal shutters in such a way that some letters are seen with only the right eye, some with only the left eye, and some with both eyes. Patients with monofixation syndrome delete letters that are imaged only in the nonftxating eye.

30. Monofixation Syndrome Diagnosis Testing stereoacuity is an important part of the monofixation syndrome evaluation. Any amount of gross stereopsis conftrms the presence of peripheral fusion. Most patients with monoftxation syndrome demonstrate 200- 3000 sec of arc stereopsis. However, because some patients with this syndrome have no demonstrable stereopsis, other tests for peripheral fusion, such as the Worth 4-dot test and Bagolini glasses, must be used in conjunction with stereoacuity measurement. Fine stereopsis (better than 67 sec of arc) is present only in patients with bifixation.

31. Amblyopia

32. Amblyopia Amblyopia is a unilateral or, less commonly, bilateral reduction of best-corrected VA that cannot be attributed directly to the effect of any structural abnormality of the eye or the posterior visual pathways. Amblyopia is caused by abnormal visual experience early in life resulting from one of the following: • strabismus • anisometropia or high bilateral refractive errors (isometropia) • stimulus deprivation

33. Amblyopia Amblyopia is responsible for more unilaterally reduced vision of childhood onset than all other causes combined, with a prevalence of 2%- 4% in the North American population. This fact is particularly distressing because, in principle, most amblyopic vision loss is preventable or reversible with timely detection and appropriate intervention.

34. Amblyopia Children with amblyopia or at risk for amblyopia should be identified at a young age, when the prognosis for successful treatment is best. Screening plays an important role in detecting amblyopia and other vision problems at an early age and can be performed in the primary care practitioner's office, allowing the primary care physician to help coordinate the care of these patients, or in community-based vision screening programs. Repeated screening is important for continuing to check for the development of vision problems and is also helpful in detecting false-positive results. A consensus about the best method and the appropriate age to screen has not yet emerged.

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

36. Amblyopia In general, the critical period for stimulus deprivation amblyopia occurs earlier than that for ocular misalignment or anisometropia. Furthermore, the time necessary for amblyopia to occur during the critical period is shorter for stimulus deprivation than for strabismus or anisometropia.

37. Amblyopia Although the neurophysiologic mechanisms that underlie amblyopia are far from clear, the study of experimental modification of visual experience in animals and laboratory testing of humans with amblyopia have provided some insights. Animal models have revealed that a variety of profound disturbances of visual system neuron function may result from abnormal early visual experience.

38. Amblyopia Cells of the primary visual cortex can completely lose their innate ability to respond to stimulation of 1 or both eyes, and cells that remain responsive may show significant functional deficiencies. Abnormalities also occur in neurons in the lateral geniculate body. Evidence concerning involvement at the retinal level remains inconclusive; if present, changes in the retina make at most a minor contribution to the overall visual defect.

39. Amblyopia Several findings from both animals and humans, such as increased spatial summation and lateral inhibition when light detection thresholds are measured using different-sized spots, suggest that the receptive fields of neurons in the amblyopic visual system are abnormally large. This disturbance may account for the crowding phenomenon (also known as contour interaction), whereby Snellen letters or equivalent symbols of a given size become more difficult to recognize if they are closely surrounded by similar forms, such as a full line or field of letters.

40. Amblyopia Classification Amblyopia has traditionally been subdivided in terms of the major disorders that may be responsible for its occurrence.

41. Strabismic Amblyopia The most common form of amblyopia develops in the consistently deviating eye of a child with strabismus. Constant, nonalternating heterotropias (typically esodeviations) are most likely to cause significant amblyopia. Strabismic amblyopia is thought to result from competitive or inhibitory interaction between neurons carrying the non fusible inputs from the 2 eyes, which leads to domination of cortical vision centers by the fixating eye and chronically reduced responsiveness to input by the nonfixating eye.

42. Strabismic Amblyopia Amblyopia itself does not as a rule prevent diplopia. Older patients with long-standing deviations might develop double vision after strabismus surgery despite the presence of substantially reduced visual acuity from amblyopia.

43. Strabismic Amblyopia Several features of typical strabismic amblyopia are uncommon in other forms of amblyopia. In strabismic amblyopia, grating acuity, the ability to detect patterns composed of uniformly spaced stripes, is often reduced considerably less than Snellen acuity.

44. Strabismic Amblyopia Apparently, the affected eye sees forms in a twisted or distorted manner that interferes more with letter recognition than with the simpler task of determining whether a grating pattern is present. This discrepancy must be considered when the results of tests based on grating detection, such as Teller card preferential looking (a method of estimating acuity in infants and toddlers), are interpreted

45. Strabismic Amblyopia • 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.

46. Ecccentric fixation Eccentric fixation refers to the consistent use of a nonfoveal region of the retina for monocular viewing by an amblyopic eye, Minor degrees of eccentric fixation, detectable only with special tests such as visuscopy, are seen in many patients with strabismic Amblyopia and relatively mild acuity loss. A visuscope projects a target with an open center surrounded by 2 concentric circles onto the retina, and the patient is asked to fixate on the target. If the target is not directed at the fovea, the degree of eccentric fixation can be measured using the concentric circles as a guide, Many ophthalmoscopes are eq uipped with a visuscope.

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

48. Anisometropic Amblyopia Second in frequency to strabismic amblyopia, anisometropic amblyopia develops when unequal refractive errors in the 2 eyes causes the image on 1 retina to be chronically defocused. This condition is thought to result partly from the direct effect of image blur on visual acuity development in the involved eye and partly from interocular competition or inhibition similar (but not necessarily identical) to that responsible for strabismic amblyopia Relatively mild degrees of hyperopic or astigmatic anisometropia (1-2 D) can induce mild amblyopia.

49. Anisometropic Amblyopia Mild myopic anisometropia (less than -3 D) usually does not cause amblyopia, but unilateral high myopia (-6 D or greater) often results in severe amblyopic vision loss, Unless strabismus is present, the eyes of a child with anisometropic Amblyopia look normal to the family and primary care physiCian, typically causing a delay in detection and treatment.

50. Ametropic Amblyopia Ametropic amblyopia, a bilateral reduction in acuity that is usually relatively mild, results from large, approximately equal, uncorrected refractive errors in both eyes of a young child. Its mechanism involves the effect of blurred retinal images alone. Hyperopia exceeding about 5 D and myopia in excess of 6 D carry a risk of inducing bilateral amblyopia. Uncorrected bilateral astigmatism in early childhood may result in loss of resolving ability limited to the chronically blurred meridians (meridional amblyopia). The degree of cylindrical ametropia necessary to produce meridional amblyopia is not known, but most ophthalmologists recommend correction of greater than 2 D of cylinder.