Ординатура / Офтальмология / Английские материалы / Manual of Squint_Ahuja_2008

angle is determined by the prism and convertest, when objective response is zero in presence of manifest squint, shows there is harmonious abnormal retinal correspondence. If subjective response is less than the objective angle means unharmonious type. If it is equal to the objective one there is normal correspondence.

Diplopia Test

In diplopia test, the patient is asked to fix a small light source. Before this objective angle of squint is determined by the prism cover test or by synoptophore. Then a red glass is placed in front of one eye and green in front of the other eye. He will as a rule readily see two light, one red another white. In normal correspondence the direction and amount of separation of two light corresponds to the amount of deviation. If there is unharmonious abnormal retinal correspondence then the separation of the two image will be smaller than angle of squint. There would be no separation in harmonious type of abnormal retinal correspondence. Diplopia test is easily performed, even in small children, but patient often shows suppression on this test and this difficulty can be overcome to come extent by:

a.Putting red filter in front of eye which patient habitually used for fixation

b.A prism may be placed base up or base down (5 to 10 prism diopter) in front of one eye. This displaces the image above or below the region of elective suppression.

MANAGEMENT

Through ARC represents an abnormal reflex development which is capable of considerable degree of fixity and is an obstacle to the development or NRC and true fusion but it does provide a from of binocular cooperation that leads to a reasonable range of fusional amplitude (through anomalous) often and a harmonious ARC is often a highly satisfactory substitute for the normal sensory relationship of the two eyes through perverted and inferior to bifoveal fixation among to the poor resolution provided at the eccentric retinal point and thus through ARC cannot contribute significantly to the binocular image to give rise to “fine stereopsis” but when the deviation is small as in “microtropia” it does give rise to a ”coarse stereopsis” never the less if the eyes are in favorable position from the cosmetic point of view, a harmonious ARC provides a useful degree of binocularity which makes the development of dense amblyopia less likely and tends to stabilize the position of eyes

throughout life so that a consecutive divergence in esotropia becomes a rarity. If ARC is found to be present it is desirable to discover the extent to which it has developed fixity. It may be unstable and variable and thus amenable to treatment or it may be stable, constant and difficulty to eradicate. It is necessary to test the patient’s power of fusion and if fusion pictures can be joined without difficulty the range of fusion can be tested on provided both control are visible presence of a range of fusion in a patient with ARC indicates a well-grounded perverted reflex which has been conditioned by time and usage in much the same way as a normal reflex. Presence of week stereoscopic vision demonstrable with large slides which requires peripheral fusion usually indicates a harmonious type of ARC. A well-established harmonious ARC carries a poor prognostic significance for treatment when treating patients with ARC it is essential to realize that not only one is aiming to overcome an abnormal reflex but one that has become accepted as correct with regard to the body although ocularly incorrect, so aim must be at establishing normal projection (binocular and uniocular) not only in relation to eyes but also to the body. Before reestablishing NRC it is essential to eradicate the ARC present which can be persued with two fold aims of treatment, viz (i) the further development and consolidation of ARC is prevented by obstructing abnormal binocular stimulus, which can be achieved by use of occlusion, orthoptic and surgical treatment as also by prism therapy, (ii) to attempt overcoming the present ARC and to restore normal correspondence and projection, (iii) when the decision is to consolidate the condition of ARC , attempts are made to make ARC usally alternative to normal binocular function by suitable orthoptic exercises the aim of treatment should be to improve the anomalous fusion range, to increase peripheral(anomalous) stereoscopic sense and to develop power of binocular convergence.

Treatment Modalities Available and their Scope

Occlusion Therapy

An ARC is a binocular condition so occlusion of one or other eye as a passive from of therapy serves the dual purpose of presentation of ARC by arresting its onward progress by interrupting the continued stimulating of noncorresponding points and curatively it also serves to reduce the stability of ARC, and to get better results it requires very often to be combinded with other form of treatment of ARC and continued uninterruptly and discarded only if it fails to serve its purpose

after being given reasonably long period of trial. The place of occlusion in preoperative period is accepted by many surgeons to be helpful in breaking the ARC, but postoperative occlusion is advocated when the residual squint is seen to gradually increase in time in an attempt to reestablish the previous angle of anomaly.

Orthoptic Treatment

Stage I

a.Full correction of refractive error under complete cycloplegia

b.As already discussed, conventional occlusion to avoid stimulation of noncorresponding points and to stimulate anatomical fovea.

Stage II: Stimulation of the fovea once the vision in the amblyopic eye was sufficient to appreciate the foveal slide on the synoptophore, the patient was given stimulation of normal fovea by stimulating at the objective angle.

Method of stimulating at the objective angle: The objective angle was measured by the simultaneous foveal perception slides and the tubes were set at the objective angle. The automatic flashing device was switched on in front of the amblyopia eye stimulation was given from 10 to 15 minutes. (A careful observation is necessary to avoid any chance of using the abnormal retinal points. This can be done by drawing the patient’s concentration to the fixation object by taping the picture in the fixing eye now and then and checking the objective angle).

a.Bi-kinetic retinal stimulation was performed using simultaneous foveal perception slides, deeping the tubes at the objective angle and the patient looking straight ahead, he was asked to superimpose the picture.

b.Appreciation of an after-image of symmetrical cross. This was done twice a day for 15-20 minutes for each sitting even while the conventional occlusion was being continued. As a result of the above stimulation, the angle of anomaly was reduced neared to normal.

c.In cases of gross amblyopia, appreciation of Hadinger’s brushes was helpful both in improving the visual acuity and strengthening the normal fovea.

A cross after image was produced fixing eye and the Hadinger’s brush placed in front of the amblyopic eye. Patient was advised to superimpose the brush in the center of the cross after image. Failure to appreciate the brush is due to any organic lesion of the macular area.

It is suitable in young children with sensory mechanism relatively plastic or in when the ARC has not been firmly entrenched and it is often useful also in tacking a persistent postoperative paradoxical diplopia in an adult if functional cause for it can be ruled out preoperative prismotherapy may appear more reasonable to establish bifoveal vision except when the squints more than 35 to 40D or in the presence of marked degree of incomitances in the horizontal or vertical plane so that angle of deviation is variable. Postoperative prismotherapy is helpful to secure and promote bifoveal stimulation during the transitory period after surgery or the relieve persistent post-operative diplopia in adults.

Surgical Treatment of ARC

It should be emphasized that the best treatment of anomalous retinal correspondence is surgery. Identifying the presence of ARC inform the surgeon that the patient will experience diplopia immediately after surgery presuming the eye alignment has been changed sufficiently to move the image of the object of regard outside the suppression scotoma. Postoperative paradoxical diplopia is more readily accepted in children than in adults.

In cases where the angle is large, the preliminary surgical correction of the angle helps easy carrying out of the orthoptic treatment usually surgery was performed into cases when the presence of ocular deviation was obvious and there were complains of diplopia and abnormal head posture. Results may follow surgical treatment of a case of esotropia with ARC (i) NRC with binocular single vision in which case, the retinal correspondence should be consolidated by further orthoptic treatment (ii) there may be a small residual deviation but with development of NAC. Further orthoptic treatment and/or further surgical treatment should be undertaken within a short interval of time in order to eradicate the deviation, (iii) the ocular appearance may be improved but ARC may be still present. Although no further orthoptic treatment is necessary. Such cases need not be kept under observation. If there is indication that NRC can be developed, further orthoptic treatment should be considered immediately, (iv) tertiary correspondence may occur in which case no further treatment is indicated, (v) the deviation may revert to the original angle of squint. Orthotic treatment including occlusion is indicated for a limited period and it may be necessary.

Postoperatively a course of orthoptic exercises were carried out in improving the binocular functions and fusional reserve.

correspondence is not firmly established so that it may vary with the method or time of testing.

V.Amblyopia with strabismus due to anisometropia, with, normal retinal correspondence.

VI. Amblyopia without demonstrable strabismus due to spontaneous reduction of a manifest deviation.

VII. Amblyopia which is usually due to congenital cataract and is said to be classical type of amblyopia exanopsia. Amblyopia is customarily divided into following groups or categories:

Congenital Amblyopia

It may involve one or both eyes. It can be of the following three types:

a.Organic amblyopia

Receptor amblyopia or due to central hemorrhage

b.Amblyopia secondary to nystagmus

i.Latent nystagmus

ii.Manifest micronystagmus

c.Amblyopia secondary to congenital achromotopia.

Ametropic Amblyopia

It occur in one or both eyes in children and adults who have significant refractive errors and have not worn their glasses previously. The vision remained poor and the retina continued to function at a subnormal levels for years. There cases are mostly of high hypermetropia or astigmatism. When prescribed spectacles in adult age, they rarely show an immediate improvement of vision. However, a regular use of correct spectacles for months or years may bring about a marked visual improvement without any other therapy.

Anisometropia Amblyopia

In there cases one eye has got a normal or near normal visual acuity and the other has a high refractive error most commonly hypermetropia, some times a high astigmatism and occasionally in high myopia where the far paint is so close to the eyes that it is not practical to use the eye even for close work. These cases specially that of the hypermetropic or astigmatic group mostly maintain straight eye and therefore a central or foveal fixation and at least some binocular function. In anisometropia the visual objects imaged on the two foveas are identical but of different sharpness, or if the anisometropia is corrected of different size

(aniseikonia). Inhibition thus occurs to keep a blurred image from the more ametropic eye from interfering with perception of sharp image from the fellow eye. The retinal images of different nature percent an obstacle to fusion therefore, anisometropia may be quite often associated with secondary strabismus.

Strabismus Amblyopia

In this condition there is a reduced visual acuity in one eye in patients with strabismus or a history of strabismus without ophthalmoscopically demonstrable anomalies of the fundus.

It is an active suppression of the reception of stimuli from certain parts of the retina by the brain. This means that the higher visual centers and related areas responsible for the reception of stimuli and perception of an image start disregarding the stimulesent by one or more areas of retina usually there are such two areas in the squinting eye, i.e. the macula which is responsible for confusion and another more peripheral area which is responsible for diplopia. This process of active inhibition of stimuli in brain is facultative in early stage but becomes obligatory in late stage. Continued suppression of the stimuli from the macular area leads to amblyopia. Which it seems reasonable to is a result of pure active inhibition in early stages but there in a combination of both active and passive processes in late or well-established cases of amblyopia, i.e. a factor of disuse becomes superimposed upon the continued and constant active inhibition, in a matter of months or more usually years(5). Stimulus Deprivation Amblyopia (Amblyopia Exanopsia) — If occurs in children with congenital total cataract, complete ptosis of corneal opacitis, etc. The amblyopia is because of lack of stimulation during the formative period. The exact nature of the lesion is not clear, head tilting is known about the site and mechanism of this defect but this amblyopia differs at least clinically from amblyopia in strabismus and anisometropia by its irreversibility and occasional bilateral occurrence. Generally visual outcome is less favorable in children with complete congenital cataract than in those with partial congenital cataract, even when the ocular media are perfectly transparent postoperatively. In some of there cases foveal hypoplasia has been assure to be the cause for the visual loss,especially if poor vision is associated with microophthalmos or other congenital anomalies. In other the history clearly indicates that disuse may have indeed been a fact in producing amblyopia.

Meridional Amblyopia

Visual acuity in astigmatism may vary according to the degree of astigmatism while astigmatism of lower degree does not affect visual acuity, higher degree may be associated with reduced vision, which may be correctable with correction of the refractive error. If, however, if may remain uncorrected for a long time, it may be associated with amblyopia. Any of there subjects may have better vision in one meridian than in the other corresponding to the axis of astigmatism even after wearing full optical correction. This condition is called Meridional amblyopia. The condition has been known clinically for a long time. In meridional amblyopia, contrast sensitivity function is reduced, neural element that process spatial frequencies are affected by meridional amblyopia. Retinal sensitivity has also been found to be reduced in the amblyopia meridian in cases of meridional amblyopia. Meridional amblyopia also found in subjects who developed astigmatism secondary to soft tissue anomalies of the orbit in their early life.

HERIDITY IN AMBLYOPIA

Amblyopia which so often accompanies concomitant squint and refractive errors is not genetically determined but because it is only a secondary characteristic inheritance may be associated only because of inheritance of myopia, hypermetropia or concomitant squint.

Ocular Dominance

Dominance may be defined as physiological pre-eminence, priority or preferential activity of one of any anatomically similar bilateral pair of structure in the body for example the hands, the feet, the eyes, ears and two cerebral hemisphere, ocular dominance is the term used for the physiological superiority of one type of an individual over the other eye during binocular through both eyes are anatomically and optically identical. The dominant eye possesses a greater sense of clarity, sharpness of outline and detail and refinement of discrimination. The role of ocular dominance in amblyopia, squint has gained importance. In treatment of amblyopia, cases with crossed dominance showed greater benefit from occlusion because the amblyopic eye is encouraged to be used better. The functional status of the fellow dominant in amblyopia also needs attention. It has been pointed out that the fellow dominant eve is also not a normal eye as compared to control normal eye with binocular fixation.

The changes related to ocular dominance can be demonstrated more clearly in layer IV of the striate cortex.

VISUAL ACUITY IN AMBLYOPIA

From a practical clinical stand point, a difference in vision of two lines on visual acuity chart is frequently used as a diagnostic criteria for amblyopia. Neutral density filters produce a profound reduction in vision in eyes with central retinal lesions and glaucoma whereas “the vision of the eyes with functional amblyopia is not reduced by such filters and occasionally even slightly improves.

Many patients with amblyopia are capable of discriminating rather small visual acuity symbols when they are presented singly against a uniform ground, whereas when there symbols are presented in a row, as on a visual acuity chart, they must be larger for a patient to be able to recognize them with amblyopia eye. Thus, most amblyopia eyes seen to have two. Acuities, which could be designated as line acuity, or ‘Snellen’ acuity and single E acuity. This is known as crowding phenomenon. At the completion of treatment, presence or absence of crowding phenomenon has significant prognostic value.

ACCOMMODATION IN AMBLYOPIA

Visual acuity in near fixation is better than in distance fixation in a number of amblyopics. There is improvement in the fixation pattern of the amblyopia eye in downward gaze and there is a weakness of accommodation of the amblyopia eye compared with that of the normally sighted fellow eye. The third of amblyopic patients have reduced accommodation in their defective eye. Accommodation in a case of amblyopia was very low both for near and distance as compared with control cases. However, it improved significantly immediately often completion of treatment by penalization. Retinal threshold and sensitivity—There is a decreased sensitivity of the foveal cones in amblyopia. Flicker fusion threshold of the foveal area of patients of amblyopia is considerably depressed. A functional defect of the foveal cones would be responsible for a reduced visual acuity.

Phenomenon of Contest

The ability of strabismus amblyopia eye to differentiate contours in varying degree of luminance of the background and found that higher degree of contrast than normal is required at ordinary levels of