Eccentric Fixation

Eccentric Fixation A failure of an eye in monocular vision to take up fixation with the fovea, but with some other point. This hardly occurs except in clinical conditions as the patient is generally not fixing with that eye anyway. It is only shown when the better eye is covered (Exception = microtropia with identity)

Four Theories as to the cause of Eccentric Fixation Suppression Theory (Worth, 1906, Bangerter,1953) Anomalous correspondence theory (Chavasse, 1939, Cuppers, 1956) Motor theory (Schor, 1978) Pickwell (1981)

Worth (1906)/Bangerter (1953) – Suppression Theory: occurs when central acuity has dropped to a level below that of the surrounding area, so that better acuity results. now thought to be unlikely as foveal VA still seems to be better than in the rest of the retina.

Duke-Elder (1973)/ Chavasse (1939)/Cuppers (1956): a change in the central area of localisation resulting from a central scotoma in the amblyopic eye EF secondary to the development of ARC Major problem with this theory is that the angle of anomaly is usually much greater than angle of EF

Schor (1978) : failure of the EOM to relax from the deviation (in strabismus) = MUSCLE POTENTIATION. This is a likely cause as habitual strabismic deviation causes an adaptive after-effect which modifies the subsequent monocular localisation

Pickwell (1981) : a sequel to an enlargement of Panums fusional area following decompensated heterophoria at an early age – eventually leads to microtropia – a loss of accurate correspondence Also a sensorimotor theory by Cuiffreda, Levi and Selenow (1991) NB One or more of these theories may apply to any one patient

Relative Localisation Based on each retinal receptor having its own “local sign”, which determines the direction of objects in visual space. Refers to localisation with reference to each eye separately.

In EF the relative localisation may be as follows: - Normal or abnormal at eccentrically fixing retinal point - Normal or abnormal at the fovea of the same eye Usually if the eccentric point continues to be localised eccentrically and the fovea centrally then patients describe objects as being slightly to one side = ECCENTRIC VIEWING. This has a better prognosis for treatment than if localisation is abnormal.

Investigation of EF is best to use two methods. EF is nearly always present in strabismic amblyopia

Ophthalmoscopic Methods A target is projected and focussed onto the retina and is seen by both the Px and the practitioner. Px is asked to look at the centre of the target and the position of the fovea is noted. Position is then recorded in diagram – also record if steady/unsteady - usually EF is slightly nasal in SOT - can calibrate using the size of the optic disc in the graticule Disc = 5 deg x 7 deg NB accommodation is usually induced using this method – change focus or cycloplegia

In amblyopia – reduced VA by one Snellen line per 0.5 degree of eccentricity (very rough guide)

Past Pointing Test related to localisation carry out test initially with good eye (checks normal ability and increases confidence) occlude amblyopic eye, hold pen 25cm in front and ask patient to touch pen with the tip of their finger repeat with the non-amblyopic eye occluded. If finger goes a few cm to the side then past pointing has been demonstrated (do not repeat too many times as PX adapt) this result indicates that fixation does not coincide with the centre of localisation

Corneal Reflex Test compare reflex position in each eye in turn (other eye occluded). The relative displacement of the reflex by 1mm = approx. 11degrees or 20 PD eccentricity is not usually this great however making EF difficult to detect by this method.

Bjerrum Screen Method In normal subjects the blind spot is the same angular distance from fixation in both eyes. Plot the blind spot carefully in both eyes and compare positions Degree of eccentricity can be measured by the difference in angular distance of blind spot from fixation in each eye Requires good co-operation

Amsler Chart 5mm square in a 10 cm square, printed in white or red and black amblyopes often have small foveal scotoma which shows up as a disturbance on Amsler occurs centrally if central localisation eccentrically if EF this is not a very convincing test

After-image Transfer Test After images are transferred to normally corresponding points in the other eye. photography flashgun that is masked to provide a very bright strip of light occlude amblyopic eye and PX fixates the centre of the strip flash then produces a central after-image occluder is then changed to the good eye and PX looks at a small fixation target (eg Snellen letter) the after image then appears after a few seconds (transferred at cortical level) Px is then asked to locate position of after-image in relation to the fixation point. If it appears at one side of the letter = EF

Haidingers Brushes an entoptic phenomenon due to characteristics of the central fovea area seen with a brightly illuminated blue polarised field when the direction of the polarisation is rotated looks like two darkened and opposing sections rotating in the central field in EF they are not seen at the point of fixation but somewhere to the side or not at all if VA < 6/30 also Maxwell’s spot

Acuity Measurement Crowding phenomena : difference of 1 line can be normal but more indicates amblyopia, especially with EF

Neutral density Filters If a ND filter is added and no reduction in VA occurs then EF is likely to be present

Speed of Accommodation Much slower in EF (?also in other amblyopes)

Assessment of Fixation Centricity of Fixation (central vs eccentric) Magnitude Quality of fixation (steady vs unsteady) Pattern of fixation (drifts, saccades, nystagmus) Percent foveation (30second visuoscopy) Directional bias (nasal, temporal etc) Subjective localisation of primary visual direction Zero retinomotor point

Treatment of EF As in amblyopia, have to encourage foveal fixation Direct Occlusion alone may improve fixation but often a slight eccentricity remains Pleoptic Treatment – desensitises eccentrically fixing area After image transfer – use to locate foveal fixation NB Established EF is hard to remove. Remember in amblyopia treatment VA will not improve beyond that expected for eccentrically fixating point

Treatment of EF Cuiffreda, Levi and Selenow (1991) 2 types of treatment strategy Patient A - direct patching - break down inhibition of dominant eye Patient B - break down the EF - fine fixation tasks under controlled conditions

Haidinger’s Brushes brain to look to side to make centre at fixation point Not usually very successful and can not be done at home.

Microtropia (Microsquint, microstrabismus) a misalignment of the eyes with an angle deviation so small (less than 5 degrees) that it would usually be controlled except on dissociation of the eyes in which case in becomes a phoria.

Certain characteristic features Frequently presents between ages 2-3 years but may be overlooked until later life where it is found on routine check as VA is slightly low. Often made evident by the crowding phenomenon. Amsler charts are useful for demonstrating the abnormal fixation pattern Presence of HARC in small angle squint is associated with eccentric fixation and amblyopia Is invariably eso, exo is rare Very subtle tests are required to discover microtropia

General Characteristics Small angle (<6∆) Anisometropia Amblyopia Eccentric Fixation Harmonious ARC Peripheral Fusion Monfixation syndrome Stereopsis

Anisometropia usually 1.50D or more. Occassionally the patient has equal refractive errors.

Amblyopia usually VA is reduced by 1 or 2 lines only (6/9 – 6/12)

Eccentric Fixation always occurs in microtropia. ANGLE OF ECCENTRICITY = ANGLE OF SQUINT - no movement is detected on the CT (the area of the retina where the image falls in binocular conditions is the same as the eccentrically fixing area) Occasionally the degree of EF < angle of squint and a very small CT movement can be seen (small relative or absolute scotoma at the fovea)

Harmonious ARC the retinal area where the image falls in Pxs habitual vision = anomalously corresponding area = area used for monocular fixation = MICROTROPIA WITH IDENTITY Most microtropia’s are of this type. this is a fully adapted strabismus in terms of both motor and sensory aspects.

Peripheral Fusion peripheral vision provides fusional impulses that help maintain the eyes in their straight position. Can be measured like prism vergences.

Monfixation syndrome in many cases the angle of deviation increases on alternating CT or if one eye is covered longer than usual. An SOP is seen superimposed on the microtropia. This is also called a monofixational heterophoria (due to the eye moving from its adapted position to the full angle of squint under the cover)

Stereopsis a low grade stereopsis has been reported.

Visual Acuity the presence of amblyopia in one eye is usually the first clue that microtropia may be found. Crowding phenomenon present and letters may be missed due to the central scotoma.

Fixation The presence of eccentric fixation should be checked for using an ophthalmoscope, visuscope. The EF may be associated with ARC in the microtropia. Two types: Eccentric fixation = angle of anomaly (no shift on CT) Eccentric fixation does not equal the angle of anomaly (shift on CT).

Cover Test Not usually a strabismic movement but may find esophoria in monofixational syndrome. This could result in microstrabismus being missed.

4-Dioptre Prism Test 4 base out prism is placed before the dominant eye the image moves across the retina and the eye moves to take up fixation. The non-dominant eye moves laterally in the same direction (Herings Law of equal innervation) as it is not fixing – VERSIONAL movement is seen. The prism is then removed and a recovery versional movement is seen. The prism is then place before the amblyopic eye. This time the image is moved across the retina within the suppression area: no movement of either eye. AMBLYOPIA + NO CT MOVEMENT + POSITIVE 4∆ TEST = MICROTROPIA

Bagolini Lens Test Should get HARC – streak passes through the spot, with or without a suppression gap.

Amsler Charts Scotoma may be demonstrated due to the eccentric fixation.

Classification of Microtropia Primary – remains constant throughout life and is rare. A primary microtropia which becomes decompensated particularly between 1 –3 years as a result of an accommodative element or superimposed phoria Secondary – follows optical or surgical correction of a concomitant squint

Lang – 3 types of microtropia (reference to fixation) Central Fixation Eccentric Fixation with ARC, where the angle of anomaly is greater than the degree of eccentricity Eccentric Fixation with ARC where these angles are the same On CT 1 and 2 will give positive results whilst 3 gives a negative CT Also 3 gives a sensory adaptation to the deviation envisaged by Cuppers in his correspondence theory for the development of eccentric fixation

Development The mechanism is not fully understood. It is likely that the condition arises as a response to a scotoma that occurs in the foveal area of one eye, usually due to a blurred retinal image caused by uncorrected anisometropia. Fixation is therefore established at the edge of the scotoma in an area of the retina that is not suppressed. This implies that it develops on the basis of eccentric viewing. It is natural that the onset of EF should be followed by the development of HARC.

Heterophoria Theory: SOP causes an expansion of Panums areas with an alteration of visual direction this results in ARC that in turn causes the development of eccentric fixation because of deep suppression at the fovea. Problems with this theory Difficult to explain why this does not happen in all patients with esophoria (unless some additional abnormality is present) Does not explain microtropia in the absence of SOP. Familial Studies – Lang claims ARC is inherited as primary congenital defect

Treatment Refractive Error Correction, especially in high anisometropia. Aniseikonia is often a problem. Contact lenses correction may help. Treat underlying amblyopia by occlusion of non-squinting eye (if patient <6 years old). Full time occlusion of the fixing eye will occasionally produce a complete cure of the esotropia with restoration of normal visual and stereo acuity. Regular review – improvement must be seen within 4 months or discontinue.

Treatment Orthoptics are not appropriate. In patients >6 years – correct refractive error, otherwise do not treat the microtropia. In most cases correction of the refractive error, if necessary, is the only profitable action. Surgery is not appropriate

Eccentric Fixation

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