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)

X

F lF r

O

P

In visual cortex Fl = X and Fr = O, Therefore results in confusion Fl and P both see the X but In Fl, X = Central In P, X = In Temporal field, Therefore results in diplopia

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

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Hess and Jacobs (1979)

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Eccentricity (deg)Temp Nasal

Blue = normalGreen = ambyope 1Red = amblyope 2

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

Visuscope

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

Patient A Patient B

Fixation Status 2 degrees nasal 2 degrees nasal

Pretherapy VA 20/200 20/50

Expected acuity based on retinaleccentricity

20/50 20/50

Percent of VA loss due to EF <30% Approx 100%

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.

Investigation and Diagnosis

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