Eccentric Fixation

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  • 1. 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)

2. 3. 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)

4. 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.

5. 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

6. 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

7. 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

8. 9. 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.

10. 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.

11. Investigation of EF

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

12. 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

13. 14. Visuscope 15. 16. In amblyopia

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

17. 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

18. 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.

19. 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

20. 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

21. 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 inrelation to the fixation point.
  • If it appears at one side of the letter = EF

22. 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 Maxwells spot

23. 24. Acuity Measurement

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

25. Neutral density Filters

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

26. Speed of Accommodation

  • Much slower in EF (?also in other amblyopes)

27. 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

28. 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

29. 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

30. Haidingers Brushes

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

31. 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.

32. 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

33. General Characteristics

  • Small angle (