Eccentric Fixation

A failure of the eye to take up fixation with the fovea and using some other point on the retina instead. In strabismic patients this is only seen when the better eye is covered (unless the patient has microtropia with identity).

Between the two eyes there is 'relative localisation' which is based on each receptor having its own 'local sign' which determines the direction of objects in visual space. It's localisation with reference to each eye separately. In eccentric fixation the relative localisation could be normal or abnormal at the eccentrically fixing point or normal/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 and this is known as eccentric viewing. This has a better prognosis for treatment that if the localisation is abnormal.

How EF is investigated

EF is nearly always present in strabismic amblyopia. The best way to investigate it in practice is with the ophthalmoscope w/the graticule on. If you get the patient to look straight into the light at the centre of the target the position of the fovea relative to that target can be noted. In esotropia the EF is usually slightly nasal. You can get a relative measurement by using the scale. NB graticule disc is 5x7 degrees. You induce accomodation using this method so cycloplegia/changing focus is useful.

The past pointing test involves touching the px's finger to the tip of a pen 25cm away, first with the good eye to increase confidence, then with the good eye occluded. If the patient's finger goes slightly to one side of the pen this indicates fixation doesn't coincide with the centre of localisation. The corneal reflexes can also be assessed by occluding one eye in turn. Relative displacement of the reflex in the bad eye by 1mm = about 11 degrees or 20PD. This is a gross test as eccentricity isn't usual that great. Other tests include the Bjerrum screen, Amsler chart, after image transfer test, Haidinger's brushes, acuity measurement and ND filters. The speed of accommodation is much slower in EF but also slower in other amblyopes.

Treatment of EF

As w/amblyopia treatment you have to encourage foveal fixation. Treatment of EF isn't done often and is far too time consuming for a mild effect.

• Direct occlusion alone may improve fixation but often a slight eccentricity remains
• Pleoptic treatment desensitises the eccentrically fixing area
• After-image transfer is used to locate foveal fixation

NB Established EF is real hard to remove. In amblyopia treatment VA won't improve beyond that expected for an eccentrically fixing point.