Fixation (eye)

With fixation is in the ophthalmology targeted viewing (fixing) denotes an object in outer space, which in the normal case with the retinal point of highest resolution , the fovea centralis , is done. Another term used for this is central or foveal fixation. This point conveys the sense of direction "straight ahead" and thus represents the main physiological direction of vision of the eye , which in physical space is called a straight line between the foveola and the object to be fixed . All other retinal points and objects in the field of view represent secondary directions which differ from the main direction of view due to a different localization of the objects. This is also called the relative localization of the eye. It remains in place as long as there is foveal fixation.

A distinction is made between the egocentric localization, which relates to one's own body and is retained even when there is no longer any foveal fixation. In addition to intact anatomical and functional structures, the development and maintenance of the main direction of view of the fovea centralis and its property as the motor zero point of the eye are prerequisites for central fixation.

Pathophysiology

If the foveola loses its property as a place of fixation, two different states can arise: the eccentric setting and the eccentric fixation .

Eccentric setting

An eccentric setting exists when the use of the foveola, for example due to a macular degeneration , is no longer possible, but its main direction of vision is still preserved. Subjectively, in such cases, the person concerned has the feeling of having to “look past” an object, as otherwise it would be covered by a central scotoma . However, the foveola remains the center of the visual field. Usually this situation only occurs in adulthood.

Eccentric fixation

The term eccentric fixation is used when the main direction of vision is no longer associated with the foveola, but with another point on the retina that is used for fixation. This usually happens with strabismus and leads to amblyopia . At the same time, the main direction of vision changes to the eccentric retinal point. Subjectively, a person affected in this situation also has the impression of looking directly at an object. Accordingly, the relative localization is also based on the new main viewing direction, which is now represented by the retinal point of the eccentric fixation.

There are basically the following types of eccentric fixation:

parafoveolar fixation (within the wall reflex up to about 2 °)
parafoveal fixation (outside the wall reflex from about 2 ° to 5 °)
peripheral fixation (> 5 °)
Lack of fixation (afixation)
For the sake of completeness, the nystagmiform fixation should also be mentioned here , an unsteady or restless variant of the mentioned forms of fixation.

It is assumed that the increasing eccentricity of the fixation is accompanied by a higher deterioration in visual acuity , but a reliable relationship between these parameters has not been proven. The test methods used are of corresponding importance here.

examination

The assessment options for examinations in free space are very limited. Although various features can indicate amblyopia, they hardly allow reliable statements about the fixation, especially not about its location on the retina. Therefore, an ophthalmoscope (visuscope) is usually a good choice, with which a small object to be fixed, usually an asterisk, is projected onto the fundus and, if necessary, an existing ametropia can be compensated by connecting appropriate corrective lenses until the object is sharply focused on the fundus . The examination is carried out by the examiner, who looks from the other side of the ophthalmoscope into the eye and onto the fundus and can thus always see where the object to be fixed is currently being imaged. Even small children can be examined with the support of an optomotor reflex, which ensures a central setting of a peripheral image stimulus. However, the inspections are not easy and the results should be assessed with appropriate caution. Older children and adults are asked to look exactly into the center of the object to be fixed. By asking questions, the examiner ascertains whether the test person actually has the impression that they are fixing the center of the object straight ahead.

If the setting is eccentric, the fixation image will first be shown on a retinal point next to the foveola , but then usually after repeated prompting to look straight ahead in the center of the same. In the case of an eccentric fixation, the object will be next to, above or below the foveola, whereby the examined person states in this situation that he is looking at it directly and straight ahead.

A haploscope equipped with a so-called Haidinger tuft offers another examination option . This is an entoptic phenomenon that can only be perceived with the Foveola. Not all patients with eccentric fixation can recognize it, as can people with organically caused central scotoma. In contrast, with eccentric fixation, the Haidinger tuft is located exactly where the viewing direction of the foveola actually points.

treatment

To actively influence pathological fixation behavior, pleoptic procedures can be used if necessary . Otherwise, occlusions of the better eye are the rule for amblyopia treatment. In special cases, the poorer eye is also occluded (inverse occlusion) with the aim of "loosening" a very stable eccentric fixation. One generally hopes for a return to a foveolar, central fixation, the restoration of the associated main direction of vision and thus an improvement in visual acuity and orientation.

Others

Scientific analyzes of the fixation process , for example when looking at a picture or when reading , are carried out through eye movement registration . So-called eye trackers are used as aids .

literature

W. Haase in: Strabismus . Edited by Herbert Kaufmann, with the collaboration of W. de Decker et al., Stuttgart: Enke, 1986, ISBN 3-432-95391-7