Fixation disparity

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Fixation disparity is an ophthalmological term translated from the English term fixation disparity . It describes a sensorimotor disorder of the two-eyed vision , which represents a measurable inaccuracy of the vergence system. Here, under fusional stress, which is triggered, for example, by holding up prisms , the facial lines of both eyes no longer meet exactly in a fixed object, but at a short distance behind (exodisparity) or in front of it (esodisparity). In this situation, however, no double images occur, since the shifted intersection of the lines of sight is still within the panum area . In contrast to the determination of a heterophoria , central fusion stimuli play a decisive role. It is therefore believed that the fixation disparity may be indicative of bifoveal fixation insufficiency .

In optometry , a similar term has been established, fixation disparity , which is defined in DIN 5340-1998-04, but relating to a single measurement method and its results, the so-called measurement and Correcting by Hans-Joachim Haase (MKH) . Nevertheless, the terms are often used synonymously.

examination

To determine the fixation disparity, it is necessary to use a special examination device ( haploscope ) to show the subject a fusion image and, at the same time, a fixation mark separately for each eye. This is best done with so-called Nonius lines. The lines can be shifted horizontally against each other by the patient and must now be aligned so that they are exactly one above the other or form a continuous line. With normal binocular vision, the test person will place the two lines on top of one another without any discernible deviation. The examiner will now trigger a fusional vergence movement by holding up a prism and let the vernier lines slide over one another again. Here, a deviation of the lines subjectively superimposed on one another for the test person from the actual, objective position of the lines becomes visible. The lines shift against each other with the result that the compensatory movement required by the prism has not reached its goal to the extent required. The facial lines intersect either in front of or behind the fixation object, with the fixation disparity corresponding to their objective deviation.

With the prism both a fusional convergence and a divergence movement can be triggered to compensate for it. It is also possible to test with a vertical orientation. The relationship between fixation disparity and the prismatic loading of vergence can be shown in diagrams and so-called fixation disparity curves.

meaning

If one assumes that vergence movements as a control system always have the goal of maintaining bifoveal fixation, then the fixation disparity represents the limits above which this goal can no longer be achieved under the influence of certain disruptive factors (fusional load due to the specification of prisms). These can be compensated up to certain areas of a prism load. In addition, however, a corresponding eso- or exodisparity will arise.

If there is also heterophoria, certain anomalies can be seen compared to orthophores . On the one hand, it happens here that a fixation disparity is already present without prism loading (rest disparity), which however disappears when prisms are in front of it. In this case, there is a so-called facultative microanomaly . This allows an exact central fusion, but gives it up again under the strain already existing by the heterophoria. On the other hand, a fixation disparity occurs without prism loading, but this does not disappear even when prisms are specified. In this case one speaks of an obligatory fixation disparity . Evaluations of the fixation disparity curves make it clear that in this case the interaction of motor and sensory fusion is disturbed and a precise central fusion is not possible. Consequently, this also means that heterophorias with an obligatory fixation disparity cannot be permanently compensated by prisms.

Vergence disorders play a special role in the investigation of specific complaints such as asthenopia . Solutions and treatment approaches are sought here, particularly from the point of view of occupational medicine (close work, screen work, etc.). The choice of inspection and test procedures, which can give different results in the assessment of fixation disparity and heterophoria, is also of decisive importance here.

See also

Retinal correspondence

literature

Individual evidence

  1. ^ Herbert Kaufmann: Strabismus. with Heimo Steffen. 4th fundamentally revised and expanded edition, Georg Thieme, Stuttgart / New York 2012, page 163 ff. ISBN 3-13-129724-7 .
  2. ↑ Convergence accuracy of the eyes: New measurement methods for practice and research, Leibniz Institute for Labor Research ( Memento from September 4, 2015 in the Internet Archive )