Deviation of the eyes – How much is too much?

AmblyoPlay vision deviation
AmblyoPlay vision deviation

When starting examining visual system at the eye care specialist it is very important to start with anamnesis. It is one of the most important starting points of the thorough eye exam. The chance that an anomaly is found is often connected to the family ocular history.

For example, if a parent used to occlude one of the eyes as a child and at the exam they are not sure which eye they had to occlude, it is most probably that the occluding therapy was successful. Occluding is needed when one eye deviates from the ideal axis in regard to the other eye. The most common eye deviations are when one eye is inclined towards the nose – nasally (ESO deviation) or outwards – temporally (EXO deviation). The less common deviation is vertical, however, it is more symptomatic. 

The deviations are categorized in the intensity of the deviation or measured in the angle or prism dioptres. The visual system is so advanced that also in the cases when the two eyes are not aligned ideally the picture on the retina and therefore in the visual cortex is perceived as one.  The problem occurs when one eye is declined in such extent that the pictures projected of the two eyes on both retinas are not merging and are not perceived as one when double vision kicks in. When the image on the retina is projected on the corresponding points on the retinae, the vision is then comfortable and single with the presence of stereopsis (depth perception). If the angle of declination is too big, then the single image is not possible. In this case, the visual system neglects the image that is declined in regard to the other to perceive only one single image.

So is there no problem anymore? When suppressing one eye, what is the most common result in the development stage of the vision (e.g. between 3 and 8 years of age), the deviations are very difficult to recognize. Small deviations are normal and acceptable.

We know a few ways how to recognize bigger deviations on our own. With the test Cover-Uncover, we might find out some deviations. The procedure is as follows:

  • Find an occlude in the shape of a small racket (e.g. Wooden spoon)
  • The test person is focused on an object more than 6 meters away (e.g. Door handle)
  • Start the test by covering one eye (with occluder) and precisely observe the un-occluded eye if there is any deviation
  • Proceed with covering the second eye and again precisely observe for any deviations
  • Repeat these steps a few times to be certain
  • If there is no deviation, the test is negative, so no obvious error or anomaly
  • If the eye is declined nasally – inwards then we get EXO deviation, in case of declination temporally – outwards then we get ESO deviation
  • The same test can be done at near also, for the target get something small (e.g. Pencil top)

In case of uncertainty or bigger declinations, we suggest you visit an eye care specialist for further and more thorough testing.

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