Covering test
The covering test and uncovering test (also: cover test / uncover test ) is the most important and most common examination method in ophthalmology for the detection of strabismus diseases . The prerequisite for its use is always that the patient is able to fixate an object. During the examination in the vicinity , if possible, no light, but an image or a letter should be presented in order to activate the close-up mechanism of the eye ( accommodation ). A round plastic disc with a short handle is usually used to cover it.
One-sided coverage test
If there is a manifest squint, an object being fixed cannot be viewed by both eyes at the same time. In this situation, the one-sided masking test covers the non-cross-eyed eye and at the same time prompts the patient to continue to fixate the same object. For this purpose, the cross-eyed eye must necessarily be aligned with the object with a corresponding saccade . This saccade is called an adjustment movement . If there is a central fixation , the adjustment movement in this case corresponds to the extent of the objective squint angle.
Alternating masking test
If the eyes are covered alternately, the two-eyed vision is interrupted and every form of latent or manifest squint is visible with the corresponding adjustment movements. Exceptions to this are a special form of microstrabismus and possibly high-grade amblyopia . In some cases it is necessary to carry out the alternating masking test for a longer time in order to bring compensatory fusion movements to a stop and to trigger adjustment movements.
Uncovered
During the uncovering test, the released eye is primarily observed. This can lead to fusion movements or joint adjustment movements with the other eye. Failure to move is also possible. However, the assessment always depends on the result of the monolateral cover test and the existing fixation conditions.
Measurement of squint angles
In addition to the purely qualifying procedure, the masking test is an important instrument for measuring squint angles. For this purpose, prisms are used in the investigation in free space, in each case in the distance and near . These prisms can be used either as single prisms or by means of a prism bar (horizontal or vertical arrangement of prisms of different strengths).
One-sided prism cover test
A prism is held in front of the non-leading, i.e. cross-eyed, eye and the leading eye is covered at intervals and released again. The prism effect is adjusted until there is no further adjustment movement.
Simultaneous prism cover test
It is necessary to estimate the extent of the squint angle as precisely as possible before covering. Then the examiner must simultaneously hold a corresponding prism (usually a single prism ) in front of the cross-eyed eye and the cover plate in front of the guide eye and watch out for adjustment movements. The prism strength is increased and the test is repeated until no further adjustment movements are made. In the case of dynamic squint angles and latent components that change their extent with the degree of dissociation of the examination method, this is a tried and tested method of determining the smallest manifest squint angle. It should therefore not have to be repeated too often, which requires appropriate experience.
Alternating prism cover test
In the alternating prism cover test , a prism bar is held in front of the eye, the deviation of which is to be measured, and at the same time the cover plate alternately covers the right and left eye. The eye behind the prism bar is observed with regard to the adjustment movements and the prism strength is adjusted until these are neutralized. Since the largest squint angles can be determined with this method, it may well happen that you have to hold prisms in front of both eyes until no more adjustment movements occur.
literature
- Herbert Kaufmann (Ed.): Strabismus. With the collaboration of Wilfried de Decker et al. Enke, Stuttgart 1986, ISBN 3-432-95391-7 .
