The examination of binocular vision provides important information about the presence, or not, of amblyopia.
Required Materials
- A toy that is easy to grab (teething ring) [Figure 50].
- A prism of 15 prism dioptrics on a handle.
- A fixation light, an ophthalmoscope.
- Stereopsis test (Lang’s test, TNO stereo test, Titmus housefly test).
- Doll of max. 5 cm in size.
Figure 50
Procedure
Examination From The Age Of 6 Months
When examining the eye position, it is essential that the child has good neck control when seated. At the age of 6 months, this is not always the case. In addition, the cooperation of the child is often not too forthcoming, which may result in an unreliable examination. However, binocular vision can be examined at this age.
- Use a toy when conducting item 6 of the Van Wiechen test. If the child has binocular vision, they will make a targeted attempt to grab the toy.
- A more reliable method is the prism fusion test.
- Place the child on the examination table with their back resting against the chest of the guardian.
- Shine the fixation light and as soon as the child fixates, move the prism down in front of one eye, with the base of the prism pointing away from the nose. Because of the effect of the prism, the light beam will be refracted from the fixation light into the temporal direction, and reach the temporal retina of the eye that is positioned behind the eye. This causes double vision.
- As a result, a fixation movement will occur into the nasal direction by the eye that is situated behind the prism.
- Repeat the procedure for the other eye.
Findings
By moving the image on the retina, the eye will make a fixation movement behind the prism in the nasal direction, moving the central fovea in the temporal direction. The other will temporarily move in a temporal direction. Hering’s law: if one eye moves over a certain distance, then the other eye will also move over the same distance, in the same direction. Because double vision is not cancelled out, it will move back to the mid-position.
- Test normal, binocular vision present: The eye behind the prism will make a fixation movement in the nasal direction and the other eye will return to the mid-position following the conjugated movement.
- Test abnormal, eye behind prism is suppressed: The eye behind the prism does nothing.
- Test abnormal, eye behind prism is dominant: The other eye will move in the temporal direction, but does not return to the mid-position.
Examination From The Age Of 9 Months
- Binocular vision can be examined at this age using the prism fusion test.
- It is also possible to use Lang’s stereoscopic test.
- If the child wears corrective glasses, they will have to use them during the examination.
- Offer the child the cards whilst they are positioned completely vertically and parallel to the frontal plane, at eye level and at reading distance.
- This is best achieved when the child is positioned in front of you on the examination table, facing backwards, allowing you to look over the child’s head. With this set-up, you cannot see the child’s eyes but you can check whether the chart is held in the correct manner.
- Ask the child to point out the images and observe if the child tries to grasp them in the air in front of them, or scratch them directly on the cards.
Findings
If the child has depth perception, they will touch the toy illustrations directly on the cards. If there is a lack of depth perception, the child will randomly touch the cards or be distracted by other things. Children who are able to speak, will be able to correctly point out the images when requested.
Examination From The Age Of 4 Years
At this age, it is still possible to examine depth perception using the Lang chart [Figure 51].
Figure 51
From the age of 4 years and older, the use of the TNO stereo test is more commonly used. It is the most difficult, yet most reliable test for depth perception. The housefly test can also be used.
TNO Depth Perception Test
The child must put on red-green glasses and point out the figures on the cards, or indicate where the cake slice has disappeared [Figure 52].
Figure 52
- If the child wears corrective glasses, they should be placed behind the red-green glasses. A non-corrected left-right difference may influence the test results.
The Housefly Test
- The child must wear polaroid glasses.
- Groups of four circles (most sensitive part of the housefly test), animals (less sensitive) and a large fly (least sensitive) are depicted on the card.
- Ask the child to grab the wings of the fly (these appear to stick out of the plane of the card), to indicate which animal is stepping away from the card, or which circle is hovering above the card in each of the groups of four circles.
One disadvantage of this test is that even without polaroid glasses, it is visible which circle or animal jumps out (when the card is viewed from an angle).