Suppression #1

Suppression is another adaptation to strabismus that occurs before 6 years of age and like ARC turns up in order to avoid diplopia or confusion. It's an extension of normal visual processing that takes place at the cortex and prevents images in the strabismic eye reaching a conscious level. Again this is good and bad - no double vision but it makes restoring normal sensorimotor function harder.

It usually occurs in tandem with ARC in smaller strabs of 25 dioptres or less and dominates proceedings in larger ones.

There is a thing which needs to be considered here and it is called Retinal Rivalry. When different images are presented to two corresponding points on the retina either the strongest image or dominant eye dominates. If the eyes are equally dominant there is what is known as an alternating rhythm of change between the two eyes ie it can't make its mind up. For example an F and an L overlapping and making an E. Or two squares with diagonal lines orientated 90 degrees to each other creating some sort of thatched pattern.

Obviously when you have a strabismus your corresponding points are always receiving different images and the retinal rivalry you would have had previously with normal BV (looking into a synoptophore or whatever) would cease to exist. You would only see one image. This loss of form in the deviated eye prevents confusion.

Suppression also occurs without strabismus in amblyopia. See later.

The optician (or more likely orthoptist?) can investigate suppression in various ways. We mainly look to determine the depth of suppression and the extent of retina being suppressed (aka size of the suppression scotoma).

Existence & Depth

A neutral density filter bar can be used to assess how deeply ingrained suppression is. If the px has strabismus we instruct him/her to look at a spot and the palest filter is put in front of the non-suppressing eye and the density increased until two lights are seen. How dark the filter is at this point is a measure of the depth of suppression. Sometimes when this point is reached fixation swaps over to the other eye as if we'd put up an occluder and the suppression swaps over again resulting in no diplopia. A stereoscope can also be used with various cards featuring images of different sizes etc. If fusion cards such as interrupted passages of text are used and can be read normally then the suppression is said to be more shallow.

The infinity balance/septum test can be useful in anisometropia, heterophoria and small angle strabismus. If one eye is found to be suppressing then it may be overcome if you occlude the other eye for a moment.

A more widely used technique for determining the size of the suppression scotoma is Bagolini striated lenses. You place the lenses with the streaks at 90 degrees to each other and ask the px if both of them are seen, for a start. Then you can ask if there is a central gap where the lines cross. If one streak is missing or there's a central gap then you can use the neutral density filter as above to test the depth of suppression. Tests like this are more widely used than stereoscopes etc partly because they don't totally dissociate the eyes and are closer to 'real life' in that respect. The maddox rod could also be used but it dissociates the eyes a lot. The polarised lines on the Mallet unit dissociate to a lesser extent.

Yet another test is often seen in orthoptics departments in hospitals: the Worth Four Dot Test.
There it is. It's viewed through red and green goggles. So with a red filter on the right eye and a green filter on the left you can get the following possible results

• Px sees all lights - normal fusion
• Px sees all light but has strabismus - has ARC
• Px sees two red lights - LE suppressing
• Px sees three green lights - RE suppressing
• Px sees two red lights and three green lights -diplopia
• Px sees green & red lights alternating - alternating suppression

This test is not 'mazing. For a start the eyes are well dissociated by the red and green goggles so if a patient with unstable but functionally useful BV tries it then he/she might exhibit a suppression response (ie a false positive). Also the dots are actually so far apart that a small scotoma could be missed in between em.

Mysterious loss of suppression in heterophoria is usually down to suppression. The TNO test has a suppression plate. I can't find a picture of said plate. So i'm just going to stop this post right now.