Suppression #3

Ok, we're back with the Whole Treatment Shebang. Treatment of suppression involves training the patient to be aware of the suppressed image. Normally corresponding points must be stimulated to do this - you don't want to be enforcing ARC. So you have to treat the strabismus and totally correct the angle before proceeding. In practice the first thing you'd need to do would be to do an accurate refraction to minimise the angle of strabismus and maximise clarity (this would make the suppression less strong and easier to overcome). Then and only then try one or more of the following:

Stereoscope. The classic 'bird and cage' setup could be used. The practitioner needs to reinforce the prescence of the bird as it won't be seen initially. A variable prism stereoscope can be used to correct angles of >10 degrees. In a Holmes stereoscope the card distance can be changed. Fusion cards (eg incomplete block of text) can be used as well as a G series card (this is a black box with a binocular lock and red/black writing inside - can be used with a red filter). It's better to use 'simultaneous vision' cards w/strabismics because it encourages normal retinal correspondence.

Wire/bar reading. This again needs to be done having already fixed the strabismus. TRY IT YOURSELF. Put a pen in front of some text just above the page. You should be able to see the text between 'two' pens. The patient practices reading the text in between the pens. You need binocular vision to see the text properly.

Red Filter method. Stick a red filter over the dominant eye and trace a picture with a red pen. This again trains the suppressing eye to see whatever you're drawing. A similar exercise would involve sorting coloured beads. The red filter distorts the colour of the beads so the patient would have to use the other eye to see the difference in colour. NB it's important that you don't reinforce ARC and the monocular acuity has to be good enough for the task you're attempting.

Physiological Diplopia. Bit of string and beads/nuts/needles/whatever. The patient finds the point at which the lines 'intersect' and is trained to recognise points further away and nearer in physiological diplopia. This is a long, drawn out process and the chance of it being done these days is fairly slim.

So that's that basically.

Suppression Things to Remember

1. Suppression can transfer from one eye to the other eg in alternating convergent strab
2. It's probably initiated cortically but there may be a retinal component.
3. It eliminates confusion and indeed diplopia if the area extends far enough.

In most people with strabismus from an early age both ARC and suppression are present. Form vision is suppressed both at the macula AND at the nasal retinal point receiving the image. The localisation of an object in space is modified across a more extensive peripheral area (ie larger scotoma) of strabismic eye to avoid diplopia.

NB ARC and Suppression are both binocular adaptations - they only occur when both eyes are being used. Normal correspondence returns and there is no suppression when the fixing eye is occluded and the strabismic eye takes up fixation.

There are two more adaptations that could be present in patients with strabismus, Amblyopia and eccentric fixation. I'll deal with them in a bit.

Suppression #2

SUPPRESSION'S GOT A HOLD O'ME! SUPPRESSION! GOTTA BREAK FREE!

There are a series of tests that can be done to measure the extent of a suppression scotoma. You have to investigate it under binocular conditions. There are two possible scenarios: one - you have two suppression scotoma - one at the fovea and one at the point receiving the image OR you have one big one because they've joined together. That happens if the strabismus is large angle or longstanding.

Here's another one of those things that you read about but never actually have to do in your life ever. It is Suppression Scotometry and it involves the Bjerrum screen. So y'all know it is old hat. The px sits in front of the Bjerrum screen in the corner of the room and a small mirror is mounted at 45 degrees in front of the fixing eye. This stops the eye seeing the screen but the eye can see a small marker to one side of the px at the same distance away as the screen. This allows the scotoma to be plotted with both eyes open throughout. You use the usual bjerrum targets and pins and crap.

You gonna see suppression of the nasal retina in esotropia and suppression of the temporal retina in exotropia. As you may have noticed the test is difficult to set up and takes ages so is not often done.

Something that is often done is the Prism Bar Method. The px fixates a spotlight with the dominant fixing eye and the prism bar is put in front of the suppressing eye. When the image of the spot has moved off the suppression scotoma the patient sees two spots. Simple yet effective. You can get a quantitative estimate of the angular extent of the scotoma by calculating the difference between the angle of strabismus (you've already got that from a prism cover test right) and the amount of prism which moves the spot out of the scotoma. You could do vertical deviations with that method anarl.

Sort Of Conclusion

The deeper the suppression and greater the extent across the retina the more difficult it is to treat. Take into account the age of the patient, the duration of the strabismus and how co-operative the patient is likely to be before even trying man. You must only treat suppression if the deviation can also be eliminated and if there's a strong chance of BSV occuring. Basically this boils down to only young kids where the strab can/has been corrected and binocularly driven cells can still develop. Otherwise you may end up with intractable diplopia which is very rubbish indeed. Suppression is an adaptation to prevent diplopia and confusion so if you try and treat it and the patient ends up with diplopia and confusion then he/she will be angry.

If it's suppression in conjunction with anisometropic amblyopia it can also be treated in a similar way in combination with Rx and amblyopia treatment. I will do the whole treatment shebang tomorrow.

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.