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.