The Joy of Abnormal Retinal Correspondence

Hello there, I thought if I blogged a wee bit about some optometry related stuff that I found a bit confusing it might make more sense. So here we are. I named the blog after a Chrome song because it had the word 'eyes' in it. If you are coming here from the music blog TURN BACK NOW.

Okay, back down to business. Binocular vision isn't innate - it develops in the first three years of life and is 'established' by the time the kid is 5 or 6. The 'plastic' period where everything could go horribly wrong ends there. The reflexes that develop in the first three years that cause development of binocularly driven cells in the cortex become unconditioned at that point.

If a strabismus develops before three years then an 'adaptation' to normal development occurs which prevents symptoms (double vision/confusion) and this leads to abnormal unconditioned reflexes developing by six. The kid won't see any symptoms and left untreated has a sort of binocular vision, but not true binocular vision. If the strabismus develops between three and six then symptoms will occur. However the system is still plastic at this point so adaptations do develop and again the child ends up with no symptoms. From six onwards a strabismus is going to cause symptoms, no doubt abouddit.

The problems that occur with this strab i'm typing about are as follows;

1. DIPLOPIA - Image of an object falls onto the fovea of the dominant eye but onto the periphery of the other eye. This gives two images. The visual system has two visual directions because the images aren't falling on corresponding points on the retina as they should be.

2. CONFUSION - The two images are superimposed but the brain can't fuse them as they are different (think about an X and and O near each other on a test chart - the brain might see the X directly on top of the O). This leads to retinal rivalry and what we're calling 'confusion'.

If it is plastic enough the brain copes with these problems in two ways - ARC and suppression.

ARC is the name of a condition in which two originally non-corresponding areas of the retina start to co-operate and produce a form of binocular single vision (not a particularly good one). So for example in the above strab the dominant eye's fovea enters into correspondence with a peripheral point on the retina of the other eye as if it was the fovea. It seems quite clever, and it is, but it's not ideal. For a start vision will obviously be worse in the non-dominant eye as peripheral VA isn't usually as good as foveal VA due to receptor cell density. Although it's preferable to diplopia it still isn't ideal. The definition of ARC is as follows:

"The two foveas have different visual directions." OR
"The fovea of the fixating eye has acquired an anomalous common visual direction with a peripheral element in the deviating eye."

That sounds fine to me. But then it gets a bit more confusing, splintering off into Harmonious and Unharmonius ARC. Harmonious ARC is when the angle of the anomaly is equal to the angle of strabismus. If you're going to have ARC this is the one to have. Unharmonious ARC has an angle of anomaly that's greater than zero but less than the angle of deviation. So patients with this still have some diplopia. Bummer. Annoying me even further at this point is a third type - Paradoxical ARC or PARC in which the angle of anomaly is in the opposite direction to the strabismus! I don't know what this ARC is trying to pull. Anyway PARC is rarely seen and is usually after strab surgery.

Back to ARC in general. I was getting at this bit before - because there is no longer 'point to point' correspondence on the retina there's a loss of resolution - we aren't 'bang on' the fovea in one eye so vision simply isn't going to be as good. The ARC may occur at various levels (people ain't sure yet) - retinal horizontal cells, the LGN or the cortex. In effect what it does is produce an enlarged 'pseudo-Panums' area in the duff eye centred on the point receiving the image. This corresponds with the fovea of the other eye.

So IN ARC THE IMAGES OF THE OBJECT OF REGARD ARE GIVEN THE SAME VISUAL DIRECTION DESPITE THE STRABISMUS. THEREFORE THERE ISN'T ANY DIPLOPIA AND SOME LOW-GRADE BV MAY BE PRESENT.

CONDITIONS FOR DEVELOPMENT OF ARC
1 Strab before about 4 years
2 Delay in treatment
3 Small constant strab angle
4 Esotropia more likely than exotropia
5 Unilateral more likely than alternating.

Theories of ARC tomorrow I think.