The Anomaly of Normal Correspondence in Strabismus


This is a nice diagram from our strabismic buddy, Squnity Josh, depicting that when the two eyes are misaligned, it is possible for the two eyes to share overlap or common visual direction in the brain’s map of space even though the eyes are physically misaligned.

It’s interesting that we refer to this as anomalous correspondence.  I want to state the case that when a person has a unilateral strabismus, in which one eye provides the primary visual direction while the fellow eye is typically deviated (for want of a better term), it is actually an anomaly for the deviated eye to maintain the primary visual direction at the fovea and develop suppression.  In short, anomalous correspondence (AC) should be the norm.  The anomaly would be for the brain to preserve normal correspondence.

Why?  Because to suppress the information from one eye deeply is a very unnatural process.  And it takes alot of incredible brain power to develop the type of anomaly that allows someone with early onset strabismus to preserve the fovea as the straight ahead direction while actively suppressing its information to avoid double vision.  It makes much more sense for the brain to preserve regions of simultaneous and parallel processing, doing what it knows how to do well.  By that I mean tapping into the relationship between our dual processing systems such as central and peripheral.  That allows you to parse the visual world like a big screen TV, with PIP or split screen capability, redeploying visual attention.

It is estimated that at least half of all patients with strabismus have AC.  I would project that figure to go much higher when we separate strabismus by its age of onset.  The younger the patient is when strabismus is in place, the higher the probability that the patient will develop AC.  The patient who has had successful strabismus surgery prior to seeing us, with a relatively small residual motor misalignment in the absence of diplopia is much more likely to have AC.

If AC is indeed a useful and successful adaptation, why would we want to work hard to disrupt it?  The only good argument I can think of is when a patient tells us that the physical appearance of their eyes is their primary concern, and we believe that unless we help them re-establish normal correspondence, acceptable alignment (that word always makes me think of balancing tires, but it’s what most folks use to describe “straight”) of the eyes won’t occur or hold.

So is stating that normal correspondence (NC) in strabismus is an anomaly just a matter of semantics?  Hardly.  Unless you can make a good argument that all the incredible time and effort required to convert an embedded AC into NC is worthwhile for a specific patient, then I side with docs like David Cook and Samantha Slotnick who lately have bolstered the argument that you begin with where fusion occurs, at whatever level it occurs, and expand it.

 

– Leonard J. Press, O.D., FCOVD, FAAO

 

 

 

 

 

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