How to Prescibe Prism for Strabismus

(If any non doctors are interested I will try to give you some information to help you follow in parenthesis)

How to prescribe prism, in less than five minutes. I had this conversation with my boss when I first started my job.  I had just graduated from school and I thought I knew everything but I was sure I knew nothing.  And like a lot of eye doctors I knew prism existed.  More importantly I knew everything to do when it went wrong but I had no idea where to begin with it.   I had learned all the formulas and theory but reality never lines up with those.

For most people who are struggling with strabismus it is intermittent and at near.  We often start by assessing a von-graffe phoria at near (latent eye turn).  We then test the vergences (reserve flexibility in alignment), both horizontal and vertical.  After those and a few other tests we generally know what the problem is.  Let’s assume it is a convergence insufficiency because that is the easiest.
 

So the next question is how do we treat it?

A doctor who is not comfortable with prism often does one of two things; he simply gives the patient a bifocal.  Or he refers them on to someone who is more comfortable treating this condition.  I much prefer they do the second, as the first may help mildly but it probably won’t solve the problem.

Personally, I would probably start with an add (bifocal) as well.  I find especially in children it helps settle down the focusing system and reduces the stress on the visual system.  Then I would go break out my scotch tape. Yes that scotch tape.  I would look at the phoria and the base out recovery.  I want the recovery to be approximately twice the phoria.  In a convergence insufficiency I need to add base in prism to the system to achieve that.  So I take out some prism from my trial lens set and tape it to the phoroptor.  Then I go retest the von-graffe phoria and vergences.  Have I achieved a recovery of twice the phroria?  If not, I add more prism until I do.

If there was any vertical misalignment initially I neutralize that first then perform the horizontal assessment.  Often times after correcting even a mild vertical the horizontal will take care of itself.

Going back though this you are asking yourself but an add causes more divergence (the eyes to separate).  It does.  But we are going to uses prism to compensate.  At the same time it will make everything clearer which makes reading much easier.

The first time you grab scotch tape in front of a patient you will feel like an idiot.  Remember that feeling when they return in six months and the parents tell you she is reading now.

“Dodge”

William Dodge Perry, OD