The College of Optometrists

what is the 4pd test meant show with micrtropia?

basically you put a 4 base in ( or out) in front of each eye in turn, with the px staring at a small target. The eye then should turn slightly towards the apex in response to maintain bino single vision. If you get close you can see it move. So this is an objective test.

The un-prismed eye should not move, of course.

In microtropia, the 'bad' eye is deeply supressed centrally and will not move in response to having a prism placed over it. The good eye will move however as it is doing all the looking.

You can other things with simple prisms - if you put the 4 vertically - it should cause diplopia - anyone with supression, amblyopia or simply joshing will not report this diplopia.

You can also quickly get to understand fusional reserves by using 4 in and then ou - this is helpful if you are considering prescribing prisms any way. This idea is the basis of the prism flipper, of course.

err ... forgot to add that you use a 8 base out for a baby or infant. hope that helps - palfi

Palfi i think you may be getting a little confused.

In a normal patient. If you put a Base out prism in front of the right eye then the eye will move towards the apex.(to the left) The left eye will make an equal conjugate movement due to herings law of equal innervation (to the left also). If normal binocular vision is available the left eye will then make a vergence movement (to the right) to maintain binocular vision.

The 4pd test is looking for central suppression and is used in the diagnosis of microtropia with identity.

If placed Base out in front of 'good' eye then that eye will converge by 4^; the affected eye will make an outward conjugate movement of 4^. If the image remains within the suppression scotoma, there is no stimulus to resume foveal fixation and that eye will stay displaced.

If the prism is then placed in front of the affected eye then no movement will be observed of either eye, as the brain is not aware of the presence of the prism.

In children and infants 20^ prism is usually used as a screening test of motor fusion.

8^ is too big to assess for central supprssion and really too small an amount to assess motor fusion.

what a wonderfully clear answer Lisa - thank you too, I was getting confused!

thanx guys that helped!

I Think I understand this now...

Microtropia's are more common in small angle esotropia's, excentric fixation is present and the true foveal area is suppressed.

The 4 base out prism will shift the object onto the suppressed true foveal area found temporally, and no movement of the eye occurs.

But what if the small angle strabismus present with a microtropia was an exotropia, would the central supression area this time be nasal?

So should you also check the absence of a response to 4 Base In prism.

Can anyone help me with this?

How confusing????

yes thats right, for a microesotropia you do the 4 base out test, and a microexotropia you do 4 base in test, you should find the same results xxx