Nystagmus and Prisms

I have congenital nystagmus and have heard that wearing base out prisms
can help reduce nystagmus. This forces convergence and nystagmus is reduced.
Has anybody heard of this? My eye doctor was not aware of this.

Mike

 

(TomKopan) wrote in

Not so odd. Many nystagmus patients will have a convergence null, or
some other eye position that works for them. If you observe the patient,
you'll often know what the null is before you even measure for it. Many
patients will try to get up close, and many will exhibit a head-turn.
They do this because they see better this way, although they may not
realize that's because the nystagmus calms down.

Remember that if you use base-outs, you might consider adding a -1 to the
correction to counteract the added accomodation. Patients also prefer to
have the base-outs edge treated somehow to keep light from sneaking in.

The contact lenses presumably work because they slightly increase
afferent feedback, but this is still a matter of some conjecture.

One other thing. There are two kinds of congenital nystagmus patients
(at least you can think of them this way). There are those who don't see
well because of the nystagmus who have otherwise healthy visual systems,
and there are those who have really bad visual systems (chicken and egg
problem-- do these guys have nystagmus because they can't see well, or
did the nystagmus in such patients prevent proper visual development?--
hard to tell) who wouldn't see well even if you stopped the nystagmus
cold. The latter group will not likely be helped by base-outs, contact
lenses, or surgery.

Scott

 

Sometimes it works dramatically.

Scott

 

How much diopter would need to be added to the base-out prism to know
if the prisms have any effect on the nystagmus?

 

Generally, it's not BASE OUT prism that's used. It's yoked prisms
that are designed to shift the image towards the null point of the
nystagmus. You need to find an eye doctor who has experience in
evaluating and prescribing for these types of problems. Generally
people who specialize in low vision have a particular skill for this
type of problem.

One poster mentioned RGP lenses. I have had only one patient out of
about 10 where that worked, but it worked tremendously. If you have
any sort of significant prescription, that might be a good place to
start.

nipidoc

 

(nipidoc) wrote in

Depends on where the null point is, and how good they are. A colleague of
mine tests about 20 or 30 CN patients a year, he's been doing this for
maybe 20 years or longer, and I assisted him for about 5 of those years.
He measures the nulls with precise eye movement recordings. Often, when
there's a null at some eccentric position, there's also a null in
convergence, and the latter often quiets the nystagmus waveform down more
than the former.

I think any bias you see toward yoked prisms might be because a head turn
is a really visible clue that there is an eccentric null, and the
convergence null might be missed. Granted, yoked prisms might be a little
easier to get used to than base outs (especially if you forget to cut extra
minus into the base outs for accomodation), but most of his patients
haven't had trouble with them.

RGPs can work quite well, just as you describe. Many patients who they
work for also have convergence, or some other null, that works better, but
some prefer to avoid the prisms if the contacts work, especially if frames
interfere with sporting activities.

Scott

 

My recollection is that biofeedback for this purpose has never really taken
off. It works, but if the patients becomes excited, the techniques just go
out the window.

Scott