Ophthalmology: tropias versus phorias

All right, next topic:
ocular misalignment.

If the eyes aren't aligned properly,

then you might have an exodeviation;

exo just means outwards,

or an esodeviation, that's inwards.

There's also a hyper and a hypo:
up and down.

So let's give a couple examples of this.

Let's say you have normal eyes,
and one of the ways

you can actually estimate
if a young child

has an eye that's turning in
or turning out,

is by using the Hirschberg test.

The idea is you shine a light
at the eyes,

and you look at the light reflex,

the corneal reflex of that light
bouncing off the eye,

and you compare its position
to the underlying pupil,

and normally that reflex ought to be

laying right on top of that pupil.

However, if you have one eye
that's deviated,

for example, this left eye
is turned inwards,

it's a left esotropia, then you can see

that the light reflex is not quite over

the pupil like it's supposed to be.

In this case, the right hypertropia,

the right eye is up a little bit,

and you can actually estimate
the amount of deviation;

and the rule is for every millimeter

that this light is off center,
so in this case,

if this was one millimeter
off of the pupil,

equals about seven degrees
in misalignment, or 15 prism diopters;

and this is the measurement
that we actually use

in clinic, prism diopters.

So we'll go over prisms in a second,

but the Herschberg test, very useful.

Now, we just talked about eso/exo,

tropia/phoria, what does that mean?

Well, tropia is when the eyes
are always deviated.

So, if you have an exotropia,
that means that the eyes

are deviated out, they're wall-eyed
and they're always deviated.

Now phoria, on the other hand,

just means that they're only deviated
sometimes.

So, example of a tropia

would be exotropia or esotropia,

and of course, the hyper and hypo.

And of course, of the phorias,

we would call it something like
an exophoria,

an esophoria, etc, etc.

So let's show an example of this:
this eye,

looks like this left eye
is deviated outwards,

and we use our paddle to cover the eye;

this is called the cover uncover test,

and you look for this eye movement,
and that tells you

that it's there all the time.

So no eye movement here,

because it looks like the right eye
is the dominant eye,

this is a left exotropia.

So let's try to correct it,
and to do that,

we use prisms to try to get things
back in alignment.

Things are still moving, so this isn't
quite enough prism

to get things back in alignment.

Let's try 20 diopters, and now,

when you do our cover uncover test,

we can see that things
are back in alignment.

This was a left exotropia,
approximately 20 degrees,

20 prism diopters, that is,
and you pick the tropias up

with the cover uncover test,
which is what we just did.

So let's try this one.

These eyes look like they're
in reasonably good alignment.

So let's do our cover uncover test.
No movement.

Things are still nice and stable.

Let's try the other eye; cover, uncover.

Things are still in perfect alignment,
wonderful.

But watch this: we'll do a cover

and we'll do a cross cover test,
look at that eye move--

Oh, it's moving again-- Oh, that one's
moving.

This is what we call phoria,

it's there some of the time,
basically when we break fusion.

Let's see if we can get rid
of this for you.

Yep, about 10 diopters a prism
is all it took to fix this phoria.

So this was an exophoria.

It's there only some of the time,

and there was about 10 prism diopters
of it.

You pick up phorias with
the cross cover test,

completely different than
the cover uncover test.

Okay.

So this is a loose prism,

and certainly with kids, loose prisms
are the way to go.

They also make prism bars,

basically put a bunch of prisms
in a single bar,

and you just dial this thing up and down

till you find the right amount
of prism to correct.

I personally hate these bars,
I like to use the loose lens prisms,

they're a lot easier to use;
my own personal preference.

Certainly with children, you want to use

the loose lenses
because they're smaller.

If you use something this big,
kid's gonna try to grab it,

it's not gonna work well.

And if you have a tropia
that can't be corrected

by correcting with glasses or patching,

then you can always go to surgery;

And basically, when we have eyes
out of alignment,

we get them back in alignment,

and we do that by either shortening

or lengthening the rectus muscles.

In this case, this rectus muscle
is cut off

from its insertion and it's reinserted

back onto the sclera using suture,
and this basically lengthens

the effective length of this muscle
and gets things back in alignment.

And you can do the opposite,
you can also shorten the muscle

and reattach it back to
its original insertion.

Not an easy surgery to do,
because you. could imagine,

you're trying to do a scleral pass here

and not perforate into the eye
and hit the retina,

and the sclera is very, very thin.

I mean, so thin. We're talking about
a third of a millimeter

right underneath this muscle insertion.

You can very easily perforate
into the eye,

and not an easy surgery,
but very effective.

So that was ocular misalignment.
Let's move on.