Welcome back. We're going to talk about kind
movement disorders, that strabismus, but
we're also going to talk about an entity
where the brain quits processing the
information coming from another eye
because it's malaligned or because of other
And again, I want to acknowledge the really
wonderful work of Dr.
Jose to made all this possible.
We'll deal first with amblyopia, so normally
for most of us,
when we see an image here of a lovely dog,
it's the same in both
eyes and the brain processes it equivalently
from both eyes.
But in amblyopia, as you can see in the
bottom, it's out of
focus. In fact, can get quite faint and out
of focus with
Why does this happen?
It is due to an abnormal development that
can be acquired due to
visual impairment that causes the central
nervous system to have
defective processing, basically the brain
learns to ignore the
images coming from one eye because it's it
been blocked or otherwise obscured.
So there are some various causes for that.
So what happens kind of in the
pathophysiology, broad strokes, there's
abnormal visual stimulation, usually
unilaterally in one eye, the
lateral geniculate nucleus and the striate
cortex of the visual center that is normally
going to be activated by that eye is
It may, we may even have damage to that.
But it's basically it learns to ignore kind
of the defective
information coming from the bad eye.
The brains ultimately suppresses the stimuli
from the affected eye.
And remember that although photons are
hitting the retina and that's
providing the images we only see with her
And if that doesn't get processed
appropriately, it's as if nothing is coming
into that eye. So the brain suppresses the
stimuli from the affected eye.
With time, the eye can be become
disused and you may actually have changes in
not so much the
structure, but in the way that it is
performing or the way that it even looks.
And you end up with cortical blindness
basically in that eye.
OK, so the types of amblyopia, refractive
is the most common type.
That means that you have a very abnormal
that is giving you just very bad scattered
light hitting the retina
on that side. And that is refractive
The eyes may look perfectly acceptable from
the front, but it is the
asymmetry that is happening either in,
again, the cornea, the lens or the
back of the eye that is responsible for this
form of amblyopia.
You can also have deprivation amblyopia, so
if you have unilateral cataracts, this would
be a severe form of the disease, but
basically now you have a completely clouted
lens, can't see in there.
And the brain says, you know what, that
information's not very useful and eventually
will shut off and you'll have cortical
blindness and occurs when the vision is
obstructed. One of the forms of amblyopia,
of the causes for amblyopia is strabismus,
strabismus simply means malalignment of the
So for looking at this child here, the visual
axes appear quite
different. So the information going from the
deviated eye that isn't staring
forward, that has a medial deviation will
tend to be going to a different
area on the retina.
So what will happen in some cases of
strabismus such as this?
We will get cortical suppression of the
image from the deviating eye because it's
imperfect information or it's not enough
information and the brain will turn that
off. It's an important point to emphasize is
doesn't always cause amblyopia.
Sometimes the strabismus is occurring, in
fact, because the eye is trying
to get the light focused exactly on the
So if this is happening as a kind of a
correction for some other maldevelopment,
then we will have
completely normal visual processing.
But that's fine.
They have accommodated to the particular
of the eye in terms of the types of
Again, these are relatively permanently
deviated in various ways.
If it is deviated inward, the term we will
use is esotropia.
OK, now if it's deviated outward and call
that exotropia, if it's
deviated upwards, it's hypertropia.
And if the eyes are deviated downward, it's
All right. So now we've got the vocabulary.
How is this happening? Why is this
So there may be orbital asymmetry.
So the actual bone around the eye or the
itself may be asymmetric.
And in order for the eye to
focus light appropriately, it may have to
There may be abnormal interpupillary
So if you have a very wide set, eyes are
very narrow eyes that may lead to some forms
of strabismus. Again, trying to provide
focus of the light
onto the fovea in the macula.
You can have abnormal, extraordinary muscle
So if you think about it, if you don't
exactly put the superior oblique where it
needs to be and it pulls from a slightly
different angle, you may have a
permanent malrotation of that eye.
There may be muscle weakness for a variety
of reasons, including
developmental abnormalities in the way that
the muscles are actually put there.
You can have variation the position of the
So normally it's going to be sitting kind of
adjacent in the
nearby vicinity to the optic disc.
But sometimes it may be just the way it
developed a little bit offset and the eye
realizing that wants to get most of the
light image information to that right
location. The macula will deviate the eye so
that it always hits
without extra effort and you can have
So you could have increased either muscle
tone or increased
nervous stimulation that drives
accommodation at least in one eye or the
other. And so that you just have a
physiologic reason why you may have