Now along this journey, light transmission can encounter some hazards. So there can be some
lesions that will block this journey and result in various forms of visual impairments. So we’re going
to take a look at various visual impairments based on the location of the lesion. So first,
we’re going to have a lesion of the entire optic nerve. That lesion is shown right here.
In this case, the lesion will produce a loss of vision in the right eye, anopia. If we look here,
this is the right eye, this is the left eye and we can see that the right eye has gone dark, black.
It’s not perceiving anything or it’s not conveying anything to the primary visual cortex.
Transmission’s been blocked completely here. What are some of the causes of this type of lesion?
There are a couple. One could be trauma or another one could be inflammation of the optic nerve,
so we have optic neuritis. Here, we’re going to shift the lesion to the left optic nerve.
It’s not going to be a complete lesion as we saw with the right optic nerve. Instead,
it’s going to be a partial lesion of the left optic nerve. It’s going to be this lateral half.
So if we take a look at what’s going on here, here’s your temporal retina. This is the path
that’s being blocked. So just the left temporal retina is being blocked in that pathway. What will
happen here is you'll have a loss of the left temporal retina. That will result in blindness
of the left nasal visual field. That is shown right in through here where this area of the left eye
has gone dark. This is left nasal hemianopsia. Causes of this visual deficit could be a tumor
just compressing the lateral side of the left optic nerve or it could be a form of arterial dysfunction
and so there’s a disruption in the blood supply to just this lateral component of your optic nerve.
This could occur also on the right side. Another form of a lesion is where the lateral portion
of the right optic nerve is damaged at the optic chiasma. That is shown right in through here.
So if we look at how the axons are travelling through this area, we will see that the nasal
retina of the right eyeball right in through here, this pathway is uninterrupted. It crosses over here
and then continues to the left primary visual field. What is disrupted, however, is the temporal
retina from the right eye. See, that pathway is blocked. You’re also going to get a blockage
of the nasal retina from the left eye. It’s crossing over here and then the lesion is
sufficient enough to interrupt its transmission of the action potentials to the primary visual cortex.
What will happen here is you’ll have a loss of right temporal retina and blindness of the right
nasal visual field. That is shown right in through here. This is right nasal hemianopsia
which are also involving the left eye as well. Its deficit is this area here which is a left upper
temporal quadrant anopsia. So this part goes black or dark as it can no longer perceive
this visual information. This disorder is kind of like a slice of a pie. So this is referred to
as pie in the sky disorder when we have this left upper temporal quadrant anopsia.
Cause of this: again, we’re looking at a structure that would lie lateral to the optic chiasm.
Our primary culprit here is our internal carotid artery. So if there’s a weakness in the wall of the
internal carotid artery, there’s an aneurysm, you have dilatation. That aneurysm can put
pressure on just this general area here and cause this kind of deficit. This could occur on the
opposite side as well. Here, we have a simpler lesion in that the optic chiasm has been
lesioned completely. We see that lesion right in through here. So any of the fibers that are
crossing are blocked at this particular point. This will cause bitemporal hemianopsia.
So, you see the temporal regions here of the left and right eyes going dark.
Causes? Again, you’ve got to think about what’s in this area. You have the pituitary gland.
So an expansion of a pituitary tumor could put pressure on the optic chiasm and cause
this type of lesion. Another type of tumor is a craniopharyngioma. That too can grow and
involve the optic chiasm causing this lesion. Now, we’re looking at a lesion of the optic tract.
This tract runs from the optic chiasm to the lateral geniculate body on each side. So here’s
the right one. This is the one that’s involved. So we see a complete lesion then through this
right optic tract. This will cause left homonymous hemianopsia. What goes dark here
would be the temporal retina of the left eye and then the nasal retina of the right eye.
Causes of this form of lesion would be a tumor of the temporal lobe because the temporal lobe
is in this area. So if the tumor grows into this right optic tract, that could cause a lesion.
If there’s an abscess of the temporal lobe, that abscess can also disrupt the transmission
of our visual pathway along the right optic tract. Of course, this can occur on the left side
as well. Another point at which a lesion can occur is along the optic radiations.
The optic radiations can be completely lesioned or they may be partially lesioned. We’re going to look
at a partial lesion, two types of partial lesions of our optic radiations. The first one here
is a lesion of the right upper optic radiations. These are a part of Baum’s loop. They’re starting
to terminate on your primary visual cortex in this area. So this could be a side of the lesion here
or it could be a little more proximal or closer to the lateral geniculate bodies. This will result
in left homonymous inferior quadrant anopsia. If we take a look here, here’s the quadrant
that’s disrupted in the left temporal retina. Then here’s a quadrant that’s disrupted on the right
side but on the nasal side. Causes here could be a parietal lobe tumor or an occipital lobe tumor
putting pressure on this part of the pathway. Lesions can also occur along the lower optic
radiations as we see in through this area here. This is also starting to include the upper,
so we kind of want to just look at this right in through here. Lesions of the lower optic
radiations, these axons that are travelling here form Meyer’s loop. They will result in a darkness
here and here of each eye. This is termed left homonymous superior quadrant anopsia.
Causes of this would be a tumor. It could be a tumor of the temporal lobe or a tumor
of the occipital lobe. Then, of course, you can have a disruption of the entire optic radiations.
In here we have a complete lesion on the right side of the optic radiations. This then results
in left homonymous hemianopsia. So you can see the darkness here in the left eye and then
the disruption here at the right eye. Causes would be tumors, could have arterial dysfunction,
interruption of blood flow to this area, or a trauma injury to the brain in this particular area.
What are some of the causes of this lesion? First, we can have tumors again disrupting this final
pathway to the primary visual cortex. Arterial dysfunction could also cause a lesion to the entire
optic radiations. Trauma is another possibility or cause to this particular visual deficit.