INO vs Partial Third Cranial Nerve Palsy

"So, today we're going to be talking about how to differentiate an internuclear ophthalmoplegia from a partial third nerve palsy. And, so you might be thinking that these would be relatively easy to differentiate but there are common features to both that kind of make it a little bit difficult. So, as you know, when you have an internuclear ophthalmoplegia, you're going to have an ad-duction deficit because the medial rectus muscle of the involved eye is having a problem. So in this example, uh, this is the right eye, let's say this is the left eye. We have a medial rectus muscle that cannot ad-duct and when they try and look to the right, we're going to have ab-ducting dissociated horizontal nystagmus. So that feature right there is one of the features that we're going to be using to differentiate the INO from third nerve palsy: a dissociated abducting horizontal nystagmus that occurs on attempted gaze away from the, um, from the affected medial rectus. So when we're looking to the right, we're going to see an adduction deficit and an ab-ducting nystagmus. The second thing that we're going to be looking for in the INO is if we can overcome the INO with convergence effort. So if we can converge the eyes, that means we can activate the medial rectus muscle from the rostral thalamo-mesencephalic junction without invoking the actual third nerve, uh, itself. I can just talk to the medial rectus with convergence without having, th-, to invoke the third nerve. So the rest of the third nerve is not involved in the convergence effort. And so, if we can converge and activate this medial rectus muscle that we know that this is an INO because if it was a third nerve palsy, both convergence and the ductions and the versions would be out if it was a third nerve palsy-related medial rectus muscle palsy. And then, of course, we're going to be looking for the other findings of third nerve palsy, which is the pupil and the lid. And so, if you have a ptosis, well then that can't be an INO or if you have, uh, pupil involvement (anisocoria, dilated pupil), that cannot occur from that INO because the internuclear ophthalmoplegia is a deficit in the medial longitudinal fasciculus, which does not carry the lid or the pupil fiber. So the presence of the pupil and the lid finding would favor third nerve palsy. And likewise, we're going to have the patient look up and down and we're going to be looking for other third nerve motility findings. Now, you would think that that would be enough to say that it's not a third nerve palsy if you don't have them, but it's not because a isolated medial rectus palsy from an INO can be associated with a vertical deviation which is called "skew". And so, you can have vertical problems with INO because of the skew deviation. So, the key features in differentiating INO from third nerve palsy: an adduction deficit with an ab-ducting dissociated horizontal abducting nystagmus on gaze to the right in the left INO, convergence effort might be able to overcome the deficit if the lesion is in the pons as opposed to the midbrain, the pupil and the lid are the things were going to be using to help differentiate from third, but a vertical deviation could be in either; in an INO that might be a skew, in the third nerve palsy that's going to be the other muscles that are involved that are innervated by three."