| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (YG) Class of 2022, Baylor College of Medicine, Houston, Texas |
| Transcript |
A reverse RAPD- there is nothing really reverse about it. A reverse RAPD happens every single time but you're not really looking at that eye. So, let me just show you what the regular RAPD looks like, so you know that there's nothing unique about it. So, in a normal person who has a defect in their pupil pathway on the afferent side, relative to the fellow eye, when we shine a light in the eye both pupils constrict because the afferent pathway is bilaterally innervated. So, the signal received by the afferent pathway at the level of the Edinger-Westphal nucleus in the midbrain is actually transmitted to both pupils. And if we swing the light to an eye, in this case- the left, that has a RAPD because it's got nerve damage- when we swing the light from the normal right pupil to the abnormal left pupil, the pupil dilates. However, the exact same time because of the bilateral involvement both pupils are actually dilating when you swing the light to the RAPD eye. So, if we ask: If you swing the light from the right to the left, in a patient who has a left RAPD, which pupil dilates- right, left, either, neither, both? The correct answer is both. And that is called is a reverse RAPD if we're looking at the uninvolved eye. So, we can look at the uninvolved eye and detect a RAPD. Normally we don't have to do that…because we can just rely on the affected pupil. However, if you have a fixed and dilated pupil and anisocoria…so this left pupil is dilated but it also can't constrict because it has an efferent damage from a 3rd nerve palsy or some other damage to the iris there. We can still swing the light from the left, to the right, and then back to the left but this time we're going to be looking at the right pupil. And if the right pupil dilates then we know that when we swing the light from the right to the left and the right pupil dilates, that we actually have a RAPD on the left side. And the reason this is important is this reverse RAPD should be tested on every patient who has an efferent pupillary problem where you can't use the regular RAPD because the pupil is fixed and dilated. So, in a 3rd nerve palsy the resident might get so excited to see the complete ptosis and the eye is down and out like this and the pupil is blown. They know that's a 3rd nerve palsy. They know that it's a pupil involved 3rd. They know that a pupil involved 3rd is an aneurysm of the posterior communicating artery until proven otherwise. They know to do a CT non-contrast to look for subarachnoid hemorrhage, followed by a CTA, then an MRI and an MRA of the head, and a catheter angiogram to look for aneurysm…but what if this patient who has the 3rd nerve palsy actually has an RAPD and loss of vision in the affected 3rd nerve eye. Then, the localization moves from a 3rd to a 2nd AND a 3rd, and the intersection of 2nd street and 3rd street is NOT at the posterior communicating artery. It is in the orbital apex and that is how you can easily miss an orbital apex lesion like the Tolosa-Hunt syndrome, or a mass, or aspergillosis in someone you have imaged with every modality you have available to you. CT, CTA, MRI, MRA, and catheter angiogram but all of the head instead of the orbit. So, you need to know that you have to have two eyes but only one working pupil is necessary to test for the RAPD. The reverse RAPD actually happens every single time, you're just not looking at that pupil and we should use this reverse RAPD to look for a 2nd nerve lesion or an RAPD in a patient who has an efferent pupillary defect but especially 3rd nerve palsy because you can miss an orbital apex lesion in a patient you've done extreme imaging on if they have an orbital apex lesion and you only image the head. So, remember the reverse RAPD- there is nothing reverse about it but use it when you need it. |
