| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (PB) Class of 2022, Baylor College of Medicine, Houston, Texas |
| Transcript |
This lesson discusses ocular myasthenia gravis, but does not cover generalized myasthenia gravis. "OMG. Ocular myasthenia gravis can present in two ways to us in ophthalmology, ptosis and diplopia, and the hallmark symptom of myasthenia gravis is variability and fatigue. So it's variable and fatigue-able ptosis or and/or diplopia that is the hallmark of ocular myasthenia gravis. Now several features of the ptosis might suggest that it's myasthenia, and so if we're confronted with ptosis, we want to do a few anatomy tests. One is called the Cogan's Lid Twitch Sign and I'll just show it to you now with my hands. So when the lid goes down in down gaze, when it comes back to primary normally it just comes back up in the same way it went down. But in the Cogan's Lid Twitch Sign, the acetylcholine is building up in down gaze, and so when they look up, the acetylcholine fires, but then it doesn't last and it goes back down and the twitching is called the Cogan's Lid Twitch Sign. It's very suggestive of myasthenia gravis. We can also have enhancement of ptosis. So if one lid is down, we can lift up the tonic lid and because Hering's law of equal innervation to both lids is the same, when we lift up the tonic lid, the innervational effort to the contralateral lid is decreased and then we'll make enhancement of ptosis. So lifting up either lid will make the other lid go down, and we call that an enhancement of ptosis. And, of course, we're gonna be looking for variability during the examination and fatigue. And the fatigue we can test with sustained up gaze by having the patient keep looking up and if they fatigue, the lid will slowly fall down over time. The eye muscle almost might fall down as well. These are fatigue symptoms, so twitch enhancement, variability, and fatigue. We also have to check the orbicularis, which as you know is innervated by cranial nerve seven. But we're testing the orbicularis oculi function by having them close their eyes real tight and if we can overcome it easily with our finger opening their eyelids that would suggest that we have orbicularis weakness and that is also a very helpful sign that we're dealing with ocular Myasthenia Gravis. The diplopia and the ophthalmoplegia of Myasthenia can mimic any pattern of ophthalmoplegia. It can look like a third, it can look like a fourth, it can look like a sixth and it can look like any combination. It could look like an INO, a pseudo-INO, it can look like a one and a half seven, can look like any pattern of ophthalmoplegia. However, you need to know about some of the P's that myasthenia cannot do. So Lee's P's of things that myasthenia cannot do. So what myasthenia cannot do is it can't produce any pain because pain is a sensory finding and myasthenia is neuromuscular junction of the motor fiber. It can't cause proptosis of course but thyroid eye disease sometimes occurs in patients with myasthenia so if we did have myasthenic patient with proptosis we'd be thinking about the concomitant autoimmune disorder thyroid eye disease. You're not allowed to have any sensory deficit including paresthesia or loss of perception, like loss of visual acuity or visual field. And the most important P- you're not allowed to have pupil involvement, so no P's. No Lee's P's allowed in myasthenia. You can't have pain, proptosis, paresthesia, perception loss, or pupil involvement if you have ptosis or diplopia from myasthenia gravis. Now once you've made the clinical diagnosis of myasthenia gravis we're gonna do the antibody testing. The antibodies are anti-acetylcholine receptor antibodies. Typically there are three that we can choose from: binding, blocking, and modulating. Most places use the binding as a screen. You get a few extra percent out of the other two. You might consider doing other antibodies as well as thyroid function studies. This is a practice option. We're gonna do an imaging study above the chest to look for a tumor in the chest called a thymoma and if the thymoma was present we would take it out. Some patients with generalized Myasthenia, even without a thymoma would benefit from a thymectomy. In general I don't do thymectomy for ocular myasthenia gravis. The treatments, because there's no cure, are pyridostigmine, Mestinon, and acetylcholinesterase inhibitors, and also we can use prednisone. Prednisone tends to work better for myasthenia gravis of the ocular form. I would recommend treating in conjunction with neurology, because if they have systemic or generalized disease they might need further immunosuppression, for example Mycophenolate. Or if they're in respiratory or other of crisis or have severe generalized disease, they might need plasma exchange or more aggressive immunomodulatory therapy for myasthenia. So the bottom line is you should be thinking about myasthenia gravis every time, in every patient with ptosis and diplopia. You're not allowed to have any Lee's P's. You're gonna be looking for and asking about variability and fatigue. We're gonna check the Cogan's Lid Twitch sign enhancement and orbicularis weakness. We're gonna refer to neurology, order laboratory studies, and do a cat scan of the chest. |
