Myasthenia Gravis Illness Script - USMLE, Internal Medicine Board Review

Overview of myasthenia gravis with special attention to comparison to Lambert Eaton syndrome!

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Transcript:

Today we are going to talk about Myasthenia Gravis. Myasthenia is an autoimmune, antibody-mediated disorder whereby the acetylcholine receptor located on the postsynaptic neurons of the muscle endplates are disrupted or destroyed.

When thinking about the ideal patient for myasthenia gravis, one must consider a bimodal distribution. The patient is likely to be a younger (20-40) woman or an older (60-80) man.

The major presenting feature of myasthenia gravis will be some form of muscle weakness that is fatigable (worsens with time and/or prolonged use of the muscle in question). Keep in mind some of the most rapidly and continuously acting muscles in the body are located within the eye. Thus, ocular manifestations are quite common with myasthenia gravis. In fact, on the board exams, the question stem will almost never describe a patient without ocular symptoms as this differs so much from the classical presentation. These manifestations will include ptosis and/or diplopia). Other muscle groups that may be involved, however, include the bulbar and proximal limb muscles. The most dreaded muscle group involved in myasthenia is the respiratory muscles as this can lead to respiratory failure and death.

Exam findings in myasthenia are quite interesting. In patients with ptosis, you can actually worsen the ptosis by supporting the eyelid on the contralateral side (the so-called curtain sign). Again, fatigability is key. Having patients hold an upward gaze will often precipitate worsening symptoms. Applying an ice pack to the eye or allowing the patient to have the eye remain closed for twenty minutes will produce a marked improvement in symptoms which is quite characteristic (and partially diagnostic) for myasthenia gravis. A test that can be considered an exam is the Tensilon (or edrophonium) test. Edrophonium is a short-acting acetylcholinesterase inhibitor which will flood the synapse with acetylcholine hoping to transiently overcome the inhibition present in myasthenia. Patients will often have a significant improvement in symptoms. Please keep in mind, however, that this marked increase in acetylcholine is not limited to just the neuromuscular junction and that it will have affects elsewhere (including AV node via vagal nerve stimulation resulting in bradycardia and sometimes syncope).

Let’s compare the presentation of myasthenia patients with that of the most-common distracting diagnosis on our differential diagnosis: Lambert-Eaton syndrome. In Lambert-Eaton syndrome, patients will similarly present with weakness with one key difference: the weakness improves as the day progresses. The disease is characterized by antibodies against the presynaptic voltage-gated calcium channels. With more recruitment of the muscle group in question, more calcium is released to out-compete for the calcium channels and the symptoms improve. As I mentioned before, the muscles of the eye are constantly working. As such, we would expect to not see ocular symptoms as much in Lambert-Eaton syndrome but rather mostly limb muscles. Lambert Eaton is often a paraneoplastic disorder so look for underlying cancer (such as lung cancer) in these patients.

Lab tests often focus on antibodies. The classic antibody is the anti-acetylcholine receptor antibody which is positive in about 80% of patients. In the seronegative patients with high pretest probability for myasthenia, the next most common antibody (and one that is increasingly being tested) is anti-muscle specific kinase (anti-MuSK) antibody.

The imaging test that is most important to know for myasthenia gravis is chest imaging (either CXR or CT) to look for thymoma. Do not forget to check for a thymoma in your myasthenia patients as resection very often leads to cure.

Taken altogether, an illness script for myasthenia gravis could read: a 30-year-old woman presents with diplopia and proximal muscle weakness with finding of ptosis which worsens with maintained upward gaze and improves following application of ice-pack for 60 seconds with positive anti-acetylcholine receptor antibody on labs and CT chest consistent with thymoma.

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