Tasty Morsels of Critical Care 057 | Myasthenia Gravis

21 Feb

Welcome back to the tasty morsels of critical care podcast.

Today we’re looking at a small section of Oh Chapter 58 covering myasthenia gravis. I don’t think I’ve ever looked after a true myasthenic crisis in the ICU. Likely because they’re well managed by their neurologists on an OPD basis or well managed from an anaesthetic perspetive when they need an operation done. I have made the diagnosis twice de novo in the ED (or at least admitted them with that as the leadning diagosis) so it is out there.

It does make excellent exam level material as there is some interesting physiology and compare and contrast type tables to be made comapring with other neuromuscular diseases.

To give a flavour of what might you see in the ED (and these patients will rarely need ICU) it’s typically some kind of cranial nerve issue, typically ptosis and diplopia complaints, sometimes with some speech and swallowing issues. The cardinal features of the neuro dysfunction is flucutating weakness. Typically this is described as fatigueability. For example the ptosis isn’t too bad in the morning but by afternoon it’s much worse.

Involvement of bulbar muscles (BTW, bulbar being an archaic name for the medulla and the cranial nerves that stem off it) should be recongised as a bit of a concern given that swallowing and airway protection fall under the remit of cranial nerves 9-12.

The edrophonium test that you may have heard about in medical school can be safely forgotten about as it is no longer recommended. On the other hand the ice test can be used as a cool demonstration of the physiology. In its essence, the ptosis that improves after having some ice on the eyelids would suggest myasthenia.

The pathophysiology of the illness is likely one of the more testable aspects here. Myasthenia is an auto immune disease where antibodies are made against acetylcholine receptors in the post synaptic neuromuscular junction. As a result Ach cannot bind to these receptors and therefore cannot complete the transmission of neurologic impulse to the muscle. The ice test works because NM transmission is apparently more efficient at lower temperatures.

Once the diagnosis is made you can look super smart by thinking about their thymus as thymomas are found in ~15% of people with antibody +ve myasthenia. Many more are found to have some kind of abnormality in the normally negleted and unloved thymus.

As an outpatient these people will typically be established on pyridostigmine, a nifty medication that potentiates the remaining ACh at the NM junction. They are usually immunosupopressed on some kind of steroid or maybe some azathioprine. Some may have had their unsuspecting thymus removed in the interim.

In the ICU we’re likely to see someone in myasthenic crisis. This is commonly seen when tapering immunosuppressives or when faced with some sort of actue stress like spending time under anaesthesia while a surgeon ectomises some part of your body. There are also a large list of drugs that we can misprescribe that can mess people up.

The fundamental feature of a myasthenic crisis will be respiratory insufficiency, this is defined as need for NIV or intubation. Remember it is unusual for myasthenia to affect respiratory muscles so if it is you’re looking at big trouble. Expect this to be a quiet, undramatic sort of respiratory failure. FVC and cough will quietly disappear without any of the usual increased work of breathing we usually use to quantify respiratory failure. Hence they look fine until they’re really not.

There are a variety of vital capacity cut offs described as reasons to intubate. But as discussed in the GBS post these are somewhat arbitary. For exams a VC of 15ml/kg is certainly a good red flag to keep in mind. Conveniently the same number can be spouted for a question on GBS.

Without getting into the weeds on NMBA in myasthenia, for a non depolarising agent like roc you can expect the effect to last much longer so people have suggested a smaller dose. Given that if they’re being intuabted in ICU the prolonged effect is much less of a concern as long as you’re runnning the sedation which of course you are.

Once they’re in ICU and ick enough to be tubed we need chart ourselves and the patient a way out of this mess. The key treatments for both real life and for examinations are going to be plasma exchange or IVIG. Plasma exchange removes all the nasty antibodies. IVIG on the other hand does something akin to witchacraft and probably binds the antibodies. There is no clear data favouring one over the other with the only RCT being neutral but the trend it seems is towards PLEX.

As is typical for these immune binding/removal type therapies they need an immunosuppressive “chaser” to stop production of more autoantibodies. Typcially this will be steroids and typically you’ll not be making the decision anyhow.

The more interesting question for us is time to resolution. A common quoted median is 2 weeks of invasive ventilation. This is much shorter than might be expected for a GBS case where it is not uncommon to intubate and tracheostomise in the same day given the expected course. For myasthenia it might be reasonable to give them some time to assess response to treatment before commiting to the tracheostomy.

In terms of meds to be cautious with, the list is long but the commonly implicated bad actors include NMBA, aminoglycosides, the fluroquinonlones and crowd favourite magnesium is more likely to cause NM weakness than usual.

Finally it’s worth knowing that there is such a thing as a “cholinergic crisis” desrcibed in myasthenia that is due to excessive cholinergic effects from too much pyridostigmine. It is vanishingly rare at this stage. It’s interest is that it forms another cause of respiratory failure in the myasthenia patient that you might mistake for a myasthenic crisis but if you’re a betting man or woman (and in medicine we all are) then if your myasthenia patient has respiratory failure it’s goint to be the myasthenia almost every time



Oh’s manual 58


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