A 68 year-old woman with HTN, NIDDM2, ESRD (on IHD), descending aortic aneurysm s/p TEVAR (one year prior), and myasthenia gravis presents to the ED with three days of worsening dyspnea. In terms of her myasthenia treatment, she's currently maintained on prednisone 7.5mg daily and azathioprine. She also takes pyridostigmine 60mg three times daily. The ED obtains an initial workup, as follows. You are consulted to evaluate the patient for possible myasthenia exacerbation.
On exam, the patient is alert and oriented. She's satting 92% on 4L NC. She has some mild conversational dyspnea. She has mild ptosis and fatigable upgaze. No flaccid dysarthria. She has 4/5 strength with shoulder abduction, biceps extension, and hip flexion; she is full strength elsewhere. She has diminished pinprick sensation up to her knees as well as forearms. She has absent vibratory sensation at the ankles, and up to 5 seconds at the patellae. Her work of breathing is increased. No visible oral secretions.
The MICU fellow arrives just as you're about to leave and asks you if you're going to admit the patient for a myasthenic crisis, pointing out the patient has impending respiratory failure.
The big question for us here is: is the clinical picture suggestive of a myasthenia exacerbation?
Before we look at the labs, we need to put a lot of stock in what the patient looks like clinically. She has some proximal weakness that we'd expect with her myasthenia. She has very mild cranial nerve deficits. She has an increased work of breathing. She's hypoxemic with her current nasal cannula settings, and needs more support with oxygenation. It's true that she has impending respiratory failure. With the mild degree of myasthenia signs and symptoms, it's not totally clear that her myasthenia is the acute problem here, though possible. Now we can look to the labs to see if there's information that would sway us in one direction.
Looking at her ABG, we can see that she has a relatively normal pH and PaCO2. Her PaO2 is pretty low considering that she's getting supplemental oxygen; we're not surprised because she's also been satting low. So, the labs and vital signs point to acute hypoxemic respiratory failure, not hypercapnic respiratory failure.
So, putting this all together, she has mild myasthenia symptoms and an isolated hypoxemic respiratory failure without hypercapnia. This is not convincing for a myasthenia exacerbation. With myasthenia, if there were respiratory difficulties, we'd expect hypercapnia first (from respiratory muscle weakness reducing effective tidal volumes). In the advanced stages of hypoventilation/hypercapnia, you can then get hypoxemia from the CO2 molecules taking up so much volume so as to displace the O2 molecules (Dalton's law of partial pressures). It'd be highly unusual to get pure hypoxemia from neuromuscular respiratory weakness, at least without the presence of other findings.
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