Acute respiratory muscle weakness

Thank you to Elisabeth Askin for presenting a case of a young woman with respiratory muscle weakness thought to have myasthenia gravis. Elan shared a veritable oyster bed of pearls with us.
Top pearls
  • When evaluating for respiratory compromise in a patient with possible neuromuscular respiratory failure, don’t rely on resting O2 sat or CPO, as hypoxemia is a late sign that signals life-threatening respiratory failure
  • The $0 MEF: ask a patient to count as high as they can with one exhalation. Healthy people can count to ~40. 20 should make you concerned for respiratory muscle failure
  • Consult neurology urgently if concerned for myasthenic crisis. IVIG in these patients can stave off intubation.
Acute Extremity + Respiratory Muscle weakness has a short differential, it localizes to…
  • brainstem (rarely)
  • spinal cord
    • cervical myelopathy, hemorrhage, tumor invasion, etc
  • anterior horn
    • infections like WNV, enteroviridae and polio
    • ALS, though rarely acute
  • motor nerves
    • guillain Barre
  • neuro-muscular junction
    • myasthenia
    • lambert eaton
    • botulism
  • myopathy (this ddx borrowed from neuro report a couple weeks ago)
    •  drug-induced
      • statins, alcohol, stimulants
    • endocrine
      • cushings
      • hypothyroid
      • vitamin D deficiency
    • inflammatory
      • primary myositis (dermatomyositis, polymyositis, inclusion body myositis)
      • scleroderma
      • SLE
      • overlap syndromes
    • metabolic
      • glycogen storage disorders etc
      • suspect these when patients report pigmenturia with exertion or other stress
    • muscular dystrophies
    • malignant
      • paraneoplastic
Respiratory muscle failure is a critical care emergency requiring emergent intubation!
How do you identify it?
  • Core pearl: hypoxemia is a late, terrible sign in respiratory muscle weakness. CPO and a normal resting O2 sat can falsely reassure you.
  • Instead, get Bedside PFTs
    • Performed by your friendly, neighborhood respiratory therapist. At our hospitals, they are available 24/7.
    • Include three measurements
    • Negative inspiratory Force or Maximal inspiratory pressure (those are the same thing, sorry physicists).
      • measured by asking a patient to inhale are hard as they can with the apparatus shown below
      • VERY effort dependent. Requires good mental status, adequate seal around the mouth piece, and full participation
      • NIF of -40 is normal. Around -20, you can consider intubation
    • Forced Vital Capacity which correlates more strongly with need for intubation than NIF
    • Maximal expiratory pressure which is the opposite of a NIF
  • Combine PFTs with your clinical assessment
    • How is their work of breathing? Do they appear fatigued?
    • Neuro exam will further localize the lesion
      • Bulbar weakness, neck flexor weakness and fatiguability raise concern for myasthenia
      •  A sensory level or mixed sensory and motor involvement raise concern for Guillain Barre and its compatriots.
    • Consider the $0 The $0 MEF: ask a patient to count as high as they can with one exhalation. Healthy people can count to ~40. 20 should make you concerned for respiratory muscle failure
  • Rule out other causes
    • Important not to anchor on a neuromuscular diagnosis in a patient with a neuromuscular disease
      • Start from the bottom of the dyspnea pyramid before hiking your way to the top
  • If you are worried, phone a friend
    • Patients with objective signs of respiratory muscle weakness should be triaged to the ICU
    • Call neurology early! Especially if concerned for myasthenic crisis. They may initiate IVIG which can stave off intubation
  • Consider Bipap or High Flow Nasal Cannula
    • Bipap can reduce work of breathing, allowing patients to optimize their existing respiratory muscle function.
Love neuromuscular weakness and want to learn more?
Check out this blog post from John Farkas, a critical care doc at UVA. I can’t vouch for every iota of the content or sassiss – he is very nihilistic about the utility of NIFs. I learned a TON of physiology and it was the primary source of wisdom for this post:

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