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Chapter 5. Neuromuscular Diseases

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A 38-year-old woman with generalized myasthenia gravis who has been well controlled on low-dose prednisone presents to the emergency department with symptoms of abdominal pain, fever, and dysuria. Over the next 12 hours, she develops dyspnea and is admitted to the intensive care unit (ICU) for further monitoring. On arrival to the ICU, the patient’s speech is hypophonic and staccato, and she has marked weakness of neck flexion. Negative inspiratory force (NIF; also known as maximal inspiratory pressure [MIP]) is 15 cm H2O, and vital capacity (VC) is 10 mL/kg. Arterial partial pressure of carbon dioxide (PCO2) is 38 mm Hg and arterial partial pressure of oxygen (PO2) is 92 mm Hg. What is the most appropriate next step in management of this patient?

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A. Urgently administer 2 mg of intravenous (IV) pyridostigmine

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B. Urgently administer 1000 mg of IV methylprednisolone

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C. Urgently administer 0.4 mg/kg of IV immunoglobulin (IVIG)

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D. Proceed with endotracheal intubation

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E. Continue to observe the patient, monitoring NIF and VC every 2 to 4 hours

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D. This patient is presenting with manifest myasthenic crisis, defined as worsening of myasthenic weakness requiring intubation or noninvasive ventilation to avoid intubation. Her abrupt symptomatic deterioration and respiratory parameters warrant intubation. In patients with markers of respiratory compromise, elective intubation is preferred over emergent intubation in response to abrupt respiratory collapse. Clinical features of impending respiratory failure in myasthenia gravis include shortness of breath, tachypnea, paradoxical breathing or use of accessory muscles, marked neck flexion weakness, and staccato speech or speaking in shortened sentences.

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In neuromuscular disorders, the “20/30/40” formula suggests mechanical ventilation should be considered in patients with the following respiratory parameters:

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  • VC <20 mL/kg

  • NIF (also called MIP) < –30 cm H2O

  • Maximal expiratory pressure <40 cm H2O or >30% or more decline in these measurements on serial testing

  • Some experts use a more stringent cutoff of VC <15 mL/kg and NIF (or MIP) < –20 cm H2O when considering elective intubation.

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Note that patients with weak bulbar muscles may be unable to form an adequate seal around the spirometer, making accurate measurements difficult. Positive-pressure ventilation may be considered in some myasthenic patients to avoid intubation during treatment of a myasthenic crisis.

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Hypercapnia and hypoxemia are relatively late findings in the course of neuromuscular respiratory failure, and this patient’s normal arterial PO2 and PCO2 should not delay intervention.

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Pyridostigmine is typically discontinued during a myasthenic crisis, as it may worsen respiratory secretions. IV pyridostigmine is primarily used in stable myasthenic patients undergoing elective surgery. IVIG and plasmapheresis (PLEX) are rapid therapies for the treatment of myasthenic crisis, but ...

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