How should you evaluate patients with myasthenia gravis who present to the Emergency Department with shortness of breath?
For instance, take the hypothetical case of a 45 year-old woman who presents to the ED with shortness of breath. She has diminished breath sounds bilaterally, audible wheezes, and is in mild distress. Her past medical history includes asthma and myasthenia gravis (MG). Is she in crisis? Are there any special bedside tests or medications to consider?
First, a little background: MG is most common in women in the 2nd-3rd decade of life but it can occur in either sex at any age. Up to 20% of patients will present to the Emergency Department for the first time (ie never having been diagnosed) in crisis.
What are the symptoms/signs?
MG is characterized by weakness and fatigue of skeletal muscles that worsens with repetitive use. More than 90% of patients will have ptosis or diplopia without pupillary involvement. Ptosis can be induced or made more pronounced by having the patient perform a sustained upgaze for more than 30 seconds. Diplopia can be brought about by a sustained horizontal gaze for more than 30 seconds. Other symptoms include dysarthria, dysphagia, chewing difficulty, neck flexor weakness, and proximal muscle weakness worse in the arms as compared to the legs.
In the Emergency Department, physicians are most likely to encounter a ‘myasthenic crisis’.
Myasthenic crisis: put simply, this is when the respiratory muscles become weak; a form of MG severe enough to require intubation and mechanical ventilation.
Identifying at-risk patients early is critical in preventing this downward spiral. Use of accessory muscles is an important sign that respiratory effort may not be sustained. Be aware that generalized muscle weakness can at times mask accessory muscle use. Difficulty laying flat and inability to speak full sentences may be indicators of diaphragm dysfunction.
Unlike other causes of respiratory failure, the pulse ox and ABG cannot be relied upon as abnormalities tend to occur later in the process, once respiratory failure has already settled in. This is a great reason to start all patients with difficulty breathing on continuous capnography.
Inservice/Boards alert: What is the next step in management of MG patients with difficulty breathing?
Answer: Spirometry; better answer: negative inspiratory force
The forced vital capacity (FVC) and negative inspiratory force (NIF) are the main parameters for monitoring severity of crisis and response to treatment. An easy bedside technique to get a rough estimate of the FVC is to have the patient take a deep breath in and count aloud. If the patient is able to reach 50, this suggests normal respiratory function, while a single breath count of less than 15 suggests a dangerously low FVC.
Use the “20/30 rule”: elective intubation should be considered if the FVC is less than 20 mL/kg or if the NIF is less than 30 cmH20.
Remember that myasthenia gravis is characterized by muscles that become weaker the more they are used: trending the FVC and NIF over time is more important than an individual value. Most recommendations are to check them Q 2 hours.
Is succinylcholine use contraindicated in patients with myasthenia gravis?
That depends: are you taking an inservice/boards exam or is this real life?
Remember the contraindications to use of succinylch0line: hyperkalemia, neuromuscular diseases, denervation of muscles, tetraplegia, long-term immobilization, extensive muscle trauma, burns, and family history of malignant hyperthermia
The thought process is that patients with MG are somewhat resistant to depolarizing neuromuscular blockers like succinylcholine. In reality, succinylcholine has been shown to be safe but patients may require higher than expected dosages (studies have shown that you will need to use about 2.5x the dose you normally would). The problem is that these higher doses can lead to an unpredictable and lengthy paralysis so a non-depolarizing agent such as rocuronium may be preferred. In conclusion: you don’t have to give different medications, just be aware that you may need to give higher doses which can lead to more unpredictable paralysis – or just use rocuronium/vecuronium.
Treatment involves corticosteroids and/or IVIG. Call neurology and let them decide.
For Emergency Department folks, it’s important to know what can precipitate a crisis: just like in all patients who take warfarin, you should think twice before prescribing any medication, the same is true for patients with MG. If a patient has a documented history of MG and comes in for something as simple as a UTI, think twice before deciding on an antibiotic. Common causes of crisis in patients with MG include fluoroquinolones, aminoglycosides, macrolides, and IV contrast dye.
