Myasthenic Crisis

Myasthenic crisis is a worsening of muscle weakness precipitating respiratory failure and need for ICU admission and intubation in patients with myasthenia gravis (MG). MG is an autoimmune disorder characterized by antibodies directed against acetylcholine receptors. This results in diffuse muscle weakness, primarily involving bulbar, peripheral and respiratory muscles. Although MG is a rare condition (~5-10/100,000), approximately 15% of patients will develop a crisis during the course of their disease. Despite advances in understanding disease pathogenesis and treatment options, myasthenic crisis carries a mortality of up to 8%. Patients clinical condition can worsen precipitously, sometimes as a result of a triggers and other times without clear cause. Precipitants include:


1. Infection -  usually pnuemonia/aspiration (~70% of causes for crisis)
2. Medications - Tapering of steroids, acetycholinesterase. Or worsening muscle dysfunction from additional medications: anaesthetics, antibiotics, anticonvulsants, antiarrhythmics, beta blockers, magnesium and many others
3. Surgery
4. Pregnancy and childbirth

The cholinergic crisis (worsening of muscle weakness from toxicity due to acetylcholinesterase inhibitors (AChEI)), should be considered in pateints as it can look similar and has a very different treatment approach. Patients on less than 120mg of pyridostigmine q3h are reported to be at low risk of developing this problem.

Investigations that can aid in the diagnosis of myasthenic crisis include: ABG, CXR, vital capacity (VC) and mean inspiratory pressure (MIP). As weakness worsens, an ABG will reveal hypoxemia and hypercarbia. Imaging can identify precipitating infections and alternative causes of dyspnea. A VC of less than 1 litre or 20cc/kg is concerning of imminent failure and warrants intubation. MIP's of less than -30cmH20 should raise concern and lead to endotracheal intubation.

There are several treatment options for myasthenic crisis. These include:

1. AChEI's
2. Steroids
3. Plasmapheresis
4. IVIg

The use of AChEI's in a crisis state is controversial. The awareness of cholinesterase crisis has led many physicians to monitor patients for several days before increasing these drugs. Many prefer to hold these medications and monitor the patient clinically to try and identify the underlying cause, while others continue the medications. There have been randomized studies comparing AChEI +/- steroids, or plasmapheresis, which did not show any additional benefit in patient outcomes.

There is not a large amount of evidence for increasing steroid dosage in acute crisis. Guidelines suggest 60-80mg of prednisone a day and monitoring for acute myopathy from this medication. This should be considered primarily in patients with severe disease in the ICU setting.

Plasmapheresis and IVIg have been promoted as both first line therapies in myasthenic crisis. This idea is that removal or disruption of autoantibodies would result in improvment of muscle function. Both of these treatment options have realatively quick onset of action (days), but do not result in persistent improvement (weeks). Head to head studies have suggested that plasmapheresis has better short term response, but increased side-effects compared to IVIg. Long-term therapy should be considered in conjunction with acute management given this can take time to work (i.e. azathioprine), and the acute therapies will have worn off.

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Myasthenic crisis