Morning Report: 10/9/2012

MR is back from a small hiatus (vacation!). Here’s Dr. McMillan with today’s edition:

Here’s the case:

You have a 47 year-old male who presents with shortness of breath and leg weakness…

Myasthenia Gravis and Myasthenic Crisis

Myasthenia gravis (Gr mya- muscle; asthenia- paralysis; gravis- grave or serious): A disorder of the neuromuscular junction wherein the body produces antibodies against the motor-endplate acetylcholine receptor (AChR) that results in a “characteristic and striking fluctuating weakness and fatigability of muscle.”

Epidemiology and Pathophysiology

Females affected most from second to third decade, and around 70. Males from fifth to eighth decades. Anti-AChR antibodies cause a reduction in the number and function of AChR at the motor endplate. This in turn results in muscle weakness. Correlation between blood levels of anti-AChR antibodies and severity of disease.

Clinical features

Generalized muscle weakness
Proximal muscle groups
Neck, bulbar and facial muscle weakness
Ptosis and diplopia most common
Can present with dyspnea

MG patients on therapy can present with myasthenic crisis — inadequate ACh present in the neuromuscular junction, OR in cholinergic crisis, where there is too much ACh in the junction. Diagnosis includes use of assays for anti-AChR antibody level, electromyographic studies, and the edrophonium test. Edrophonium a diagnostic option, but comes with significant risks and not frequently implemented in ER setting.

The most critical complication is respiratory failure. Status best measured with pulmonary function tests:

Table: Pulmonary Function Tests in Patients with Myasthenic Crisis

	Normal	Criteria for Intubation
Vital capacity	> 60 mL/kg	15 mL/kg
Negative inspiratory force	> 70 cm H₂O	< 20 cm H₂O
Positive expiratory force	> 100 cm H₂O	< 40 cm H₂O

Adapted from: Mayer SA. Intensive care of the myasthenic patient. NEUROLOGY 1997;48 (Suppl 5): S70-S75.

Acute management

Supportive: Manage respiratory failure and poor tolerance of oral secretions
Medical: Cover for any concomitant infection and treat fever, IVIG vs plasmapheresis

Pitfalls

Not recognizing or addressing threat of impending respiratory failure
Not recognizing/treating concomitant infection

References:

Mayer SA. Intensive care of the myasthenic patient. NEUROLOGY 1997;48 (Suppl 5): S70-S75.

Ropper AH, Samuels MA. Chapter 53. Myasthenia Gravis and Related Disorders of the Neuromuscular Junction. In: Ropper AH, Samuels MA, eds. Adams and Victor’s Principles of Neurology. 9th ed. New York: McGraw-Hill; 2009. http://www.accessmedicine.com/content.aspx?aID=3642849. Accessed September 16, 2012.

Sloan EP, Handel DA, Gaines SA. Chapter 167. Chronic Neurologic Disorders. In: Tintinalli JE, Kelen GD, Stapczynski JS, eds. Tintinalli’s Emergency Medicine: A Comprehensive Study Guide. 7th ed. New York: McGraw-Hill; 2011. http://www.accessemergencymedicine.com/content.aspx?aID=6366264. Accessed September 16, 2012.

Yavagal DR and Mayer SA. Respiratory Complications of Rapidly Progressive Neuromuscular Syndromes: Guillain-Barré Syndrome and Myasthenia Gravis. Seminars in Respiratory and Critical Care Medicine. Vol 23 (3) 2002, p221-229.

Thanks Dr. McMillan! Leave any comments below.