Shortness of breath: Looking beyond the usual suspects

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Applied Evidence

Shortness of breath: Looking beyond the usual suspects

J Fam Pract. 2016 August;65(8):526-533

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References

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Neuromuscular diseases

Dyspnea occurs when respiratory muscles are weakened by neuromuscular diseases such as myasthenia gravis (discussed here), multiple sclerosis, or muscular dystrophy. Such diseases can cause respiratory insufficiency, increased rates of infection, or complete respiratory failure. Respiratory involvement is usually a manifestation of advanced disease. Suspect neuromuscular causes of dyspnea when you are seeing a patient admitted to the nursing home for long-term care because of profound weakness affecting their ability to do activities of daily living.

Myasthenia gravis

Myasthenia gravis, an autoimmune-mediated destruction of the postsynaptic acetylcholine receptors of the neuromuscular junction, is the most common disorder of neuromuscular transmission. It often affects the ocular (>50%; ptosis, diplopia), bulbar (15%; dysarthria, dysphagia, fatigable chewing), limb (<5%; usually proximal weakness), and respiratory muscles. Weakness typically fluctuates and worsens with muscle fatigue. Myasthenic crisis, an acute respiratory failure that occurs in 15% to 20% of patients, is often precipitated by an event such as surgery, an infection, or a medication change.²²

Suspect neuromuscular causes of dyspnea when a patient is admitted to a nursing home because of profound weakness affecting his or her ability to perform activities of daily living.

Diagnosis. Myasthenia gravis is diagnosed by a clinical history and exam suggestive of the disease. Suspect it if signs and symptoms include weakness worse with fatigue especially of the ocular muscles (ptosis or diplopia), dysphagia, dysphonia, chewing difficulty, or limb weakness. Consider laboratory testing with an anti-acetylcholine receptor (AChR) antibody assay, an assay for muscle-specific kinase (MuSK) antibody, or an anti-striated muscle (anti-SM) antibody assay if the history and exam are suggestive of the disorder.

The most reliable test is the anti-AChR antibody assay, which is positive in 50% to 90% of patients with the disease.²² Less reliable is the anti-MuSK antibody assay, which can be positive in 40% to 60% of patients who are AChR-seronegative.²³ An anti-striated muscle antibody assay is only helpful in patients with thymoma or onset of disease after age 40 years.²⁴

Consider electrophysiologic tests, including repetitive nerve stimulation studies and single-fiber electromyography, if the above laboratory tests are inconclusive.²⁵

Treatment depends on symptom severity and frequency. It can range from observation for mild occasional symptoms to chronic steroids and immunosuppressant medications in severe cases.

CASE › You see Ms. C in the intensive care unit the next day. She is intubated and has been responding poorly to the diuresis and breathing treatments used overnight. Her biopsy pathology results return and show recurrence of her small-cell lung cancer. She begins chemotherapy immediately and is extubated a few days later. She is discharged from the hospital a week later. Her shortness of breath is mild at this time, although she does require 2 liters of continuous oxygen.

CORRESPONDENCE
Christopher Taggart, MD, St. Mary’s Medical Center, Department of Family Medicine, 2698 Patterson Rd, Grand Junction, CO 81506; christopher.taggart@sclhs.net.