Diagnostic Testing

If a patient presents with symptoms suggestive of MG, tests are available to support the diagnosis.9 The ice pack test is a simple, safe, and cost-effective bedside test to start the diagnostic process for determining whether patients with ptosis have MG, but it lacks specificity and cannot confirm the diagnosis.10 Ice packs are placed on the eyelids, causing increased neuromuscular transmission due to lower muscle temperatures. After 2 minutes, the ice packs are removed; an improvement in ptosis indicates a diagnosis of MG.9-11

Serologic testing for anti-AChR or anti-MuSK antibodies provides reliable diagnostic confirmation with up to 99% specificity.12 Laboratory confirmation of AChR antibodies can include binding, blocking, and modulating assays, but the binding assay is the most sensitive and specific for MG. Titer levels solely indicate disease presence; they do not correlate with disease severity. A negative antibody result does not rule out MG.12

Electrophysiologic studies provide confirmation of MG.9 An electrode is placed over the muscle endplate, stimulating the motor nerve. The stimulation is repeated while measuring the compound muscle action potential amplitude (CMAP). If MG is present, a progressive CMAP decline may occur with repeated stimulation.9 In patients without MG, CMAP values should not change. Single fiber electromyography (SFEMG) is highly sensitive for MG (95%).9 However, SFEMG is more complicated and less available than other electrophysiologic studies. SFEMG records action potentials of 2 muscle fibers

innervated by the same motor axon via needle electrodes. The variation in the time interval between the 2 action potenial responses is termed the neuromuscular jitter. In MG, the neuromuscular jitter interval is increased compared with patients without MG.9


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Early Diagnosis Is Vital

Diagnosing MG is challenging; the diagnosis often is missed, delaying treatment and patient education. An early diagnosis is vital to improve patient prognosis and lower the risk for myasthenic crises.13 With advances in diagnosis and treatment, MG is no longer a guarantee of disability and mortality. Therapy is highly individualized. Treatment plans are built around 4 treatment categories: symptomatic, chronic immunomodulating, rapid immunomodulating, and surgical.14

  • Symptomatic treatment can be achieved with oral acetylcholinesteraseinhibitors.15 Pyridostigmine is the most commonly used anticholinesterase. Symptoms lessen as the degradation of acetylcholine in the neuromuscular junction is slowed or stopped. This medication has a greater effect on limb and bulbar symptoms than ocular symptoms. Pyridostigmine dose is titrated to avoid cholinergic side effects, especially gastrointestinal effects.14
  • Chronic immunomodulating treatments include corticosteroids and other immunotherapies that interfere with T- and B-cell production.15 Prednisone is first-line therapy for shorter-term immunosuppression. For long-term immunosuppression, azathioprine is the first choice, typically combined with low-dose corticosteroids. These medications lessen the autoimmune reaction and decrease attack on AChRs.16
  • Rapid immunomodulating MG usually is treated with shortterm plasmapheresis; IV immune globulin (IVIG) is used for emergent therapy. These treatments have a faster onset of action of mere days but a shorter duration of action that only lasts for 3 to 6 weeks.17 These typically are reserved for myasthenic crisis, bridge therapy when initiating chronic immunomodulators, or as an addition to chronic treatment in refractory MG. Plasmapheresis removes AChR antibodies from circulation. IVIG provides normal antibodies that modify the immune system temporarily.18
  • Surgical treatment involves a thymectomy. Patients with thymomas benefit from the removal of the thymus, and research shows possible benefit for nonthymomatous MG.18 Thymectomy can reduce the immune reaction, relieve disease symptoms and progression, lessen the need for immunosuppression, and decrease hospitalizations. Surgery typically is limited to patients younger than 60 years.18

Outcome of Case

After the patient’s diagnosis of MG, she began treatment with pyridostigmine; azathioprine was added later, along with periodic tapers of prednisone. She was not a surgical candidate due to her advanced age.

After the patient underwent a hysterectomy, her symptoms drastically increased. She now has weakness in her proximal limb muscles, dysphagia, dyspnea, dysarthria, weakness with chewing, diplopia, and ptosis that all fluctuate and worsen with increased activity throughout the day. She sleeps more than 12 hours each night without improvement in fatigue. The patient has undergone additional plasmapheresis and IVIG treatments, with minor symptom improvement.

Lauren Blessing, PA-C, is a certified Physician Assistant at Northwest Georgia Oncology Centers in Atlanta. Sara Haddow Liebel, MSA, PA-C, recently retired as an associate professor in the Physician Assistant Department at Augusta University,  Georgia.

References

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