Health Questions and Answers

Neuromuscular Junction

Name the most common neuromuscular junction disease seen on the medical ward. Summarize its prevalence and distribution.
Myasthenia gravis (MG). MG has a prevalence of 1 case/10,000 population and a bimodal age distribution, occurring in young women in their teens and 20s and older men aged 60 and above.

What causes MG?
MG is an autoimmune disease in which patients produce antibodies that destroy the acetylcholine receptors on muscle. Acetylcholine is the neurotransmitter that makes muscles contract.

How does MG present?
MG presents with proximal weakness, especially ptosis and diplopia, with fatigue on use and recovery with rest. Because MG can involve the respiratory muscles, pulmonary failure is the most feared complication.

How is MG treated?
Treatment consists of acetylcholinesterase inhibitors, which block the enzymatic breakdown of acetylcholine, thus allowing greater concentrations of acetylcholine at the receptor. Pyridostigmine (Mestinon) is the drug of choice, but immunosuppressive drugs, including prednisone, azathioprine, and cyclosporine, are often necessary to attack the underlying autoimmune process. Plasmapheresis and IVIG have also been shown to help. Surgical thymectomy is probably beneficial, but its role in treating MG remains controversial.

What other disease may cause neuromuscular junction problems?
Lambert-Eaton myasthenic syndrome (LEMS).

What causes LEMS?
Like MG, LEMS is an autoimmune condition, although its target is the presynaptic voltage-gated calcium channel involved in acetylcholine release, not the receptor. It is commonly seen in the association with occult carcinoma, especially small-cell carcinoma of the lung. LEMS clinically resembles MG because of fluctuating proximal weakness.

How is LEMS treated?
It is generally treated by therapy for the underlying neoplasm, sometimes accompanied by plasmapheresis and other immune suppressors, especially in cases where no occult cancer can be found. Guanidine may provide symptomatic relief.

Which drugs may worsen neuromuscular junction diseases?

  • Aminoglycosides
  • Tetracycline antibiotics
  • Corticosteroids (acutely)
  • Thyroid hormone
  • Phenothiazines (e.g., chlorpromazine)
  • Quinidine
  • Lidocaine
  • Propranolol
  • Lithium
  • Dilantin

References
WEB SITES
www.neuroguide.com
www.aan.com (American Academy of Neurology)
www.medmatrix.org
www.internets.com/mednets/sneurolo.htm
www.medwebplus.com/subject/Neurology.html

BIBLIOGRAPHY

  • Gronseth GS, Barohn RJ: Practice Parameter: Thymectomy for autoimmune myasthenia gravis. Neurology 55: 7-15, 2000.
  • Neusom-Davis J: A treatment algorithm for Lambert-Eaton myasthenic syndrome. Ann NY Acad Sci 841: 817-822, 1998.
  • Aminoff M: Neurology and General Medicine, 3rd ed. Philadelphia, Churchill-Livingstone, 2001.
  • Noseworthy JH (ed): Neurologic Therapeutics. London, Martin Dunitz, 2003.
  • Bradley WG, Daroff RB, Fenichel GM, Jankovic J: Neurology in Clinical Practice, 4th ed. Philadelphia, Butterworth-Heinemann, 2004.
  • Caplan LR: Stroke: A Clinical Approach, 3rd ed. New York, Butterworth-Heinemann, 2000.
  • Samuels MA, Feske S (eds): Office Practice of Neurology, 2nd ed. Boston, Churchill-Livingstone, 2003.
  • Johnson RT, Griffin JW: Current Therapy in Neurologic Disease, 6th ed. St. Louis, Mosby, 2002.
  • Rolak LA (ed): Neurology Secrets, 4th ed. Philadelphia, Hanley & Belfus, 2005.
  • Victor M, Ropper AH: Prinicples of Neurology, 7th ed. New York, McGraw-Hill, 2001.

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