34 - Neuromuscular disorders

Summary

Myasthenia gravis

Of all the neuromuscular diseases, myasthenia gravis probably has the most significant implications for surgical patients. It is caused by an autoimmune attack on the acetylcholine receptors of the postsynaptic (muscle) side of the neuromuscular junction. Characteristic clinical features include fluctuating weakness and fatigue, usually involving the extraocular muscles and eyelids (producing diplopia and ptosis). Weakness of the limbs can be severe, sometimes resulting in almost total paralysis. Sensation and deep tendon reflexes are normal. Respiratory muscle weakness is common and can be fatal. The introduction of practical mechanical ventilation has resulted in a dramatic decrease in the mortality rate.

Although the clinical features of myasthenia gravis are sufficiently characteristic in some cases, confirmatory tests are usually necessary. The acetylcholine receptor (AchR)-antibody level is elevated in over 80% of patients with myasthenia gravis. Elevated levels of this antibody are extremely specific for this disease. As a result the AchR-antibody serum test is typically the first step in confirming the diagnosis, and the presence of elevated levels eliminates the need for additional confirmatory testing. In antibody negative patients or when faced with an acutely symptomatic patient, the edrophonium test is often used. The strength of a specific weak muscle should be determined before and after the intravenous administration of 8 mg of edrophonium. A test dose of 2 mg of edrophonium should always be given first and atropine should be available in case significant bradycardia develops.