USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What distinguishes a cholinergic crisis from myasthenia gravis during the Tensilon Test?

1. Increased salivation

2. Excessive sweating

3. Miosis

4. Flaccid paralysis worsened by edrophonium

The correct answer is: Flaccid paralysis worsened by edrophonium

During the Tensilon Test, which involves the administration of edrophonium (a short-acting cholinesterase inhibitor), one can differentiate between a cholinergic crisis and myasthenia gravis by observing the response to the medication. A cholinergic crisis occurs when there is excessive stimulation of acetylcholine receptors due to an overabundance of acetylcholine resulting from cholinesterase inhibition. This can lead to symptoms such as muscle weakness and flaccid paralysis. In myasthenia gravis, edrophonium temporarily improves muscle strength because it increases the availability of acetylcholine at the neuromuscular junction, where there are fewer functional receptors due to autoimmune attack. In contrast, during a cholinergic crisis, administering edrophonium exacerbates flaccid paralysis because it further enhances the already excessive levels of acetylcholine, leading to more severe receptor overstimulation. Thus, the key distinguishing factor during the Tensilon Test is that flaccid paralysis worsened by edrophonium indicates a cholinergic crisis, whereas a transient improvement in muscle strength suggests myasthenia gravis. Recognizing this difference is critical for determining the appropriate management strategy for the patient.