A patient develops numbness in the arm after an interscalene brachial plexus block. The most likely cause is which?

A patient develops numbness in the arm after an interscalene brachial plexus block. The most likely cause is which?

• A. Excessive retraction by the surgeon
• B. Prolonged pressure on the brachial plexus from malpositioning
• C. Residual anesthesia
• D. Prolonged pressure against the posterior surface of the humerus

Answer: (C)

The main idea is that numbness after a regional block is most often due to the lingering effect of the local anesthetic itself. In an interscalene brachial plexus block, the anesthetic diffuses around the upper trunk and roots, producing sensory loss that mirrors the distribution blocked. The duration depends on the agent used—shorter-acting drugs wear off in a couple of hours, longer-acting ones can last several hours. So seeing numbness soon after the block is placed fits with residual anesthesia, and you’d expect it to gradually fade as the drug is metabolized and cleared. If the numbness were caused by excessive retraction or malpositioning, you’d anticipate signs of nerve injury such as motor weakness or a sensory deficit in a distribution that doesn’t align with the block’s typical area, and the deficit could persist beyond the drug’s duration. Prolonged pressure against the posterior humerus would affect the radial nerve distribution, not the shoulder/arm area targeted by an interscalene block, making that explanation less likely for this scenario. So the most likely reason for numbness in this context is residual anesthesia from the local anesthetic used in the block.

The main idea is that numbness after a regional block is most often due to the lingering effect of the local anesthetic itself. In an interscalene brachial plexus block, the anesthetic diffuses around the upper trunk and roots, producing sensory loss that mirrors the distribution blocked. The duration depends on the agent used—shorter-acting drugs wear off in a couple of hours, longer-acting ones can last several hours. So seeing numbness soon after the block is placed fits with residual anesthesia, and you’d expect it to gradually fade as the drug is metabolized and cleared.

If the numbness were caused by excessive retraction or malpositioning, you’d anticipate signs of nerve injury such as motor weakness or a sensory deficit in a distribution that doesn’t align with the block’s typical area, and the deficit could persist beyond the drug’s duration. Prolonged pressure against the posterior humerus would affect the radial nerve distribution, not the shoulder/arm area targeted by an interscalene block, making that explanation less likely for this scenario.

So the most likely reason for numbness in this context is residual anesthesia from the local anesthetic used in the block.