During onset of a peripheral nerve block, which fibre type loses function FIRST, and which recovers LAST?

• A Large motor A-alpha fibres first, thin C fibres last
• B All fibre types are blocked simultaneously; recovery follows reverse size order
• C Unmyelinated C pain fibres first, autonomic B fibres last
• D Small myelinated B fibres (preganglionic sympathetic) first, large motor A-alpha fibres last ✓

Explanation

Susceptibility to local anaesthetic block depends on fibre diameter, myelination, and firing frequency. Small myelinated D fibres carrying preganglionic sympathetic impulses are blocked first, followed by A-delta pain and temperature fibres, then touch and proprioception, while large heavily myelinated A-alpha motor fibres are blocked last. Clinically this produces the sequence: sympathetic, sensory, then motor blockade, and recovery occurs in the reverse order. This explains why vasodilation and skin warming precede loss of pinprick sensation during block onset.

Reference: Morgan and Mikhail's Clinical Anesthesiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.