Why the Posterior Cutaneous Nerve of the Arm is Missed in Supraclavicular and Interscalene Blocks
Anatomical Explanation
The posterior cutaneous nerve of the arm is consistently missed by both supraclavicular and interscalene blocks because it branches directly from the radial nerve in the axilla, well distal to where local anesthetic is deposited in these proximal brachial plexus approaches.
Key Anatomical Points
The posterior cutaneous nerve of the arm originates from the radial nerve after the brachial plexus has already divided into its terminal branches in the axillary region 1
Both interscalene and supraclavicular blocks target the brachial plexus at levels proximal to the axilla - the interscalene approach targets the nerve roots/trunks between the scalene muscles at the C5-C7 level, while the supraclavicular approach targets the divisions/trunks just above the clavicle 2
Local anesthetic spread from these proximal injection sites does not reliably extend distally enough to reach the posterior cutaneous nerve of the arm, which has already separated as an independent branch in the axilla 1
Clinical Implications
Coverage Limitations
Interscalene and supraclavicular blocks provide excellent anesthesia for shoulder surgery but have predictable gaps in coverage for the posterior arm 3, 4
The axillary approach is the only brachial plexus block that reliably covers the posterior cutaneous nerve of the arm and the medial cutaneous nerve of the arm, as the injection site is at or distal to where these nerves branch 1
When This Matters Clinically
For shoulder procedures, this missed coverage is typically not clinically significant since the surgical field is primarily innervated by the suprascapular and axillary nerves, which are reliably blocked by interscalene approaches 5, 4
However, for procedures involving the posterior aspect of the upper arm or when tourniquet pain is a concern, supplemental blockade or alternative approaches may be needed 1
Practical Considerations
If complete arm anesthesia including the posterior cutaneous nerve territory is required, choose an axillary or infraclavicular approach rather than interscalene or supraclavicular blocks 1, 2
The anatomical separation of this nerve from the main plexus occurs at approximately 14.2 cm proximal to the lateral epicondyle, well beyond the reach of proximal plexus blocks 6