What is the technique for performing an ultrasound‑guided block of the medial cutaneous nerve of the arm (MCNA)?

Ultrasound-Guided Medial Cutaneous Nerve of Arm Block

The medial cutaneous nerve of the arm (MCNA) is most effectively blocked using ultrasound guidance with a subcutaneous injection technique in the axillary region, typically requiring only 1-3 mL of local anesthetic when visualized directly, though the nerve is often blocked as part of an axillary brachial plexus approach rather than as an isolated target.

Anatomical Considerations

The MCNA arises from the medial cord of the brachial plexus and provides sensory innervation to the medial aspect of the upper arm. This nerve must be blocked for:

• Superficial surgery on the anteromedial surface of the upper arm
• Tourniquet tolerance during upper extremity procedures
• Complete anesthesia when performing axillary brachial plexus blocks 1

The nerve runs superficially in the subcutaneous tissue of the axilla and medial upper arm, making it accessible to ultrasound visualization, though image quality can be variable 1.

Ultrasound-Guided Technique

Equipment and Patient Positioning

• Use a high-frequency linear ultrasound probe (10-15 MHz or higher) for optimal visualization of superficial structures 2
• Position the patient supine with the arm abducted 90 degrees
• In-plane needle guidance is preferred for direct visualization of needle trajectory and local anesthetic spread 2

Scanning Approach

Start scanning in the axillary region, moving the probe along the medial aspect of the upper arm. The MCNA appears as a small hypoechoic structure with hyperechoic rim in the subcutaneous tissue, though visualization quality varies significantly—good quality images are achieved in only approximately 48% of cases 1.

Injection Technique

• Target the subcutaneous tissue where the MCNA is identified
• Use 1% mepivacaine or equivalent local anesthetic
• Volume requirements are minimal when using ultrasound guidance—typically 1-3 mL is sufficient for isolated MCNA block
• Inject under direct visualization to confirm spread around the nerve 1

Clinical Outcomes and Efficacy

Ultrasound guidance dramatically improves block success compared to conventional landmark techniques. In a randomized trial, 88% of patients achieved complete sensory block at 20 minutes with ultrasound guidance versus only 19% with conventional subcutaneous infiltration (P < 0.001) 1. When complete anesthesia was achieved, onset occurred within 10 minutes in over 90% of patients in both groups 1.

Critically, 97.6% of patients who received ultrasound-guided MCNA block tolerated tourniquet placement versus only 38.1% in the conventional group (P < 0.001), demonstrating the clinical importance of this block for upper extremity surgery 1.

Integration with Axillary Brachial Plexus Block

The MCNA is frequently blocked as part of a comprehensive axillary approach rather than in isolation. The traditional axillary technique described by Dalens included one subcutaneous injection over the axillary artery specifically targeting the MCNA and intercostobrachial nerve 2.

Modern ultrasound-guided axillary blocks should include:

• Direct visualization and blockade of the major brachial plexus nerves
• Supplemental subcutaneous infiltration in the axilla for MCNA and intercostobrachial nerve coverage
• This ensures complete anesthesia of the medial upper arm, which is often missed with proximal approaches 3

Pediatric Considerations

In pediatric patients, the same ultrasound-guided principles apply with dosing adjustments 2:

• Calculate maximum safe local anesthetic dose based on weight
• Ropivacaine is preferred due to lower systemic toxicity
• Smaller volumes are required due to closer skin-to-nerve distances
• Use 0.15 ml/mm² of cross-sectional nerve area as a general guideline for peripheral nerve blocks in children 2

Common Pitfalls

• Inadequate ultrasound image quality: The MCNA can be difficult to visualize consistently; if not clearly seen, use a field block approach with subcutaneous infiltration along the medial upper arm
• Insufficient coverage: Blocking only the brachial plexus terminal branches without addressing the MCNA leads to tourniquet intolerance and incomplete anesthesia
• Excessive volume: When the nerve is visualized, small volumes (1-3 mL) suffice; larger volumes increase systemic absorption risk without improving efficacy
• Confusing MCNA with medial cutaneous nerve of forearm: These are distinct nerves with different distributions; both may need blocking depending on surgical site

The ultrasound-guided selective approach provides superior efficacy, faster onset, and better tourniquet tolerance compared to conventional field block techniques 1.