Axillary Nerve Block: Techniques and Clinical Applications

Axillary nerve block is a widely used regional anesthesia technique that provides effective analgesia for procedures involving the lower arm, wrist, and hand. By targeting the brachial plexus at the axilla, it minimizes the need for general anesthesia while improving postoperative pain control and reducing opioid use. Success depends on precise anatomical knowledge, appropriate anesthetic selection, and accurate guidance techniques.

Relevant Axillary Anatomy

The axillary region serves as a conduit for the brachial plexus as it courses toward the upper limb, making it a prime location for regional anesthesia. The brachial plexus in this area consists of the lateral, medial, and posterior cords, which give rise to the musculocutaneous, median, ulnar, and radial nerves. These structures are enveloped by the axillary sheath, a fascial extension of the deep cervical fascia, which facilitates anesthetic spread when properly administered. Understanding the spatial relationships of these nerves within the axilla is key to achieving a successful block while minimizing complications.

The neurovascular bundle, positioned within the axillary fossa, is closely associated with the axillary artery, a key landmark for nerve localization. The musculocutaneous nerve typically lies outside the neurovascular sheath, embedded within the coracobrachialis muscle, requiring a separate injection for complete anesthesia. The median, ulnar, and radial nerves are more consistently found encircling the axillary artery, making them more accessible for blockade. Variations in nerve positioning, particularly in relation to the artery, have been documented in cadaveric studies and clinical imaging, underscoring the importance of individualized assessment.

The surrounding musculature, including the pectoralis major and minor anteriorly and the latissimus dorsi and teres major posteriorly, influences needle trajectory and anesthetic diffusion. The axillary vein, positioned medial to the artery, presents a potential site for inadvertent vascular puncture, emphasizing the need for careful technique. Additionally, the intercostobrachial nerve, a sensory branch of the second intercostal nerve, is not part of the brachial plexus but contributes to medial upper arm sensation. Failure to anesthetize this nerve can result in residual discomfort, particularly with tourniquet use.

Nerve Distribution in the Region

The axillary nerve block targets the terminal branches of the brachial plexus as they traverse the axilla, each contributing distinct sensory and motor functions to the upper limb. The median, ulnar, and radial nerves encircle the axillary artery within the neurovascular bundle, positioning them for efficient anesthetic spread. The musculocutaneous nerve, however, often lies outside this sheath, requiring a separate injection to ensure complete blockade. These anatomical nuances influence both the efficacy of the block and the potential for incomplete anesthesia.

The median nerve, arising from the lateral and medial cords, provides motor innervation to most anterior forearm muscles and sensory input to the palmar aspect of the lateral three and a half fingers. The ulnar nerve, from the medial cord, governs intrinsic hand muscle control and sensation to the medial one and a half fingers. The radial nerve, from the posterior cord, supplies the extensor muscles of the forearm and dorsal sensory innervation of the hand. Incomplete blockade of any of these nerves can lead to inadequate pain relief or residual motor function, particularly in procedures requiring extensive forearm or hand immobilization.

Beyond the primary nerves of the brachial plexus, additional sensory contributors must be considered. The medial cutaneous nerve of the forearm, from the medial cord, provides sensation to the medial forearm and is often blocked alongside the main neural structures. Similarly, the intercostobrachial nerve, a branch of the second intercostal nerve, carries sensation from the upper medial arm and is not inherently anesthetized during a standard axillary block. This omission can cause unexpected discomfort, particularly in patients requiring prolonged tourniquet use, necessitating a supplementary subcutaneous injection.

Local Anesthetic Preparations

Selecting the appropriate local anesthetic for an axillary nerve block requires balancing onset time, duration, and potential systemic toxicity. The choice depends on the procedure and anticipated postoperative pain management needs. Short-acting agents like lidocaine (1-2%) provide rapid onset within 5-15 minutes but wear off within 2-3 hours, making them suitable for brief interventions. In contrast, longer-acting anesthetics such as bupivacaine (0.25-0.5%) or ropivacaine (0.5%) extend analgesia for 8-16 hours, reducing the need for additional pain control.

Lidocaine, with its fast diffusion and intermediate protein binding, enables quick sensory blockade but has a limited duration. Bupivacaine, known for its high lipid solubility and strong sodium channel affinity, provides prolonged anesthesia but carries an increased risk of cardiotoxicity if injected intravascularly. Ropivacaine, a structurally similar alternative, offers a safer profile with reduced cardiac effects while maintaining effective analgesia. The selection process involves weighing these factors against patient-specific considerations such as comorbidities and surgical complexity.

Adjuvants can enhance anesthetic performance. Epinephrine (1:200,000 or 1:400,000) is frequently added to lidocaine or bupivacaine to induce vasoconstriction, slowing systemic absorption and prolonging nerve blockade. Dexamethasone can extend analgesia by 6-10 hours. Clonidine and dexmedetomidine, alpha-2 agonists, have also been explored for their ability to enhance block duration while providing mild sedation, though their routine use remains debated due to potential hemodynamic effects.

Common Methods of Guidance

Precise needle placement is essential for maximizing anesthetic efficacy while minimizing complications. Traditionally, nerve localization relied on anatomical landmarks and transarterial techniques, where the needle is advanced through the axillary artery to deposit anesthetic posteriorly. While effective, this approach carries an increased risk of vascular puncture and inconsistent spread, leading to variable success rates. Advances in guidance techniques have significantly improved accuracy and reliability.

Peripheral nerve stimulation (PNS) uses electrical impulses to elicit muscle contractions corresponding to specific nerve distributions. Adjusting needle positioning until an appropriate motor response—such as thumb flexion for the median nerve or wrist extension for the radial nerve—is observed ensures targeted anesthetic delivery. Despite its utility, PNS does not provide direct visualization of anatomical structures, leaving room for misplacement, particularly in patients with anatomical variations or altered neuromuscular responses.

Ultrasound guidance has largely supplanted older techniques due to its superior precision and real-time visualization of neural structures, surrounding vasculature, and the spread of local anesthetic. High-frequency linear probes allow for detailed imaging of the axillary neurovascular bundle, enabling dynamic adjustments during injection. Studies show that ultrasound-guided axillary blocks achieve higher success rates, shorter procedural times, and lower complication risks compared to landmark-based or PNS methods. Additionally, the ability to visualize the musculocutaneous nerve separately ensures a more complete block, addressing a common pitfall of non-ultrasound techniques.

Clinical Applications

Axillary nerve blocks play a significant role in upper limb surgeries, offering effective anesthesia and prolonged postoperative analgesia while reducing reliance on systemic opioids. This technique is frequently used for procedures involving the forearm, wrist, and hand, such as carpal tunnel release, tendon repairs, and fracture fixations. By providing targeted nerve blockade, it minimizes patient discomfort and facilitates a bloodless surgical field when combined with a tourniquet. It is also a preferred choice for patients with contraindications to general anesthesia, such as those with severe pulmonary disease or high aspiration risk, as it avoids airway manipulation and systemic sedative effects.

Beyond surgical applications, axillary nerve blocks are increasingly used in pain management for complex regional pain syndrome (CRPS) and other neuropathic conditions affecting the upper limb. By interrupting nociceptive signaling at the brachial plexus, these blocks can provide substantial relief in chronic pain patients, improving physical therapy participation and functional recovery. Additionally, continuous catheter techniques have been explored for prolonged pain control, particularly postoperatively or for individuals requiring extended analgesia due to trauma. Studies show that catheter-based axillary blocks can significantly reduce opioid consumption and improve patient satisfaction compared to systemic pain management strategies.