An axillary nerve block is a regional anesthesia technique that involves injecting local anesthetic around specific nerves in the axilla, also known as the armpit. This procedure numbs the upper limb for surgical procedures or pain management.
What is an Axillary Nerve Block?
An axillary nerve block works by blocking nerve signals in the axillary region, preventing pain sensations from reaching the brain. This regional anesthesia offers an alternative to general anesthesia for various procedures. The block specifically targets the terminal branches of the brachial plexus, a network of nerves that extends down the arm. It provides pain control for upper limb surgeries with fewer systemic risks compared to general anesthesia.
Understanding the Numbed Areas
The axillary nerve block primarily anesthetizes the upper limb from the mid-arm down to and including the hand. This broad coverage is achieved by targeting several key nerves: the musculocutaneous, median, ulnar, and radial nerves. These nerves are responsible for both sensory perception and motor control in the forearm, wrist, and hand.
The block results in a loss of sensation across the lateral half of the hand, the dorsum of the first three digits, and half of the fourth finger down to the nail bed, due to the median nerve blockade. The ulnar nerve blockade affects sensation in the middle third of the palm, the dorsum of the fifth finger, and the medial aspect of the fourth finger. The radial nerve blockade provides sensation to the posterior and lateral aspects of the arm and forearm down to the wrist, and the lateral aspect of the dorsum of the hand.
Beyond sensory effects, the block also impacts motor function, leading to an inability to move the affected areas. The ulnar nerve, for example, innervates muscles like the flexor carpi ulnaris and several small muscles in the hand, excluding those of the thenar eminence and the first two lumbricals. The radial nerve controls the triceps and brachioradialis muscles, along with the extensors in the posterior forearm. The musculocutaneous nerve, which provides innervation to the biceps and brachialis muscles, continues distally as the lateral cutaneous nerve of the forearm, supplying sensation to the lateral forearm. The axillary nerve itself is not blocked by this procedure, so the skin over the deltoid muscle retains sensation.
Common Uses of the Block
An axillary nerve block is frequently employed for surgical procedures involving the elbow, forearm, and hand. This includes operations such as carpal tunnel release, tendon repairs, and fracture repairs in the hand and wrist. It is also suitable for procedures like joint replacements in the forearm and elbow.
Beyond surgery, the block is also used for pain management, particularly for acute postoperative pain or certain chronic pain conditions affecting the upper extremity, such as complex regional pain syndrome. The block offers targeted pain relief, reducing the need for systemic pain medications. Its safety profile and ease of administration make it suitable for many distal upper limb operations.
The Procedure and What to Expect
Before the procedure, patients are positioned lying on their back with the arm abducted and externally rotated to expose the armpit area. The skin is cleaned in a sterile manner, and a local anesthetic is injected into the skin at the needle entry site to minimize discomfort.
Ultrasound guidance is used during the procedure to visualize the nerves and surrounding structures in real-time, which allows for precise placement of the local anesthetic. A thin needle is advanced towards the target nerves (median, ulnar, radial, and musculocutaneous nerves) located around the axillary artery. The local anesthetic is then injected in small amounts around each individual nerve.
Patients may feel a brief sensation of “pins and needles” or a mild discomfort as the needle approaches the nerves. The onset of numbness occurs within minutes to about half an hour after the injection, with full numbness developing within 30 minutes. The duration of the block’s effects can vary depending on the type and concentration of local anesthetic used, lasting between 9 to 12 hours with anesthetics like ropivacaine. For shorter procedures, lidocaine or mepivacaine can be used, providing anesthesia for 2.5 to 3 hours.
Important Considerations and Potential Side Effects
While an axillary nerve block is considered a safe regional anesthesia technique, patients should be aware of potential side effects. Mild effects include bruising or temporary soreness at the injection site. These resolve on their own.
More serious risks include nerve injury, which can manifest as numbness, tingling, muscle weakness, or pain in the affected area. Most nerve injuries are temporary and resolve within a few days to three months, but permanent injury is possible, though rare. Other infrequent complications include infection at the injection site or bleeding, which could lead to a hematoma that presses on a nerve. Allergic reactions to the anesthetic are also possible, though uncommon. Discussing these potential outcomes with a healthcare provider helps ensure a comprehensive understanding before the procedure.