Median Nerve Block Techniques - Health

Median Nerve Block Techniques/ A median nerve block is a simple, safe, and effective method of obtaining anesthesia to the palmar aspect of the thumb, index finger, middle finger, a radial portion of the palm and ring finger. Landmark-based techniques have been utilized for decades with success. However, since the introduction and widespread use of ultrasound, clinicians can obtain more consistent anesthesia with smaller volumes and fewer complications. [rx][rx] Peripheral nerve blockade of the hand has been used primarily in the peri-operative and post-operative settings by anesthesiologists and hand surgeons, although emergency physicians and traumatologists have become more familiar with these blocks in the setting of acute pain and trauma.[rx][rx]

Anatomy and Physiology of Median Nerve Block

The median nerve is a terminal branch of the brachial plexus, formed by the medial (C5, C6) and lateral cords (C8, T1). It branches from the brachial plexus at the axilla and courses through the upper arm along with the brachial artery, between brachialis and biceps brachii distally. Proximal to the elbow, the median nerve is located lateral to the brachial artery, and at the level of the elbow, the nerve is located medial to the artery.

Coursing distally, the median nerve enters the forearm between the biceps and pronator teres. In the proximal forearm, the nerve gives off the anterior interosseous nerve (AIN) branch, the terminal motor branch of the median nerve. AIN and the median nerve continuation both course deep to flexor digitorum superficialis and superficial to flexor digitorum profundus. The median nerve emerges in the distal forearm between FPL and FDS before transitioning to the wrist and hand via the carpal tunnel. [rx]

The median nerve travels in the carpal tunnel along with the FDS, FDP, and FPL tendons. The boundaries of the carpal tunnel include the transverse carpal ligament (the roof), the scaphoid tubercle and trapezium (radial border), the hook of hamate and pisiform (ulnar border), and the proximal row of carpal bones (the floor). In the tunnel, the median nerve is medial/ulnar to the tendons of FDS and FD. The nerve can be blocked at any point in the arm or forearm. However, its location in the forearm is quite superficial, and it is no longer adjacent to the brachial artery which makes inadvertent vascular puncture or injection less likely. [rx]

Carpal tunnel blocks are often performed, but studies have noted the wide range of anatomic variations and heterogeneous outcomes and success of blockade in the carpal tunnel. A 2015 study proposed a novel technique utilizing wrist width to aid in injection accuracy at the wrist. [rx] The technique involves the placement of the volar injection at the level of the carpal tunnel either on the relative radial (volar/radial) or relative ulnar (volar/ulnar) in a patient-specific approach to minimize complications and injury to surrounding tendons and neurovascular structures. Using the most radial side of the wrist at the “zero” position in terms total wrist width, the radial (30% position along the total width) and ulnar (60% position along the total width) were reliable injection techniques. The volar/radial position was safer than the ulnar position as well.

Types of Median Nerve Block

Nerve blocks are also useful in the emergency department for the following indications: Acute pain management of the extremities.[rx][rx][rx]

Anesthesia of the extremity for procedures
Alternative to procedural sedation
Alternative to opioids in certain patient populations (e.g., a head injury patient, patients with the concomitant mental status change, patients given buprenorphine)

Many nerves can be blocked depending on the injury. These include the following upper or lower extremities

Brachial plexus roots at the interscalene location block the shoulder, upper arm, elbow, and forearm
Brachial plexus trunks at the supraclavicular location block the upper arm, elbow, wrist, and hand
Brachial plexus cords at the infraclavicular location block the upper arm, elbow, wrist, and hand
Brachial plexus branches at the axillary location block the forearm, wrist, hand, and elbow including the musculocutaneous nerve
Median nerve at the elbow blocks the hand and forearm
The radial nerve at the elbow blocks the hand and forearm
Ulnar nerve at the elbow blocks the hand and forearm
The femoral nerve at the femoral crease blocks the anterior thigh, femur, knee, and skin anesthesia over the medial aspect of the leg below the knee
The sciatic nerve at the subgluteal location or anterior approach below the femoral crease blocks the posterior aspect of the thigh, and anterior, lateral, and posterior lower leg, ankle, and foot
The sciatic nerve at the popliteal location blocks the anterior, lateral, and posterior lower leg, ankle, and foot
Ankle block of five separate nerves to the ankle and foot (saphenous nerve, deep peroneal nerve, superficial peroneal nerve, posterior tibial nerve, and sural nerve) blocks the entire foot

Indications of Median Nerve Block

Blockade of the median nerve provides anesthesia to the palmar aspect of the thumb, index finger, middle finger, the radial aspect of the ring finger, or radial half of the palm including skin and underlying metacarpals and phalanges.
This block may be used as the sole anesthetic, or as an adjunct for reduction or surgical repair of fractures and dislocations, repair of complex lacerations or performing incision and drainage of cysts and abscesses. The use of peripheral nerve blocks for postoperative analgesia have demonstrated shorter recovery times and decreased opioid use.
A median nerve block is also an excellent opioid-sparing analgesic option for burns involving tissue in the median nerve distribution. A terminal median nerve block is a valuable rescue technique for incomplete brachial plexus blocks.
Although compartment syndrome is frequently a concern with high energy injuries, there are reports of using forearm median, ulnar, and radial nerve blocks in blast injuries and other high energy injuries without serious sequelae. [rx] One should always assess the patient and mechanism of injury along with the treating physician before block placement.

Contraindications of Median Nerve Block

Contraindications to a nerve block include:

Patient refusal
Cellulitis or abscess over the site of injection
Anaphylactic reaction to local anesthetics
Maybe relatively contraindicated in high-energy injuries at increased risk for compartment syndrome, discuss with consultants first.

Equipment

Equipment for the procedure includes the following:

Local anesthetic – type and quantity vary depending on the intended duration of the block. 5mL of lidocaine 2% for a short-medium duration block (1-2 hours), 5mL of bupivacaine 0.5% or ropivacaine 0.5% (2-4 hours)
Block needle – short bevel 6cm or longer
Chlorhexidine 2% or povidone-iodine
High frequency (8MHz or greater) linear ultrasound probe
Sterile ultrasound gel
Sterile probe cover
Access to intrepid 20% (1.5mL/kg bolus, 0.25mg/kg/hr drip) in the case of cardiac arrest due to LAST

Personnel

A practitioner trained in peripheral nerve blocks. Nurse able to administer resuscitation medications if needed.

Preparation

Preparatory steps include the following:

Obtain consent for nerve block including risks, benefits, and alternative treatments
Verify the patient’s identity and site to be blocked
Perform and document a detailed neurovascular exam of the affected extremity
Position the affected arm on a stand, with the arm extended at the elbow and supinated distally
Clean the arm from the elbow crease to about the mid-forearm with chlorhexidine 2% or povidone-iodine and allow the solution to dry
Place ultrasound machine within line of sight
Apply a sterile covering and sterile gel to a high-frequency linear ultrasound probe
Draw local anesthetic solution into a sterile syringe.

Technique

A nerve block should be performed in the following steps:

Place the probe in the elbow crease over the proximal radius.
Identify the radius, with pronator teres lying medially.
Identify the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) medial to pronator teres.
The median nerve should be easily identifiable as a hyperechoic structure with a honeycomb appearance between FDS and FDP.
Insert a block needle in-plane and parallel it to the probe surface. Ensure that the needle tip is visible at all times.
Aim the needle tip at the fascial plane between FDS and FDP.
Aspirate to confirm the needle tip is not within a blood vessel.
Inject a small amount of local anesthetic to confirm needle tip placement.
The needle tip may need to be advanced, or withdrawn slightly for optimal placement in the fascial plane, so that spread of local anesthetic around the nerve is achieved.
Once injectate is confirmed to be within the fascial plane containing the median nerve, continue aspiration followed by incremental injection of 1 mL to 2 mL of a local anesthetic until the satisfactory spread has occurred.

Complications

Complications can include

Bleeding from the puncture site
Intramuscular hematoma
Infection at the site of injection
Allergic reaction to local anesthetic
Vascular puncture
Intravascular injection
Local anesthetic systemic toxicity (LAST)
Nerve damage including neuropraxia or neurolysis
Unsuccessful block

References

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