- Median nerve blocks at the wrist, either alone or in combination with blockade of the ulnar and radial nerves, are useful emergency department (ED) procedures. Compared to local anesthesia, nerve blocks provide greater efficacy and coverage of anesthesia useful for more complicated wounds or procedures involving the hand. In general, adequate anesthesia is a prerequisite to proper irrigation, examination, and repair of all wounds.
- The median nerve can be blocked at multiple sites along its passage through the upper extremity. However, only the wrist block is commonly used in the ED setting.
- Safe and effective application of the median nerve block requires a thorough understanding of the regional anatomy (see images below).
Wrist anatomy cross-section.
- A median nerve block at the wrist provides anesthesia and analgesia to the palmar surfaces of the lateral two-thirds of the palm, the thumb, the index and middle fingers, and one half of the ring finger (see image below).
Sensory innervation of the median nerve on the palmar hand.
- The median nerve passes through the carpal tunnel and gives rise to the palmar digital nerves supplying sensation to the lateral digits and motor innervation of the lateral two lumbricals. Sensation in the palm is supplied by superficial branches of the median nerve arising in the distal forearm. A recurrent branch of the median nerve also provides motor function to the thenar muscles.
- For simple lacerations, nerve blocks at the wrist may be slower and less reliable than local infiltration or digital block. However, they can be particularly useful in the following ED applications:
- Simultaneous injury to multiple digits
- Large abrasions or avulsions of the hand that require thorough irrigation, debridement, or both
- Avoiding distortion of anatomy in areas with limited subcutaneous tissue or tissue that is already excessively swollen
- Simultaneously anesthetizing several lacerations in close proximity
- In many of these situations, the median nerve block can be combined with ulnar or radial blocks at the wrist to achieve the desired coverage.
- Allergy to anesthesia: Consider using procaine (an ester anesthetic) in patients with true allergy to lidocaine or bupivacaine (amide anesthetics).
- Anatomic variation: Median nerve block may be contraindicated in the presence of prior surgery or injury at the wrist, proximal vascular grafts, or arteriovenous (AV) fistula.
- Additional injuries: The presence of additional injuries proximal to the wrist may necessitate a more proximal nerve block. Bier blocks or nerve blocks at the brachial plexus are more effective but require additional expertise to employ.
- Hepatic failure: Amide anesthetics, including lidocaine and bupivacaine, are metabolized by the liver. Take extra care to minimize systemic toxicity in patients with hepatic failure.
- The choice of anesthetic depends upon the specific procedure to be performed and the desired duration of effect.
- The most commonly used agent is 2% lidocaine. The higher concentration increases the concentration gradient in the tissue, allowing for faster spread of the agent from a smaller volume.
- Bupivacaine (0.25%-0.5%) can also be used for longer duration of anesthesia. Bupivacaine is not advised by the manufacturer for children younger than 12 years because of a lack of safety data.
- Hypersensitivity reactions are rare and may be due to preservatives rather than the anesthetic itself. Lidocaine and bupivacaine are both amide anesthetics and may have allergic cross-sensitivity. Procaine, an ester anesthetic, may be tolerated in patients with true allergic reactions to lidocaine.
- The maximum dose should be considered but is not frequently an issue for minor injuries in adults. Nerve blocks may also require less total dose than local injection.
- As with digital nerve blocks, epinephrine-containing solutions have been traditionally avoided because of the theoretical risk of extremity ischemia from arterial constriction. However, reliable reports of this complication, even in digital nerve blocks, are lacking, and recent studies of epinephrine-containing solutions have shown more benefit than harm for that application. In the case of median nerve blocks, epinephrine is unlikely to decrease bleeding at the site of injury and cannot be advised for this purpose.
- As lidocaine solutions are preserved at a low pH level, they may also be buffered with sodium bicarbonate (NaHCO 3 ) to decrease the pain of administration.
- The details of 2% lidocaine (plus NaHCO 3 ) are as follows:
- 2% lidocaine contains 20 mg/mL
- Onset: 10-20 min
- Duration: 2-5 hours
- Maximum dose: 4 mg/kg; not to exceed 300 mg
- The details of 0.5% bupivacaine are as follows:
- 0.5% bupivacaine contains 5 mg/mL
- Onset: 10-30 min
- Duration: 5-15 hours
- Maximum dose: 2.5 mg/kg; not to exceed 175 mg
For information comparing local anesthetics and a thorough discussion regarding allergic reactions to local anesthetics, see Local Anesthetic Agents, Infiltrative Administration.
- The field should be sterilely prepared. Chlorhexidine solution is the preferred agent for many practitioners and is recommended by the CDC for optimal sterility in other applications.
- A 25- to 27-gauge needle is appropriate.
- A smaller needle is less likely to cause direct nerve damage and causes less pain with injection.
- However, smaller needles do have disadvantages. With a smaller needle, confirmation of intravascular location by pulling back the plunger is more difficult. Also, a smaller needle may also elicit less paresthesia prior to injection, if placed intraneuronally.
- Positioning is critical to all procedures in the ED. Attention to practitioner and patient comfort maximizes success.
- Elevate the bed and place the patient’s arm on a Mayo stand or other support.
- Place a small roll under the wrist to extend the wrist slightly.
- Prior to any nerve block, perform and document a thorough neurovascular examination. Two-point discrimination is performed in the region to be blocked, with comparison to the contralateral limb.
- At the wrist, the median nerve lies within the carpal tunnel, deep and radial to the palmaris longus (PL) tendon and medial to the flexor carpi radialis (FCR) tendon.
- Injection is performed by insertion of the needle perpendicular to the skin between the PL and FCR tendons, angled slightly to place the needle tip directly beneath the PL (see image below). The injection is made 2-3 cm proximal to the distal crease of the wrist to avoid injecting within the carpal tunnel.
Median nerve block: Injection is performed between the tendons of the flexor carpi radialis (FCR) and the palmaris longus (PL), proximal to the crease of the wrist.
- The location of the PL and FCR tendons can be highlighted by having the patient flex the middle finger against resistance or pinch the thumb and little finger together.
- Some individuals lack the PL tendon, in which case the injection is made 5 mm ulnar (medial) to the FCR tendon, or approximately at the midpoint of the wrist.
- The needle is inserted to a depth of 1-2 cm. Passage through the fibrous flexor retinaculum may be felt. The nerve lies more superficial than one may expect. However, injecting in a space too deep is better than in a space too shallow, as the retinaculum prevents anesthetic from reaching the nerve if injected too superficially.
- An anesthetic volume of 3-5 mL is appropriate. Aspiration to prevent intravascular injection is generally advised to prevent systemic toxicity. If resistance to injection is met or paresthesia is elicited on needle insertion, the needle should be repositioned slightly more medial to avoid intraneuronal injection.
- To include the peripheral (palmar) branches, a small wheal of anesthetic is placed subcutaneously as the needle is withdrawn and massaged gently over the volar wrist.
- After injection, adequate time must be allowed for full effect (at least 10 minutes) before the desired procedure is performed on the hand.
- An alternative method for performing these blocks under dynamic ultrasonographic guidance has recently been evaluated for use in the ED.
- As with most procedures, proper positioning of the patient and practitioner is critical to success and safety.
- Give the patient instructions on care for the anesthetized hand and fingers, especially when using long-acting anesthetics. Anesthetized extremities are at greater risk of injury.
- Infection: This complication should be extremely rare when proper technique is used.
- Hematoma: This complication is more likely to occur if multiple punctures are made. Proper technique should keep the needle far from the radial or ulnar arteries.
- Nerve injury: Reposition the needle if increased pain or paresthesia is noted, and avoid injection against resistance.
- Vascular compromise: Avoidance of epinephrine-containing solutions is generally recommended, although this risk may be exaggerated.