A median nerve block, done at the wrist, anesthetizes the volar surface of the thenar half of the hand (from the thumb through the radial half of the ring finger) as well as the dorsal surfaces of the corresponding fingertips, excluding the thumb.
Median nerve block can be done using anatomic landmarks or ultrasonographic guidance. Ultrasonographic guidance increases the likelihood of successful peripheral nerve blockade and reduces the risk of complications but requires equipment and trained personnel.
(See also Local anesthesia for laceration treatment.)
Indications
Laceration or other surgically treated lesion of the volar surface of the radial (lateral) half of the hand from the thumb through the radial half of the ring finger and including the corresponding dorsal fingertip surfaces (excluding the thumb)*
Ring removal from the index, middle, or ring finger
Pain control for hand or digit fractures
A nerve block has advantages over local anesthetic infiltration because it can cause less pain (eg, in palmar skin repair) and does not distort tissue.
* To anesthetize the entire palm, also do an ulnar nerve block.
Contraindications
Absolute contraindications
History of allergy to the anesthetic agent or delivery vehicle (choose a different anesthetic)
Absence of anatomic landmarks needed to guide needle insertion (eg, due to trauma)
Relative contraindications
Infection in the path of needle insertion: Use procedural sedation or other anesthesia.
Coagulopathy*: When feasible, correct prior to procedure, or use a different means of analgesia.
* Therapeutic anticoagulation (eg, for pulmonary embolism) increases the risk of bleeding with nerve blocks, but this must be balanced against the increased risk of thrombosis (eg, stroke) if anticoagulation is reversed. Discuss any contemplated reversal with the clinician managing the patient's anticoagulation and then with the patient.
Complications
Adverse reaction to the anesthetic or delivery vehicle (eg, allergic reaction to the anesthetic [rare] or to methylparaben [a preservative])
Hematoma
Neuritis
Spread of infection, by passing the needle through an infected area
Most complications result from inaccurate needle placement.
Equipment
Nonsterile gloves
Barrier precautions as indicated (eg, face mask, safety glasses or face shield, cap and gown)
Syringe (eg, 5 to 10 mL) and needle (eg, 25 or 27 gauge, 3.5 cm long) for anesthetic injection
For ultrasonography: Ultrasound machine with high-frequency (eg, 7.5 MHz or higher) linear array probe (transducer); probe cover (eg, transparent sterile dressing, single-use probe cover); sterile, water-based lubricant, single-use packet (preferred over multi-use bottle of ultrasound gel)
* Local anesthetics are discussed in Lacerations.
Additional Considerations
Document any preexisting nerve deficit before doing a nerve block.
Stop the nerve block procedure if you are unsure where the needle is or if the patient is uncooperative. Consider sedation for patients who are unable to cooperate or remain still.
Relevant Anatomy
The median nerve lies deep to the flexor retinaculum and about one centimeter under the skin of the volar wrist.
The palmaris longus tendon lies superficial to the retinaculum and is absent in up to 20% of patients.
The median nerve is located slightly lateral (radial) to the palmaris longus tendon and medial (ulnar) to the flexor carpi radialis tendon.
Positioning
Position the patient with the arm resting with the palm facing up and the wrist extended about 30 degrees. A rolled towel placed under the dorsal aspect of the wrist may assist in positioning.
Step-by-Step Description of Procedure
Check sensation and motor function of the median nerve.
Wear gloves and use appropriate barrier precautions.
Locate the flexor carpi radialis and palmaris longus tendons, which become prominent when the patient flexes the wrist against resistance. The palmaris longus tendon is usually the more prominent of the two tendons.
Needle-entry site: The needle will be inserted adjacent to the radial (lateral) border of the palmaris longus tendon just proximal to the proximal wrist crease. If the palmaris longus tendon is absent, the needle-entry site is about 1 cm ulnar to the flexor carpi radialis tendon.
Cleanse the site with antiseptic solution.
Place a skin wheal of anesthetic, if one is being used, at the needle-entry site.
Insert the needle perpendicularly through the skin and advance it slowly until a slight pop is felt as the needle penetrates the flexor retinaculum. When paresthesia in the distribution of the median nerve confirms proper needle placement, withdraw the needle 1 to 2 mm.
Aspirate to exclude intravascular placement and then slowly (ie, over 30 to 60 seconds) inject about 3 mL of anesthetic.
If the patient does not feel paresthesia, redirect the needle in an ulnar direction, under the palmaris longus tendon. If paresthesia is still not felt, slowly inject 3 to 5 mL of anesthetic in the proximity of the nerve 1 cm deep to the tendon.
Allow about 5 to 10 minutes for the anesthetic to take effect.
Median nerve block, ultrasound-guided
Set the ultrasound machine to 2-D mode or B mode. Adjust the screen settings and probe position if needed to attain an accurate left-right orientation. This almost always means orienting the side-mark on the probe to the operator's left side (corresponding to the left-sided marker dot/symbol on the ultrasound screen).
Cleanse the sides and volar surface of the wrist with antiseptic solution.
Cover the probe tip with a layer of gel, then cover the tip with a sterile transparent dressing tightly (to eliminate air bubbles underneath). Apply sterile lubricant to the covered tip.
Place the probe tip transversely (short-axis cross-sectional view) on the proximal wrist crease.
Adjust the gain on the console so that the blood vessels are hypoechoic (appear black on the ultrasound screen) and the surrounding tissues are gray. Nerves appear as an echogenic (white), honeycombed, triangular shape, often adjacent to an artery (the median nerve, however, is not accompanied by an artery).
Adjust ultrasound maximum depth to about 3 cm.
Identify the median nerve about 1 cm deep to the radial border of the palmaris longus tendon.
Slide the probe slightly up the wrist to more clearly see the nerve. Do not move the probe from this spot.
Insert the needle and slightly tilt/rotate the probe to view the needle on the ultrasound screen (an in-plane, longitudinal image).
Maintaining the entire longitudinal needle image on the screen, advance the needle tip close to the nerve.
Inject a small test dose of anesthetic (about 0.25 mL) to see whether it spreads around the nerve. If not, move the needle closer to the nerve and inject another test dose.
When the needle tip is properly positioned, inject 1 to 2 mL of anesthetic solution to further surround the nerve. If necessary, reposition the needle tip and inject more small amounts; however, the donut sign—nerve completely surrounded by anesthetic—is not required.
Aftercare
Ensure hemostasis at the injection site.
Instruct patient regarding anticipated time to anesthesia resolution.
Warnings and Common Errors
Do not insert the needle more than 1 cm deep to the palmaris longus tendon, because injecting anesthetic too deeply is a common cause of failure of median nerve block at the wrist.
To minimize the risk of needle breakage, do not do any of the following: bend the needle, insert it to its full depth (ie, to the hub), or attempt to change direction of the needle while it is inserted.
To help prevent nerve injury or intraneural injection, instruct patients to report paresthesias or pain during the nerve block procedure.
To help prevent intravascular injections, aspirate before injecting.
If using ultrasound, always maintain ultrasound visualization of the needle tip during insertion.
Tricks and Tips
Minimize the pain of injection by injecting slowly (eg, 30 to 60 seconds), warming the anesthetic solution to body temperature, and buffering the anesthetic.
