Digital Nerve Block

Digital nerve blocks are important tools for the emergency medicine clinician. Injuries or infections of the digits are extremely common. Adequate analgesia is essential to properly address the presenting condition and to minimize the patient's discomfort. Digital blocks are useful in many scenarios in which local infiltration of an anesthetic would require several injections into the already painful site of injury. Furthermore, local infiltration around the wound may create increased swelling, making the repair more difficult. Several techniques are available for performing digital blocks.

Relevant anatomy

Each digit is innervated by 4 digital nerves. In the upper extremity, the digital nerves arise from the median, ulnar, and radial nerves. The 2 palmar digital nerves innervate the palmar aspect of the digit and the nail bed, whereas the dorsal nerves innervate the dorsum of the digit (see images below). The tibial and peroneal nerves branch off into the digital nerves of the lower extremities, which follow a pattern of distribution analogous to those of the upper extremity.[1]

Digital blocks are indicated for any minor surgery or procedure of the digits. These include, but are not limited to, the following:

• Large irregular lacerations

• Lacerations involving the nail or the nail bed

• Ingrown nails

• Felon or paronychia

• Trephination of subungual hematoma

• Digit dislocations or fractures[2]

Contraindications to digital nerve block include:

• Compromised digit circulation

• Infected injection site

• Known allergy to anesthetic

Best practices for digital nerve block include:

• Avoid epinephrine use in the digits; a clamped Penrose drain can be used to limit bleeding.

• Limit the patient’s discomfort by using a smaller needle, injecting slowly, and using small amounts of anesthetics.

• Anesthesia of the great toe is more difficult to achieve and requires 3-sided/4-sided ring blocks.

• Use of sterile technique is essential to limit the risk of introducing infections (especially with the transthecal block).

The equipment necessary for performing a digital nerve block includes:

• Sterile gloves, drapes, and gauze pads

• Povidone-iodine (Betadine) solution

• Syringe, 5-10 mL, with an 18-gauge needle for drawing up the anesthetic and a 25- to 30-gauge needle for injection

• Local anesthetic of choice

Local anesthetic agents have the basic structure of an aromatic and a hydrophilic, separated in the middle by an amino-ester or an amino-amide. This forms the basis of classification of local anesthetics into 2 groups: the ester-type agents (eg, procaine) and the amide-type agents (eg, lidocaine).[3]

The choice of agent is based on the desired duration of analgesia and the patient’s allergy profile. Lidocaine is the most commonly used anesthetic. If longer anesthesia is required, another amide anesthetic, such as bupivacaine, can be used. If the patient is known to be allergic to lidocaine, an ester-type anesthetic, such as procaine, can be substituted. Table 1 summarizes the properties of commonly used agents. For more information, see Local Anesthetic Agents, Infiltrative Administration.

Table 1. Commonly Used Local Anesthetics and Their Properties ^[4] (Open Table in a new window)

Local anesthetics are to be used without epinephrine in the digits to avoid vasoconstriction of adjacent arteries, which may lead to ischemia or infarction of local tissues. Despite studies that have shown epinephrine to be safe in these circumstances,[5] epinephrine is traditionally avoided in the digits. A study by Sonohata et al showed that a single injection (such as in the transthecal block) of 3 mL anesthetic with epinephrine was effective in achieving adequate analgesia. Also, the time to achieving analgesia was shorter and the effects were longer. Ischemic injury was not reported.[6]

The block should be performed cautiously in areas where nerve function is compromised. Small volumes of anesthetic should be used to minimize local swelling, especially in cases in which compartment syndrome is being considered.

Topical vapocoolant spray during minor procedures can be used to reduce needle penetration pain associated with digital nerve block and pain associated with local anesthetic infiltration.[7]

Numerous potential complications and local anesthetic toxicities have been described in the literature, including the following:[8]

• Pain at injection site

• Infection at injection site, especially with transthecal block

• Wound infection: Local anesthetics have been shown to possess antimicrobial properties. Although studies have shown that use of local anesthetics does not alter incidence of wound infection, their use may produce false-negative wound cultures.[9]

• Local injuries: Injuries to nerves and tendons can result in long-term complications such as neuropathies and tendonitis.

• Wound healing: Several studies have shown that local anesthetics inhibit wound healing by decreasing the tensile strength of wounds;[10] another study showed that local anesthetics decrease local inflammatory response.[11]

• Inadvertent intravascular injection: This increases the risk of cardiotoxicity and neurotoxicity.[8]

• Allergic reactions

• Vasovagal syncope

Several different techniques can be used to anesthetize the digits: the web-space block, the transthecal block, the 3-sided digital block, and the 4-sided ring block. Standard sterile precautions should be followed for all of the described procedures.

This method is very effective in achieving adequate anesthesia and is probably the least painful.

• Place the patient’s hand on a sterile field with the palm down.

• Hold the syringe perpendicular to the digit and insert the needle into the web space, just distal to the metacarpal-phalangeal (MP) joint (see image below).

  Needle position for web-space block.

• Slowly inject the anesthetic in the dorsal aspect of the web space.

• Slowly advance the needle straight down toward the volar aspect of the web space, slowly infiltrating the surrounding tissues of the web space (see video below). The needle should not pierce the volar aspect of the web space.

  Web-space block technique.

• Withdraw the needle and repeat the procedure on the other web space of the involved digit.

• The toes (except the great toe) can be effectively anesthetized in the same manner.

Originally described by Chiu in 1990,[12] this technique is also known as the flexor tendon sheath digital block. While treating trigger finger by injecting steroids and lidocaine into the tendon sheath, Chiu noted that anesthesia of the entire digit was achieved. Although adequate anesthesia is achieved with a single injection, this injection is painful because the needle pierces the very sensitive skin of the palm. Studies have shown that this type of block is as effective as traditional ring blocks in achieving adequate anesthesia.[13, 14, 15]

• Place the patient’s hand on the sterile field with the palm up.

• Locate the flexor tendon sheath by palpating it at the distal palmar crease.

• Insert the needle at a 45-degree angle just distal to the distal palmar crease (see image below).

  Needle position for transthecal block.

• Inject the anesthetic, it should flow freely. If resistance is met, reposition the needle by slowly withdrawing it.

A modified version of this technique can also be used effectively.[16]

• Position the patient’s hand with the palm facing up.

• Insert the needle at a 90-degree angle at the metacarpal crease until bone is hit.

• Withdraw the needle slightly and inject the anesthetic.

• During the injection, use the nondominant hand to apply pressure just proximal to the injection site, to direct the flow distally (see image below).

  Modified transthecal block.

This type of digital block is effective in anesthetizing the great toe, but it can be used for any digit.

• Place the patient’s extremity volar/plantar side down.

• Insert the needle at a 90-degree angle at the medial aspect of the digit, just distal to the metatarsal-phalangeal joint (see image below).

  Medial injection for 3-sided digital block.

• Slowly inject the anesthetic as the needle is advanced toward the volar/plantar side, without piercing the volar skin.

• Slowly withdraw the needle and redirect it medially.

• Advance the needle slowly from medial to lateral side while the anesthetic is injected (see image below).

  Medial to lateral injection for 3-sided digital block.

• Withdraw the needle.

• Make another injection over the already anesthetized skin at the lateral aspect of the digit, with the needle at 90 degrees, advancing it from the dorsal to ventral aspect, as was done medially (see image below).

  Lateral injection for 3-sided digital block.

This method is an extension of the 3-sided block. After the 3-sided block is performed, a third injection is performed. Insert the needle at the lateral aspect of the digit on the volar/plantar side and advance it medially as the anesthetic is slowly injected. This technique is less favored because of the potential for ischemic complications.

When only the distal part of the digit is involved (eg, nail injury), a wing block procedure can be used instead of a digital block.

• Position the extremity with the volar/plantar side down.

• Hold the needle perpendicular to the long axis of the digit and at 45 degrees to the plane of the sterile field.

• Insert the needle 3 mm proximal to an imaginary point where a linear extension of the lateral and proximal nail folds would intersect (see image below).

  Wing block technique.

• Inject the anesthetic along the proximal nail fold.

• Slowly withdraw the needle and redirect it toward the lateral nail fold.

• This may be performed on the opposite side of the nail as necessary.[17]