Medical

Digital nerve block

Digital nerve block

Authors
Robert Baldor, MD
Barbara M Mathes, MD, FACP, FAAD
Section Editor
Allan B Wolfson, MD
Deputy Editor
Jonathan Grayzel, MD, FAAEM

Disclosures

Last literature review version 19.3: Fri Sep 30 00:00:00 GMT 2011 | This topic last updated: Mon Mar 21 00:00:00 GMT 2011 (More)

INTRODUCTION — The digital nerve block is a procedure in which an anesthetic solution is injected into the base of a finger or toe to provide regional anesthesia. Other methods to anesthetize locally the tissues of the digits vary from applications of topical agents to subcutaneous injections of anesthetic solutions. Due to the extreme sensitivity of the palmar surfaces of the digits, a local injection may be painful and ineffective at anesthetizing the tissues. The injection sites for the digital nerve block are typically less painful; furthermore, they provide a larger area of consistent anesthesia throughout the operative region.

A digital nerve block is indicated for the treatment and repair of many acute conditions, including finger or toe lacerations beyond the mid-proximal phalanx, nail bed injuries, paronychias, nail avulsions, and foreign bodies of the digit.

CONTRAINDICATIONS AND PRECAUTIONS — Contraindications include:

  • Compromised digital circulation
  • Infection of the skin or tissues through which the needle will pass
  • Allergy to the anesthetic agent

Additional cautions include:

  • Avoid using an anesthetic with epinephrine in patients at risk for ischemic injury. (See 'Use of epinephrine' below.)
  • Consider other techniques when neurologic function of the digit has been previously compromised by an injury.
  • Use small volumes of anesthetic to decrease the mechanical compression on nerves and blood vessels.
  • Avoid injecting directly into nerves.
  • Avoid prolonged use of tourniquets.

ANATOMY — Each digit is innervated by four digital nerves. In order to achieve a complete anesthetic effect, it is necessary to block all four nerves. The digital nerves of the fingers arise from either the median or ulnar nerves and divide in the palm into palmar (or volar) branches. These nerves are accompanied by digital blood vessels as they run on both sides of the flexor tendon sheath to innervate each finger. The palmar nerves dominate by innervating all of the finger and the nail bed except for the dorsum of the finger, which is innervated by the dorsal digital nerves that run along the dorsolateral aspect of each finger (figure 1A-B). The digital nerves of the toes run on both sides of each toe and represent the terminal branches of the tibial and peroneal nerves (figure 2A-B) [1] .

ANESTHETIC AGENTS

General issues — Most local anesthetics are classified as amides or esters. (See "Infiltration of local anesthetics".) These pharmacologic agents act by inhibiting nerve impulses. Commonly used preparations include the amide class agent lidocaine (Xylocaine) and the ester class agent procaine (Novocain) in concentrations of 1 or 2 percent. Use of higher concentrations can reduce the total volume of fluid injected, which may reduce discomfort and complications from tissue compression. Onset of anesthesia with a digital block typically occurs in about five minutes, but full anesthetic effect takes longer (10 to 15 minutes). Individual variation is to be expected.

The effect of lidocaine or procaine may last only 30 to 40 minutes, but anesthesia with bupivacaine (Marcaine) 0.25 percent typically lasts several hours [2] . The duration of effect of all agents is longer if epinephrine is added, but bupivacaine remains longer-acting [3] . Bupivacaine has been used in combination with shorter-acting agents in patients where prolonged anesthesia is required to try to achieve rapid onset of anesthesia with a prolonged effect, but this is probably not necessary; one small randomized trial found that bupivacaine alone had a similar onset of action to that of lidocaine plus bupivacaine (mean time to anesthesia 5.35 versus 5 minutes) [4] .

Before choosing an agent, check to see that the patient does not have any allergies to the preparation to be used. Allergies to lidocaine are rare, but they do occur. Treat allergic reactions aggressively.

Use of epinephrine — Some local anesthetics are combined with epinephrine in commercially available solutions (eg, lidocaine hydrochloride 1 percent with epinephrine 1:100,000). Epinephrine causes local vasoconstriction, thereby reducing bleeding, and maintains the anesthetic in the tissues for a longer period of time. However, the digital arteries run in close proximity to the digital nerves and injecting epinephrine into the base of a digit causes vasoconstriction of these vessels [5] , which may increase the risk of digital ischemia in susceptible individuals.

In most cases, the use of epinephrine for digital blocks is effective and safe when used for short procedures, such as treating ingrown nails or paronychia, repairing a skin laceration, or performing a skin biopsy. In patients at increased risk for digital ischemia or infarction, such as those with peripheral vascular disease, it is best to avoid epinephrine.

Comprehensive reviews of the medical literature and large observational studies have failed to find any reports of gangrene or major ischemic complications solely attributable to lidocaine with epinephrine [6-11]  or with the use of commercial lidocaine/epinephrine preparations [12] . Gangrene had occurred in 1948, prior to the introduction of commercial lidocaine/epinephrine preparations [12,13] . A Doppler study of 24 patients treated with digital blocks containing epinephrine found that the drug's vasoconstrictive effects resolve within 90 minutes [5] .

One alternative to using epinephrine to control bleeding when performing a digital block is to place a small Penrose drain clamped around the base of the digit during the procedure. This method is used for short procedures; prolonged digit ischemia (over 60 minutes) must be avoided. Be certain to remove the drain once the procedure is completed.

Pain with administration — Administration of anesthetics can be painful. A number of measures have been recommended to reduce the pain of anesthesia although data on efficacy are somewhat limited [14] . These include (see "Infiltration of local anesthetics"):

  • Buffering the solution with sodium bicarbonate [15,16]
  • Using small needles (27 or 30 gauge)
  • Injecting slowly
  • Injecting subcutaneously instead of intradermally
  • Warming the anesthetic to body temperature [17]

The standard solution of lidocaine has a pH of 5.0 to 7.0. It can be buffered to a pH of 7.0 to 7.2 by adding one part of 1 mEq/mL of sodium bicarbonate to 9 or 10 parts of 1 percent lidocaine. (See "Infiltration of local anesthetics".)

EMLA topical cream (lidocaine/prilocaine) has been used to decrease the pain caused by the initial injection of the anesthetic agent. The effectiveness of such agents for decreasing the pain of a digital block is uncertain, as two studies in patients with ingrown toenails produced conflicting results [18,19] . The reason for these differing results is unclear. For EMLA to provide effective albeit superficial anesthesia it must be applied for one hour prior to injection.

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PREPARATION

Informed consent — Informed consent is always advisable when pursuing a procedure, regardless of how minor. Explain the benefits and risks of the procedure, and make sure that the patient understands the issues. Common risks for digital blocks include infection and bleeding. There is also the risk of distal paresthesia, if the nerve is damaged and the possibility of distal infarct with vasospasm. However, these risks are small and can be avoided with careful attention to technique. Prior to proceeding with the block, perform a neurologic exam to detect sensory abnormalities.

Materials — Gather all the materials necessary to perform the procedure and place them within easy reach. Prepare all supplies in a sterile fashion. For some procedures (eg, removal of in-grown nails, wart treatment), a clean field, rather than sterile technique, may be sufficient. Prepare the anesthetic vial with alcohol prior to drawing up the solution. Usual materials needed for the digital block are:

  • A 3-mL syringe with an 18 to 22 gauge needle for drawing up anesthetic.
  • Local anesthetic, buffered with sodium bicarbonate if possible. Digital block is usually achieved with 2 to 3 mL of the anesthetic solution.
  • A 27 to 30 gauge needle for performing the anesthetic injection.

Skin preparation — Prepare the skin with a povidone iodine or chlorhexidine solution to decrease the risk of infection. Perform three separate scrubbings with the antibacterial solution to include both the injured area and the injection sites. The ideal technique is to begin at the area of attention and scrub outwards in a circular fashion. This may be difficult with toes, and an alternative is to soak the digit for five minutes in the antibacterial solution. We suggest soaking rather than scrubbing for inflamed, painful digits, such as those with an ingrown nail or paronychia.

DIGITAL BLOCK PROCEDURES — Performing a digital block is relatively straightforward but requires attention to specific precautions. The basic approach is the same for both toes and fingers. Various techniques have been described for anesthetizing the digits. These include the traditional four-sided "ring block," the three-sided digital block, the finger web space block, the transthecal block, and the subcutaneous block [20,21] .

General considerations

  • Regardless of the technique chosen, advance and inject slowly to minimize pain being caused by distension of the tissues from infiltration of the fluid. Avoid using more than 3 to 4 mL of anesthetic solution.
  • Use a small gauge needle; 27 to 30 gauge is recommended.
  • Before injecting the solution, slowly retract on the plunger of the syringe to avoid injecting into one of the digital blood vessels that accompany the digital nerves.
  • While the procedure is somewhat uncomfortable, it should not cause undue pain. Excessive pain or paresthesias suggests that the needle is against or in a nerve. Withdraw 2 mm and reinject; the goal is to bathe the surrounding tissue and nerve with anesthetic, rather than injecting directly into the nerve.
  • It can take 10 to 15 minutes for the anesthetic to take complete effect. Therefore, it is recommended to wait for at least 10 minutes after injection to ensure that an adequate block will be achieved. If sensation is present after 10 minutes, wait an additional five minutes.
  • Test for anesthesia by pinching the tissues with forceps or with a needle prick. Begin testing with areas most likely to be numb (ie, proximal tissue). Always test the area to be operated upon. For nail removal, test the tip, the proximal and lateral nail folds, and the nail bed.
  • The techniques below describe bilateral blocks to achieve complete anesthesia. If anesthesia is only required on one side of a digit, the block may be limited to that side.
  • If sensation persists at the nail tip despite adequate injection, a wing block can be used. (See 'Wing block procedure' below.)

Fingers

Finger web space block (traditional digital block) — The technique is performed as follows:

  • Once the area has been prepped (see 'Skin preparation' above), place the patient's hand flat and palm-side down on a sterile drape.
  • Hold the syringe perpendicular to the finger and insert the needle into the subcutaneous tissue of the web space at the base of the finger, just distal to the MCP (metacarpal/phalangeal) joint (picture 1).
  • Inject the anesthetic into the subcutaneous dorsal tissue, and infiltrate the tissues surrounding the dorsal nerve.
  • Slowly advance the needle straight through the web space toward the palmar surface, injecting as the needle is advanced, and infiltrating the tissues surrounding the palmar nerve. Avoid pushing the needle through the palmar skin surface. One to 1.5 mL of anesthetic should be sufficient.
  • Withdraw the needle completely and repeat the procedure on the opposite side of the finger.

Transthecal block — The transthecal block (or flexor tendon sheath digital block) is performed by infusing the flexor tendon sheath with an anesthetic solution through a single injection:

  • Locate the flexor tendon by palpating at the level of the distal palmar crease. Flexing the digit can help to identify the tendon.
  • Once the area has been prepped (see 'Skin preparation' above), place the patient's hand flat and palm-side up on a sterile drape.
  • Using a 25-gauge needle, hold the syringe at a 45 degree angle and pierce the skin surface just distal to the distal palmar crease (picture 2). Advance the needle into the flexor tendon sheath.
  • Once in the sheath, slowly inject 1 to 2 mL of anesthetic solution (1 percent lidocaine, buffered if possible). The solution should flow freely into the synovial space between the tendon and sheath. If resistance is met, it is likely that the needle tip is against the tendon. Withdraw slightly, until the injection proceeds smoothly. Distend the sheath slightly with the anesthetic solution so that the medication diffuses throughout the synovial sheath.

An alternative method can be used to ensure that the needle is in the tendon sheath:

  • Insert a 25-gauge needle (not attached to a syringe) into the flexor tendon sheath in the same manner as described above.
  • Check the needle placement by having the patient flex and extend the finger. If the needle is in the tendon sheath, it should swing in a wide arc with tendon movement.
  • Once needle placement is confirmed, attach the syringe to the needle and inject the anesthetic slowly.

Subcutaneous block — The subcutaneous block is performed by injecting an anaesthetic solution just below the skin in the middle of flexor crease at the base of the finger and then massaging the solution into the tissues.

  • Locate the proximal skin crease at the base of the palmar side of the injured finger.
  • Once the area has been prepped (see 'Skin preparation' above), place the patient's hand flat and palm-side up on a sterile drape.
  • Wearing sterile gloves, pinch together the soft tissue just distal to the proximal skin crease using your nondominant hand.
  • With the dominant hand, insert a 30 gauge needle at the midpoint of the skin crease just below the skin. If a 30 gauge needle is unavailable, a 27 or 25 gauge needle may be used; smaller needles reduce pain.
  • Slowly inject 1 to 2 mL of anesthetic solution.
  • Massage the anesthetic solution into the tissues.
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Choice of technique — The theoretical advantage of a transthecal or subcutaneous block is that anesthesia of an entire digit can be achieved with a small volume of solution via a single needle stick, thus putting less direct mechanical pressure on the neurovascular structures.

Several small randomized trials comparing transthecal blocks with traditional digital blocks have found them to be equally effective, with similar onset and duration of anesthesia, but the transthecal technique is somewhat more painful and technically more difficult [21-23] . The authors of one small observational study suggest that the transthecal block works via subcutaneous administration of anaesthetic [24] . There is a theoretical risk of flexor tendon injury from the transthecal approach, although this is unlikely. According to one case series, the technique is effective in children [25] .

Several small randomized trials comparing subcutaneous blocks with both transthecal and traditional blocks have found the subcutaneous approach to be equally effective and less painful [22,26-28] . The subcutaneous approach may be less effective at providing anaesthesia over the dorsum of the proximal phalanx.

Thumb — In most instances, digital blocks used for fingers can be used to provide anesthesia for the thumb. Techniques used for the great toe are also readily adapted for use in the thumb. (See 'Great toe' below.)

Great toe — While the technique is similar for fingers and toes, the great toe is approached somewhat differently as there are nerve branches that come up the dorsal aspect of the great toe that can be missed if anesthetic is only injected laterally and medially.

The great toe can be anesthetized with a four-sided ring block or a three-sided toe block. The three-sided block is preferred to avoid the potential ischemic risk from the ring block.

According to one small randomized trial, pain can be reduced when performing a block of the great toe by using a 2-stage injection [29] . This approach involves injecting a small amount (about 0.2 mL) of anaesthetic solution along the standard injection track adjacent to the toe, waiting two minutes, and then reinserting the needle into the same site and injecting the remainder of the solution (about 0.8 mL).

Three-sided toe block

  • Once the area has been prepped (see 'Skin preparation' above), place the patient's foot flat and plantar-side down on a sterile drape. Alternatively, place the heel on the drape and stabilize the toe with the other hand.
  • Hold the syringe perpendicular to the toe and insert the needle just distal to the MTP (metatarsal/phalangeal) joint at the lateral edge of the toe (picture 3).
  • Inject anesthetic into the subcutaneous dorsal tissue.
  • Slowly advance the needle straight from the dorsal to the plantar surface, injecting as the needle is advanced (picture 4). Avoid pushing the needle through the plantar surface.
  • One to 2 mL of anesthetic should be sufficient.
  • Inject over the dorsum of the toe by partially withdrawing the needle and redirecting it across the dorsal aspect of the toe, injecting solution from the lateral to the medial aspect of the toe.
  • Inject the medial aspect of the toe by inserting the needle perpendicular to the medial aspect, entering via an area of previously anesthetized skin. Inject 1 to 2 mL from the dorsal to the plantar surface.

Four-sided ring block — This block is essentially an extension of the three-sided block, except that it requires an additional injection through the plantar surface of the toe.

  • Perform a three-sided block (see 'Three-sided toe block' above).
  • Withdraw the needle completely, and insert the needle into the medial or lateral aspect of the plantar surface of the toe, entering through a previously anesthetized area.
  • Inject 1 mL of anesthetic across the plantar surface of the toe, either going laterally to medially or medially to laterally.

Other toes

Toe digital block

  • Once the area has been prepped (see 'Skin preparation' above), place the patient's foot flat and plantar-side down on a sterile drape. Alternatively, place the heel on the drape and stabilize the toe with the other hand.
  • Holding the syringe perpendicular to the finger, insert the needle into the subcutaneous tissue of the web space at the base of the toe, just distal to the MTP (metatarsal/phalangeal) joint in the same manner as described for the finger web space block (see 'Finger web space block (traditional digital block)' above).
  • Inject the anesthetic into the subcutaneous dorsal tissue and infiltrate the tissues surrounding the dorsal nerve.
  • Slowly advance the needle straight through the web space toward the plantar surface, injecting as the needle is advanced, and infiltrating the tissues surrounding the plantar nerve. Avoid pushing the needle through the plantar skin surface. One to 1.5 mL of anesthetic should be sufficient.
  • Withdraw the needle completely and repeat the procedure on the opposite side of the toe.

Unilateral digital block — Ingrown nails sometimes occur on only one side of a toe or finger. In such cases, a unilateral digital block may be used to provide anaesthesia for removal of the affected nail segment. This involves injecting anesthetic either at the base of the digit or at the area between phalanges on either the medial or lateral side. To perform the procedure, insert the needle midway between the dorsal and volar surfaces of the digit perpendicular to the skin, pointing directly toward the phalanx. The needle tip should be near the bone (if you touch the bone, withdraw the needle 1 or 2 mm). Then inject 0.5 mL of anesthetic.

Wing block procedure — An alternative procedure for achieving anesthesia of a distal digit (as might be needed for nail surgery) is to perform a wing block [30] . A wing block is technically relatively simple and may achieve more rapid anesthesia than a digital block. It can be used for fingers or toes and, if anesthesia is only needed on one side of a digit (as for a partial nail removal), can be performed unilaterally.

  • Once the area has been prepped (see 'Skin preparation' above), place the patient's finger or toe flat and volar-side down on a sterile drape or stabilize the digit with the other hand.
  • Holding the syringe at a 45 degree angle to the plane of the table and perpendicular to the long axis of the digit, insert the needle a short distance to enter the deep intradermal tissue of the dorsum of the digit at a point approximately 3 mm proximal to an imaginary point where a linear extension of the lateral and proximal nail folds would intersect (picture 5).
  • Inject the anesthetic into the intradermal tissue, first infiltrating the proximal nail fold; the needle can be advanced to allow infiltration along the proximal nail fold. The needle is then partially withdrawn and redirected to allow infiltration of the intradermal tissue along the lateral nail fold. As the anesthetic is injected, the folds blanch and distend creating a wing-like appearance.
  • When bilateral anesthesia is desired, anesthetic must be infiltrated similarly on the opposite lateral nail fold and along the entire proximal nail fold. For small digits, it may be possible to achieve this by advancing the needle without requiring a second skin puncture.
  • If sensation persists at the nail tip, additional anesthetic may be injected into the dermis of the sensate skin using a 30 gauge needle. Anesthesia should be almost immediate.
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SUMMARY — Several safe and equally effective techniques are available to achieve a digital block. These are described in detail in the text. The least invasive technique that the clinician is comfortable with should be chosen. Regardless of the technique, pay careful attention to details such as using aseptic techniques and minimizing the volume of anesthetic that is injected. Check the adequacy of anesthesia before starting any procedure and administer additional anesthetic directly into sensate skin when necessary.

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REFERENCES

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