Digital Blocks

When it comes to regional anesthesia, digital block can be one of the most satisfying (or frustrating) techniques out there.  When done well, it renders the patient completely pain-free for the duration of a potentially pretty excruciating procedure–whether it be a simple finger laceration repair, a meticulous nail bed exploration, or even reduction of a fractured or dislocated phalanx.  When done incorrectly, it seems like a whole lot of effort with little reward–and often a pretty upset patient.

In this post I’ll describe two ways to perform a digital block, with accompanying video:  firstly, the “can’t fail” method, for beginners; then, for more advanced practitioners looking to expand their repertoire, a more elegant approach to digital blockade.

Digital anatomy

But first, a little primer/reminder of the anatomy of the digit.  This helps us visualize what we are trying to accomplish with a digital block.  At the level of the base of the finger (or toe), there are actually 4 digital nerve branches: two dorsal digital nerves, and two palmar digital nerves.  Be mindful that each of these nerves run adjacent to a digital artery branch.

Digital nerves and arteries, depicted in cross section.

Digital nerves and arteries, depicted in cross section.

The Ring Block: the “Can’t Fail” Method

Though the anatomy of the digit remains unchanged after millennia, it seems there are at least half a dozen descriptions of “correct” ways to accomplish digital block anesthesia.  After ten years of performing digital blocks, I’ve come to the conclusion that the less precise you attempt to be, and the more anesthetic you are willing to instill, the more likely your block will work.  Hence, the “can’t fail” method, as shown in the video below.

The keys to this block are:

  • The hand is initially positioned with the palm face down resting on a steady surface. The injection site is prepared with isopropyl alcohol (or other sterilizing agent per local protocols/practice standards).  The hand will need to be flipped at the end of the procedure. Application of EMLA for 45 minutes before starting (if time allows) or ethyl chloride spray just prior to the procedure (if available) can ease the pain of injection.
  • Create a ring of anesthesia around the base of the digit. Don’t concern yourself too much with instilling near the nerves, because you’ll be using enough anesthetic in such a small space it is almost impossible not to get it there.
  • I use a very generous helping of local anesthetic, sometimes as much as 6-8 cc.  My go-to anesthetic of choice is typically 0.5% bupivicaine; I prefer this longer acting anesthetic over lidocaine simply because I often don’t know how long the interval will be between performing the block and performing the procedure in a busy ED.  Its time of onset is 2-5 minutes, similar to lidocaine, so there is no downside if you plan to begin your procedure promptly.  Despite a fair amount of myth-busting evidence out there that lidocaine with epinephrine is safe for use in the digit, I don’t–not because I don’t believe the data (or lack thereof).  Rather, because there is no need.  The block performed in this fashion (almost) can’t fail.
  • The duration of action of bupivicaine is cited as being 4-8 hours, but keep in mind that when using generous volumes, this can be even longer.  This being the case, I would warn the patient that the block can sometimes last a very long time.  I once had a patient with residual numbness/parasthesia 24 hours later (which I learned from a follow up call back–it resolved by 36 hours).

Transthecal Digital Block: The “Finesse Method”

In sharp contrast to the ring block, the transthecal block is a more refined method of obtaining digital anesthesia, but also a more challenging one to master.  The technique was first described in 1990 by Dr. Chiu, a hand surgeon.  Subsequently, it was studied for use in the ED and found to be an effective technique with low rates of failure.  The premise of the technique is to inject anesthetic within the flexor tendon sheath, which then diffuses around the proximal phalanx and to each of the digital nerves.  The technique is essentially identical to that used for release of a trigger finger.

It’s most useful in the ED in situations where a ring block can’t be used.  A great example of a time when I use it is to anesthetize a finger with an actual ring causing a tourniquet-like effect and digital compartment syndrome.  This facilitates painless removal of the ring, without adding to the edema of the proximal digit.

Keys to the technique:

  • The hand is positioned with the palm face up.  The injection site is prepared with isopropyl alcohol (or other sterilizing agent per local protocols/practice standards).
  • Palpate the flexor tendon on the palmar surface just proximal to the metacarpophalangeal joint.  Gently flex the finger to help it become more obvious, if in doubt.
  • Using a 3 cc syringe attached to a 25 or 27 gauge needle, inject lidocaine at a 45 degree angle at the distal palmar skin crease, aimed towards the finger.
  • If within the tendon sheath, the anesthetic should flow freely.  If not, you may be within the tendon itself.  This can be confirmed by asking the patient to slightly flex the finger, which will produce movement of the syringe. If this is the case, withdraw the needle while maintaining pressure on the syringe, and once in the appropriate location, free-flow of anesthetic should occur.  It is normal for the patient to complain of some pressure or discomfort within the finger during injection.
  • The optimal volume of 1% lidocaine in terms of rapidity of onset, completeness, and duration appears to be 3 ml, based on a study of healthy volunteer subjects.
  • A modification of the technique, with injection in to the tendon sheath more distally between the proximal digital and PIP creases, appears to be effective in children.

Closing thoughts

Digital blockade is exceptionally useful, but takes some practice.  I recommend getting used to the ring block technique, which will be effective in most situations, before trying to master the transthecal block, which takes a bit more finesse.  Once you are facile with this basic emergency medicine technique and have your blocks working effectively, you’ll be able to approach injuries of the finger with greater confidence!

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