Genesis of brachial plexus at midhumeral level

Many studies have shown that success rate with tha axillary block is definitely related to multiple injections of local anesthetics in order to achieve a reliable spread over the main nerves in the axilla. The reasons that led to perform a brachial plexus block at the midhumeral level in 1994 are the following:

• The definition of success is variable and confusing in the litterature. Indeed surgical anesthesia remains a vague definition since some surgical procedures involve only 2 or 3 nerves and not the 4 main terminal nerves of the brachial plexus. In addition, the administration of sedatives and even supplemental local anesthesia were not considered in the assessment of success in a number of studies. Finally some authors have defined success as a complete sensory and motor block involving the 4 main nerves whatever the surgical procedure was.
• The real criteria for the evaluation of a brachial plexus block must be the loss of sensitivity to pain (and even loss of motricity) of the main four nerve trunks. With such strict criteria, the conventional axillary approach to the brachial plexus block, using a single injection technique, is effective in only 50-80 percent of cases, irrespective of the localization procedure used, paresthesia, nerve stimulation, or perception of sheath penetration.

• The multiple stimulation concept, introduced in 1987 by Sherbrooke's anesthesiologists (Canada), was attractive. This concept of multiple stimulation and multiple injections made outmoded the old concept of the sheath and septaes. The potential risk of multiple injections is to damage a non-reactive root that has already been infiltrated.

• The midhumeral block can be considered as a more distal axillary block where the four main nerves are distant enough from the vessels. Indeed, at the upper-middle third junction of the arm, these four nerves diverge progressively from the humeral artery making their selective electrical location easier. The humeral block which is very popular in France since its initial description by LJ Dupré in 1994 represents a selective location and infiltration of each of the 4 nerves through a single puncture site using a peripheral nerve stimulator.

This reflection led to carry out a first study on the brachial plexus block at midhumeral level, aiming at showing that it was possible to locate the main four brachial plexus trunks, from a single cutaneous puncture site. The initial block technique at midhumeral level has therefore been described.

Anatomy of brachial plexus at midhumeral level

The humeral channel contains the humeral artery and both its satellite veins, as well as the median nerve, ulnar nerve, and medial cutaneous nerve of the forearm. In the upper part of the channel, the median nerve is located on the antero-lateral side of the artery and the ulnar nerve on the medial and slightly posterior side of the artery, whereas the medial cutaneous nerve of the forearm is located on the medial side of the artery, in front of the internal humeral vein. In the lower part, the internal intermuscular septum is interposed between the median nerve at the front and ulnar nerve at the back whereas the medial cutaneous nerve of the forearm exits the tunnel with the basilic vein.

The radial and musculocutaneous (lateral cutaneous nerve of the forearm) nerves do not form part of the humeral channel. The musculocutaneous nerve comes along the coracobrachialis muscle and then descends the fossa between the biceps brachii and anterior brachialis muscles. The radial nerve is located posterior to the artery, close to the humeral bone.

Equipment

• Skin pencil,
• Nerve stimulator; short-bevelled, insulated needle (50 mm),
• Two 20-ml syringes filled with the local anaesthetic chosen.

Anaesthetics

A total volume of 40 ml is largely sufficient. In elderly patients, 20ml have been shown to be effective for surgery.
The choice of local anesthetic should be tailored to the patient’s needs. Long-acting drugs are used when postoperative pain is expected to last 8- 24 hours (fractures, bone surgery, extensive surgery …).

• 1.5% lidocaine with epinephrine is used for surgery. With this solution, observed plasma concentrations (3.4 ± 0.5 µg/ml) are comparable to those obtained following an axillary block (46), far from the toxic threshold values.
• 1.5% mepivacaine can also be used, particularly to avoid the use of epinephrine. Duration of the motor block with lidocaine containing epinephrine is 3-5 hours. Addition of clonidine to lidocaine prolongs block duration, providing 5-7 hours of sensory block.
• 0.5% bupivacaine combined with 2% lidocaine provides a prolonged sensory block of 10-16 hours.

However, Combining local anaesthetics, which is commonly used in regional anaesthesia, is not justified since blood toxicity is additive in such a setting.

Nevertheless, the selective location and infiltration of these 4 nerves and the relative distance between them (when compared to the axillary approach) can be used to administer different solutions on each of these nerves for some specific indications particularly in ambulatory surgery. The objective is to provide a short blockade of the musculocutaneous territory (short acting local anesthtetic) in order to achieve a fast recovery of arm motion and a prolonged blockade of the other territories (long acting local anesthetic) in order to benefit from longer postoperative analgesia.

Indications

Anaesthesia

All distal surgery of the upper limb: soft tissue surgery of the elbow, forearm surgery, wrist and hand surgery, can be performed with a midhumeral block.

Analgesia

Long-acting local anesthetics (bupivacaine, ropivacaine or levobupivacaine) easily provides analgesic blocks lasting 12-24 hours after a single shot humeral block. However, this block is not suitable for catheter placement, in view of the dispersion of the nerves at this level.

Contra-indications

Irrespective of the efficacy and reliability of the brachial plexus block at the midhumeral level, some contra-indications must be kept in mind:

Contra-indications common to regional anaesthesia of the upper limb:

• Patient's refusal,
• Known allergy to local anaesthetics,
• Dyscrasia,
• Evolutive demyelinating diseases.

Contra-indications specific to the brachial plexus block at midhumeral level:

• Unavailability of nerve stimulators
• Arm lymphangitis is a formal contra-indication since lymphatic drainage of the arm goes though the humeral channel. However, peripheral infections are not a contra-indication to this block.
• History of axillary lumpectomy (risk of lymphoedema).
• In Trauma surgery, when a large haematoma at the elbow level spreads to the arm, acute pain making upper limb mobilization impossible and extension of the haematoma, (modifying anatomical landmarks and hindering stimulation), may prevent the performance of the block.

Block performance

The patient lies in the supine position, the arm abducted at 80° and the forearm extended.

LANDMARKS

At the junction between the upper and middle third of the arm, the humeral pulse can be felt and a line drawn over the humeral artery using a skin pencil. If the humeral pulse is not perceptible, the artery is drawn by a line joining the top of the axillary fossa and middle of the cubital folds.

Standard safety conditions (venous access as well as available anaesthesia and resuscitation equipment), the cutaneous region concerned is prepared according to usual aseptic procedures.

For each of these 4 mixed nerves location, the initial stimulating current is set to 1.5- 2 mAmp. Once the corresponding motor response is found, the current intensity is decreased progressively until a good motor response is remains present with the lowest intensity (0.3- 0.5 mAmp). At this stage, a 1 ml injection test should lead to a complete disappearance of the motor response which confirms the proximity of the needle tip to the nerve. Then, the injection of the remaining dose should be done incrementally with frequent aspiration tests.

1- Infiltration of the median nerve. The median nerve is quite superficial and lies antero-superior to the artery. The needle is inserted almost tangentially to the skin, parallel to and slightly lateral to the brachial artery, until perforation of the fascia. Stimulation of the median nerve is characterized by flexion of the wrist and pronation of the forearm (contraction of flexor carpi radialis muscle). Before injecting, remove the palpating hand and release the needle: if the motor response disappears, it means that the needle is still superficial to the sheath and thus the operator should advance and perforate the fascia to get closer to the median nerve which is situated deep to the fascia. A local anesthetic solution (6- 10 ml) is then slowly injected after careful aspiration test.

2- Infiltration of the ulnar nerve. Afetr withdrawing the needle to the subcutaneous tissue, the needle is now redirected medially to the humeral artery toward the ulnar nerve to look for contraction of flexor carpi ulnaris muscle. This nerve is located inferior to the artery. Successful stimulation results in flexion of the 4th and 5th fingers and adduction of the thumb. Hereagain, a 5- 10 ml solution is injected.
At this level, the risk of vascular injection is greater since the ulnar nerve is surrounded by the humeral vein, basilic vein and its branches. In most cases, the medial cutaneous nerve of the forearm has not yet perforated the overlapping aponeurosis and is simultaneously anaesthetized by this injection.

3- Infiltration of the radial nerve. The radial nerve lies posterior to the humeral artery at this level, near the shaft of the humerus bone and triceps muscle. Once withdrawn again to the subcutaneous tissue, the needle is directed posterior to the artery toward the humeral bone. The needle passes beneath the artery, towards the lower border the humerus, and locates the radial nerve (contraction of extensor muscles of fingers and/or wrist). Successful stimulation induces extension of the wrist, fingers and thumb.
Hereagain, 8 to 10 ml are injected.

4- Infiltration of the musculocutaneous nerve. Using the same puncture site, the needle is withdrawn almost to the skin, directed toward the coracobrachialis muscle and looking for the musculocutaneous nerve by advancing the needle beneath the biceps muscle (contraction of the biceps brachii muscle. Correct needle placement is confirmed by flexion of the elbow. An additional volume is injected (5- 10 ml) slowly.

5- Infiltration of median cutaneous nerves. These nerves are sensory and do not require stimulation. Subcutaneous infiltration of 2-3 ml of local anaesthetis medially to the artery is sufficient to block the terminal branches of the medial cutaneous nerves of the arm and forearm, if they have already perforated the fascia brachialis.

6- Infiltration of the terminal branches of the axillary nerve. 3 to 4 ml of local anaesthetics are injected subcutaneously and transversally, on the posterior surface of the arm, at the junction of the upper and middle thirds of the arm. This infiltration, which requires another puncture site, is only performed for elbow surgery when the incision, or drip hole, is located behind the elbow.

RESPONSES TO STIMULATION

Block quality depends on the quality of evaluation of motor response to stimulation.

Satisfactory responses can be obtained, according to the nerve to be blocked and to the needle position in relation to the humeral artery, are presented in the table below.

Differentiation between median nerve and ulnar nerve responses can be sometimes confusing when watching flexion of the fingers. Indeed, assessment should be based on palpation of the tendons of flexor carpi radialis and flexor carpi ulnaris muscles for the median and ulnar nerves respectively.

Martin-Gruber anastomosis, corresponding to very frequent anastomosis at the level of the arm and forearm, between the median and ulnar nerves. Anastomotic fibers run parallel to the median nerve and then ulnar nerve and end in the median or ulnar territories. Irrespective of the type of anastomosis, it never involves the flexor carpi ulnaris muscle, which represents the reference response for ulnar nerve stimulation.

Extension of the elbow may be observed following direct stimulation of the triceps brachii muscle while locating the radial nerve. Evidence of radial nerve location is only given by the active contraction of extensor muscles of the wrist or hand.

Complications

• Blood toxicity has been reported during the performance of a brachial plexus block at midhumeral level. Important veins and artery may expose to the risk of accidental intravascular injection. This complication can be avoided by frequent aspiration and slow incremental injection of local anesthetics.

• Risk of damaging a non-reactive nerve which has already been infiltrated in the area. This potential complication is mainly theoretical consdiering that the onset of anaesthesia is longer than block performance time.

• Small haematomas at the puncture site may be observed up to 24 hours postoperatively (5% of patients) and last few days.
  

Tips

1. Efficacy:

• The success rate, defined as the patient's operability without using general anaesthesia or complementary analgesia by IV route, is usually between 92- 97%.  A large study inclusing 1468 patients and performed on training physicians has shown that the observed failure rate was 5%.
• Total disappearance of sensitivity and motricity of the four nerve territories (median, ulnar, radial and lateral cutaneous) was observed in more than 95% of cases.
• Only one study compared the conventional axillary block to the midhumeral block. The success rate, defined as the loss of sensitivity to touch on each nerve territory, was significantly greater with the midhumeral block (80% vs 54%). In addition, a motor block was more frequently achieved (90% vs 70%).

2. Duration of block performance:

• As opposed to the classic axillary block, the brachial plexus block at midhumeral level requires locating the four main nerves of the upper limb, which explains a longer period for block performance. Practically speaking, the duration of block performance is not greater than 11 minutes and is not significantly different from the duration of the axillary block performance. Although this method is time-consuming during the learning phase, time performance rapidly decreases with experience.

3. Onset time:

• The time of onset for a complete block using 1.5% lidocaine with epinephrine is included between 11 and 25 minutes, depending on studies.

4. The advantage

• The advanatge of the humeral block remains its efficacy. The success rate is greater than 95%, with a constant motor block. The quality of anaesthesia provides total tourniquet tolerance, too. Selective blocks can also be performed depending on surgical needs. Although block performance appears to be complex, it is easy to learn and allows to quickly achieve a notable success rate. All four nerves can be approached from a single injection site. The usual safety precautions when performing regional anesthesia also apply with the humeral block.