FORELIMB NERVE BLOCKS
Intraoperative brachial plexus block
Blocking the nerves of the brachial plexus can be performed intraoperatively by depositing lidocaine onto the visualized nerve
or by injecting directly into the perineurium, the connective tissue sheath that surrounds a bundle of nerve fibers, three
to five minutes before transection. This technique can provide excellent postoperative analgesia after forelimb amputation.
The duration can be extended by adding bupivacaine and other adjunctive drugs such as opioids and alpha2 agonists. A similar blockade of the lumbar plexus nerves can be performed for hindlimb amputation.
Paravertebral and traditional brachial plexus blocks
For regional forelimb blockade when the nerves will not be visualized during the surgical procedure, a paravertebral technique
or a traditional brachial plexus block technique is used. Local anesthetics can be used alone or in combination with opioids,
alpha2 agonists, or both. Either brachial plexus block technique could result in blockade of the phrenic nerve (C5), and performance
of bilateral brachial plexus blocks may inadvertently paralyze the diaphragm, although this may not cause clinically significant
respiratory depression.11,12 Pneumothorax can also occur with both types of blocks. In addition, inadvertent injection into the thoracic dural sheath
could occur with the paravertebral technique, which could result in high sympathetic blockade, systemic hypotension, and severe
Paravertebral brachial plexus block. Traditional brachial plexus techniques (see next section) consistently provide analgesia distal to the elbow. Recently, the paravertebral brachial plexus block (Figure 4) has been recommended as an alternative to a traditional brachial plexus block.1,11,13 The cervical spinal nerves C6, C7, and C8 and the thoracic spinal nerve T1 are blocked as close as possible to the intervertebral
foramina, instead of in the axillary region, providing analgesia to the entire forelimb with lower volumes of local anesthetic.
This technique is best performed in anesthetized or heavily sedated patients.
Figure 4. For a paravertebral brachial plexus block, nerves C6-C8 and T1 are blocked as close to the intervertebral foramina
as possible, providing analgesia to the entire forelimb.
With the patient in lateral recumbency and using sterile technique, move the scapula caudally to expose the large transverse
process of the sixth cervical vertebra and the first rib. Block the ventral branches of C6 and C7 as they cross the dorsal
surface of the transverse process of C6 by inserting a needle dorsal to the process and directing it toward the cranial and
then caudal margins of the process. Block the ventral branches of C8 and T1 on the lateral surface of the first rib by directing
the needle to the cranial and caudal border of the dorsal part of the first rib, close to the articulation with the vertebra.
A modified technique has also been described.11 With this method, C6 and C7 are blocked as described above; however, C8 and T1 are approached by identification of the axillary
artery and costochondral junction of the first rib. Local anesthetic is deposited along the cranial margin of the first rib,
1 to 2 cm dorsal to the costochondral junction. Paravertebral techniques are technically more difficult than the traditional
brachial plexus block, particularly in obese animals.
Traditional brachial plexus block. A preoperative block using the traditional technique provides analgesia to the antebrachium, and possibly the elbow, by blocking
the radial, median, ulnar, musculocutaneous, and axillary nerves (Figure 5).1,2 This technique is best performed in heavily sedated or anesthetized animals.
Figure 5. A traditional brachial plexus block performed medial to the scapulohumeral joint provides analgesia distal to the
Place the patient in lateral recumbency, and after preparing the skin using aseptic technique, insert a spinal needle into
the axillary region, medial to and at the level of the shoulder joint, directed toward the costochondral junction and parallel
to the vertebral column. The needle's distal end should lie just caudal to the spine of the scapula. Aspirate the syringe
to avoid intravascular administration, and then inject two-thirds of the dose. Inject the remaining one-third as you slowly
withdraw the needle. Increasing the volume of local anesthetic used by diluting it with sterile saline solution up to 50%
can improve the degree of blockade by increasing the volume injected.
Radial, median, ulnar, and musculocutaneous nerve blocks
For procedures of the elbow and antebrachium, the radial, median, ulnar, and musculocutaneous nerves may be blocked proximal
to the humeral epicondyles. These nerves can often be palpated, making this technique relatively straightforward to perform.
After sterile preparation, approach the radial nerve on the lateral aspect of the distal humerus. Palpable just proximal to
the lateral epicondyle, the radial nerve is located between the brachialis muscle and the lateral head of the triceps.11 The median, ulnar, and musculocutaneous nerves are located close to one another on the medial aspect of the forelimb, proximal
to the medial epicondyle. The brachial artery is situated among these nerves. The musculocutaneous nerve is cranial to the
artery while the median and ulnar nerves are located caudally. Identify the artery with palpation, and aspirate the syringe
before depositing local anesthetic and adjunct drugs near these nerves, avoiding intra-arterial, intravenous, and intraneural
The success rate of both brachial plexus block techniques, as well as other nerve blocks, may be improved with the use of
a nerve locator, which helps to accurately locate the peripheral nerves.14-16 The nerve locator consists of a constant current generator, a grounding patient electrode, an electrode attached to an insulated
stimulating needle, and an extension set attached to a syringe for injection of local anesthetic (see boxed text "Nerve location through electrical nerve stimulation").
Nerve location through electrical nerve stimulation
The use of nerve stimulators to locate peripheral nerves is routine in human surgical procedures, and the body of literature
is extensive. Several meta-analyses of the human literature indicate that peripheral nerve blockade supplies equivalent postoperative
analgesia and an improved side effect profile compared with epidural analgesia.17,18
A recent study in dogs demonstrated the efficacy of a nerve stimulation-guided blockade of the brachial plexus for analgesia
distal to the shoulder.16 Another report described the successful use of a nerve stimulator to perform a brachial plexus block for carpal arthrodesis
in a dog.19 Subsequently, the use of a nerve stimulator to identify the brachial plexus of 20 dogs undergoing distal thoracic limb procedures
was described.20 Ten dogs received a mixture of lidocaine and bupivacaine, and 10 dogs received an equal volume of saline solution. Intraoperative
and postoperative opioid requirements were significantly lower in dogs that received the local anesthetics. The use of a nerve
stimulator to assist in the placement of an indwelling perineural catheter near the brachial plexus of a dog with severe trauma
of the distal thoracic limb has also been reported.15
A recent abstract compared the use of a nerve locator to achieve lumbar plexus blockade with bupivacaine vs. systemic administration
of methadone to dogs undergoing hindlimb orthopedic procedures.21 Intraoperative isoflurane requirements were lower in the locoregional group, and cardiovascular parameters were improved.21 The use of a nerve locator to facilitate mandibular nerve blocks in crocodilians has been described.22 A recent study described dissection of the brachial plexus, sciatic and femoral nerves, identification of relevant anatomical
landmarks, and the use of a nerve stimulator to locate these nerves in four anesthetized dogs.14 Disappointingly, the use of a nerve locator did not result in increased staining of the brachial plexus with new methylene
blue when compared with a blind technique in one study.23
The volumes of local anesthetic for use in brachial plexus, lumbar plexus, and sciatic nerve blocks, based on distribution
of an injected lidocaine and methylene blue solution, have been investigated.24 Local anesthetic volumes of 0.3 ml/kg were found to adequately stain the nerves of the brachial plexus, whereas 0.05 ml/kg
was judged appropriate for sciatic nerve block. The lumbar plexus appeared to require a volume of 0.4 ml/kg, but statistical
significance was not reached.
For a traditional brachial plexus block, when the musculocutaneous nerve is stimulated, the biceps muscle will contract, causing
flexion of the elbow.25 Injecting local anesthetic should cause the muscle movement to immediately cease.9,14-16,19,25 Refer to Table 1 for drug choices and dosages.
The use of a nerve locator for the paravertebral block has been described11 ; however, the exact muscle movements that should be elicited to achieve a successful block are still being defined. Various
motor movements may be elicited from stimulation of individual nerves within the brachial plexus and range from dorsal displacement
of the scapula to extension of the triceps.11 An in-depth review of the paravertebral blockade is provided elsewhere.11 If a hiccupping movement of the diaphragm is elicited, the phrenic nerve has been located, and the needle should be repositioned
before injection of local anesthetic.
By adopting the above techniques, you can greatly reduce your patients' pain after surgery or trauma. These techniques do
not require expensive equipment or drugs, yet can provide effective analgesia of prolonged duration, reducing the requirements
for systemic analgesics to control pain. Look for upcoming articles on maxillary and mandibular nerve blocks and epidural
anesthesia and analgesia.
The authors wish to thank Gregory Hirshoren, Instructional Resources, College of Veterinary Medicine, The University of Tennessee,
for the photos that accompany this article.
Christine Egger, DVM, MVSc, DACVA
Lydia Love, DVM
Department of Small Animal Clinical Sciences
College of Veterinary Medicine
The University of Tennessee
Knoxville, TN 37996