Interscalene Nerve Block

Background

Interscalene nerve block refers to the placement of local anesthetic around the roots or trunks of the brachial plexus at the level of the C6 vertebral body between the anterior and middle scalene muscles. The procedure was first well described and popularized by Alon Winnie in 1970.[1]

Interscalene nerve block is typically performed to provide anesthesia or analgesia for surgery of the shoulder and upper arm.[2, 3, 4] The interscalene block is not effective for surgery of the hand or forearm that involves the ulnar nerve distribution of C8-T1. Although shoulder surgery can sometimes be performed under interscalene block and sedation, many practitioners prefer to use it in conjunction with general anesthesia.

Indications

Indications for interscalene nerve block include the following:

• Shoulder surgery (arthroscopic or open) including total shoulder replacement, hemiarthroplasty, rotator cuff repair, labral repair, and acromioplasty

• Humerus fracture

• Clavicle fracture

• Other arm surgery that does not involve the medial aspect of the forearm or hand

Contraindications

Contraindications for interscalene nerve block include the following:

• Patient refusal

• Infection at planned injection site

• Preexisting neurologic defects

• Local anesthetic allergy

• Coagulopathy

• Contralateral phrenic nerve dysfunction

• Severe chronic obstructive pulmonary disease

Anatomy

The five roots (anterior rami) of the brachial plexus originate from the spinal nerves of C5-T1. These pass out of the intervertebral foramina, over the superior aspects of the transverse processes, and run downward in the neck towards the first rib. These roots will join and divide several times to form trunks, divisions, cords, and then finally emerge as terminal nerve branches. See the image below.

The roots combine to form trunks as they cross the first rib. The superior trunk is formed by C5 and C6, the median trunk consists of C7, and the inferior trunk is formed from C8 and T1. During an interscalene block, the nerves are anesthetized at the root or trunk level.

The anterior and middle scalene muscles arise off the anterior and posterior tubercles of the transverse process of the cervical vertebrae and insert on the first rib. The subclavian artery and brachial plexus both cross over the first rib between these two muscles, with the artery being medial to the plexus. The roots of the plexus lie within a fascial sheath, which is formed from the fascia of the surrounding scalene muscles. The roots combine into trunks as they pass over the first rib. The plexus, artery, and fascial sheath continue down under the clavicle into the axilla.

Although the above is the classic description of the anatomy, it is well known that there are not infrequent anatomic variations. Gutton et al examined 146 different brachial plexuses with ultrasound and found the following: 36% had an intramuscular passage of a root, 8% had a C5 root ahead of the anterior scalene muscle, and 23% had an artery crossing the roots or trunks.[5] Muhly and Orebaugh used ultrasound to examine variations in vasculature anatomy around the brachial plexus in the neck.[6] Scanning revealed an arterial branch adjacent to, or passing directly through, the brachial plexus in the supraclavicular region in 86% of patients. Within the interscalene region, an artery was identified coursing in a lateral direction in 90% of cases, while a corresponding small vein, coursing medial to lateral in this area, was noted in 46% of cases.

For more information about the relevant anatomy, see Brachial Plexus Anatomy.

Equipment

Equipment for interscalene nerve block includes the following:

• Antiseptic solution: chlorhexidine gluconate

• Block tray with sterile drape, 1% lidocaine ampule, 3-mL syringe, and 25-gauge needle

• A 22-gauge 35–50-mm insulated nerve block needle (short bevel)

• Peripheral nerve stimulator

• Ultrasound machine and transducer cover

• Sterile gloves

• Local anesthetic

• Local anesthetic additives

Patient preparation

Anesthesia

Typical local anesthetic solutions for peripheral nerve blocks include ropivacaine 0.5%, bupivacaine 0.25–0.5%, and mepivacaine 1.5%. The author typically prepares 30 mL of local anesthetic for use. Since this block is commonly performed for postoperative analgesia, ropivacaine and bupivacaine solutions are commonly used and will provide 12–24 hours of postoperative pain relief.

In April 2018, the Food and Drug Administration (FDA) approved bupivacaine liposome injectable suspension (Exparel) for interscalene brachial plexus nerve block to produce postsurgical regional analgesia following shoulder surgery in adults. In a phase 3 trial (n=156), total postsurgical opioid consumption (P < 0.0001), opioid-free patients (P < 0.01), and time to first opioid rescue (P < 0.001) through 48 hours were all statistically superior to placebo. The suspension is intended for use as a nerve block to relieve pain associated with shoulder surgery for a period of 48 to 72 hours following administration.[7]

Epinephrine is frequently added to possibly prolong the duration of the block, as well as to add a marker of intravascular injection. Typical ranges of epinephrine are 1:200,000 to 1:600,000. The more dilute 1:600,000 mix may be preferred because of concerns of epinephrine-induced neural toxicity in the case of an intraneural injection.

Positioning

The patient should be positioned supine with the back mildly elevated and head rotated to the nonoperative side. If ultrasound is to be used, it is helpful to place a blanket behind the operative shoulder to elevate it off the bed.

Approach considerations

When examining a patient prior to an interscalene block, it is helpful to palpate key landmarks. The head should be turned towards the contralateral side.

The sternocleidomastoid muscle is easily visualized and palpated. It overlies the superior aspects of the scalene muscles. Asking the patient to lift his or her head off the table will help define the lateral border of the sternocleidomastoid muscle.

The anterior scalene muscle emerges from under the sternocleidomastoid muscle and runs inferiorly and laterally towards the first rib. At the level of the cricoid cartilage, the examiner should place his or her fingers under the sternocleidomastoid muscle and inferior to the external jugular vein, sliding them laterally to encounter the anterior scalene muscle.

The interscalene groove separates the anterior from the middle scalene muscle. This groove sometimes is very obvious during palpation, but frequently is subtle and more like a cleft. See the image below.

The groove generally starts at the intersection of external jugular vein and the sternocleidomastoid muscle and runs down inferiorly and laterally towards the midpoint of the clavicle. See the image below.

In patients with short and thick necks, it is helpful to know where to expect to find the groove before palpating for it.

Once the groove is palpated, one can perform several maneuvers to try to verify correct identification. During deep inspiration, the groove often is accentuated because the scalenes are accessory muscles of respiration, which tense during inspiration. Also, the groove can be followed down towards the first rib to try palpating the subclavian artery, which emerges between the scalenes. Identification of this artery in the inferior aspect of the groove ensures that the correct groove is being palpitated. However, the subclavian artery is not always palpable, either because it is too deep or because the inferior aspect of the groove is covered by the omohyoid muscle.

Verify the correct procedure side prior to performance of block.

Nerve stimulation technique

This procedure is performed with the help of an assistant who will monitor the patient, adjust the nerve stimulator, and inject the local anesthetic.

Monitoring devices such as pulse oximetry, ECG and blood pressure cuff are placed on the patient. The patient is properly positioned. The nerve stimulator ground electrode is attached to the patient via an electrocardiogram patch. Oxygen is administered via a nasal cannula. A mild level of sedation can be provided with small increments of midazolam and fentanyl.

Typically, a block needle is added to a sterile block support tray that also contains local anesthetic for skin infiltration.

The patient's skin is prepped and draped. The block needle extension tubing is passed off to an assistant, who attaches the local anesthetic solution and flushes the needle. The block needle electrical connecting cable is also passed off and attached to the nerve stimulator.

The interscalene groove is located as previously discussed, by sliding the fingers laterally from under the sternocleidomastoid muscle. The index and middle finger of the left hand remain in the groove, and a small skin wheal is made over the groove and between these fingers. Although the classic description is to enter the groove at C6, some practitioners instead choose to enter wherever they feel the groove best.

The nerve stimulator is usually set at 1.0 to 1.5mA just before entering the skin. The block needle is inserted perpendicularly to all planes and slightly caudal. The needle is slowly advanced through the sheath, at which time a fascial "pop" may be felt. As the needle approaches the roots of the plexus, the muscle supplied by that root will begin to contract. 

Once an appropriate muscle twitch is visible, the current is decreased slowly to determine the threshold (the lowest current at which stimulation still occurs). The closer the needle is to a nerve, the lower the threshold will be. Most practitioners attempt to obtain a current at 0.5 mA or less. If the threshold current obtained is higher than desired, the needle is repositioned. Once the desired response is found, the needle is stabilized and 30 mL of local anesthetic is injected.

In an effort to prevent intraneural injection, the needle should be withdrawn slightly if the threshold current is less then 0.2mA, the injection pressure is high, or if the patient reports a paresthesia during needle placement or injection. As with all nerve blocks, injection should be slow with frequent incremental aspiration every 3 to 5mL.

During the interscalene block, motor responses involving the muscles of the biceps, triceps, deltoid, or any muscle in the forearm or hand muscle would all be appropriate. If the needle is placed lateral and posterior to the middle scalene, it is possible to stimulate the accessory, dorsal scapular, and long thoracic nerves, which results in stimulation of the trapezius, rhomboids, and serratus anterior muscles, respectively. If the needle is placed too anterior to the brachial plexus, phrenic nerve stimulation will cause contraction of the diaphragm (hiccups). Understanding these undesired twitches can help the practitioner reposition the needle.

Ultrasound guidance

The use of ultrasound provides easier identification of the brachial plexus, especially in obese patients. It also allows practitioners to see if there is an appropriate spread of local anesthetic around the nerves. For example, it is possible with nerve stimulation techniques to stimulate the plexus while outside the fascial compartment; thus, the local anesthetic injection will not surround the plexus appropriately. It is possible to reduce the total volume of local anesthetic injected if adequate spread is seen, or to redirect the needle if inadequate spread is seen.

An ultrasound probe is placed in a sterile sheath. The ultrasound probe is placed above and parallel to the clavicle to locate an image of the subclavian artery and brachial plexus. See the image below.

The plexus is traced towards the head, to a point where it can be seen between the scalene muscles, but away from the subclavian artery and lung. See the image below.

With the in-plane approach, the needle enters the skin on the lateral aspect of the ultrasound transducer and is directed within the ultrasound plane towards the brachial plexus. See the images below.

An out-of-plane approach is also possible by centering the transducer over the plexus, and advancing the needle towards the plexus. With this technique, the needle is not actually visualized, however, tissue distortion combined with nerve stimulation can provide feedback where the needle is located. See the image below.

Ultrasound and nerve stimulation can be used in conjunction. Once practitioners are comfortable with ultrasound guidance, they tend to be less reliant on nerve stimulation. There are some circumstances where it is desirable not to stimulate the patient so as to reduce movement-caused pain, such as fractures and in the postoperative setting.

When using ultrasound guidance, a lower volume of local anesthetic may be used (ie, 15–25 mL). Visualization of the adequacy of the spread of local anesthetic helps to determine the volume used.

An interesting concern is what constitutes adequate ultrasound location and spread of local of local anesthetic. Spence et al compared the effectiveness of an injection between the middle scalene muscle and brachial plexus sheath (periplexus) with an injection within the brachial plexus sheath (intraplexus) in 170 patients.[8] There was no difference between the 2 groups in block onset times or block quality. After adjusting for sex, age, and volume injected, intraplexus blocks lasted a mean of 2.6 hours (16%) longer (95% confidence interval, 0.25-5.01, P =.03) than periplexus blocks.

Interscalene blocks are generally very safe. They have the same potential complications as any injection of local anesthetic (eg, infection, hematoma, allergic reaction). However, there are other side effects or complications that are more specific to the interscalene location of injection. When other nearby nerves are contacted by local anesthetic, they may become anesthetized with resultant paresis of innervated structures. This effect is transient, with duration of about the same length as the brachial plexus block. It is possible that low-volume techniques performed with ultrasound may reduce the incidence of these side effects.

Postoperative paresthesia or nerve deficit

Short-term paresthesia or dysesthesia may occur in 5–10% of patients, but most cases resolve within weeks to months. Motor deficits are exceedingly rare. By 6 months postprocedure, the incidence of continuing nerve deficits ranges from 0.02% to 0.2%.[9, 10] These cases are thought to be related to nerve trauma during needle placement or from local anesthetic being injected intraneurally.

Severe pain during injection may indicate an intraneural injection and require needle repositioning. Mild pain or aching at the site of injection is normal during injection of local anesthetic. Remember that postoperative deficits can also be secondary to surgical trauma, compression, or stretch of the plexus during the procedure, preexisting deficits, or improper positioning.[11]

The patient’s arm should be placed in a sling postoperatively and protected from trauma or malpositioning. If postoperative nerve deficits are found, a neurology consultation is appropriate. Electromyography testing and nerve conduction studies may be indicated to document the location of the nerve deficit.

Local anesthetic systemic toxicity

Local anesthetic systemic toxicity can occur either secondary to direct vascular injection or from slow absorption of the local anesthetic. Injection into either the external jugular vein or vertebral artery is possible with poor technique; this can result in seizures and unconsciousness. A test dose should always be injected, with the rest of the local anesthetic given in increments with frequent aspiration to avoid these occurrences.

Blood levels secondary to local anesthetic absorption usually peak at 30 minutes after injection. The total load of local anesthetic given must be monitored and limited to prevent toxic reactions in a given patient.

Transient diaphragmatic paralysis

In 1991, we learned that about 40 ml of local anesthetic injected at the C6 interscalene block location resulted in a close to 100% incidence of ipsilateral hemidiaphragmatic paresis.[12]   This effect is due to dysfunction of the phrenic nerve. There are two possibilities why phrenic nerve dysfunction occurs. Local anesthetic can spread up the brachial plexus sheath to the roots that form the phrenic nerve (C3-C5). Alternatively, local anesthetic can spread from the point of injection, and spread to directly contact the phrenic nerve. The phrenic nerve lies on top of the anterior scalene muscle, and at the C6 level, may only be a few centimeters away from the brachial plexus.      

Phrenic nerve dysfunction is typically diagnosed by using ultrasound to observe how a diaphragm moves. (Fluoroscopy has also been used.) Normally, a diaphragm will descend with inspiration. When paralyzed, it will ascend instead. This is because the accessory muscles of respiration are causing rib cage expansion, and the diaphragm passively goes along for the ride.

What is the clinical significance? In a study of 10 patients receiving interscalene block, forced vital capacity decreased 27%, forced expiratory volume in 1 second decreased 26%, and peak expiratory flow rate decreased by 15%.[13] Most patients have enough pulmonary reserve that unilateral diaphragmatic weakness is not clinically significant as long as they are at rest. However, clinically significant palsy may occur in up to 10% of interscalene blocks,[14]  which manifests as an inability to take a deep breath or a feeling of dyspnea that develops as the block onsets. This typically occurs in the patient with severe chronic obstructive pulmonary disease or other condition with minimal respiratory reserve. The phrenic nerve block will wear off as the local anesthetic wears off.  

When a patient complains of dyspnea, how is this managed?  First of all, we always elevate the back of the bed – the more upright the patient is, the easier it is for them to breathe. Next, supplemental oxygen is administered and titrated to a reasonable saturation level. Most patients will then require time for the block to wear off. On occasion, a patient will require assisted ventilation with noninvasive or invasive techniques.

Several strategies have been used to attempt to reduce the incidence of phrenic nerve dysfunction, including using more diluted local anesthetic solutions, lower volume of injectate (5–10 mL), and a lower site of injection (C7) or at trunk level. These strategies have been summarized by Verelst and van Zundert,[15] and El-Boghdadly et al.[16]  Another strategy is to block the main shoulder sensory nerves more distally, and thus completely avoid phrenic nerve involvment. One technique being utilized and investigated is to block both the suprascapular nerve and the axillary nerves. Dhir et al. randomized patients to recieve either interscalene block or combined suprascapular and axillary blocks for arthroscopic shoulder surgery.[17]  The patients receiving interscalene block were more comfortable in the recovery room, but the combined suprascapular/axillary group had better pain relief at 24 hrs. They concluded that for arthroscopic shoulder surgery, combined suprascapular and axillary blocks can be a clinically acceptable analgesic option with different analgesic profile compared with interscalene block.

Prolonged diaphragmatic paralysis

In 2013, Pakala et al. reported a review of 9 patients who had been referred to departmental QA because of prolonged phrenic nerve dysfunction.[18]  In the effected pateints, a relatively high volume of local anesthetic was used, and the blocks were performed with either paresthesia technique or nerve stimulator. They calculated their departmental incidence of this effect to be 1 in 2069 (or 0.048%). Seven of the 9 patients had improvement or complete resolution over a two-year period.

Kaufmann et al. reported their experience with 14 patients suffering permanent diaphragm paralysis after ISB evaluated and treated between 2009 and 2012 at a tertiary referral center for peripheral nerve injuries.[19]  Thirteen of the 14 patients underwent surgical exploration of the phrenic nerve. Surgery demonstrated adherence and entrapment of the phrenic nerve within dense scar tissue consistent with chronic inflammation. Segmental atrophy was also noted in some nerves. The phrenic nerves were dissected free of entrapment and compression. Sural nerve grafts were used to bypass areas of atrophy. With this treatment, 9 out of 13 patients had improvement or resolution.

The mechanism of this injury may be multifactorial. In some patients it may have resulted from trauma to the phrenic nerve by the needle. There is also the possibility of stretch injury to the nerve (or brachial plexus in general) during surgical procedures. Hogan has also proposed a mechanism for  the inflammatory process seen in patients—local anesthetic myotoxicity.[20]  Local anesthetics that are injected or leak into the anterior scalene muscle under the phrenic nerve could set up an inflammatory process that leads to nerve entrapment.

Hoarseness

Hoarseness occurs in up to about 30% of patients.[14] It may occur due to vasodilation of blood flow to larynx or from the spread of local anesthetic to the recurrent laryngeal nerve.

Horner syndrome

Horner syndrome consists of miosis, ptosis, and anhidrosis. In addition, other possible features include unilateral flushing of the conjunctiva and nasal congestion. This occurs secondary to blockade of the sympathetic chain in the neck, with an incidence of 50–75%.[21] In rare cases, this sympathetic block has been associated with bronchospasm.[22]

Pneumothorax

When the traditional interscalene technique is used, the incidence of pneumothorax is close to 0%. When the needle is placed at the level of C6, it should be well above the dome of the pleura. If dyspnea occurs after interscalene block, phrenic nerve palsy is statistically the more likely culprit. Dyspnea secondary to pneumothorax usually does not occur immediately but rather develops over time.

Epidural or intrathecal injection

On extremely rare occasions, local anesthetic may track back along the plexus roots into the epidural space. Intrathecal injection can occur from either needle placement through an intervertebral foramina or via injection into a dural cuff (which sometimes accompanies nerve roots out of the foramina).[23]  Franco et al. performed dissections of the brachial plexus and reported frequent macroscopic splitting of roots C6 and C7.[24]  An injection of local anesthetic between the upper fascicle of C6 and lower fascicle of C6 instead of the gap between C6 and C7 would be an intraneural injection at C6 with possible spread to the epidural space.

The effect of epidural or intrathecal injection depends on the amount of local anesthetic that reaches those locations. Effects may range from dyspnea and bilateral arm weakness to apnea and hemodynamic collapse. The treatment is supportive care, including resuscitation, until the local anesthetic effects wear off.

Miscellaneous trauma

There is a case report of spinal cord injury during interscalene block placement under general anesthesia using nerve stimulation technique.[25] It may be best to perform these blocks on patients who are awake and sedated (and not under general anesthesia), so that the patient’s ability to feel and respond to a paresthesia is intact.