Interscalene Nerve Block

Updated: Oct 29, 2015
  • Author: Raymond Graber, MD; Chief Editor: Meda Raghavendra (Raghu), MD  more...
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Interscalene nerve block refers to the technique of anesthetizing the roots or trunks of the brachial plexus in the neck 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] It is not as effective for surgery that involves the C8-T1 nerve roots (ulnar nerve distribution). Although it is possible to do shoulder surgery with interscalene block alone, many practitioners prefer to use it in conjunction with a light general anesthetic.


Indications for interscalene nerve block include the following:

  • Shoulder surgery, such as rotator cuff repair, acromioplasty, hemiarthroplasty, and total shoulder replacement
  • Humerus fracture
  • Other arm surgery that does not involve the medial aspect of the forearm or hand


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


The roots of the brachial plexus are the anterior divisions of C5-C8 and T1 spinal nerves. 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 the terminal branches. See the image below.

The brachial plexus and nearby structures in the n The brachial plexus and nearby structures in the neck.

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.


Periprocedural Care


Equipment for interscalene nerve block includes the following:

  • Antiseptic solution: povidone iodine or chlorhexidine gluconate with skin swabs
  • Block tray with sterile drape, 1.5% lidocaine ampule, 3-mL syringe, and 25-G needle
  • A 22-G 40- to 50-mm insulated nerve block needle with attached injection tubing
  • Nerve stimulator
  • Ultrasound machine and transducer cover
  • Sterile gloves
  • Local anesthetic
  • Local anesthetic additives

Patient Preparation


Typical local anesthetic solutions for peripheral nerve blocks include lidocaine 1-1.5%, mepivacaine 1-1.5%, bupivacaine 0.25-0.5%, and ropivacaine 0.5%. The author typically prepares 30 mL of local anesthetic for use. Because this block is commonly performed to provide postoperative analgesia, ropivacaine and bupivacaine solutions are more commonly used. They will provide 12-24 hours of postoperative pain relief.

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.


The patient should be positioned with the back mildly elevated and head rotated away. If ultrasound is to be used, it is helpful to put 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 certain landmarks. The head first should be turned towards the contralateral side.

The sternocleidomastoid muscle is easily seen and palpated. It overlies the superior aspects of the scalene muscles. Having the patient lift his or her head 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. 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 sternocleidomastoid muscle (SCM) and interscal The sternocleidomastoid muscle (SCM) and interscalene (IS) groove.

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.

Palpating the interscalene groove. Palpating the interscalene groove.

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 generally performed with the help of an assistant. The assistant will be nonsterile and will help to monitor the patient, run the nerve stimulator, and inject the local anesthetic.

Monitoring devices such as pulse oximetry 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.

A sterile field is used. Typically, a block tray is opened up, the block needle is added to it, and local anesthetic for skin wheal is drawn up.

The patient is prepped and draped. The block needle extension tubing is passed off to an assistant, who attaches the local anesthetic mix 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 block needle is inserted perpendicularly to all planes and slightly caudal. The needle is advanced through the sheath, at which time a fascial "pop" may be felt. As one of the roots of the plexus is neared, the muscles supplied by that root will be stimulated to contract. Usually a current of 1-1.5 mA is adequate to start searching.

Once an appropriate muscle contraction is seen, 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. Some practitioners generally search for a current of less than 0.5 mA. 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.

As with all nerve blocks, injection should be slow, in increments and with frequent aspiration. If the threshold current is 0.2 mA or less, if injection pressure is high, or if the patient has a paresthesia during needle placement or injection, the needle is pulled back slightly because of concern of intraneural placement.

An acceptable motor response would be one involving the deltoid muscle or any muscle in the arm or hand. 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 between the anterior scalene and the sternocleidomastoid muscle (ie, too anterior), the phrenic nerve is frequently stimulated, causing contraction of the diaphragm (ie, hiccupping). Thus, these undesired twitches can help guide the practitioner in repositioning of the needle.

Ultrasound Guidance

The use of ultrasound can make it easier to locate the brachial plexus, especially in obese patients. It also allows practitioners to see that the local anesthetic is going where they intend it to. 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 brachial plexus (BP) and subclavian artery (SC The brachial plexus (BP) and subclavian artery (SCA).

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

The middle scalene (MS) muscle, anterior scalene ( The middle scalene (MS) muscle, anterior scalene (AS) muscle, and brachial plexus (BP).

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

The in-plane approach to ultrasound-guided intersc The in-plane approach to ultrasound-guided interscalene block.
Needle is adjacent to lateral aspect of the brachi Needle is adjacent to lateral aspect of the brachial plexus (BP). Local anesthetic (LA) is seen surrounding the plexus.

It is also possible to do an out-of-plane approach, by centering the transducer over the plexus, and advancing the needle towards the plexus. With this variation, the needle is not actually visualized, but tissue distortion helps give feedback where the needle is located. See the image below.

The out-of-plane approach to ultrasound-guided int The out-of-plane approach to ultrasound-guided interscalene block.

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. [7] 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%. [8, 9] 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. However, mild pain or aching during injection is normal. 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. [10]

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 site of the lesion.

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.

Diaphragmatic Paralysis

Phrenic nerve dysfunction occurs probably because the roots of the phrenic nerve (C3-C5) are anesthetized as they pass through the interscalene sheath. Alternatively, there may be direct spread of local anesthetic to the phrenic nerve. Clinically significant palsy may occur in up to 10% of interscalene blocks, [11] which manifests as an inability to take a deep breath or a feeling of dyspnea that develops as the block onsets.

Fluoroscopy has demonstrated palsy in up to 100% of patients having interscalene blocks. [12] 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. However, patients with severe chronic obstructive pulmonary disease or bronchospasm may not tolerate this complication.

Several strategies have been used to attempt to reduce the incidence of phrenic nerve dysfunction, including using more dilute local anesthetic solutions, lower volume of injectate (5-10 mL), and a lower site of injection (C7). These strategies have been summarized by Verelst and van Zundert. [14]


Hoarseness occurs in up to about 30% of patients. [11] 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%. [15] In rare cases, this sympathetic block has been associated with bronchospasm. [16]


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

It appears that 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). [17]

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. [18] 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.