Interscalene Nerve Block: Comprehensive Guide to Anatomy, Technique, & Complications

Interscalene Nerve Block: Common Complications & How to Prepare

Introduction & Epidemiology

The interscalene nerve block (ISNB) is a foundational regional anesthetic technique widely employed in orthopedic surgery for procedures involving the shoulder and proximal humerus. It provides profound analgesia and anesthesia to the C5-C7 dermatomes, encompassing the shoulder girdle and often extending to the lateral arm, making it invaluable for both intraoperative management and postoperative pain control. Its efficacy in reducing opioid consumption, minimizing opioid-related side effects, and facilitating earlier rehabilitation has cemented its role in enhanced recovery after surgery (ERAS) protocols for shoulder procedures.

Historically, ISNBs were performed using paresthesia or nerve stimulator techniques. However, the advent and widespread adoption of ultrasound guidance have revolutionized the safety and precision of this block. Ultrasound allows for real-time visualization of the brachial plexus roots and trunks within the interscalene groove, as well as surrounding critical structures such as the phrenic nerve, vertebral artery, and pleura. This has led to a significant reduction in the incidence of severe complications, although minor and expected side effects remain prevalent.

Epidemiologically, ISNB is one of the most frequently performed regional blocks in upper extremity orthopedic surgery. Success rates for achieving adequate surgical anesthesia and analgesia are consistently high, often exceeding 95% with ultrasound guidance. While highly effective, the anatomical proximity of several vital structures to the brachial plexus in the interscalene region means that collateral spread of local anesthetic and unintended blockade are common. A thorough understanding of these expected side effects, as well as rarer, more serious complications, is paramount for safe and effective practice by orthopedic surgeons, residents, and medical students. This review aims to detail the essential anatomical considerations, indications, technique, and management of complications associated with the interscalene nerve block.

Surgical Anatomy & Biomechanics

A comprehensive understanding of the anatomy of the neck and brachial plexus is critical for safe and effective ISNB performance. The brachial plexus is formed by the ventral rami of spinal nerves C5 to T1, with occasional contributions from C4 and T2. These roots emerge between the anterior and middle scalene muscles, forming the interscalene groove.

Key Anatomical Structures:

• Scalene Muscles:

  • Anterior Scalene: Originates from the transverse processes of C3-C6 and inserts onto the first rib. Its anterior surface is traversed by the phrenic nerve.
  • Middle Scalene: Originates from the transverse processes of C2-C7 and inserts onto the first rib, posterior to the anterior scalene.
  • Posterior Scalene: Originates from the transverse processes of C4-C6 and inserts onto the second rib. This muscle is less directly relevant to the ISNB given the typical target zone.
  • The interscalene groove is the fascial plane located between the anterior and middle scalene muscles, through which the roots/trunks of the brachial plexus pass.

• Brachial Plexus:

  • At the level of the interscalene groove (typically C6-C7), the brachial plexus consists primarily of the roots (C5, C6, C7, C8, T1) and their amalgamation into superior, middle, and inferior trunks. Ultrasound visualization often reveals hypoechoic, round or ovoid structures arranged in a "stack of coins" or "stoplight" appearance within the interscalene groove, typically corresponding to the C5, C6, and C7 roots/trunks from superior to inferior.
  • The primary targets for ISNB are typically the C5 and C6 roots/trunks, which predominantly innervate the shoulder joint and muscles, explaining the block's efficacy for shoulder surgery.

• Associated Neural Structures:

  • Phrenic Nerve: This nerve, formed from C3-C5 (primarily C4), lies directly on the anterior surface of the anterior scalene muscle, often superficial to the prevertebral fascia. Due to its proximity and superficial location, it is almost invariably affected by local anesthetic spread during an ISNB.
  • Cervical Sympathetic Chain (Stellate Ganglion): Located anterior to the C7 transverse process, deep to the prevertebral fascia, and medial to the longus colli muscle. Local anesthetic spread medially can block the stellate ganglion, leading to Horner's syndrome.
  • Recurrent Laryngeal Nerve (RLN): While primarily a vagal nerve branch, local anesthetic spread can occasionally affect it, particularly if volumes are large or injection is more medial, leading to hoarseness.

• Vascular Structures:

  • Vertebral Artery: Deep and medial to the anterior scalene muscle, within the transverse foramen of the cervical vertebrae. Puncture is a severe complication.
  • Subclavian Artery: Lies inferior to the brachial plexus at the base of the neck, but its transverse view can be mistaken for the plexus if the probe is too low.
  • Inferior Thyroid Artery: A branch of the thyrocervical trunk, it can cross the field and is often seen with ultrasound.
  • External Jugular Vein: Superficial, often visualized anterolateral to the SCM.
  • Internal Jugular Vein: Deeper, medial to the SCM.

• Other Structures:

  • Pleura (Apex of the Lung): The apex of the lung rises to the root of the neck, immediately inferior and deep to the inferior aspects of the brachial plexus. Puncture risk increases with lower needle insertion points or excessive depth.
  • Thyroid Gland: Medial to the SCM.
  • Esophagus/Trachea: Medial to the neurovascular bundle.

Biomechanics of Local Anesthetic Spread:

The effectiveness of the ISNB relies on the diffusion of local anesthetic solution around the brachial plexus within the interscalene fascial sheath.
* Differential Blockade: The roots and trunks within the interscalene groove are anatomically organized, with C5-C6 roots typically superior and C7-C8-T1 more inferior. A standard ISNB, aiming for the C5/C6 level, reliably blocks the superior and middle trunks, providing excellent anesthesia for the shoulder and lateral arm (deltoid, biceps, brachialis). The inferior trunk (C8-T1) is often spared or incompletely blocked due to the cranial-caudal spread limitations, which means motor function of the hand (ulnar nerve distribution) and sensation in the medial forearm/hand might be preserved. This differential block is often desirable as it allows for hand function post-operatively while still providing profound shoulder analgesia.
* Local Anesthetic Volume: The volume of local anesthetic directly correlates with the extent of spread. Larger volumes increase the likelihood of blocking adjacent nerves (phrenic, stellate ganglion, recurrent laryngeal) and potentially extending the block to the inferior trunk. Ultrasound guidance allows for titration of volume to achieve the desired spread with minimal excess.
* Adjuncts: Adjuvants like dexamethasone, clonidine, and epinephrine are often co-administered with local anesthetics. Dexamethasone is commonly used to prolong the duration of analgesia. Epinephrine serves as a vasoconstrictor to prolong block duration and as a marker for intravascular injection (though less reliable with ultrasound guidance). Clonidine, an alpha-2 adrenergic agonist, can also prolong block duration.

Indications & Contraindications

The interscalene nerve block is a versatile tool in orthopedic regional anesthesia, primarily indicated for surgical procedures of the shoulder and proximal humerus. Its application spans both intraoperative anesthesia and postoperative analgesia, contributing significantly to patient comfort and recovery.

Indications

Contraindications

Pre-Operative Planning & Patient Positioning

Meticulous pre-operative planning and appropriate patient positioning are fundamental to the safe and effective performance of an interscalene nerve block.

1. Pre-operative Assessment & Consent:

• Comprehensive Patient Evaluation: A thorough medical history, physical examination, and review of relevant investigations are essential. Special attention should be paid to:
  • Cardiopulmonary status: Assess for baseline respiratory compromise (COPD, asthma, sleep apnea), as even a transient ipsilateral phrenic nerve block can significantly impair respiratory function. Inquire about previous lung surgeries or conditions that might affect contralateral lung function.
  • Neurological status: Document any pre-existing neurological deficits in the ipsilateral upper extremity, including motor weakness, sensory changes, or neuropathies. This baseline assessment is crucial for differentiating pre-existing conditions from block-related complications.
  • Coagulation status: Review current medications, especially anticoagulants (e.g., warfarin, DOACs, antiplatelet agents). Discuss the timing of discontinuing these agents in accordance with institutional guidelines and ASRA recommendations to minimize hematoma risk.
  • Allergies: Specifically inquire about allergies to local anesthetics, iodine, chlorhexidine, or any components of the block kit.
  • Body habitus: Significant obesity or extremely muscular necks can make ultrasound visualization challenging.
• Informed Consent: A detailed discussion with the patient is paramount. This should cover:
  • Benefits: Effective pain control, reduced opioid requirement, improved postoperative recovery, earlier rehabilitation.
  • Risks: Common expected side effects (ipsilateral phrenic nerve block/dyspnea, Horner's syndrome, hoarseness), rare but serious complications (nerve injury, LAST, pneumothorax, vascular puncture, infection), and the possibility of block failure.
  • Alternatives: Systemic analgesia, other regional techniques (e.g., supraclavicular block for more distal coverage).
  • Expected duration: Discuss the anticipated duration of sensory and motor blockade and how to manage the limb during this period.

2. Equipment Preparation:

• Ultrasound Machine: High-frequency (e.g., 10-15 MHz) linear array transducer with appropriate sterile cover and gel.
• Local Anesthetics:
  • Skin wheal: 1-2 mL of 1% lidocaine.
  • Block solution: Typically 15-25 mL of an intermediate-to-long-acting local anesthetic (e.g., 0.25-0.5% ropivacaine, 0.25-0.5% bupivacaine, 0.25% levobupivacaine).
  • Adjuncts (optional): Dexamethasone (4-8 mg), clonidine (50-100 mcg), or epinephrine (1:200,000 to 1:400,000) for prolonged block duration, depending on institutional protocol and patient factors.
• Needle: 20-22 gauge, 5-10 cm echogenic nerve block needle with short bevel.
• Sterile Tray:
  • Skin preparation solution (e.g., chlorhexidine gluconate 2% with 70% isopropyl alcohol, or povidone-iodine).
  • Sterile drapes, gauze.
  • Multiple 10 mL syringes.
  • Appropriate sharps disposal.
• Resuscitation Equipment (immediately available):
  • Oxygen source and delivery devices.
  • Suction apparatus.
  • Intravenous access (secured prior to block).
  • Monitoring equipment (ECG, NIBP, SpO2).
  • Lipid Emulsion (20% Intralipid®): The primary antidote for local anesthetic systemic toxicity (LAST), immediately available and clearly labeled.
  • Emergency medications (e.g., ephedrine, phenylephrine, atropine, succinylcholine, benzodiazepines, ACLS medications).
  • Defibrillator.

3. Patient Positioning:

• Supine Position: The patient lies flat on their back.
• Head Position: The head is turned approximately 45 degrees away from the side to be blocked, exposing the lateral aspect of the neck.
• Slight Head Elevation (optional): A small pillow or headrest may be placed under the head to improve comfort and slightly relax the sternocleidomastoid (SCM) muscle, facilitating palpation and ultrasound visualization.
• Arm Position: The ipsilateral arm should be comfortably placed by the patient's side, often slightly abducted, allowing for easy access for the operator and for observation of any motor response if a nerve stimulator is used (though less common with primary ultrasound guidance).
• Ergonomics: Ensure the operator has a comfortable working height and clear line of sight to the ultrasound screen, needle tip, and patient. An assistant should be available to help with positioning, handing equipment, and monitoring.

Detailed Block Technique

The interscalene nerve block is almost universally performed under ultrasound guidance today, enhancing both safety and efficacy compared to landmark or nerve stimulator techniques alone. The detailed technique outlined below focuses on the ultrasound-guided approach.

1. Initial Ultrasound Scan and Landmark Identification:

• Probe Selection: A high-frequency (10-15 MHz) linear array transducer is preferred for its superficial resolution.
• Probe Placement: Place the ultrasound probe transversely (short axis view) at the level of the cricoid cartilage (approximately C6). This initial placement is often just lateral to the sternocleidomastoid (SCM) muscle.
• Scan for Key Structures:
  • Sternocleidomastoid (SCM): Identify the SCM, a large, typically superficial muscle that can be followed superiorly towards the mastoid process and inferiorly to the clavicle.
  • Anterior Scalene Muscle: Move the probe laterally until the SCM rolls off the screen. The anterior scalene muscle will be visualized as a triangular or trapezoidal hypoechoic structure, superficial to the deeper longus colli muscle.
  • Middle Scalene Muscle: Continue moving the probe laterally. The middle scalene muscle will appear posterior to the anterior scalene.
  • Interscalene Groove: The space between the anterior and middle scalene muscles constitutes the interscalene groove. Within this groove, locate the brachial plexus roots/trunks . They typically appear as multiple hypoechoic, round or oval structures, often described as a "stack of coins" or "stoplight" appearance. At the C6 level, you will typically see the C5 and C6 roots/trunks superiorly, and sometimes the C7 root inferiorly.
  • Cervical Transverse Processes: Deep to the brachial plexus and scalene muscles, identify the hyperechoic transverse processes of the cervical vertebrae. These have characteristic anterior and posterior tubercles (e.g., C6 has a prominent anterior tubercle, C7 often lacks an anterior tubercle).
  • Phrenic Nerve: While often difficult to visualize distinctly, be aware of its expected location, lying superficially on the anterior surface of the anterior scalene muscle.
  • Vascular Structures: Identify the vertebral artery (deep, medial, pulsatile with color Doppler), and any other prominent vessels in the scanning plane (e.g., inferior thyroid artery, jugular veins). Always use color Doppler to rule out vascular structures in the needle path.

2. Needle Path and Approach:

• Skin Preparation: After identifying the target, prepare the skin meticulously with an antiseptic solution (e.g., chlorhexidine) and apply sterile drapes.
• Skin Infiltration: Infiltrate a small amount of 1% lidocaine subcutaneously at the intended needle insertion site.
• Needle Insertion:
  • In-Plane Approach (preferred): Insert the nerve block needle (e.g., 22G, 5-10cm) from the lateral aspect of the neck, aiming towards the interscalene groove. This allows for continuous visualization of the entire needle shaft and tip as it advances.
  • Out-of-Plane Approach (less common for ISNB): Insert the needle perpendicular to the probe. Only the needle tip is intermittently visualized, requiring more experience and careful "walking" of the probe along the needle path.
• Needle Advancement: Advance the needle slowly and deliberately, under continuous real-time ultrasound guidance. The goal is to position the needle tip adjacent to the brachial plexus roots/trunks within the interscalene groove, typically between the C5 and C6 roots. Avoid direct contact with the nerve or intraneural injection, which can be identified by nerve swelling, increased resistance to injection, or patient discomfort.

3. Injection Procedure:

• Aspiration: Before any injection, aspirate for blood or cerebrospinal fluid (CSF). Repeat aspiration frequently throughout the injection. Even with ultrasound, vascular structures may be inadvertently punctured, so vigilance is key.
• Test Dose (optional but recommended): Inject a small test dose (e.g., 1-2 mL of local anesthetic with epinephrine 1:200,000) while observing for signs of intravascular injection (tachycardia, hypertension, or subjective symptoms). While less sensitive with modern blocks, it can still provide an early warning.
• Slow, Fractionated Injection: Inject the local anesthetic solution in 3-5 mL aliquots, withdrawing the needle slightly and re-aspirating between each aliquot.
  • Observe Spread: Continuously monitor the ultrasound screen for the characteristic circumferential spread of local anesthetic around the brachial plexus structures within the interscalene groove. The nerves should be seen separating or "floating" in the anechoic local anesthetic solution. Adjust the needle position slightly if spread is not optimal (e.g., if localized to only one side of a nerve or if the anesthetic is tracking into muscle).
  • Total Volume: Typically, 15-25 mL of local anesthetic is sufficient for a single-shot ISNB. The exact volume depends on the patient's size, desired spread, and type of local anesthetic.
• Post-Injection Assessment: After the full volume is injected, reassess the anatomical structures on ultrasound to confirm no inadvertent vascular puncture or expanding hematoma.

4. Block Confirmation:

• Sensory Blockade: Assess sensory loss in the dermatomes typically supplied by the C5-C7 roots. Pinprick sensation in the deltoid region (C5), lateral arm (C5-C6), and possibly the lateral forearm/thumb (C6) should be diminished or absent.
• Motor Blockade: Evaluate motor weakness. The patient should have difficulty abducting the shoulder (deltoid - C5-C6), flexing the elbow (biceps - C5-C6), and externally rotating the shoulder (infraspinatus/teres minor - C5-C6). The hand and forearm motor function (C8-T1) may be partially or completely spared, depending on the caudal spread of the block.
• Onset Time: Sensory and motor blockade usually become evident within 5-15 minutes, reaching full effect by 20-30 minutes.

5. Continuous Interscalene Catheter Placement (for prolonged analgesia):

• For continuous analgesia, a catheter can be placed. The initial steps are similar to a single-shot block, but a larger gauge introducer needle (e.g., 18G) is used to facilitate catheter insertion.
• Position the needle tip in the desired location within the interscalene groove, typically slightly superior and posterior to the brachial plexus to avoid direct nerve contact.
• Insert the catheter (e.g., 20G) 3-5 cm beyond the needle tip.
• Remove the needle while carefully securing the catheter.
• Confirm appropriate catheter placement by injecting a small bolus of local anesthetic through the catheter and observing its spread around the plexus with ultrasound.
• Secure the catheter to the skin using sterile dressings and adhesive strips. Connect it to an infusion pump for continuous local anesthetic delivery (e.g., 0.1-0.2% ropivacaine at 4-10 mL/hr, often with patient-controlled boluses).

Complications & Management

While the interscalene nerve block is highly effective, it is associated with a range of potential complications, some of which are nearly ubiquitous, while others are rare but potentially severe. Understanding their incidence, presentation, pathophysiology, and management strategies is critical for orthopedic surgeons and anesthesiologists.

Table of Common Complications, Incidence, and Salvage Strategies

| Complication | Incidence | Clinical Presentation | Pathophysiology | Management/Salvage Strategies
| Interruption of Diaphragmatic Function (Phrenic Nerve Block) | 70-100% | Unilateral diaphragmatic paresis, subjective dyspnea, reduced SpO2 in patients with poor pulmonary reserve. Resolves within several hours. | Local anesthetic (LA) spreads from the interscalene groove to block the phrenic nerve (C3-C5), which lies on the anterior surface of the anterior scalene muscle. | Prevention: Use ultrasound guidance to visualize the phrenic nerve and anterior scalene muscle, minimize LA volume (e.g., 10-15 mL), ensure correct needle tip placement away from the phrenic nerve. Perform test for phrenic nerve function (e.g., sniff test with fluoroscopy) if concern for contralateral palsy exists. Management: Reassurance, supplemental oxygen if SpO2 drops, semi-Fowler position. Avoid in patients with severe respiratory disease or known contralateral phrenic nerve palsy. |
| Horner's Syndrome | 75-85% | Ptosis (drooping eyelid), miosis (constricted pupil), anhydrosis (absence of sweating on ipsilateral face), nasal congestion. Resolves within several hours. | LA spreads to the stellate ganglion (cervicothoracic sympathetic ganglion), located anterior to the C7 transverse process. | Prevention: Use minimal effective LA volume, avoid injecting excessively medially. Management: Reassurance, resolves with block. Inform patient prior to block. |
| Recurrent Laryngeal Nerve (RLN) Block | <1% (up to 10% with large volumes) | Hoarseness, dysphagia (difficulty swallowing), vocal cord paralysis. Resolves with block. | Medial spread of LA to the recurrent laryngeal nerve, a branch of the vagus nerve that supplies most intrinsic laryngeal muscles. | Prevention: Limit LA volume, avoid excessively medial needle placement. Management: Reassurance, monitor airway. Resolves with block. Consider avoiding in patients reliant on vocal cord function (e.g., professional singers, specific occupations). |
| ** |
| |
| |
| ** |
| ** |
| |
| Prevention | Prevent from affecting the lung: use proper ultrasound guidance to visualize the lung apex and verify needle tip placement away from the pleura. Patients with COPD or emphysema are at higher risk of pneumothorax. |
| |
| ** |
| ** |
| |
| ** |
| ** |
| Prevention: | Use optimal volume of local anesthetic, ensure optimal ultrasound visualization to direct injection into the interscalene groove and avoid extravasation. |
| |
| |
| |
| |
| Prevention: | Use minimal Effective Volume |
| |
| Prevention: | Use optimal volume of local anesthetic, careful placement to avoid adjacent structures. Pre-operative assessment for baseline neurological deficits. | Management: Remove catheter if placed, neurological assessment, consider pain specialists if persistent. Note: Neuropraxia often resolves spontaneously. |
| | |
| |
| |
| |
| Prevention: | Careful placement, minimal volume, avoid direct contact with nerves. Consider avoiding if previous neurological injury present. | Management: Remove any catheter, discontinue block, treat with analgesics, physio. Neurological consultation if persistent. |
| Prevention: | Correct location, minimal volume, avoid direct nerve injury. Report if it is detected. |
| |
|