Interscalene Block

Anatomy

• The interscalene block targets the ventral rami of C5, C6 (often seen split into two distinct neural structures), and sometimes C7 at the upper roots of the brachial plexus (C5-T1). As the nerves exit the intervertebral foramina, they travel anterolaterally and inferiorly and pass between the anterior and middle scalene muscles.¹
• The area between the anterior and middle scalene muscles is referred to as the “interscalene groove” and is the target of the interscalene block. It is lateral to the anterior scalene muscle and medial to the middle scalene muscle.¹
• The interscalene groove is found lateral to the internal jugular vein, carotid artery, and vertebral artery.
• The nerves can form a pattern on ultrasound, colloquially referred to as the “stoplight sign.”

Patient Positioning

• The most ergonomic position for the interscalene block is semi-supine, with the ipsilateral side up and the patient’s head turned to the contralateral side.
• Other positions that can be used to achieve an interscalene block include supine and semi-lateral decubitus, with the ipsilateral side slightly raised.

Needle Positioning

Ultrasound Technique

• With the patient in the optimum position and head turned in the opposite direction, a linear high-frequency ultrasound probe is placed transversely along the neck, approximately 2–4 cm cephalad to the clavicle in the supraclavicular fossa, at the level of the cricoid cartilage.
• The target nerves appear as hypoechoic structures between the anterior and middle scalene muscles. The number of nerves seen ranges from 3 to 5, depending on the individual’s anatomy.^1,2
• Color-doppler can be used to identify and avoid vasculature in the field.
• An alternative approach to establishing the interscalene view is to start with the supraclavicular brachial plexus view and translate the probe cephalad until the plexus coalesces to form a series of hypoechoic structures between the anterior and middle scalene muscles.
• Once the interscalene view has been confirmed and the entry point has been anesthetized, a block needle can be guided to the interscalene groove using either an in-plane or an out-of-plane technique. The in-plane technique involves the needle entering laterally and traveling medially through the middle scalene muscle before exiting the middle scalene fascia until it lies adjacent to the lateral side of the interscalene groove. The out-of-plane technique can be achieved by guiding the needle between the anterior and middle scalene muscles to the interscalene groove.¹
• The needle passage through the middle scalene fascia sometimes produces a tactile or visual effect for the provider.

Nerve Stimulation Technique

• Nerve stimulation, particularly when combined with ultrasound, can help identify nerves at risk of needle trauma. The long thoracic and dorsal scapular nerves commonly pass through the middle scalene muscle and should be carefully avoided to prevent injury (Figure 3).
• With the patient in the optimum position and their head turned in the opposite direction, the posterior border of the sternocleidomastoid is palpated and followed posterolaterally to find the interscalene groove between the bellies of the anterior and middle scalene muscles. A line can be traced laterally from the cricoid cartilage to the interscalene groove, which usually corresponds to the C6 vertebrae. These reference points are typical insertion points for the interscalene block.^1-3
• After local anesthesia has been established at the intended entry point, an insulated stimulating needle with initial current settings of 1-2 mA, pulse duration of 0.1 ms, and a frequency of 2 Hz can be inserted at a slightly caudad angle and advanced in-plane from posterior to anterior. The needle is advanced until a motor response is generated in the distribution of the C5-C6 nerve roots, as demonstrated by muscular contraction of the deltoid, biceps, or brachioradialis.1 Avoid eliciting a motor response below 0.2 mA, as this may suggest intraneural needle placement and result in a posttraumatic inflammatory response.⁴

Considerations

Injectate Volume

• Typical volumes used range from 10-20 mL when performed under ultrasound guidance, and up to 40 mL for non-ultrasound-guided techniques.
• A catheter may also be placed for prolonged delivery of local anesthetics and extended analgesia.
• Hydrodissection with nonactive fluid, such as normal saline, is recommended to ensure proper needle placement and to minimize unnecessary local anesthetic loss. Negative aspiration should also be confirmed prior to injection and intermittently upon administering the local anesthetic.

Side Effects

• The phrenic nerve lies along the anterior surface of the anterior scalene muscle, deep to the sternocleidomastoid, which is routinely blocked with this technique. Ipsilateral blockade of the phrenic nerve leads to hemidiaphragmatic paralysis, which may not be tolerated in patients with pulmonary disease, obesity, or uncontrolled anxiety.¹
• Ipsilateral Horner syndrome can result from blockade of the cervical sympathetic chain, leading to ptosis, miosis, anhidrosis, conjunctival injection, and nasal congestion.¹

Complications^1-3

• Potential complications should be considered when performing the interscalene block: infection, bleeding, nerve damage (including spinal cord, spinal nerves, brachial plexus, and the long thoracic and dorsal scapular nerves as they pass through the middle scalene muscle; avoid injection against high resistance [> 15 psi] to minimize risk⁵); arterial injection (including the vertebral artery as it passes deep to the anterior scalene muscle or branches of the thyrocervical trunk); epidural or intrathecal injection (avoid advancing needle more than 2.5 cm to minimize risk); pneumothorax, medication reaction, and localized pain at the site of injection.

Technique Comparison

• When comparing ultrasound to nerve stimulation technique, ultrasound allowed for fewer needle passes and faster onset of motor blockade after injection. However, there were no clinically significant differences in block failure, patient satisfaction, or complications.²
• When comparing nerve stimulation to paresthesia-based techniques, there were no clinically significant differences in onset time, block failure, patient satisfaction, or complications.³