Interscalene Block

CLINICAL PEARLS

Nerve Localization

1. The Trace Back Method

If the nerve roots in the interscalene region are difficult to visualize, scan inferiorly in the supraclavicular region to identify the trunks/divisions of the brachial plexus (often seen as “a bunch of grapes”). Next move the transducer cephalad to trace the nerve roots more proximally in the interscalene region.

Arrowheads = nerves   SA = subclavian artery

Arrowhead = nerve   SA = subclavian artery

ASM = anterior scalene muscle   Arrowhead = nerve   MSM = middle scalene muscle   SA = subclavian artery

ASM = anterior scalene muscle   Arrowheads = nerves   MSM = middle scalene muscle   SA = subclavian artery

2. Identifying a Specific Nerve Root

One way to more accurately identify the nerve root and its corresponding level is to examine the shape of the transverse processes. The transverse processes at and above C6 cast a bony shadow that is shaped like a “U”, representing the anterior and posterior tubercles. Also, the vertebral artery is generally visualized below C6.

Sonogram showing C5 nerve root entering the neural foramen   Note the U shaped transverse process (TP) showing prominent anterior and posterior tubercles at C5.

Sonogram showing C6 nerve root entering the neural foramen   Note the U shaped transverse process (TP) showing prominent anterior and posterior tubercles at C6.

Sonogram showing C7 nerve root entering the neural foramen   Note the transverse process (TP) is less prominent (without an anterior tubercle at C7).

The vertebral artery is usually visualized below.C6.

3. The Phrenic Nerve

• Because the phrenic nerve is commonly affected during interscalene block, it may be worthwhile to identify the phrenic nerve and inject local anesthetic at a distance far away from the phrenic nerve.
• However, the small phrenic nerve (1-1.5 mm) often lies in a narrow plane between the sternocleidomastoid and anterior scalene muscles (Pictures: courtesy of Dr. Andre Ceccoli, France). Its identity can be confirmed by nerve stimulation.
• Color Doppler may be necessary to differentiate small hypoechoic vessels e.g., branches of the transverse cervical artery from the hypoechoic phrenic nerve, both lying in the same intermuscular plane.

The small hypoechoic structure in the plane between the sternocleidomastoid muscle (SCM) and the anterior scalene muscle (ASM) is suspected to be the phrenic nerve (arrowhead).   MSM = middle scalene muscle

To confirm its identity, a needle (arrows) is inserted in a medial to lateral direction to enter the intermuscular plane and approach the hypoechoic structure (arrowhead).   ASM = anterior scalene muscle   MSM = middle scalene muscle   SCM = sternocleidomastoid muscle

A small amount of fluid (indicated by the asterisk) is injected to “hydro dissect” and open up the plane. This allows clearer visualization of the small hypoechoic structure (arrowhead).   ASM = anterior scalene muscle   MSM = middle scalene muscle   SCM = sternocleidomastoid muscle

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