Continuous Interscalene Brachial Plexus Blocks: An Anatomical Challenge between Scylla and Charybdis?

Abstract

Brachial plexus blocks at the interscalene level are frequently chosen by physicians and recommended by textbooks for providing regional anesthesia and analgesia to patients scheduled for shoulder surgery. Published data concerning interscalene single-injection or continuous brachial plexus blocks report good analgesic effects. The principle of interscalene catheters is to extend analgesia beyond the duration of the local anesthetic's effect through continuous infusion, as opposed to a single injection. However, in addition to the recognized beneficial effects of interscalene blocks, whether administered as a single injection or through a catheter, there have been reports of consequences ranging from minor side effects to severe, life-threatening complications. Both can be simply explained by direct mispuncture, as well as undesired local anesthetic spread or misplaced catheters. In particular, catheters pose a high risk when advanced or placed uncontrollably, a fact confirmed by reports of fatal outcomes. Secondary catheter dislocations explain side effects or loss of effectiveness that may occur hours or days after the initial correct function has been observed. From an anatomical and physiological perspective, this appears logical: the catheter tip must be placed near the plexus in an anatomically tight and confined space. Thus, the catheter's position may be altered with the movement of the neck or shoulder, e.g., during physiotherapy. The safe use of interscalene catheters is therefore a balance between high analgesia quality and the control of side effects and complications, much like the passage between Scylla and Charybdis. We are convinced that the anatomical basis crucial for the brachial plexus block procedure at the interscalene level is not sufficiently depicted in the common regional anesthesia literature or textbooks. We would like to provide a comprehensive anatomical survey of the lateral neck, with special attention paid to the safe placement of interscalene catheters.

Keywords: catheter malposition; interscalene brachial plexus block; interscalene catheter; scalenovertebral triangle; shoulder surgery; ultrasound.

Figure 1

Transverse section of the lateral neck at the level of the larynx illustrating the close relationship between the spinal canal and the ventral rami of C5, C6, and C7. Catheter advancement along the nerve course toward the neuroforamen may explain intrathecal misplacements. The black circle indicates the close relationship of the ventral rami to the epidural space and the spinal cord. Dotted black arrows mark the dura mater. Non-dotted black arrow: vertebral artery; C5: ventral ramus of the 5th spinal nerve; C6: ventral ramus of the 6th spinal nerve; C7: ventral ramus of the 7th spinal nerve; CCA: common carotid artery; ES: epidural space; aSM: anterior scalene muscle; mSM: middle scalene muscle; SCM: sternocleidomastoid muscle; SC: spinal cord; TP C6: transverse process of the 6th cervical vertebra; iJV: internal jugular vein. The details of the body donor’s embalmment, preparation, and dissection are provided in Appendix A.

Figure 2

Transverse section of the lateral neck region showing the anatomical relationships within the compartment of the brachial plexus. A muscular connection separates the ventral rami from C5, C6, and C7. The phrenic nerve is depicted deep to the prevertebral fascia within the same compartment as the brachial plexus. The gap is narrowed by fat and connective tissue. Red arrows mark the prevertebral fascia; black arrows mark the vertebral artery; white arrows mark the sympathetic trunk; white circle: phrenic nerve; black circle: dorsal ramus of the 6th spinal nerve; C5: ventral ramus of the 5th spinal nerve; C6: ventral ramus of the 6th spinal nerve; C7: ventral ramus of the 7th spinal nerve; CCA: common carotid artery; Mb: muscle bridges; LCM: longus colli muscle; aSM: anterior scalene muscle; mSM: middle scalene muscle; SCM: sternocleidomastoid muscle; TP C7: transverse process of the 7th cervical vertebra; TG: thyroid gland; iJV: internal jugular vein. Note: the continuity of the prevertebral fascia of the body donor was accidentally interrupted via dissection (dotted red arrows). This location represents the connection to the Danger Space [27]. Details of the body donor’s embalmment, preparation, and dissection are provided in Appendix A.

Figure 3

Sonogram of the lateral neck region at the level of the 7th cervical vertebra captured to demonstrate problems of LA distribution. Nerves of the brachial plexus are deep to the prevertebral fascia (red arrows), and the supraclavicular nerves from the cervical plexus (not depicted) are superficial to it. Muscle bridges between anterior (aSM) and medius scalene muscle (mSM) act as mechanical barriers to LA spread (blue arrows). In this patient, local anesthetic would likely cause phrenic nerve (white circle) palsy if injected close to the ventral ramus of C5 due to its direct proximity to the phrenic nerve within the same compartment. The yellow circle marks a supraclavicular nerve. The vertebral vessels (VA: vertebral artery; VV: vertebral vein) are in close proximity to the 7th spinal nerve (C7). They and the spinal nerve at the level of the neuroforamen will be subject to injuries if a block with a needle trajectory planned according to Winnie is carried out. C6: ventral ramus of the 6th spinal nerve; SCM: sternocleidomastoid muscle; TPC7: transverse process of the 7th cervical vertebra; pTC7: posterior tubercle of the 7th transverse process; LN: lymph node; CCA: common carotid artery; iJV: internal jugular vein.

Figure 4

Sonogram of the lateral neck region at the level of the 6th cervical vertebra, demonstrating the variable course of the ventral rami (spinal nerves). In this patient, the ventral ramus of the 5th spinal nerve (C5) passes superficially to the anterior scalene muscle (aSM), while the ventral ramus of the 6th spinal nerve (C6) does not. Nerves of the brachial plexus were deep to the prevertebral fascia (red arrows), with the supraclavicular nerves from C4 running superficially to it (yellow circle). The phrenic nerve (white circle) is located deep to the prevertebral fascia on top of the fascia of the aSM. A phrenic-nerve-sparing brachial plexus block at this level is simply not possible. The ventral ramus of the 6th spinal nerve (C6) is depicted at its course out of the neuroforamen between the anterior tubercle (aT C6) and posterior tubercle (pT C6). Any needle advancement toward the neuroforamen carries the risk of nerve injury since the nerve at this position is not movable between the bony structures. SCM: sternocleidomastoid muscle; TPC6: transverse process of the 6th cervical vertebra; mSM: middle scalene muscle; iJV: internal jugular vein; CCA: common carotid artery.

Figure 5

Sagittal section through the left lateral neck region (scalenovertebral triangle) including the pleural cavity. The phrenic nerve (white circles) is deep to the prevertebral fascia (red arrows). White stars mark the ventral rami of the spinal nerves, pointing out their close vicinity to the phrenic nerve, which explains why phrenic-nerve-sparing injection techniques are so difficult and why ventro-medial LA distributions should be avoided. SA: subclavian artery; P: pleura; aSM: anterior scalene muscle; SCM: sternocleidomastoid muscle. Details of the body donor’s embalmment, preparation, and dissection are provided in Appendix A.

Figure 6

The dissection of the right lateral neck (ventrolateral view) demonstrates the peripheral course of the supraclavicular nerves after their exit through the prevertebral fascia (PVF). The PVF is depicted at the lateral border of the sternocleidomastoid muscle (SCM). This image clearly depicts the different compartments of the cervical and brachial plexus. Analgesia for skin incision or closure in shoulder surgery can only be achieved if the supraclavicular nerves are blocked superficial to the PVF, provided that the phrenic nerve can be spared. eJV: external jugular vein; GAN: greater auricular nerve; SCF: superficial cervical fascia; SCN: common trunk of the supraclavicular nerves; TCN: transverse nerve of the neck. Details of the body donor’s embalmment, preparation, and dissection are provided in Appendix A.

Figure 7

Anatomically plausible pathway of an interscalene-placed catheter directed toward the infraclavicular region demonstrated with a measuring pin. Yellow arrowheads mark the measuring pin that can be advanced from the interscalene to infraclavicular regions without significant resistance along the course of the brachial plexus in a preformed space. AA: axillary artery; LF: lateral fasciculus of brachial plexus; PmajM: pectoralis major muscle; PminM: pectoralis minor muscle; SCM: sternocleidomastoid muscle; SSM: subscapularis muscle; ST: superior trunk; TAT: thoracoacromial trunk. Details of the body donor’s embalmment, preparation, and dissection are provided in Appendix A.

Figure 8

Ultrasound image captured at the level of the first rib, where spinal nerves C5 and C6 form the superior trunk, demonstrating the instant control of LA spread observation. The needle tip position can be controlled by the “double-dot sign”, which represents the orifice of the needle with the bevel up (white oval circle). In this patient, the out-of-plane approach was used to place a catheter in the direction of the plexus and sidestep the dorsal scapular nerve (yellow dotted circle), which could interfere with the in-plane approach from the posterior–lateral direction. LA injection can be stopped if one observes medial spreading toward the phrenic nerve (white circle). One branch of the supraclavicular nerve is marked by a yellow circle. If the supraclavicular nerves need to be blocked, the needle must be withdrawn and redirected because the prevertebral fascia (red arrows) represents a mechanical barrier for LA. aSM: anterior scalene muscle; mSM: middle scalene muscle; iJV: internal jugular vein; VA: vertebral artery; C7 and C8: ventral rami of the 7th and 8th spinal nerves, respectively. SCM: sternocleidomastoid muscle.

Figure 9

Sonogram of the left lateral neck region after the injection of 1 mL of local anesthetic via an interscalene catheter. White stars: brachial plexus; red arrows: prevertebral fascia; white arrowheads mark the catheter in situ; white circle: phrenic nerve; LA: local anesthetic; aSM: anterior scalene muscle; SCM sternocleidomastoid muscle; mSM: middle scalene muscle.