Peripheral Nerve Interventions

Anatomy

The SSN arises as the first branch from the superior trunk (formed from the fifth and sixth cervical root) of the brachial plexus. After its origin, it travels inferior to the trapezius to enter the suprascapular notch, a semicircular depression seen on the superolateral border of the scapula, where it is covered by the transverse scapular ligament. The nerve further courses under the supraspinatus muscle to curve along the lateral border of the spine of the scapula (at the spinoglenoid notch) and then terminates by supplying the infraspinatus muscle ( Fig. 111.1 ). The branches from the SSN includes motor innervation to supraspinatus and infraspinatus muscles as well as articular branches to the shoulder joint ( Fig. 111.2 ). It has been postulated that the SSN provides 70% of the sensory output to the shoulder joint complex. The SSN is accompanied by the suprascapular artery and vein along its course. The vessels pass above the transverse scapular ligament to run along the SSN in the supraspinous fossa. This is an excellent location for the US-guided technique to block the nerve. The nerve is enclosed in a compartment here in the supraspinous fossa, whose boundaries are the supraspinous fascia superiorly, the spine of the scapula posteriorly, and the plate of the scapula anteriorly.

Conventional Block Technique

Traditional anatomic landmark- or fluoroscopy-guided techniques target the SSN at the suprascapular notch. Reaching the target site without the use of image guidance puts the subject at risk of not only failure but also complications. The most dreaded complication of pneumothorax is seen in approximately 1%, and usually arises from the needle directed too anteriorly or too deep. A computed tomography-guided study demonstrated poor needle location if a landmark-guided technique is used. Although fluoroscopy might assure a correct needle placement, the possibility of spillage of injectate to the brachial plexus cannot be prevented. Furthermore, the needle could be placed vertically into suprascapular fossa by fluoroscopy guidance which still might need larger injectate volumes (10 mL or more) with a minor chance of spreading to axillary fossa.

The optimal site to target the SSN is at the floor of suprascapular fossa, midway between the suprascapular and spinoglenoid notches ( Fig. 111.3 ). The advantage of this site is elimination of risk of inadvertent pleural puncture. The injectate is limited within the compartment formed by the boundaries of the suprascapular fossa. US provides a feasible technique to visualize this site.

Ultrasound-Guided Intervention

The patient is positioned in a sitting or prone position as tolerated. Prominent landmarks include the coracoid process, the acromion process, and spine of the scapula. Because SSN block is a superficial procedure, a high-frequency linear ultrasound transducer (15-6 MHz) is used and a depth of 6 cm is set on the US screen. The transducer is placed with a slight anterior tilt in the coronal plane over the suprascapular fossa. The probe is placed perpendicular to the imaginary line joining the acromion and coracoid process, denoting the course of the SSN. This placement helps visualize trapezius, supraspinatus, and the bony fossa forming the base of the image from superficial to deep levels. (see Fig. 111.3 ). Further fine adjustments are made in the angle of the transducer in a cephalocaudal direction, to image the SSN and artery. An in-plane technique is used to advance a 80-mm-long needle under direct vision from the medial end of the transducer because the acromion covers the lateral end of the image, making it difficult to introduce the needle. Because of the closed compartment around the SSN, an injectate volume of 5–8 mL is adequate.

Anatomy

The 12 thoracic spinal nerves divide into a posterior and anterior branch after they leave the respective intervertebral foramina. The posterior cutaneous branch innervates the skin and muscles of the paravertebral region. The anterior branch continues as the respective ICN. The ICN is initially located between the pleura and the posterior intercostal membrane. It further courses through this membrane to lie deep to the internal intercostal muscle. The intercostal vessels pass along this groove, close but often cephalad to the ICN ( Fig. 111.4 ). The neurovascular bundle lies in the intercostal space but runs deep to the subcostal groove at the angle of rib. At a distance of about 5–8 cm anterior to the angle of the rib, the groove ends and blends into the surface of the lower edge of the rib. The initial lateral cutaneous nerve branches off and pierces the external intercostal muscle between the posterior and midaxillary line to innervate the skin of the chest. The ICNs continues anteriorly to end as the anterior cutaneous branch after exiting at the level of midclavicular line.

There are exceptions to this usual anatomy of ICN. The first ICN has no anterior or lateral cutaneous branch, with most of its fibers leaving the intercostal space to join with the eighth cervical nerve root (C8) to form the lower trunk of the brachial plexus. The remainder minor part courses as the first ICN. A part of the second and third ICN forms the intercostobrachial nerve supplying axilla and medial arm. The nomenclature of the 12th ICN is subcostal nerve , because it is bordered superiorly only by a rib .

Conventional Block Technique

The traditional landmark technique can be done with the patient prone or sitting. The block is done at the angle to the rib before the lateral cutaneous nerve taking off. The needle is directed cephalad and walked off the inferior margin of the corresponding rib into the subcostal groove. The needle is further advanced by 2–3 mm. The rib’s inferior margin and the pleura is separated by distance as small as 0.5 cm. After a negative aspiration check, the injection is performed. The incidence of pneumothorax ranges from 0.09% to 8.7% with the landmark-guided technique.

The needle can also be advanced under fluoroscopy, with the needle contacting the inferior margin of the rib. After negative aspiration, contrast is injected to visualize spread along the intercostal space. However, this does not decrease the risk of pneumothorax because pleura cannot be visualized on fluoroscopy. The technical advantage of using US for ICN intervention has been shown in cadavers as well as in subjects with postthoracotomy pain.