Abstract
Regional anesthesia has been widely used for intraoperative and postoperative pain management in shoulder, clavicle, and humerus surgery. Interscalene brachial plexus block (IBPB) is a common regional anesthesia method that is currently performed under ultrasound guidance. Posterior approach is more commonly used because of the risk of phrenic nerve injury from the anterior approach.1 However, the operator needs to be cautious to avoid the dorsal scapular nerve (DSN) and long thoracic nerve (LTN), during this approach.
DSN is mainly derived from the 5th cervical nerve root and supplies motor innervation of levator scapulae muscle and rhomboid muscle. DSN passes through the middle scalene muscle at an average of 3 cm (1.8–4.5 cm) from its origin.2 For identification of DSN, the transducer is positioned in the transverse plane (short-axis view) to visualize the carotid artery. Subsequently, sliding the transducer slightly laterally across the neck, and then scanning craniocaudally from 5th cervical root to 7th or 8th cervical root. DSN is identified within or superficial to the middle scalene muscle, usually found in the cross-sectional image between the levels where 5th and 6th cervical roots come out. DSN appears as an oval-shaped hyperechoic line, with small areas of hypoechoic fascicles within (Video 1). While scanning caudally, DSN is more easily identified within middle scalene muscle at the 6th cervical root level.
The following is the supplementary data related to this article:
Video 1.
Twitching of levator scapulae muscle is seen (Video 2) and a twitching of rhomboid muscle is palpable using nerve stimulator. Occasionally, the needle is right beside the DSN even when the nerve is not visible on ultrasound (Video 3). Thus, if muscle twitching occurs, the needle should not be inserted without re-examining the surrounding needle site with ultrasound; and if DSN is identified, the needle should be inserted at a different angle.
The following is the supplementary data related to this article:
Video 2
Video 3.
Twitching can occasionally be caused by contact of the stimulating needle with the skin (Video 4), since DSN is located superficially and runs posteriorly. In order to avoid risk of DSN injury, another needle insertion site that is more posterior to ultrasound-DSN imaging using posterior approach is required.
The following is the supplementary data related to this article:
Video 4.
DSN syndrome is a chronic pain syndrome that occurs due to injury by the needle approach. It is characterized by pain along the medial border of scapula that radiates to the lateral surface of upper extremities and causes weakness of the rhomboid and/or levator scapulae muscles.3 Patients with DSN syndrome usually complain of a not-well-defined shoulder pain and dysfunction with different degrees of functional impairment.
LTN is reportedly derived from the 5th and 6th cervical roots, which innervates the serratus anterior muscle. It runs within or next to the middle scalene muscle often in proximity to DSN (Video 1). LTN can be identified with the same method used for DSN. However, LTN is located deeper to the DSN, usually between 6th and 7th cervical root level. LTN appears as an oval-shaped hyperechoic line, with small areas of hypoechoic fascicles within (Video 1). While scanning caudally, LTN is easy to be identified at the level of 7th cervical root. Twitching of serratus anterior muscle is seen using nerve stimulator (Video 5).
The following is the supplementary data related to this article:
Video 5.
LTN can be encountered during IBPB by the posterior approach, causing risk of nerve injury.4 LTN syndrome is a chronic pain syndrome of the shoulder associated with different degree of serratus anterior muscle palsy, impairment of shoulder elevation, and characteristic scapula winging with medial translation and rotation of the inferior angle towards the midline.
Kim et al reported that in total 70 cases who underwent ultrasound-guided IBPB with nerve stimulator, DSN was encountered in 44 cases (62.8%), LTN in 15 cases (21.4%), both nerves in 10 cases (14.3%), and neither was encountered in 21 cases (30.4%); moreover, the average current measured immediately before the disappearance of muscle twitches was 0.44 mA and 0.50 mA at DSN and LTN, respectively.5 The risk of encountering nerves is avoidable if the stimulation currents are started at 1.5 mA.
In conclusion, careful examination of anatomical structures, including DSN and LTN, is essential to safe procedure. Nerve stimulator is recommended for ultrasound-guided IBPB with posterior approach. In the presence of motor response, it is considerably safer to deflect the needle and find an alternate path. If performed under general anesthesia, it is recommended to avoid oval-shaped hyperechoic line containing small areas of hypoechoic fascicles in the needle plane.
Conflict of interest
There is no conflict of interest.