A supraclavicular brachial plexus block is being performed. The anaesthetist must identify the muscle over whose anterior surface the phrenic nerve descends before entering the thoracic inlet. Which muscle is this?
- A Middle scalene
- B Anterior scalene ✓
- C Posterior scalene
- D Levator scapulae
Correct answer: B. Anterior scalene
Explanation
The phrenic nerve (B3, B4, B5 chiefly B4) forms on the surface of anterior scalene and descends vertically across its anterior aspect under the prevertebral fascia before entering the chest between the subclavian artery and vein. The roots of the plexus emerge between anterior and middle scalene, so depositing local anaesthetic too medially on anterior scalene risks diaphragmatic paralysis.
Reference: Gray's Anatomy for Students, 4th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.