A patient undergoing interscalene brachial plexus block develops transient ipsilateral diaphragmatic paralysis. Which structure lying on the anterior surface of the scalenus anterior muscle was affected?
- A Vagus nerve
- B Phrenic nerve ✓
- C Sympathetic trunk
- D Ascending cervical artery
Correct answer: B. Phrenic nerve
Explanation
The phrenic nerve (roots C3, C4, C5) descends vertically on the anterior surface of scalenus anterior under the prevertebral fascia before entering the thorax behind the subclavian vein. Local anaesthetic spread from an interscalene block can anaesthetise it here, producing hemidiaphragmatic paresis. The vagus lies within the carotid sheath medial to scalenus anterior, and the sympathetic trunk lies posterior to the carotid sheath, both deeper than the injection plane.
Reference: Gray's Anatomy, 42nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.