The structure least likely to be encountered during placement of an interscalene block is

The structure least likely to be encountered during placement of an interscalene block is

• A. Phrenic nerve
• B. Vertebral artery
• C. Recurrent laryngeal nerve
• D. Vagus nerve

Answer: (D)

When performing an interscalene block, you aim for the brachial plexus roots/trunks in the space between the anterior and middle scalene muscles at about the C5–C7 level. The structures most likely to be near the needle path include the phrenic nerve, which runs along the anterior surface of the anterior scalene and can be affected, producing diaphragmatic paresis; the vertebral artery, which ascends through the cervical transverse foramina and can be at risk if the needle tracks medially toward deep structures; and the recurrent laryngeal nerve, a branch of the vagus that travels in the tracheoesophageal groove and can be affected by spread of local anesthetic near the lower neck. The vagus nerve itself lies within the carotid sheath, more medial to the interscalene region, and is not in the usual trajectory of the needle or the typical spread of anesthetic for this block. Therefore it is least likely to be encountered during placement.

When performing an interscalene block, you aim for the brachial plexus roots/trunks in the space between the anterior and middle scalene muscles at about the C5–C7 level. The structures most likely to be near the needle path include the phrenic nerve, which runs along the anterior surface of the anterior scalene and can be affected, producing diaphragmatic paresis; the vertebral artery, which ascends through the cervical transverse foramina and can be at risk if the needle tracks medially toward deep structures; and the recurrent laryngeal nerve, a branch of the vagus that travels in the tracheoesophageal groove and can be affected by spread of local anesthetic near the lower neck.

The vagus nerve itself lies within the carotid sheath, more medial to the interscalene region, and is not in the usual trajectory of the needle or the typical spread of anesthetic for this block. Therefore it is least likely to be encountered during placement.