A 55-year-old man is being evaluated for a 4-week history of right shoulder pain. The pain worsens when actively abducting the shoulder or when lying on the shoulder at night. The patient also has weakness with abduction and external rotation of the right shoulder. He has attempted treatment with over-the-counter analgesics with limited relief. MRI of the shoulder shows a partial rotator cuff tear, and the patient is scheduled for surgical repair. Immediately before the surgery, he receives anesthesia with an injection between the right anterior and middle scalene muscles in order to block the C3 through C7 nerve roots of the cervical and brachial plexuses. Which of the following muscles is most likely to be paralyzed due to the anesthesia?

Question

A 55-year-old man is being evaluated for a 4-week history of right shoulder pain.  The pain worsens when actively abducting the shoulder or when lying on the shoulder at night.  The patient also has weakness with abduction and external rotation of the right shoulder.  He has attempted treatment with over-the-counter analgesics with limited relief.  MRI of the shoulder shows a partial rotator cuff tear, and the patient is scheduled for surgical repair.  Immediately before the surgery, he receives anesthesia with an injection between the right anterior and middle scalene muscles in order to block the C3 through C7 nerve roots of the cervical and brachial plexuses.  Which of the following muscles is most likely to be paralyzed due to the anesthesia?

Quizplus · Accepted Answer

11ec160b_3599_dcbc_9249_55ce2788a9d9_MD0001_00 The brachial plexus originates from the C5-T1 spinal nerve roots, which combine into 3 trunks and pass between the anterior and middle scalene muscles (scalene triangle) in the posterior neck. The trunks then differentiate into divisions, cords, and terminal nerve branches. The brachial plexus supplies all sensory and motor innervation to the upper extremity, except for an area of skin near the axilla which is innervated by the intercostobrachial nerve and the trapezius muscle which is innervated by the accessory nerve (CN XI). An interscalene nerve block is a regional anesthesia technique used for procedures involving the shoulder and upper arm. In this technique, anesthetic is administered to the brachial plexus roots and trunks between the anterior and middle scalene muscles. Anesthetic also traverses along the interscalene sheath and affects the roots of the phrenic nerve (C3-C5), which causes transient ipsilateral diaphragmatic paralysis. Therefore, an interscalene nerve block should be avoided in patients with conditions that could be worsened by unilateral diaphragmatic paralysis (eg, chronic lung disease, contralateral phrenic nerve dysfunction). (Choice A) The cricothyroid muscle is innervated by the external branch of the superior laryngeal nerve (vagus nerve [CN X]) and helps tense the vocal cords, allowing higher-pitched speech. (Choice C) The intercostal muscles aid in expansion and contraction of the chest cavity during breathing. They are innervated by the intercostal nerves (ventral rami of thoracic spinal nerves). (Choice D) The platysma is a superficial neck muscle that covers the anterior neck from the clavicle to the mandible and tenses the skin of the neck and lower face. It is innervated by the cervical branch of the facial nerve (CN VII). (Choice E) The sternocleidomastoid, a superficial neck muscle controlling neck flexion and head rotation, is innervated by the accessory nerve (CN XI). Educational objective: An interscalene nerve block is used to provide anesthesia for the shoulder and upper arm by anesthetizing the upper brachial plexus (C5-C7) as it passes between the anterior and middle scalene muscles. Anesthetic also transverses along the interscalene sheath, frequently resulting in transient ipsilateral diaphragmatic paralysis due to involvement of the phrenic nerve roots (C3-C5). __________ References: Sudden severe postoperative dyspnea following shoulder surgery: remember inadvertent phrenic nerve block due to interscalene brachial plexus block. (http://www.ncbi.nlm.nih.gov/pubmed/25114416)

A 55-Year-Old Man Is Being Evaluated for a 4-Week History