From Grand Rounds from HSS: Management of Complex Cases | Volume 7, Issue 2
A 41-year-old woman with a diagnosis of biceps tendinitis was scheduled to undergo arthroscopic biceps tenodesis. She also had a 7-year history of myasthenia gravis and autoimmune-mediated asthma with frequent exacerbations.
Myasthenia gravis is an autoimmune disease characterized by skeletal muscle weakness and fatigability. Anesthetic goals of care for these patients include recognition of disease severity and cautious use of non-depolarizing neuromuscular blockers and anticholinesterases—medications typical of general anesthetics. Preoperative risk assessment for postoperative mechanical ventilation—particularly in the case of concomitant pulmonary disease—and expeditious extubation are important elements of care. Plans for multimodal analgesia focus on minimizing respiratory depression, often through the use of regional anesthetic techniques.
The patient’s primary myasthenia gravis symptoms were ocular and respiratory in nature and worsened over the course of each day. Symptoms were managed with pyridostigmine, methotrexate, and prednisone, requiring increased steroid doses during asthma exacerbations, in addition to daily use of bronchodilators. Her medical history also included obesity, hypertension, and chronic kidney insufficiency.
This patient underwent arthroscopic biceps tenodesis under regional anesthesia supplemented with intravenous sedation. Before surgery, she received clearance from her rheumatologist and her pulmonologist, with recommendations to administer stress-dose steroids and continue the medications she took at home for myasthenia gravis and asthma perioperatively.
The shoulder joint and the rotator cuff are innervated by a confluence of nerves originating from the C5 and C6 nerve roots, in addition to skin innervation by the supraclavicular nerve from the superficial cervical plexus . Therefore, the anesthesia plan was to (1) utilize a superior trunk block in conjunction with intravenous sedation intraoperatively and (2) rely on the superior trunk block and nonopioid medications for postoperative analgesia. The goal was to minimize the patient’s opioid requirements in the perioperative period, thereby decreasing risk of respiratory depression.
Standard monitors were applied. The patient was initially sedated with midazolam 5 mg and fentanyl 100 mcg, which were titrated intravenously over 10 minutes. Subsequently, an ultrasound-guided, superior trunk block was performed using a linear, high-frequency probe  and a Chiba 22 G × 2-3/8” block needle. Local anesthetic injection included a combination of 1.5% mepivacaine 10 mL and 0.5% bupivacaine 10mL. Adequate anesthesia was achieved within 15 minutes, and no signs of respiratory depression were noted. The surgery proceeded without incident under conscious sedation with a propofol infusion at 58 mcg/kg/min.
The patient was discharged on the day of surgery without delay. She reported that analgesia from the block lasted for 18 hours, after which she managed her pain with nonopioid analgesics.
Although employing brachial plexus blocks has been favored for many patients undergoing shoulder arthroscopy, it can also benefit those with tenuous respiratory status. For a patient at high risk for respiratory depression associated with general anesthesia and opioid analgesia, a functioning brachial plexus block can significantly minimize the amount of opioids required intra- and postoperatively, a particularly important concern for ambulatory surgeries.
Traditionally, interscalene blocks have been favored for shoulder surgeries, as targeting the C5 and C6 nerve roots adequately anesthetizes the shoulder. Supraclavicular blocks are also reasonable, although the suprascapular nerve may not necessarily be anesthetized with lower volumes of local anesthetic deposited at that location. In such a situation, there would be inadequate anesthesia of the supraspinatus and infraspinatus muscles during surgical manipulation .
Selection of the appropriate peripheral nerve block requires consideration of the side-effect profiles of each block. Interscalene blocks provide adequate anesthesia and postoperative analgesia for shoulder surgery, but they have the highest incidence of permanent neurologic complications of all peripheral nerve blocks  and 100% incidence of phrenic nerve paresis . While supraclavicular blocks are performed at a more distal position of the brachial plexus, posing less risk for neurologic injury, at least 50% of patients still exhibit ipsilateral diaphragmatic paresis [7, 8]. The resultant respiratory compromise can have significant repercussions in patients with concomitant pulmonary disease or decreased functional reserve capacity. Careful consideration must therefore be given to the type of brachial plexus block selected and the volume of local anesthetic used in ensuring adequate anesthesia and analgesia, while minimizing spread to other structures.
With these goals in mind, for this patient we relied on intravenous sedation in conjunction with an ultrasound-guided, long-acting, brachial plexus block, with a low volume of local anesthetic directed specifically at the superior trunk—distal to where the C5 and C6 nerve roots unite, but proximal to where the suprascapular nerve branches off (Fig. 1) [1, 2]. At this level, the phrenic nerve has diverged from the brachial plexus, potentially reducing the risk of its involvement when a low volume of injectate is used.
Figure 1: Ultrasound-guided superior trunk block anesthesia. The superior trunk (C5, C6) is isolated at the takeoff of the suprascapular nerve. The needle approaches the structures posterolaterally, with local anesthetic injection surrounding the structures.
As this case illustrates, minimizing the risk of respiratory depression is a prime goal of anesthetic management of patients with respiratory compromise resulting from myasthenia gravis. Ultrasound-guided regional anesthesia has utility for these patients as part of a multimodal analgesic plan. The goals of care include optimizing respiratory mechanics and promoting early mobilization in order to facilitate quick return to baseline respiratory status and to reduce cardiopulmonary complications. A low-volume superior trunk block, as described here, can achieve these objectives, while potentially preserving diaphragm function.
Investigation currently under way at HSS will further elucidate phrenic nerve involvement with the superior trunk block.
Patricia Pang, MD
Regional Anesthesiology and Acute Pain Medicine Fellow