A 40-year-old woman receives an interscalene brachial plexus block for arthroscopic shoulder surgery. Shortly after the injection, she develops dyspnea and decreased breath sounds on the ipsilateral side. What is the most likely cause?

Phrenic nerve palsy is correct because the phrenic nerve runs on the anterior scalene muscle in the same fascial compartment as the brachial plexus at the interscalene level, so local anaesthetic spread causes ipsilateral diaphragmatic paresis in nearly all cases.

Why phrenic nerve palsy is the answer

The phrenic nerve arises from the C3, C4, and C5 ventral rami, with C4 providing the dominant contribution. After forming at the lateral border of the anterior scalene muscle, it descends obliquely across the anterior surface of that muscle, covered only by the prevertebral fascia. The interscalene brachial plexus block targets the roots and trunks of the brachial plexus as they pass between the anterior and middle scalene muscles at the C6 level. Because the phrenic nerve lies on the anterior scalene within the same fascial compartment, local anaesthetic injected into the interscalene groove spreads to involve the phrenic nerve in virtually all cases. Ultrasound and fluoroscopic studies have confirmed that phrenic nerve blockade occurs in nearly 100 percent of interscalene blocks when conventional volumes of 20 to 30 mL are used. The resulting ipsilateral diaphragmatic paresis reduces forced vital capacity by approximately 25 to 30 percent. In a healthy patient this is well tolerated, but it manifests clinically as dyspnea and decreased breath sounds on the blocked side, exactly as described in the stem. The key discriminating feature is the temporal relationship: symptoms appear shortly after injection, are limited to the ipsilateral hemithorax, and are not accompanied by signs of local anaesthetic systemic toxicity or pneumothorax.

The anatomy that makes this unavoidable

The brachial plexus at the interscalene level consists of the C5 to T1 roots, with C5 and C6 already forming the upper trunk. These structures lie in the interscalene groove, a depression between the anterior scalene muscle anteriorly and the middle scalene muscle posteriorly. The phrenic nerve, formed by the ventral rami of C3, C4, and C5, runs vertically down the anterior surface of the anterior scalene muscle, invested by the prevertebral fascia. This fascial layer is continuous with the fascia surrounding the brachial plexus sheath. When local anaesthetic is deposited into the interscalene space, it spreads along this fascial plane and reaches the phrenic nerve. The supraclavicular portion of the lung apex lies inferior to this region, separated from the brachial plexus by the dome of the pleura and the suprapleural membrane. A pneumothorax would require the needle to traverse the pleural cavity, which is not in the standard needle path for an interscalene block performed at the C6 level. The recurrent laryngeal nerve, a branch of the vagus, loops around the subclavian artery on the right and the aortic arch on the left before ascending in the tracheoesophageal groove. It is not in the path of an interscalene injection. The stellate ganglion lies anterior to the transverse process of C7, posterior to the carotid sheath, and can be blocked during interscalene injection, producing Horner syndrome, but this does not cause dyspnea.

Clinical significance and risk stratification

Phrenic nerve palsy after interscalene block produces a predictable pattern of respiratory compromise. The ipsilateral hemidiaphragm becomes paralyzed and elevates on chest radiograph. Fluoroscopic sniff testing shows paradoxical upward movement of the affected diaphragm during inspiration. Ultrasound assessment of diaphragmatic excursion confirms reduced or absent movement. Forced vital capacity drops by 25 to 30 percent, and the patient reports dyspnea, particularly when supine. Decreased breath sounds at the ipsilateral base reflect basal atelectasis from diaphragmatic dysfunction. The clinical significance depends heavily on preexisting respiratory status. A healthy 40 year old woman with normal pulmonary function will compensate with the contralateral diaphragm and accessory muscles. However, patients with contralateral diaphragmatic paralysis, severe COPD, or morbid obesity may decompensate significantly. This is why interscalene block is relatively contraindicated in patients with preexisting respiratory compromise. Reducing the injectate volume to 5 to 10 mL or performing a supraclavicular or infraclavicular block can spare the phrenic nerve while still providing surgical anaesthesia for shoulder procedures. The posterior approach to the brachial plexus at the cervical level also reduces the incidence of phrenic nerve block.

This table summarizes the clinical findings that distinguish phrenic nerve palsy from other complications of interscalene block.

How this topic appears in the exam

The differential diagnosis of dyspnea after interscalene block includes pneumothorax, local anaesthetic toxicity, high neuraxial block, and phrenic nerve palsy. Pneumothorax presents with sudden pleuritic chest pain, dyspnea, and decreased breath sounds, but it is rare with interscalene block because the needle does not approach the pleural apex. It is more common with supraclavicular block. Local anaesthetic systemic toxicity produces perioral numbness, tinnitus, metallic taste, seizures, and cardiovascular collapse, not isolated ipsilateral decreased breath sounds. Recurrent laryngeal nerve palsy causes hoarseness and vocal cord paralysis, not decreased breath sounds. It can occur with interscalene block due to spread to the vagus nerve or its branches, but the presentation is different. Horner syndrome, from stellate ganglion block, produces ptosis, miosis, and anhidrosis. Total spinal anaesthesia from inadvertent intrathecal or epidural injection causes bilateral motor block, hypotension, and respiratory arrest. The combination of dyspnea with ipsilateral decreased breath sounds, appearing shortly after injection in a patient who is otherwise stable, is pathognomonic for phrenic nerve palsy. Examiners frequently test this by asking which nerve is responsible for respiratory compromise after interscalene block, or by presenting a scenario where the student must choose between phrenic nerve block and pneumothorax. The key discriminator is that pneumothorax is a complication of supraclavicular block, not interscalene block, and phrenic nerve palsy is an expected consequence of interscalene block in nearly all patients.

Why the other options fail

One-glance recall table

Mnemonics

What the exam actually asks

• Phrenic nerve palsy occurs in nearly 100 percent of interscalene blocks with conventional volumes of 20 to 30 mL
• Pneumothorax is associated with supraclavicular block, not interscalene block
• Recurrent laryngeal nerve palsy causes hoarseness, not decreased breath sounds
• Local anaesthetic toxicity presents with CNS and cardiovascular signs, not unilateral respiratory findings
• Reducing injectate volume to 5 to 10 mL or using a posterior approach can spare the phrenic nerve
• Interscalene block is relatively contraindicated in patients with preexisting respiratory compromise

Traps that cost marks

• Confusing interscalene block complications with supraclavicular block complications, particularly pneumothorax
• Assuming that dyspnea after any nerve block must be local anaesthetic toxicity
• Missing that phrenic nerve palsy is an expected consequence of interscalene block, not a rare complication
• Failing to recognize that decreased breath sounds at the base reflect diaphragmatic dysfunction, not pulmonary pathology

Frequently asked

Why does interscalene block cause phrenic nerve palsy in nearly all cases?

The phrenic nerve runs on the anterior surface of the anterior scalene muscle, covered by the prevertebral fascia. This fascia is continuous with the sheath surrounding the brachial plexus in the interscalene groove. When local anaesthetic is injected into the interscalene space, it spreads along this fascial plane and reaches the phrenic nerve. Because the phrenic nerve is in the same fascial compartment as the brachial plexus at this level, it is virtually impossible to block the brachial plexus without also blocking the phrenic nerve. This is why phrenic nerve palsy occurs in nearly 100 percent of interscalene blocks when conventional volumes are used.

Is phrenic nerve palsy after interscalene block dangerous?

In a healthy patient with normal pulmonary function, unilateral diaphragmatic paresis is well tolerated. The contralateral diaphragm and accessory muscles compensate, and the patient may only notice mild dyspnea when supine. However, in patients with preexisting respiratory compromise, such as severe COPD, contralateral diaphragmatic paralysis, or morbid obesity, the loss of one hemidiaphragm can cause significant respiratory decompensation. This is why interscalene block is relatively contraindicated in these patients. Reducing the injectate volume to 5 to 10 mL or using alternative approaches like supraclavicular or infraclavicular block can spare the phrenic nerve while still providing adequate surgical anaesthesia.

How can phrenic nerve block be avoided during shoulder surgery?

Several strategies reduce the incidence of phrenic nerve block. Reducing the local anaesthetic volume to 5 to 10 mL decreases the spread to the phrenic nerve while still providing brachial plexus anaesthesia. The posterior approach to the brachial plexus at the cervical level targets the nerve roots farther from the phrenic nerve. Supraclavicular and infraclavicular blocks avoid the phrenic nerve entirely and are preferred for patients with respiratory compromise. Anterior suprascapular nerve block combined with axillary nerve block can provide shoulder anaesthesia without phrenic involvement for selected procedures.

References

• Miller's Anesthesia, 9th. chapter on peripheral nerve blocks, complications of brachial plexus block
• Morgan and Mikhail's Clinical Anesthesiology, 6th. chapter on regional anesthesia, upper extremity blocks
• Harrison's Principles of Internal Medicine, 21st. chapter on respiratory physiology, diaphragmatic function