What volume of local anesthetic injection in interscalene block is associated with an increased risk of neuraxial (epidural/spinal) spread?

Neuraxial Spread During Interscalene Block: Volume Considerations

Volumes of 20 mL or greater of local anesthetic during interscalene block are associated with increased risk of neuraxial (epidural/spinal) spread, while volumes of 5-10 mL significantly reduce this risk while maintaining efficacy.

Risk Factors for Neuraxial Spread

• Interscalene block performed at higher cervical levels (C4 and above) increases the risk of neuraxial spread due to proximity to the neuraxis 1
• Traditional approaches using large volumes (20+ mL) of local anesthetic increase the risk of spread to the epidural or subarachnoid space 1
• Ultrasound guidance allows for significant volume reduction while maintaining efficacy, thereby reducing risk of neuraxial complications 2, 3

Evidence for Minimum Effective Volume

• Studies demonstrate that as little as 5 mL of 0.75% ropivacaine can provide successful surgical anesthesia for shoulder procedures when using ultrasound guidance 3
• The minimum effective volume for 50% of patients (MEV50) in peripheral nerve blocks has been shown to be significantly lower than traditional volumes, suggesting similar principles apply to interscalene blocks 4
• Multiple small deposits (5-10 mL total) are more effective than single large volume injections and reduce risk of complications 5

Recommendations for Safe Practice

• Use ultrasound guidance to visualize needle placement and local anesthetic spread, allowing for volume reduction 1
• Limit total volume to 5-10 mL of local anesthetic when using ultrasound guidance for interscalene block 3, 6
• When performing interscalene block without ultrasound guidance, consider using multiple small deposits rather than a single large volume injection 5
• Choose alternative approaches when appropriate - axillary or infraclavicular approaches may be safer than high interscalene approaches in patients at risk for neuraxial spread 1

Monitoring and Safety Considerations

• Thorough testing for block success is essential to prevent the need for supplemental blocks or general anesthesia 1
• Monitor for signs of high block (respiratory compromise, hypotension, bradycardia) for at least 30 minutes after block placement 1
• Have resuscitation equipment immediately available when performing any interscalene block 1
• Calculate safe dose of local anesthetic based on patient weight to prevent local anesthetic systemic toxicity 1

Common Pitfalls and Caveats

• Failure to recognize that an interscalene catheter has migrated into the epidural or intrathecal space can lead to inadvertent administration of excessive local anesthetic doses 1
• Patients with anatomical variations or previous neck surgery may be at higher risk for neuraxial spread even with lower volumes 1
• Sedation during block placement may mask early symptoms of neuraxial spread, so minimal sedation is recommended 1
• Even with ultrasound guidance and reduced volumes, vigilance for signs of neuraxial spread remains essential 1