- Ipsilateral phrenic nerve block resulting in diaphragmatic paresis occurs in 100% of patients undergoing interscalene blockade, even with dilute solutions of local anesthetics, and is associated with a 25% reduction in pulmonary function.
- Involvement of the vagus, recurrent laryngeal, and cervical sympathetic nerves is rarely significant, but a patient experiencing symptoms related to these side effects may require reassurance.
- The incidence of pneumothorax is low when the needle is correctly placed at the C5 or C6 level because of the distance from the dome of the pleura.
- Severe hypotension and bradycardia (i.e., the Bezold-Jarisch reflex) have been reported in awake, sitting patients undergoing shoulder surgery under an interscalene block. The cause is presumed to be stimulation of intracardiac mechanoreceptors by decreased venous return, which produces an abrupt withdrawal of sympathetic tone and enhanced parasympathetic output. This effect results in bradycardia, hypotension, and syncope. The frequency of this reflex is decreased when prophylactic β-blockers are administered.
- Nerve damage or neuritis can occur with any peripheral nerve block, but it is uncommon and is usually self-limited.
- Epidural and intrathecal injections have been reported with this block, a finding emphasizing the importance of inserting the needle in a caudad direction.
- Local anesthetic toxicity as a result of intravascular injection should be guarded against by careful aspiration and incremental injection. Seizure activity from this complication is particularly undesirable after rotator cuff surgery because the repair can be compromised by the associated muscular activity.