Successful interscalene block (ISB) will produce an ipsilateral phrenic nerve block.

The phrenic nerve is the sole motor supply to the diaphragm, and ipsilateral hemidiaphragmatic paresis occurs in up to 100% of patients receiving ISBs. Usually, phrenic nerve palsy is well tolerated, and is often unnoticed by healthy patients. However, forced vital capacity decreases by approximately 25%, which can produce ventilatory compromise in patients with limited pulmonary reserve, requiring assisted ventilation.

If the recurrent laryngeal nerve is inadvertently blocked, vocal cord palsy occurs with symptoms of hoarseness and possibly acute respiratory insufficiency. This complication is ordinarily of little consequence unless bilateral laryngeal nerve palsy results, which may produce severe laryngeal obstruction.

Cranial nerve X and XII palsy (Tapia's syndrome), may also occur following ISB. Symptoms include one-sided cord paralysis, aphonia, and the patient's tongue deviating toward the side of the block.

Horner's syndrome may occur when the local anaesthetic spreads to the stellate ganglion with its cervical sympathetic nerves. Symptoms include ptosis of the eyelid, miosis, and anhidrosis of the face. However, the existence of Horner's syndrome may not indicate that the brachial plexus is adequately blocked.