The angle of approach is important to avoid accidental intravascular or intrathecal injection. The needle is then advanced carefully until a paraesthesia is elicited. A click may be detected as the needle passes through the prevertebral fascia. This usually occurs at the superficial level. The use of a nerve stimulator with a special insulated needle is very helpful in confirming the correct placement of the needle and performing the interscalene block accurately.
Correct stimulation produces twitching below the shoulder.
Stimulation of the diaphragm indicates too anterior an approach. Other local anaesthetic agents like lignocaine or prilocaine may be used. Complications Inadvertent epidural or subarachnoid injection is a potentially serious complication resulting from incorrect needle placement. Vertebral artery injection, this can result in convulsions and loss of consciousness. Phrenic nerve block is frequently produced, this complication precludes bilateral use of this technique.