Intramuscular nerve blockade is used for diagnostic, prognostic, and therapeutic treatment of non–velocity-dependent muscle tone, flexor spasm, and dystonia.
After informed consent is obtained, the patient is positioned comfortably to allow optimal access to the muscles involved. The patient is prepared in a standard aseptic fashion over an area large enough to allow palpation of landmarks, and sterile technique is used throughout the procedure. A skin wheal is raised over the main muscle bulk of the muscles to be injected. A 1½- to 4-in. (4- to 10-cm) insulated needle is advanced through the wheal, with a nerve stimulator used to localize the motor nerve branches or motor points. The current is reduced until the minimum current is required to elicit muscle contraction. When the needle tip is within 1 mm of the motor nerve, and after negative aspiration, 1 to 2 mL of 4% to 6% phenol is injected for neurolysis (see Figure).
Intramuscular nerve or motor point blockade is reported to have duration of effect from 1 to 36 months (median, 11.5 months). No dose-response or dose-duration of effects relationship has been demonstrated for motor point blocks (51,52). The needle is positioned to produce the maximal twitch at the lowest stimulus. The needle is usually adjacent to the nerve when 0.5 to 0.1 mA produces motor stimulation with an insulated needle, and 1 mA with an uninsulated needle. The motor points of each muscle cluster at the midpoint of the muscle fibers.
Significant complications are rare with intramuscular nerve injections, and transitory side effects include pain of mild intensity, tenderness and swelling at injection sites, and dysesthesia. Inadvertent neurolysis of a mixed nerve results in painful paresthesia in about 11% of patients.