Long-term observations of the masseter muscle following single or repeated injections of lidocaine hydrochloride into developing mice.

Effects of lidocaine-HCl on extrafusal muscle fibers in the masseter muscle of developing mice were studied histologically and morphometrically. In the affected region, many extrafusal muscle fibers were circular and smaller in diameter than unaffected extrafusal muscle fibers, and the SDH activity of the affected fibers was very irregular. In lidocaine-HCl injected groups, the ratios of extrafusal muscle fibers with central nuclei to total extrafusal muscle fibers were higher than those in the saline solution-injected groups. The diameters of extrafusal muscle fibers in the masseter muscle treated with lidocaine-HCl were smaller than those in the saline solution-injected groups. These changes in the muscle with lidocaine-HCl continued for 45 days after a single injection. Five injections of lidocaine-HCl into developing mice caused long-term degeneration of the masseter muscle. Thus, the present study suggests that a local anesthetic agent caused degeneration of immature muscle fibers of the masseter muscle in developing mice and may result in long-term decrease of masticatory capacity. Therefore, lidocaine-HCl may inhibit the synchronized development of masticatory organs in developing animals.