REGIONAL DIFFERENCES IN THE SUBACUTE RESPONSE OF RABBIT ORBICULARIS OCULI TO BUPIVACAINE-INDUCED MYOTOXICITY AS QUANTIFIED WITH A NEURAL CELL-ADHESION MOLECULE IMMUNOHISTOCHEMICAL MARKER

Purpose. This study examined the subacute myotoxic effects of injectio n of the local anesthetic bupivacaine on the orbicularis oculi muscle in the rabbit eyelid. In contrast to other muscles, the orbicularis oc uli was resistant to injury by the usual anesthetic doses of bupivacai ne when local infiltration is used. An attempt was made to assess the sensitivity of orbicularis oculi muscle to bupivacaine using a variety of increased bupivacaine concentrations and in combination with hyalu ronidase. Methods. Bupivacaine was injected into rabbit lower eyelids at a variety of doses with and without the injection of hyaluronidase before bupivacaine treatment. Muscle injury was assessed immunohistoch emically using an antibody to an isoform of neural cell adhesion molec ule, anti-leu-19, a molecule shown to localize on the surface of regen erating muscle fibers. The number of neural cell adhesion molecule-pos itive muscle fibers was assessed 4 and 7 days after bupivacaine inject ion to determine the number of fibers that were injured. Results. When bupivacaine was injected into the lower eyelid at a dose of 1.5 mg, o nly 10% of the orbicularis oculi muscle was injured. The most effectiv e injury involved either multiple injections of 3 mg bupivacaine or of hyaluronidase 20 minutes before the injection of 3 mg bupivacaine, re sulting in injury of up to 58% of the muscle fibers. In all cases, the preseptal region of the orbicularis oculi showed a greater percentage of injury than the pretarsal portions of the muscle. Conclusions. Alt hough multiple injections of bupivacaine and bupivacaine combined with hyaluronidase significantly increased the percentage of muscle cells injured, total destruction of the orbicularis oculi muscle was never s een. Neural cell adhesion molecule was a good marker for the quantific ation of the regenerating muscle fibers. It is proposed that the tight fasciculation of the orbicularis oculi muscle may play a role in prev enting access of the local anesthetic to the individual muscle fibers. This demonstrates the relative clinical safety of local anesthetic in jection into the eyelid.