Passive length-tensile properties of extraocular muscles under botulinum toxin type C.

PURPOSE Passive length-tension of the extraocular muscle was measured after injection of botulinum neurotoxin type C (BoNT-C) to evaluate its chemo-denervation effect. METHODS BoNT-C was injected unilaterally at 5.0 or 2.5 units in the superior rectus muscle of the albino rabbit. The muscle was separated several times at its original insertion between 3 days to 8 weeks after injection, and the passive length-tension produced by stretching the muscle from its physiologic length was measured. The length-tension curve was analyzed, and the passive load was determined from early components. The compliance was determined by approximating the entire ascending curve by an exponential function. RESULTS In the 5.0-unit group, the passive load increased significantly 2 weeks after injection but decreased to the level of the control group after 4 weeks and remained at that level until after 8 weeks after injection. In the 2.5-unit group, also, there were changes similar to those in the 5.0-unit group, but the changes were not significant compared to the control group. No significant change was observed in the compliance in either group. CONCLUSIONS Persistent chemo-denervation effect was not observed in passive length-tension at the doses of BoNT-C used in this study. The increase in the passive load soon after injection was suggested to have been caused by the same mechanism as BoNT-A.