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Passive length-tensile properties of extraocular muscles under botulinum toxin type C
Takashi Furuse1, Satoshi Hasebe, Hiroshi Ohtsuki
1Department of Ophthalmology, Okayama University Graduate School of Medicine and Dentistry, Okayama, Japan.
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.