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P. A. Stahler, B. C. Anderson, A. R. Harrison, L. K. McLoon; Localization and Effect of Botulinum Toxin on Neuromuscular Junction Density in Rabbit Orbicularis Muscle. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5263. doi: https://doi.org/.
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Direct injections of botulinum toxin into the orbicularis oculi muscles are routinely performed for the treatment of patients with eyelid spasms. The effect of these botulinum toxin injections on the neuromuscular junction (NMJ) density and three-dimensional location in adult orbicularis oculi muscle are unknown. This study examined normal and botulinum-treated adult rabbit orbicularis oculi muscle for neuromuscular junction density.
Average density of NMJs within the pretarsal and preseptal regions of adult rabbits was determined morphometrically for normal adult rabbit eyelids and for eyelids that had been injected with 5 Units botulinum toxin A one, two, or 4 weeks prior to sacrifice. The rabbit eyelids were removed, embedded, frozen, sectioned in the longitudinally oriented plane, and immunostained for NMJ localization with fluoroscein-labeled alpha-bungarotoxin. NMJ locations for the four groups were reconstructed, and density of NMJs was determined for the pretarsal and preseptal regions of each orbicularis oculi muscle.
In normal orbicularis oculi, the pretarsal and preseptal NMJ densities are 254 and 168 junctions per mm3, respectively. Botulinum toxin treatment significantly increased the mean NMJ density in both the pretarsal and preseptal regions at all post-injection time points. There were approximately two-fold increases in NMJ density after each set of botulinum toxin injections compared to age-matched control orbicularis oculi muscles.
Botulinum toxin results in a significant increase of NMJ density when examined one, two, and four weeks after treatment. Quantification of NMJ localization and density in normal orbicularis muscle, as well as after botulinum toxin treatment, has ramifications for understanding surgical or pharmacologic interventions used for the treatment of blepharospasm.
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