Morphological Changes After Subtenon Botulinum Toxin a Injection Into Rabbit Extraocular Muscles

Abstract

Abstract: : Purpose: Since its early experimental use on primate extraocular muscles in 1973 (Scott et al. 1973), botulinum toxin has been used to treat strabismus and other neuromuscular diseases. Botulinum toxin A is a potent presynaptic neuromuscular blocking agent, which induces denervation–like alterations in the motor innervation of skeletal muscle fibers. Previous clinical and experimental reports were mostly about intramuscular injection of botulinum toxin with electromyography (EMG) guidance. Because EMG guidance is a prolonged procedure, is noisy, and involves the placement of a long needle in front of the patient’s face, patients invariably feel anxious. In this study, we examined the effect of subtenon botulinum toxin injection without EMG guidance by quantifying the morphological changes of the rabbit extraocular muscle fibers. Methods: Experiments were performed with 10 white rabbits (2.0–2.5 kg). Ten units of botulinum toxin A was injected into subtenon space of ipsilateral superior rectus muscles. 0.1 ml of normal saline, as a control, was injected into subtenon space of contralateral superior rectus muscles. Sections were stained with Masson’s trichrome and observed by light microscopy. Using an Image–Pro Plus software, the diameter of myofibers of the orbital layer was measured on histological sections 3 weeks after injection. The statistical analysis was done with Kruskal–Wallis test. Results: Compared with global layer, the orbital layer was predominantly affected by botulinum toxin. Quantitative changes were noted in the muscle fibers of the orbital layer following the injection of 10 units of botulinum toxin. The average diameter of the fiber cells in orbital layer after botulinum toxin injection was 11.5±2.26 um at 3 weeks, whereas that of the control was 14.4±3.77 um. The difference was statistically significant (p<0.01). Conclusions: At 3 weeks after subtenon botulinum injection, the diameter of myofibers in the orbital layer of the rabbit superior rectus muscle was reduced significantly compared with that in the control. The result of subtenon botulinum toxin injection may be comparable to that of intramuscular injection. Further studies about long–term changes would be needed.

Keywords: extraocular muscles: structure