Effects of botulinum a toxin injection on the extraocular muscle fiber layers: Comparison between subtenon injection and intramuscular injection

To compare the morphological changes following injection of botulinum A toxin to the extraocular muscle fiber layers with those following injection to the subtenon intramuscular system. Twelve New Zealand white rabbits were divided into two groups. In the first group (six rabbits), four received injections of 10 units of botulinum A toxin into the subtenon space of the superior rectus muscle OD, and the same dose of botulinum A toxin was injected directly into the superior rectus muscle OS. The other two rabbits in the first group were labeled as control animals; 0.1 ml of normal saline was injected into the subtenon space OD, and direct intramuscular injection was performed on the opposite eye. The animals in the first group were killed after 4 weeks to measure the average diameter of the muscle fibers in both the orbital and global layer. In animals of the second group (six rabbits), the same procedures were performed and the animals were killed 12 weeks after treatment. The average diameter of muscle fibers in both the orbital layer and global layer was markedly reduced in all of the botulinum A toxin-injected groups at 4 weeks after treatment compared with the controls. There was no difference in the average diameter of the muscle fibers between the orbital layer of the subtenon-injected group and that of the intramuscular-injected group at 4 weeks after treatment, but the average diameter of the muscle fibers in the global layer was significantly reduced in the intramuscular-injected group compared with the subtenon-injected group at 4 weeks after treatment. At 12 weeks following treatment, there was no difference in the average diameter between the botulinum A toxin-injected group and the control group regardless of where the toxin was injected. Subtenon injection of botulinum A toxin induced similar morphological changes as direct intramuscular injection in the extraocular muscle fiber layers. These results suggest the possibility of clinical applications of subtenon botulinum A toxin injection for the treatment of strabismus.