Morphological changes in the orbital surface layer muscle of the rabbit eye produced by botulinum toxin.

We quantitated the morphological changes in the orbital surface layer muscles in the rabbit following the single injection of botulinum A toxin. Experiments were performed in 9 white rabbits (1.6-3.0 kg). They were administered 5 units (5 rabbits) or 10 units (4 rabbits) of botulinum toxin injected into the superior rectus muscle of one eye. The diameter of myofibers of the orbital and intermediate layer zones was measured with an ocular micrometer on histological sections 3 days and 1, 3 and 5 weeks after injection. Quantitative changes were noted in the muscle fibers of the orbital surface layer zone following the injection of 10 units of botulinum toxin. At 1 week, the diameter of myofibers in the orbital layer was reduced, but it was increased at 5 weeks compared to that in the control eyes; in contrast, no change in the diameter of muscle fibers was found in the intermediate layer zone.