Morphological Changes in the Orbital Surface Layer Muscle of the Rabbit Eye Produced by Botulinum Toxin

Abstract

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.

Acute and long-term effects of botulinum neurotoxin on the function and structure of developing extraocular muscles

“…In the 2-week old chicks (P14), the size and distribution of mitochondria changed dramatically after treatment with BoNT, as we expected based on previous work in other species (Spencer et al, 1992;Ohtsuki et al, 1998). The average area, diameter, and perimeter of mitochondria in orbital and global muscle fibers of types I, II, and IV was significantly decreased (to 30-55%, Table 1).…”

“…Based on previous studies, the reduction of myofiber size seen within the orbital layer was expected. Prior investigations using adult EOM have shown that BoNT does not alter the size of all myofiber types equally, but instead tends to have its greatest morphological effect on the orbital, singly innervated muscle fibers (equivalent to our fiber type II) (Spencer and McNeer, 1987;Spencer et al, 1992;Ohtsuki et al, 1998;Kranjc et al, 2001). Such alterations in this particular fiber type have been postulated to play an important role in eye alignment during the treatment of strabismus (Porter et al, 1995).…”

“…Depending on the method of classification, EOMs contain six to eight distinct fiber types differentially located within these layers and they are either multiply innervated (slow tonic contracting) en grappe fibers or singly innervated (fast contracting) en plaque fibers (Mayr, 1971;Wasicky et al, 2000). For adult EOM, there is some consensus that BoNT specifically affects the morphology of myofibers within the orbital layer and does not significantly alter the size of myofibers within the global layer (Spencer and McNeer, 1987;Ohtsuki et al, 1998, Stahl et al, 1998Kranjc et al, 2001). However, for developing EOM, little is known about the effects of BoNT on myofiber morphology.…”

Ultrastructural changes in myotendinous nerve endings induced by injection of botulinum toxin into the extraocular muscle

“…The dose of botulinum toxin used in experimental studies varies, though it is typically in the range of 2-10 IU [17,25,30,31]. Although the ultrastructures of nerve endings might be affected differently according to the dose of botox, we used 5 IU in 0.1 ml because 1.25-5 IU of botulinum toxin is usually used for the treatment of strabismus.…”

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

“…8,9 It has also been shown that botulinum A toxin induces morphological changes in the orbital surface layer fi bers. 10,11 Botulinum A toxin is now used to weaken extraocular muscles because it is considered a safe and effective therapy for various muscle-related disorders. To alter eye alignment in clinical practice, injections of purifi ed botulinum A toxin to the extraocular muscles are performed under audible electromyographic guidance, using a unipolar Tefl on-coated 27-gauge needle to ensure accuracy.…”