• To evaluate muscle histologic features in humans following therapeutic botulinum toxin injections, we studied orbicularis oculi from 11 patients with blepharospasm; nine had previously received botulinum toxin injections and two had not. All muscles had comparable variability in muscle fiber diameter, with no necrosis, inflammation, denervation, or consistent alterations in muscle fiber internal architecture. Botulinum toxin produces no persistent histologic changes in human muscle fibers.
Harris CP, Alderson K, Nebeker J, Holds JB, Anderson RL. Histologic Features of Human Orbicularis Oculi Treated With Botulinum A Toxin. Arch Ophthalmol. 1991;109(3):393-395. doi:10.1001/archopht.1991.01080030095046