The pupillary effects of retrobulbar injection of botulinum toxin A (oculinum) in albino rats

Botulinum toxin (BoTx) has been clinically used in the treatment of localized dystonic states such as blepharospasm, as well as in strabismus. Reported side effects have included primary excessive weakness of neighboring extraocular muscles. To evaluate possible involvement of the iris, we injected BoTx into the retrobular space of albino rats. Ipsilateral mydriasis with cholinomimetic supersensitivity developed in the treated animals. There was no apparent optic nerve dysfunction. The authors observed these effects using BoTx doses insufficient to cause clinical weakness or electrophysiological evidence of generalized neuromuscular dysfunction. The mydriasis disappeared spontaneously within 2-3 weeks. Higher BoTx doses resulted in severe neuromuscular paralysis and death. These findings were consistent with clinical botulism, which may include autonomic paralysis. The site of BoTx action could be the ciliary ganglion or cholinergic terminals in the iris. The authors concluded that side effects of BoTx were not necessarily limited to striated muscle weakness.