CLINICAL-EVIDENCE OF EXTRAOCULAR-MUSCLE FIBER-TYPE SPECIFICITY OF BOTULINUM TOXIN

Objective: To compare the effects of botulinum toxin on static and dyn amic aspects of eye movements, and thereby elucidate the mechanisms of its action on eye muscles. Background: Laboratory evidence indicates that static alignment and saccades are subserved by different extraocu lar muscle fiber types, and botulinum toxin may cause specific dysfunc tion of the fibers controlling static alignment. Diplopia is a well-kn own side effect of periorbital botulinum toxin injections in humans, a nd may be a clinical correlate of the laboratory findings. Methods: Se arch coil recording of eye movements was performed in one patient with systemic botulism, and in three patients with diplopia following peri orbital injection of botulinum toxin A. Results: In the patient with a cute botulism, eye movement alignment, range, and saccadic velocity pr ofiles were abnormal. In three patients with iatrogenic diplopia, stat ic alignment was abnormal but movement range and saccadic velocities w ere within normal limits. Edrophonium improved the range of movements and saccadic velocities in the patient with systemic botulism but was ineffective in reversing ocular misalignment in the one iatrogenic pat ient to whom it was administered. Conclusions: Precise alignment is su bserved by orbital singly innervated muscle fibers, and the effects of botulinum toxin are greatest on these fibers. This predilection is ap parent when the toxin dose is very small, as must have been the case i n our patients with iatrogenic diplopia. The lack of a response to edr ophonium probably reflects structural damage to muscle fibers. In cont rast, larger doses of toxin produce an acute dysfunction of all extrao cular muscle fiber types, which is responsive to edrophonium and conse quently reflects partial blockade at the neuromuscular junction.