Botulinum toxin paralysis of adult monkey extraocular muscle. Structural alterations in orbital, singly innervated muscle fibers.

Botulinum toxin was injected into the medial rectus muscles of adult rhesus monkeys, with postinjection survival periods of three to 56 days. Light and electron microscopic changes were most apparent in the orbital, singly innervated muscle fibers, which during the acute stage (seven to 28 days), exhibited denervationlike hypertrophy with dispersion of the central mitochondria toward the periphery of the fibers. Withdrawal of the capillary network on which this fiber type normally is dependent for oxidative function was a secondary adaptive response to the disuse resulting from botulinum toxin paralysis of neuromuscular transmission. Neuromuscular junctions still were present on all muscle fiber types, although evidence of sprouting was apparent. In the long term (following return of function at 42 to 56 days), the muscle fibers appeared normal and the vasculature recovered in proportion to the decreased cross-sectional area of the muscle fibers.