Background: Reports of acquired strabismus caused by injection of local anesthetics during cataract surgery have increased recently. The authors proposed a mechanism to explain the occurrence of strabismus with apparent overactive muscles after cataract surgery. Methods: The authors studied 19 patients in whom strabismus developed after cataract surgery. Prism and cover test in the diagnostic positions of gaze and forced-duction testing were used to identify the affected muscles. Results: The deviation was greater in the field of action of the presumed tight muscle in 16 of 19 patients. An ipsilateral hypertropia with superior rectus muscle overaction subsequently developed in two patients with an initial hypotropia. An overaction of the ipsilateral lateral rectus muscle causing an exotropia developed in one patient with initially limited abduction. Conclusions: Myotoxicity from direct injection of local anesthetics into an extra-ocular muscle probably causes transient paresis followed by segmental contracture of the involved muscle. Mild contractures result in strabismus with a motility pattern of an overactive muscle. Larger amounts of contracture lead to restrictive strabismus. The risk of strabismus may be decreased by administering the local anesthetic into sub-Tenon space using a blunt-tipped cannula when performing cataract surgery.
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