Persistent diplopia after retrobulbar anesthesia (2005)

Reply: We thank Chan and coauthors for their interest in our article. They noticed the predominance of superior rectus involvement in our study and suggest that our use of a bridle suture may be the potential cause of diplopia. Bridle suture injury to the superior or inferior rectus muscle was believed to cause strabismus after cataract surgery.1 However, recent studies indicate that myotoxicity from local anesthetics may play an important role in the development of postoperative strabismus.2–7 Rainin and Carlson2 first suggested the myotoxic effects of local anesthesia on extraocular muscles, and their report was followed by many others.3–7 Munoz and coauthors3 reported 7 cases with superior rectus overaction and 4 cases of superior rectus paresis; a bridle suture was not used in cataract surgery in any patient. Esswein and von Noorden4 demonstrated no correlation between pareses and the use of a bridle suture, antibiotic or corticosteroid injection, or surgical technique and concluded that permanent paresis of a vertical rectus muscle may be caused by the myotoxic effect of the local anesthetic. Chan and coauthors mentioned 3 references for the bridle suture as a “well-recognized” cause of persistent postoperative vertical diplopia. However, no patient in the first study had bridle sutures8 and the third reference states that “our data do not support the theory that bridle suture injury to the superior rectus muscle has a major pathogenic role in vertical strabismus after cataract surgery.”9 One author of the third reference later found that some patients had received a misdiagnosis of bridle suture injury.10 In the remaining reference, Catalano et al.1 found scarring around the superior rectus and superior oblique muscles at surgery in 2 of 8 patients and believed the complications were related to bridle suture placement. Their patients showed hypertropia that worsened with down gaze or contralateral gaze, restriction on forced duction test, and, finally, fibrosis at surgery. On the other hand, our patients showed hypertropia that increased with up gaze and ipsilateral gaze and no marked restriction on forced duction test; inspection of the muscle at surgery did not disclose fibrosis (except in the same case), thus satisfying the criteria of overactive strabismus pattern.11,12 Besides, most of our patients typically reported they saw double on patch removal, which might be more compatible with paresis by anesthetics. Capó et al.6 demonstrated how the tip of a needle can touch the superior rectus muscle in cadaveric dissection. Because only the tip of the needle touches the superior rectus muscle, limited segmental damage might cause an overaction, whereas diffuse contracture from more extensive muscle damage produces restrictive patterns.6,7 In fact, overaction is more common when the superior rectus muscle is involved, and restriction is more common when the inferior rectus muscle is involved.3,6 The affected muscle in local anesthesia depends to some extent on the type of anesthesia. The muscle struck during a peribulbar block is usually the inferior rather than the superior rectus muscle, but for a retrobulbar injection, the 2 muscles are equally likely to be struck.6 Because most reports of diplopia after local anesthesia enrolled mixed populations with retrobulbar and peribulbar anesthesia and none of our patients had peribulbar anesthesia, we found a lower incidence of inferior rectus muscle involvement with retrobulbar anesthesia only. Grimmett and Lambert13 also identified 4 patients with superior rectus muscle overaction after retrobulbar injection among 7 cases with strabismus that developed after cataract extraction. Muñoz and coauthors3 found 11 cases of superior rectus overaction or paresis, and 10 cases of inferior rectus fibrosis or overaction, but did not describe the type of anesthesia used. As we mentioned in the Discussion,11 the smaller anatomical orbit dimensions in Asians, especially the shorter vertical diameter,14–16 might contribute to the remarkable frequency of superior rectus involvement in our study compared with the frequency in previous studies of white patients. We believe that the predominance of superior rectus involvement in our study may be attributed to the application of retrobulbar anesthesia and an anatomical difference of orbit. We appreciate that Chan and coauthors gave us the opportunity to clarify 1 of the points in our article. Soo Kyung Han MD Jeong Hun Kim MD Jeong-Min Hwang MD