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Luisa Moreira Hopker, Juliana Neves, Daiane Nascimento, Tomas Mendonça, Edmar Zanoteli, Norma Allemann; Cross-sectional area measurement and fiber type distribution after bupivacaine injection in the rabbit extraocular muscle. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3436.
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Bupivacaine (BUP) injection in extraocular muscles (EOM) has been used to correct small angle strabismus in adults. The mechanism behind its effect is not completely understood. It has been demonstrated that it causes an initial necrosis followed by regeneration with an increase in strength. No previous study reported fiber composition changes after its injection. This experimental study aims to show changes in cross-sectional area of the myofibers and distribution of fiber types after injection of BUP in the EOM of rabbits.
40 male New Zealand rabbits were selected, 8 were age-matched controls. 32 rabbits received 0.3 ml of Bupivacaine 1.5% in the superior rectus (SR) of right eye (OD) and were sacrificed at 7,28,60 and 92 days. Haematoxylin & Eosin stain was performed and ImageJ was used to measure cross-sectional area. Immunohistochemistry was performed to analyze the proportion of fibers types 1 (Myo1) and 2 (Myo2), as well as the fibers with high proportion of embryonic myosin (MyoEmb) expression. Cross-sections of each muscle fiber type were analyzed by manual tracing. Comparisons between groups were made by Kruskal-Wallis test.
There was a difference in the area measurement between groups 7-day vs 60-day (909±255μm2;1,492±404, p=0.002), 7-day vs 92-day (909±255μm2;1,317±334μm2;p=0.012) and 28-day vs 60-day (1,062±363 μm2; 492±404μm2;p=0.043). The proportion of Myo1 was increased in the 60-day (88.5±16.2%;) group when compared to following groups: Control (56.4±16.3%;p=0.005), 7-day (51.6±15.9%;p=0.002) and 92-day (53.3±10.2%;p=0.005). There was no statistical significant difference in Myo2. There was a decrease in MyoEmb in the 60-day group (46.7±21.8%) when compared to control (79±14.2%;p=0.011), 7-day (82±13.4%; p=0.003) and 92-day (81.8±21.8%;p=0.003) groups.
Bupivacaine, when injected in the SR of rabbit decreases the fibers area around 7 days probably due to necrosis followed by a transient increasing trend and recovery to its initial size. The transient increase of Myo1 and decrease in MyoEMB at 60 days might play a role in force generation observed after BUP injection in clinical studies. Further research in rabbits analyzing fiber composition and EOM strength is needed to understand how increased type 1 fibers proportion could possibly correlate to strabismus correction after BUP administration seen in the clinical practice.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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