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L.K. McLoon, B. Becker, S.P. Christiansen; Increasing Extraocular Muscle Strength with Insulin Growth Factor-II Injections in Rabbits . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3125.
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Purpose: The ability to strengthen an underacting extraocular muscle (EOM) is as important as weakening an overacting EOM in the treatment of strabismus. Currently this can only be accomplished surgically. We hypothesized that since EOM has the potential to add myonuclei in the adult, it should be possible to increase the rate of this process resulting in a larger, stronger muscle. Previous work demonstrated that adult rabbit EOM continues to express IGF and IGF-receptor. The presence of IGF-receptor in normal EOM suggested that treatment with IGF might result in muscle hypertrophy that, in turn, should increase force generation in the treated muscle.Methods: Single superior rectus muscles from rabbits were injected with either 10 or 20 micrograms IGF-II, and the contralateral superior rectus received an injection of saline only. One week after injection, muscle morphology and muscle force were quantified for both the IGF and sham injected muscles. Muscle force was assessed using an in vivo preparation. Results: One week after injection with 10 micrograms IGF-II, the treated muscle showed an increase in myofiber cross-sectional area. In the IGF-treated muscles mean developed tension (10Hz) was 0.747 mN/cm3 compared to 0.422 in controls, and at 200 Hz the mean tetanic tension of the treated muscles was 4.752 mN/cm3 compared with 3.342 mN/cm3 in saline-injected controls. Similar increases were seen at all stimulation frequencies. The muscles injected with 20 micrograms IGF showed an even greater increase in force compared to that seen after injection of 10 µg IGF. Conclusions: EOM appears to be very responsive to direct injection of insulin growth factor. It is important to demonstrate that these increases in both muscle mass and muscle force can be maintained for a sufficient duration to be effective in the treatment of strabismus. Injection of growth factors into EOM may prove to be a viable means of increasing extraocular muscle strength and may add to the expanding array of new pharmacological treatments for strabismus currently in development.
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