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L. K. McLoon, S. C. Christiansen, M. J. Mustari; Effect of Sustained Release IGF-I on the Medial Rectus of Infant Non-Human Primates. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2841.
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Primary forms of strabismus are caused by extraocular muscle (EOM) underaction, overaction, or a combination of both. There is currently no pharmacologic treatment for underacting muscles. In previous studies in rabbits we demonstrated that prolonged exposure of adult superior rectus muscles to insulin growth factor-I (IGF-I) results in a significant increase in EOM cross-sectional area and muscle force generation lasting over 3 months. The feasibility and efficacy of a pharmacologic approach to increasing muscle size and altering eye position were tested using the medial rectus muscles of infant non-human primates.
Sustained-release pellets of IGF-I were implanted contiguous to the medial rectus muscles in two infant non-human primates. The infants were assessed for retention of the pellets at the implant site by MRI and for eye position abnormalities using Hirschberg testing. After 3 months, the animals were euthanized. Histological and morphometric analyses of myofiber cross-sectional area in treated muscles were compared to age-matched control medial rectus muscles.
The implanted slow-release pellets stayed in place in all 4 medial rectus muscles treated, as evidenced by MRI analysis and pellet position during muscle removal. No eye position changes were seen in these infant monkeys. Mean myofiber cross-sectional area was increased almost 3-fold compared to age-matched control medial rectus muscles.
Because the infant extraocular motor control system is very adaptable, it is not surprising that strabismus did not develop in these normal infant monkeys. However, the significant increase in mean myofiber cross-sectional area suggests that sustained exposure of the EOM to IGF-I has potential as an approach for strengthening underacting muscles in strabismic children. Future studies will be directed at treating strabismus in this non-human primate model.
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