I commend Dr Mims for his thoughtful comments about IOOA. He believes that primary IOOA is caused by muscle hypertrophy and secondary IOOA could be explained by contracture. He bases this on his anecdotal observation at surgery that IOs that are primarily overacting typically appear larger than IOs that are overacting secondary to fourth-nerve palsy.1 Dr Mims feels that contracture would not account for the hypertropia being larger in gaze up and in than in horizontal sidegaze.
Kushner BJ. Alternate Explanations for Inferior Oblique Muscle “Overaction”—Reply. Arch Ophthalmol. 2006;124(12):1798. doi:10.1001/archopht.124.12.1798-a