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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.
A hypertrophied muscle is one that is enlarged owing to individual muscle fibers being larger than normal.2 The term hypertrophy merely describes anatomy and does not denote an alteration of function a priori. It is not clear to me why an IO that is larger than normal should necessarily cause a hypertropia that is greatest gaze up and in, unless it also generates an increased contractile force. Reading between the lines of Dr Mims' letter, I believe he is using the term hypertrophy to imply an increase in contractile force as opposed to contracture, which is associated with an increased elastic force. Numerous unpublished written and oral communications I have exchanged with Dr Mims over the past year confirm my belief. I feel that this dichotomy is overly simplistic. Muscles can be enlarged yet have subnormal contractile force. Graves orbitopathy represents 1 example of this.3 Conversely, muscles may have normal size but increased contractile force. This can occur if a normal muscle receives increased innervation or if there is a change in the proportion of the different fiber types within a muscle.2
I hypothesized that the initial process resulting in primary IOOA was increased stimulation to the IO that in turn leads to contracture.2 I feel that this increased innervation can account for the largest hypertropia being in gaze up and in, which could not be explained by contracture alone.
Dr Mims' claim that primary IOOA is characterized by a larger muscle than is found with secondary IOOA is a testable hypothesis, as pointed out by Demer.4 I would be interested in what a rigorous investigation of this issue would show. Demer also gave other explanations for Mims' observation of enlarged IOs in primary IOOA. In addition, I routinely observe that although IOOA is associated with abnormal exaggerated forced traction testing results,5 I note similar findings in primary and secondary IOOA. David Guyton, MD, indicated that his experience is similar to mine (written communication, July 8, 2006). If primary IOOA is indeed characterized by hypertrophy of the muscle, one might expect that to be discernible by the exaggerated forced traction test.
Correspondence: Dr Kushner, Department of Ophthalmology and Visual Sciences, University of Wisconsin Hospital and Clinics, 2870 University Ave, Suite 206, Madison, WI 53705 (bkushner@facstaff.wisc.edu).
Financial Disclosure: None reported.
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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