Re: “Lateral Rectus and Medial Rectus Expansion Following Orbital Decompression”

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We read with interest the recent article by Gupta et al.1 examining rectus muscle expansion following balanced orbital decompression. We commend the authors on studying these radiologic changes and would like to discuss and comment on several ideas raised.
First, the authors measured “change in maximal axial muscle width” for the medial rectus (MR) and lateral rectus (LR) and found that axial muscle width increased more for the LR. However, they concluded that extraocular muscles “expand” and “enlarge,” which implies 3-dimensional volumetric expansion. From a geometric standpoint, change in axial muscle width could serve as a proxy for volume if measuring a cylinder or prism of constant length. The extraocular muscles are neither a cylinder nor a geometric prism. Moreover, they are not of constant length, because proptosis reduction from orbital decompression results in an accordion-like shortening of the extraocular muscles. Conclusions about volume changes thus cannot be made based on linear axial muscle widths, and rigorous comparison of results to those of prior studies2,3 that measured actual muscle volume is not an apples-to-apples comparison.
Second, the precise method by which “axial muscle width” was measured is unclear. The axial muscle width could be construed as a horizontal width perpendicular to the midline. The orthogonal muscle width is orthogonal to the direction of the muscle fibers. Orthogonal muscle width is a more accurate measure of true cross-sectional width of the muscle belly. If using axial muscle width, the LR would appear wider postoperatively in part due to the more bowed, horizontal configuration of the muscle at its widest point. Therefore, the authors could be comparing a more orthogonal preoperative cross-sectional width to a more oblique postoperative width, which may have partially accounted for the LR’s increased width postoperatively without associated true volumetric expansion.
Third, there appears to be a discrepancy between the representative figure shown and the calculated results. From Figure, we digitally measured the preoperative and postoperative “orthogonal” muscle widths in millimeters (axial muscle widths would have yielded less accurate results given the horizontal LR configuration): preop: right medial rectus (RMR) 7, right lateral rectus (RLR) 4, left medial rectus (LMR) 4, left lateral rectus (LLR) 6; postop: RMR 12, RLR 7, LMR 7, LLR 10. The change in widths are RMR +5, RLR +3, LMR +3, and LLR +4. Based on these measurements, the average width increased more for MR than LR, with RMR having the greatest width increase, thereby seemingly to contradict the conclusion that muscle width increases more for LR.
Fourth, based on Figure 1, extensive lateral wall decompression was performed while medial wall decompression was only partial in nature, leaving most middle and posterior ethmoid air cells intact. The term “balanced” decompression implies that the medial and lateral decompressive effect should be roughly equal. Had a total ethmoidectomy and complete medial wall decompression been performed, MR may have had greater room to expand medially, assumed a more bowed configuration, and increased more in its measured axial widths. Thus, the study’s finding that muscle width increases more for LR than MR may reflect the laterally skewed surgical technique, while a truly balanced decompression may have yielded more comparable changes in LR and MR widths.
Fifth, we question whether aggressive exposure of the dura mater (Fig., bottom) with direct apposition of the LR belly with the meninges may itself affect muscle volume. Dura mater is not an inert tissue but rather a biologically active tissue.4 When performing deep lateral wall decompression, we prefer to leave a thin bony shell to avoid dura-rectus muscle contact.
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