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Editors Selection IGR 21-2

Clinical Examination Methods: Retinal Vasculature in Glaucoma

Comment by Alex Huang on:

78032 Pilot study assessing the structural changes in posttrabecular aqueous humor outflow pathway after trabecular meshwork surgery using swept-source optical coherence tomography, Yoshikawa M; Akagi T; Uji A et al., PLoS ONE, 2018; 13: e0199739

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In this paper, the authors compared post-trabecular aqueous humor outflow (AHO) pathway anatomy using anterior segment swept-source optical coherence tomography before and after minimally invasive glaucoma surgeries (MIGS). The authors reconstructed a 3-D representation of distal-AHO pathways using methods they previously developed. The constructs were compared qualitatively as pathways as being 'increased,' 'non-significantly changed,' or 'decreased' after surgery. Quantitatively, AHO pathways were measured from the collector channel to episcleral vein when the entire pathway was observable. After successful MIGS and IOP lowering, all of the qualitative outcomes were seen. There was no significant difference in AHO pathway area before (3155 ± 1633 pixels) and after (3212 ± 1684 pixels; p = 0.50) surgery.

The immediate impression was that there was no consistent structural association between distal AHO pathway anatomy (qualitatively or quantitatively) and IOP reduction. The authors noted considerable prior AHO structural and tracer-based research, spurned in large part by MIGS. Put together, the only consistent conclusion has been that the AHO biological system is much more complex than originally appreciated.

The immediate impression was that there was no consistent structural association between distal AHO pathway anatomy (qualitatively or quantitatively) and IOP reduction

The TM has long been held to be the main source of AHO resistance. In this case, the observations in this report are not surprising. TM was ablated, IOP was lowered, and thus the distal outflow pathways did not have to change. However, we simultaneously know that distal AHO can change. Phase-sensitive OCT,¹ muscarinic agonists, and laser trabeculoplasty have shown distal AHO pathway structural alterations. Aqueous angiography has shown the ability of the eye to dynamically increase and decrease AHO.^2,3 Thus, the overall 'no-change' in this paper was unlikely because distal AHO pathways simply did not change but instead because the eye was responding differently to different circumstances ('increased,' 'decreased', etc.). There was no 'net' change. Potential reasons for various ocular responses would be to prevent blood reflux into the eye or to maintain stable IOP for stable optics.

Ultimately, this paper showed that distal AHO anatomy change did not predict MIGS outcomes. What remains to be seen is whether pre-operative characteristics (structural or flow-based) can predict MIGS outcomes.

Ultimately, this paper showed that distal AHO anatomy change did not predict MIGS outcomes

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