Understanding Schlemm canal to improve the results of stent surgery

The author provides an overview of how having a better understanding of the anatomy and physiology of Schlemm canal, in addition to considering the clinical details revealed by slit lamp examination and gonioscopy, can help to improve the results of trabecular stent surgery. (Image courtesy of ©Belus / stock.adobe.com)

We do not yet have a clear understanding of the anatomy and physiology of Schlemm canal, which leads to imperfect results in implantation of trabecular microbypass devices, although some studies have focused on evaluating and understanding its function and anatomy. It is believed that the outflow of the aqueous humour is segmental and pulsatile,¹ and studies have illustrated the pump-like function of Schlemm canal.²

The aqueous humour flows more nasally, but there is no pattern for the flow, with many possible variations between individuals.¹ This makes it difficult to delineate the functional sectors of Schlemm canal and to decide where best to implant a stent (iStent; Glaukos) (Figure 1). We may easily implant the stent in a ‘silent’ area, in which case is it ineffective. The diameter of Schlemm canal is between 50.8 μm and 393.5 μm, and it is narrower in older patients and those with glaucoma.³

In comparison, the central outlet of the iStent is 80 μm; the central outlet may be thicker than Schlemm canal in some cases, which means stent implantation will not improve outflow (Figure 2).

In a previously published case series,⁵ we evaluated blood reflux after implanting iStents. Blood reflux was seen in a few cases, from one stent only, not from both.⁵ Some studies have implanted 3 stents to try to maximise outflow, but no significant difference in outcomes was reported.⁶ We suggest that this was the result of implanting the stents in a silent area where there is no outflow. This information led to the question of how we improve the results of trabecular stent surgery.

We make use of two suggestions:

1. Evaluating blood reflux in Schlemm canal via gonioscopy before surgery to find the best location to implant the stent. There are three patterns of blood reflux, as follows:

• No reflux, which indicates collapse of Schlemm canal (Figure 3)
• Patchy filling of Schlemm canal, illustrating its partial collapse (Figure 4)
• Full filling of Schlemm canal, indicating that it and the collector channels are healthy⁷ (Figure 5)

It may be not effective to implant a stent if blood reflux is absent.

2. Taking time to locate the aqueous veins by examining the conjunctiva nasally. This may suggest an approximate best position for implanting the stent (Figure 6).

In conclusion, we need a better understanding of the anatomy and physiology of Schlemm canal, in addition to considering the clinical details revealed by slit lamp examination and gonioscopy, to improve the results of trabecular stent surgery.

Abdelwahhab Azzawi, MD, FEBO, MRCPS (Glasg)

E: info@azzawi-augenpraxis.com

Dr Azzawi is an ophthalmic consultant in Großenhain, Germany. He has no financial disclosures.

References

1. Huang AS, Penteado RC, Papoyan V, Voskanyan L, Weinreb RN. Aqueous angiographic outflow improvement after trabecular microbypass in glaucoma patients. Ophthalmol Glaucoma. 2019;2(1):11-21. doi:10.1016/j.ogla.2018.11.010

2. Johnstone M, Xin C, Tan J, Martin E, Wen J, Wang RK. Aqueous outflow regulation - 21st century concepts. Prog Retin Eye Res. 2021;83:100917. doi:10.1016/j.preteyeres.2020.100917

3. Chen Z, Sun J, Li M, et al. Effect of age on the morphologies of the human Schlemm’s canal and trabecular meshwork measured with swept‑source optical coherence tomography. Eye (Lond). 2018;32(10):1621-1628. doi:10.1038/s41433-018-0148-6

4. Akiyama G, Huang AS. Aqueous humor outflow imaging: see where the aqueous is flowing. Glaucoma Physician. March 2019. https://www.glaucomaphy sician.net/issues/2019/march-2019/aqueous-humor-outflow-imaging

5. Azzawi A. Monitoring blood reflux during gonioscopy in glaucoma. Ophthalmology Times Europe. September 2020. https://europe.ophthalmologytimes.com/view/monitoring-blood-re flux-during-gonioscopy-in-glaucoma

6. Katz LJ, Erb C, Carceller GA, et al. Prospective, randomized study of one, two, or three trabecular bypass stents in open-angle glaucoma subjects on topical hypotensive medication. Clin Ophthalmol. 2015;9:2313-2320. doi:10.2147/OPTH.S96695

7. Grieshaber MC, Pienaar A, Olivier J, Stegmann R. Clinical evaluation of the aqueous outflow system in primary open-angle glaucoma for canaloplasty. Invest Ophthalmol Vis Sci. 2010;51(3):1498-1504. doi:10.1167/iovs.09-4327