Playing matchmaker with Schlemm canal surgery

(Image Credit: AdobeStock/Claudia Nass)

Reviewed by Sheng Lim, MD, FRCOphth; and Karl Mercieca, MD, FRCOphth, FEBOS-GL

During the European Society of Cataract and Refractive Surgeons meeting, Sheng Lim, MD, FRCOphth, professor of glaucoma studies at St Thomas’ Hospital and King’s College London, both in England, and Karl Mercieca, MD, FRCOphth, FEBOS-GL, director of glaucoma services and lead surgeon at the University of Bonn Eye Clinic, in Germany, explored characteristics of the Schlemm canal.

Visualizing TM outflow

In his introductory presentation,¹ Lim explored trabecular outflow as the key to understanding glaucoma’s progression and therapeutic avenues. He cited trabecular meshwork (TM) outflow as a critical component in the regulation of IOP, and its dysfunction is associated with “almost all” glaucoma. The TM functions as a filtration site for aqueous humor, where the fluid drains into Schlemm canal before entering the systemic circulation.

The concept of segmental flow within the TM suggests that certain regions may facilitate higher rates of aqueous outflow compared with others. The presence of pigmented areas on the TM has been observed to correlate with areas of increased flow, potentially indicating that these regions are more efficient at facilitating drainage into the Schlemm canal. Understanding these concepts is therefore essential in achieving successful TM surgeries.

Ruling out Schlemm canal

Mercieca characterized Schlemm canal surgery as a suitable intervention for a wide variety of patients. Those who fall in the 75% of glaucoma patients with mild to moderate iterations of the disease are prime candidates for conventional outflow methods using Schlemm canal.² For some of that patient group, and for many of the 25% of patients with severe glaucoma, bypassing the Schlemm canal completely may be more effective.

These patients should, instead, undergo procedures that utilize the subconjunctival or suprachoroidal space. Patients with advanced glaucoma or ocular comorbidities are more likely to need bypass devices.

“Schlemm canal devices are considered by many to have a more favorable safety profile compared to subconjunctival and suprachoroidal devices,” Mercieca noted.

To decide which surgery is the right fit for a patient, Mercieca said clinicians should consider which anatomical area they want to affect. A bypass stent, canaloplasty, or trabeculotomy will be the correct answer, depending on whether a surgeon wishes to improve flow in the TM, the Schlemm canal, or collector channels.

A stand-alone procedure

To illustrate the complicated nature of fitting Schlemm canal surgeries to patients, Lim cited the OMNI outflow study³ from the King’s College London Frost Eye Research Department at St Thomas’ Hospital. The 30 patients in this cohort were aged 18 to 85 years. Unlike in other studies, all eyes with primary open-angle glaucoma (POAG) or ocular hypertension (OHT) were pseudophakic and had stand-alone surgery. Contralateral eyes served as a control.

Participants underwent medication washout for 28 days before the baseline measurements of IOP, and Schiotz tonographic outflow facility. The washout and measurements were repeated at 3 and 12 months postsurgery.

All patients received 360° viscocanaloplasty and 160° to 180° trabeculotomy, using the OMNI Surgical System (Sight Sciences). At
3 months, there were improvements in outflow facility with corresponding reduction in IOP, vastly superior to a prior, similar study conducted with the iStent (Glaukos Corporation), Lim noted. However, these improvements became statistically not significant by 12 months.

Significant reductions in glaucoma medications were found at both time points but only 19% were reported as a “complete success” at 1 year, a metric that denoted a less than 21 mm Hg pressure reading, reflecting a 20% IOP reduction and total discontinuation of medication. A further 8% of patients were a “qualified success,” with a pressure less than 21 mm Hg and 20% IOP reduction, but still required medication.

For the remaining 19 patients (73%), treatment was considered to have failed. Adverse effects included IOP spikes (7 patients),
cyclodialysis clefts (5 patients), hyphema
(6 patients), uveitis (2 patients), and iridodialysis and hypotony (1 patient each).

Lim said the OMNI Surgical System appeared efficacious at 3 months but had minimal effect at the 12-month mark. An initial gain in the trabecular outflow facility mirrored the reduction in IOP, though these results were not maintained. This suggests a larger opening in trans-TM surgery may be more effective, but, ultimately, scarring is likely to compromise the longevity of surgical results following any TM surgeries, Lim explained.

Real-world case studies

Mercieca presented surgical videos featuring patients who underwent different types of Schlemm canal surgery. The first 2 patients were twin brothers, aged 75 years. One patient, a former actor, had POAG for 10 years and experienced significant symptoms from a cataract in his right eye. In the latter, visual acuity was 6/24 with IOP of 18 mm Hg. The other patient, a former pilot, experienced 15 years of OHT in his left eye. He experienced significant cataract symptoms, with a visual acuity of 6/18 and IOP of 19 mm Hg in the affected eye. Both men used 2 topical drops per day.

Mercieca performed a trabecular bypass with the Hydrus Microstent (Alcon) for the patient who was an ex-pilot. Because his twin, the ex-actor, may have needed traditional surgery later on, Mercieca chose to utilize the iStent inject for his right eye that may need a superior deep sclerectomy in the future. “These patients looked identical, but their glaucoma was different,” he said. “Different eyes have different needs.”

In another example, Mercieca described a female patient, aged 46 years, who was recently diagnosed with OHT. She had previously undergone an unsuccessful selective laser trabeculoplasty procedure and she remained phakic. She specifically expressed that she did not want an implant. Mercieca performed an ab interno canaloplasty using the iTrack Advance (Nova Eye).

Another female patient, aged 38 years, reported a family history of glaucoma, including a myocilin gene mutation, a known genetic cause of POAG. The patient had an IOP of 20 mm Hg, was on 2 ocular drops and was phakic. Using the OMNI Surgical System, Mercieca performed a combined ab interno trabeculoplasty and 180° trabeculotomy. Patients with the myocilin gene mutation tend to have better outcomes with procedures that cut the TM, he said.

Finally, Mercieca described a male patient, aged 49 years, who was working as an IT technician. This patient had high OHT, with an IOP greater than 23 mm Hg, and 3 daily ocular drops.

The patient was myopic and phakic. He also had a nickel allergy, a limiting factor
for some implanted devices. Mercieca performed an excimer laser trabeculostomy (Elios Vision).

Conclusion

Mercieca emphasized the importance of matching a particular patient profile to a specific surgery.

“We are spoiled for choice, in a way,” he said, “but we can tailor our treatment to the patient standing in front of us.”

References

1. Lim S. Why Schlemm’s canal: anatomy and physiology of outflow. Presented at: European Society of Cataract and Refractive Surgeons Congress;
   September 6-10, 2024; Barcelona, Spain.

2. Mercieca, K. When to do Schlemm’s surgery: real world examples. Presented at: European Society of Cataract and Refractive Surgeons Congress;
   September 6-10, 2024; Barcelona, Spain.

3. Sherman T, Swampillai A, Goyal S, et al. Changes in intraocular pressure and tonographic outflow facility following combined viscocanaloplasty and trabeculotomy with the OMNI surgical system. Invest Ophthalmol Vis Sci. 2023;64(8):3480. https://iovs.arvojournals.org/article.aspx?articleid=2787738