Playing matchmaker with Schlemm canal surgery

Two glaucoma specialists advocate for matching a Schlemm canal surgery to the patient based on ocular needs, lifestyle factors and more. Image credit: ©Carles – stock.adobe.com

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

Visualising TM outflow

In his introductory presentation,¹ Prof 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 humour, 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 to 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 Schlemm's canal. Understanding these concepts is therefore essential in achieving successful TM surgeries.

Ruling out Schlemm canal

Dr Mercieca characterised 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 which utilise 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,” Dr Mercieca noted.

To decide which surgery is the right fit for a patient, Dr Mercieca said clinicians should consider which particular 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, Schlemm canal or collector channels. Factors to consider when choosing a surgical approach include the following:

• The patient’s age
• The longevity and progression of the glaucoma
• The patient’s lens status
• Lifestyle factors, including mobility, support system and recovery needs
• Prior glaucoma treatments and other surgeries
• Preference for, or request to avoid, implanted devices
• Availability and timing of procedure
• Insurance aspects including approval and reimbursement

A stand-alone Schlemm canal procedure

To illustrate the complicated nature of fitting Schlemm canal surgeries to patients, Prof 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 between the ages of 18 and 85. Unlike other studies, all eyes with primary open-angle glaucoma (POAG) or ocular hypertension (OHT) were pseudophakic and had standalone surgery. Contralateral eyes served as a control. Participants underwent medication washout for 28 days before the baseline measurements of IOP, Schiotz tonographic outflow facility. The washout and measurements were repeated at 3 and 12 months post-surgery.

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 Corp.), Professor Lim noted. However, these improvements became statistically not significant by 12 months.

Significant reduction in glaucoma medications were found in both time points but only 19% were reported as a “complete success” at 1 year, a metric which denoted 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 under 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 (seven patients), cyclodialysis clefts (five patients), hyphema (six patients), uveitis (two patients), and iridodialysis and hypotony (one patient each).

Prof Lim said the OMNI Surgical System appeared efficacious at 3 months but had minimal effect at the 12-month mark. An initial gain in 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, Prof Lim explained.

Real-world case studies

Dr Mercieca presented surgical videos featuring patients who underwent different types of Schlemm canal surgery. The first two patients were twin brothers, both 75 years old. 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 catarct symptoms, with a visual acuity of 6/18 and IOP of 19 mm Hg in the effected eye. Both men used two topical drops per day.

Dr 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, Dr Mercieca chose to utilise the iStent Inject for a right eye which 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, Dr Mercieca described a female patient, 46 years old, recently diagnosed with OHT. She had previously undergone an unsuccesful selective laser trabeculoplasty procedure and she remained phakic. She specifically expressed that she did not want an implant. Dr Mercieca performed an ab-interno canaloplasty using the iTrack Advance (Nova Eye).

Another female patient, 38 years old, 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 two ocular drops and was phakic. Using the OMNI Surgical System, Dr 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, Dr Mercieca described a male patient, 49 years old, who was working as an IT technician. This patient had high OHT, with an IOP of greater than 23 mm Hg, and three daily ocular drops. The patient was myopic and phakic. He also had a nickel allergy, a limiting factor for some implanted devices. Dr Mercieca performed excimer laser trabeculostomy (Elios Vision).

In conclusion, Dr Mercieca emphasised 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.