Trabecular Meshwork Bypass by Stent

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Article initiated by:

Albert S Khouri, MD, Fiona Kim

All contributors:

Albert S Khouri, MD, Ohoud A Owaidhah, MD

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Trabecular Micro-bypass Stents

Introduction

Prevalence & Impact

1. Glaucoma is the second leading cause of blindness in the world
   • Worldwide population aged 40-80 years approx. 3.54% are affected
   • Projected to affect 76 million people by 2020
   • Highest prevalence in Africa and Asia
2. Glaucoma is progressive, chronic, and irreversible disease despite treatment
3. Ab-interno microbypass stent
   • bypass through the Schlemm's Canal increases outflow through channels in nasal quadrant

Background Info

1. Intraocular Pressure
   • IOP is determined by inflow and outflow
   • Lower IOP prevents slow down of glaucomatous progression
2. Outflow Dysfunction
   • Function of aqueous humor is to provide nutrients and hydration of the Anterior Chamber while removing waste products
   • Aqueous humor is produced by the ciliary body and flows through the Trabecular meshwork, enters the Schlemm’s Canal, passes into collector channel entrances to the external wall of the Schlemm’s canal.

AH then enters the deep scleral plexus into vessels called aqueous veins that disperse through to episcleral veins to the venous system

1. • Inability of the outflow leads to a build up of aqueous humor in the anterior segment pressing onto the vitreous humour and the optic nerve
2. Resistance in Trabecular Meshwork
   • Trabecular meshwork is a filter like structure that regulates aqueous humour outflow 360˚ circumference divided into segments of high and low flow regions
   • TM is the most important structure for regulating aqueous outflow.

Indications

The iStent Trabecular Micro-Bypass Stent is FDA approved in the U.S indicated for use in conjunction with cataract surgery for the reduction of intraocular Pressure (IOP) in adult patients with mild to moderate open-angle glaucoma currently treated with ocular hypotensive medication.

Types

• Primary open angle/Chronic open angle
• Secondary Open angle - Buchara one-year study of iStent microbypass on secondary glaucoma showed effective decrease in IOP
• Pseudoexfoliation Glaucoma
• Pigmentary Glaucoma
• Steroid induced Glaucoma
• Traumatic glaucoma
• Mild to moderate cases
• must already be on topical treatment

Contraindications

Other elevated IOP disease -

1. Primary or secondary angle closure glaucoma
2. Neovascular glaucoma
3. Uveitic glaucoma

Elevated Episcleral diseases

1. Thyroid eye disease
2. Retrobulbar tumor
3. Sturge-Weber Syndrome

Other

• Children
• Chronic inflammatory disease
• Abnormal anterior segment
• Pseudophakic/aphakic patients

iStent inject – can be used as stand alone but is off-label use Third generation istent

Evaluation

Gonioscopy - to determine

1. Peripheral anterior synechiae
2. Rubeosis
3. Pigmentation in TM – outflow channels

Optical Coherence Tomography of Optic Nerve Head

• Noninvasive imaging technique to view cross-sectional layers
• Detects glaucomatous thinning in four quadrants to determine severity of disease in comparison to cross-normative data based on age

Design

Mechanism of Action -

• Two multidirectional outflow

First generation – Istent

1. U.S FDA approved 2012
2. Lentgh: 1mm, height: 0.33mm, snokel: 0.25mm x120 micom

weight: 60microgram sugrical grade nonfeerromagnetic titanium heparin-coated

1. Fit : fit within 270 micrometer, three retention arches
2. Inserter system: preloaded single use inserter w/rotator grip two versions for left/right eye

Second Generation- iStent Inject

1. U.S FDA approved 2018
2. Design
   • Head: in Schlemm's canal
   • Side flow four outlets (50micometers each)
   • Thorax: in Trabecular meshwork
   • Flange: in AC
   • Central Inlet
3. Relationship between number of istents and efficacy data
   • Greater IOP and med reducing efficacy
     • Katz experiment – 1 vs 2 vs 3 iStent in 42 month clinical trial
       • Findings: placement of first iStent is the most effective with subsequent iStents providing incremental decrease in IOP
         • One iStent – avg 7.6mmHg decrease (30% reduction)
         • Two iStents – avg 9.2mmHg decrease (37% reduction)
         • Three iStents – avg 10.9mmHg decrease (43% reduction)

Trabecular Micro-bypass Stent Pipeline Products

1. Third generation - iStent Stand alone
   • Estimated date 2022-2023
   • Without Cataract extraction surgery
2. Fourth Generation - iStent Supra
   • Estimated date 2020
   • In combination with CE for moderate cases
3. Fifth Generation - iStent infinite
   • Estimated date 2020-2021
   • For Advanced/Refractory stages

Surgical Techniques

Insertion

1. First gen - through clear cornea phaco incision physiological preservation of 8 to 11 mmHg
2. Second gen - on pre=loaded inject inserter placed two to three clock hours apart in inferonasal quadrant

Complications

Operative complications

• Corneal edema
• Hypotony
• BCVA loss of +/- 1 line,
• Iritis
• PCO
• Stent obstruction

Glaucoma-related or stent 2nd surgeries

• Stent repositioning or LPI
• Trabeculoplasty
• Deep sclerotomy/sclerostomy

Other complications

• Obstruction in Schlemm’s canal
• Scarring around transluminal portion of iStent – important for long term effectiveness

Perioperative management

1. • Similar to CE postoperative treatment
   • Postop drops: Anti-inflammatory & Antibiotic

Outcomes

Benefits

• Reduce IOP while reducing or eliminating medication use
• Spare conjunctiva and reduce risk of iatrogenic hypotony and bleb formation
• Decrease risk of large IOP fluctuations associated w/noncomplaince
• Minimize risk of iatrogenic hypotony and bleb formation

Evidence of Benefits - Clinical trials for first gen iStent

1. US IDE Trial
   • Efficacy - 68% of patients with 21mmHg or less IOP w/o use of medication
2. Dr. Randy Craven study
   • Better IOP control on no medication through 24 month
3. Independent Physician-sponsored trial - Dr. Antonio Fea
   • After 15 months, mean IOP in iStent patients lower than CE alone
   • 67% of patients were able to be medication free vs. 27% of CE only patients

Evidence of Benefits- Clinical trials for 2nd gen iStent

1. Five year iStent inject study
   • Mean IOP at baseline 20mmHg. At five years mean IOP 16.3mmHg
2. Pivotal Trial
   • 75.8% IOP reduction in unmedicated GIOP after 24 months
   • 31% medication reduction from baseline
3. Herenger 3 year long term follow up study
   • 37% reduction in IOP after 3 years, 100% patients with IOP under 18mmHg
4. Harasymowycz Study - large retrospective study (Germany)
   • Mean IOP in 6 months-1 year = 13.8mm Hg
5. Clement (Australia) - multi surgeon study
   • 66% medication free after stent implant