Trabecular Bypass: Hydrus

Husam Ansari and Reay H. Brown

The Hydrus microstent (Ivantis, Inc., Irvine, CA) is a minimally invasive glaucoma surgery (MIGS) device designed to lower the intraocular pressure (IOP) while avoiding the complications of traditional glaucoma filtration surgeries. The Hydrus microstent is unique among the MIGS devices in that it provides both trabecular bypass and scaffolding of a three-clock-position arc of Schlemm’s canal, placing the trabecular meshwork on stretch and providing enhanced access of aqueous humor to downstream collector channels. This chapter reviews the Hydrus surgical technique, patient selection, and initial clinical results.

Case Presentation

A 79-year-old woman with primary open-angle glaucoma (POAG) presented complaining of decreased vision in her left eye. She had difficulty with reading and driving at night due to glare from oncoming headlights. Her glaucoma was controlled with the use of a topical β-blocker and prostaglandin analogue. She desired cataract surgery to improve her visual function, and despite maintaining adherence to medical therapy, she was very interested in reducing her need for topical medications for glaucoma control. Preoperatively, her best-corrected visual acuity (BCVA) with mild myopic, astigmatic correction was 20/60, and IOP was 15 mm Hg. She had moderate nuclear sclerosis and her angle was open to grade 4, with moderate pigmentation of the trabecular meshwork (TM). There was inferior notching of the optic nerve rim and superior nasal step on visual field testing. With a visually significant cataract and moderate POAG controlled on two medications, it was determined that the patient was a good candidate for cataract extraction combined with implantation of a Hydrus microstent.

The patient underwent uncomplicated cataract extraction, intraocular lens implantation, and Hydrus microstent implantation in the left eye. The Hydrus microstent was deployed into the nasal angle. Her postoperative course was uncomplicated, with steady improvement in her BCVA and immediate cessation of topical glaucoma medications. Two years after surgery, her visual acuity was 20/20 without correction, her IOP was 15 mm Hg without any glaucoma medications, and the Hydrus microstent was in its original position.

The Hydrus Microstent

The Hydrus microstent is an 8-mm nitinol implant that enhances aqueous humor outflow via three mechanisms (Fig. 30.1). First, the distal end of the device pierces through the TM and rests in Schlemm’s canal (SC) while the proximal end of the device (the inlet) remains in the anterior chamber (AC), providing unobstructed flow of aqueous humor from the AC to SC. Second, the three windows on the anterior face of the device stretch the TM, providing an alternate pathway for aqueous through the TM along the three-clock-position length of the device. Third, the device scaffolds SC, facilitating access of aqueous humor in the canal to downstream collector channels along the length of the device. The extended scaffolding concept distinguishes the Hydrus from the iStent (Glaukos Corp., Laguna Hills, CA), a previously approved MIGS device that also targets the trabecular bypass. The iStent also has a nonluminal section placed into the SC but with a shorter span of scaffolding.

In preclinical studies, the microstent has been shown to have excellent long-term biocompatibility in nonhuman primates and rabbits, and has been shown to increase outflow facility, decrease outflow resistance, and maintain patency of collector channel ostia in human cadaver eyes.^1–5 The device is implanted through a clear corneal incision under gonioscopic visualization using a preloaded handheld inserter and is readily combined with phacoemulsification cataract extraction (Fig. 30.2).

Patient Selection

The ideal initial patient for the Hydrus microstent has mild-to-moderate open-angle glaucoma controlled on one or two topical medications, requires IOP in the mid-teens, has moderate to dense pigmentation of the TM, and needs cataract surgery. These patients already have decided to have surgery to improve their vision and can be counseled that the Hydrus procedure adds minimal risk to the cataract surgery with the goal of decreasing their IOP and reducing or eliminating their need for glaucoma medications. The TM pigmentation will enhance the surgeon’s view of the AC angle target tissue, making it easier to place the Hydrus within the SC.

Surgical Technique

Patients undergoing Hydrus implantation combined with cataract surgery may receive the surgeon’s usual preoperative eyedrop regimen for cataract surgery. Topical or retrobulbar anesthesia may be utilized. After cataract extraction and intraocular lens implantation, intracameral miotics are helpful to improve the view of the angle. The AC is subsequently filled with viscoelastic.

Next, correct positioning of the patient and the operating microscope must be attained. The surgeon usually is seated temporally, although working superiorly is also possible. The microscope should be tilted 30 to 40 degrees toward the surgeon while the patient’s head is tilted 30 to 40 degrees away from the surgeon (Fig. 30.3). With the patient and microscope positioned this way, a handheld direct gonioprism coupled to the cornea with viscoelastic provides an excellent view of the nasal AC angle.

Prior to entering the eye with the inserter, the surgeon must take several steps to ensure a smooth deployment of the device. The surgeon’s index finger is placed on the advancement wheel on the inserter. The surgeon may pronate or supinate his or her wrist to whatever position is most comfortable and then rotate the cannula tip of the inserter on its long axis to achieve the optimal angle of deployment. The microstent should be lubricated by dipping the cannula tip in viscoelastic and then partially advancing and then retracting the microstent by rotating the advancement wheel. Immediately prior to entering the eye, the surgeon should ensure that the distal end of the microstent is visible in the cannula tip but is not protruding from the cannula (Fig. 30.4). Placement of the distal end of the device in this “ready” position minimizes the amount of wheel rotation needed to initiate deployment once the cannula incises the TM.