Shunts, tubes, blebs, oh my! [TPG]

As we expand our scope of practice, our duties as optometrists increase with new responsibilities in regards to treating and managing glaucoma. In most cases, we continue to monitor glaucoma suspects and treat early glaucomatous cases.  Unfortunately even if we hit target eye pressure, glaucoma can still significantly progress requiring ophthalmological consult and surgical intervention.  Many of these advanced cases may never return but in truth, these patients still require routine optical tune-up and maintenance, frequent eye pressure checks, and sometimes, acute care. These patients can potentially walk through any private practice doors in desperate times. It is therefore critical to be able to co-manage and properly triage these complicated cases when we elect to be on the glaucoma playing field. 

Common treatments for mild and moderate glaucoma include new procedures and devices collectively termed as “minimally/micro-invasive glaucoma surgery (MIGS)”.  The main advantage of MIGS is that they are non-penetrating and/or bleb-independent procedures and avoid major complications related to blebs.  Selective laser trabeculoplasty (SLT) is an efficient and commonly used procedure that carries the lowest risk among glaucoma surgeries.  Possible complications are minimal but include temporary increase in IOP (experienced in 20% of patients but typically resolve in 24hours), transient blurring of vision, head/brow ache (if pilocarpine is used), eye pain and soreness, and mild inflammation.  Rare cases of macular edema and hyphema can occur.^{1, 2}

Surgical treatment is an option when topical medications and/or laser procedures are not tolerated and/or do not sufficiently reduce IOP.   Although trabeculectomy is still regarded as the “gold standard” in glaucoma surgery, the advent of MIGS have gained preference among patients and surgeons due to favorable safety profiles, ease of combination with cataract surgeries, and shorter surgical and recovery times. Eye pressures in the majority of MIGS clinical trials are typically in the mid-teens and therefore most ophthalmologists recommend these procedures for mild-moderate glaucoma cases.³

MIGS:

The Trabectome (NeoMedix Inc., Tustin, CA, USA) and the iStent Trabecular Micro-Bypass Stent (Glaukos Corporation, Laguna Hills, CA, USA) were the first MIGS products with FDA approval in the US.

The Trabectome was designed to reestablish access to the eye’s natural drainage pathway by removing a 60-to 120-degree strip of the trabecular meshwork and the inner wall of Schlemm’s canal with electrocautery. The goal is to achieve direct flow of aqueous into the canal and then into the collector channels.  The most common side effects in Trabectome include surgical failure, transient IOP spikes, hyphema, and goniosynchiae leading to scarring or membrane formation over the surgical site. Postoperative transient IOP spikes of 10mmHg or higher have been observed in 4-10% of patients but typically resolve without further surgical intervention.⁴ The iStent® is inserted into the Schlemm’s canal and provides an effective and safe procedure to treat coexisting open angle glaucoma and cataract cases. Common side effects for the iStent also include hyphema, transient IOP spike, inflammation, and inability to implant the stent or stent malpositioning.  Occasionally there may be obstruction of the stent lumen by blood clot or by the iris.  Multiple iStents may be implanted for better IOP control.^{4, 5}

Tip #1: Typically these surgeries are combined with cataract surgery, phacoemulsification alone have shown to decrease IOP between 2-4mmHg⁶

Tip #2: View the angle with a gonioscopic lens. Treat the transient IOP spike and inflammation accordingly with glaucoma medications and steroids

Tip #3: Hyphema is generally limited and self-resolving

Trabectome

iStent

A vast majority of patients who have mild to moderate glaucoma are often controlled with topical therapy and/or are great candidates for MIGS.  However, patients who have advanced uncontrolled glaucomatous disease may require filtering surgeries with trabeculectomy and/or drainage implants.  Modern glaucoma drainage implants consist of a tube that shunts aqueous humor to an end plate (or implant). The Ex-PRESS (Optonol Ltd., Neve Ilan, Israel) shunt is a hybrid of an aqueous shunt and a trabeculectomy.

Trabeculectomy: The bleb starts at a small dip at the superior limbus followed by a diffuse elevation of the conjunctiva.  Antimetabolites such as 5-fluorouracil (5-FU) or mitomycin-C (MMC) injections may be given intra/post-operatively to prevent scar formation.  Key features on examination include: area (diffuse vs demarcated), elevation/height, vascularity, and if the bleb is cystic or encysted (wall thickness). Increased area can result in over-filtration and subsequent hypotony.  Increased vascularity signifies vessel inflammation and concern for bleb failures, infection, and has also been correlated with loss of IOP control.  Avascular blebs are indicative of conjunctival thinning and are at risk for late leaks and rupture.  Although there are many studies that have described bleb morphology, there is no gold standard bleb grading.^{7, 8}

Tip #1: MMC & 5FU injections given post-operatively can be very toxic to the cornea causing significant punctate epithelial defects and potentially corneal melt in severe cases.  Patients should lubricate aggressively with preservative-free artificial tears
Tip #2: Although challenging use fluorescein to look for any bleb leaks utilizing the Seidel test. Some leaks percolate very slowly and are difficult to see. Start an antibiotic and immediately and refer back to the ophthalmologist if a leak is suspected.  Conservative measures of treatment include monitoring, aqueous suppressants, antibiotics, or use of a large diameter bandage contact lens.  Gentamicin is the antibiotic of choice for dual purposes: 1) broad coverage to help prevent infection which is always a concern with bleb leaks; 2) irritates the surface of the bleb which stimulates epithelial cells to proliferate causing scarring to help seal the leak.
Tip #3: In the presence of hypotony, patients can develop maculopathy and are susceptible to choroidal hemorrhages and detachments as well as optic disc edema. Hypotony maculopathy can lead to permanent vision loss. However, the duration of hypotony required to lead to maculopathy remains unknown.  Visual acuity is typically restored when IOP increases.  Macular OCT is a great tool to assess for macular changes.⁹
Tip #4: Refer immediately if blebitis is suspected.  Start patients on flouroquinolone antibiotics every hour while awake. Be aware that blebitis can quickly progress to endophthalmitis with severe eye pain and significant decreased vision.

Tip #5: Some patients do ocular digital massage as part of their regimen. Digital massage can improve aqueous flow through the bleb, reducing IOP.  Measure the IOP before and after digital massage.

Tube-shunt implants: Common implants include Ahmed valve (New World Medical, Inc., Rancho Cucamonga, CA), non-valved Baerveldt  (Abbott Medical Optics, Inc., Santa Ana, CA), and less commonly non-valved Molteno (Molteno Ophthalmic Limited, Dunedin, New Zealand) and Krupin  slit-valve (Hood Laboratories, Pembroke, Massachusetts, USA).  Examination requires assessing the implant, bleb, and the tube. The implant is sutured to the sclera under the conjunctiva and tenon near the muscle insertion and a bleb of fibrous tissue and collagen forms around the plate of the implant. Examination of the bleb is similar to that of a bleb in a trabeculectomy. The tube most commonly lies in the anterior chamber but can also be located in the posterior chamber with concurrent vitrectomy. Sometimes a surgical iridotomy is also performed to visualize the tube if it’s inserted in the sulcus.  Ideal length of the tube is short enough to be visible in the anterior chamber with no corneal or iris touch.  If the tube lies across the iris plane and comes into contact with the iris mild complications such as iritis or occlusion of the tube could occur. If the tube lies too close to the corneal endothelium, the cornea can fail over time and a second surgery for tube repositioning or even corneal transplant may be required. There should be careful examination to look for tube exposure or extrusion of the explant. In some practices, anti-metabolite injections are also used post-operatively to maintain adequate bleb formation.

Tip #4: Multiple tubes are becoming more frequent. Once a tube is implanted other surgical options are very limited if the IOP remains uncontrolled.

Tip #5: With the placement of the implant diplopia can be a post-operative complication. Prismatic glasses may be needed.

Alternative surgical options include non-penetrating glaucoma surgeries: canaloplasty, deep sclerectomy, and cyclophotocoagulation. In canaloplasty, an ophthalmic microcannula is used to enlarge the main drainage channel. After 360 degrees of canal cannulation, a suture is inserted into the entire circumference of the canal to provide tension to the inner wall of the canal. This allows the trabecular meshwork to facilitate better aqueous outflow into the canal. In deep sclerectomy, the aqueous outflow is enhanced by removing the inner wall of Schlemm's canal and juxta-canalicular trabecular meshwork, which are the structures responsible for outflow resistance.3   Cyclophotocoagulation (CPC) is typically indicated for refractory glaucoma in eyes with poor visual potential or blind and painful eyes. Cyclophotocoagulation ablates the ciliary processes subsequently lowering the production of aqueous humor and the eye pressure. In transcleral (TS) CPC the diode laser is applied superficially at the peri-limbal site and in endocyclophotocoagulation (ECP) the endolaser probe is applied intraoperatively through the pars plana to ablate the ciliary processes.  Newer treatment options include combined cataract extraction and ECP for moderate glaucoma cases.11

In general, immediate referral is indicated when there is significant increase in eye pressure and if there is inflammation and/or infection. If the tube is exposed or if there is any infection to the tube shunt, the issue needs to be addressed immediately.  Most surgical treatment options are reserved for advanced cases where the target eye pressures are typically in the low teens. Post-operative patients are also susceptible to infections which can occur at any time.

Surgical intervention for glaucoma continues to evolve with novel treatment options.  It is important for optometrists to be up to date and knowledgeable in order to co-manage these complicated cases. Patients often require general follow-ups and refraction post-operatively or need frequent eye pressure checks. Most importantly, patients may seek urgent care post- operatively and immediate treatment and referral may be warranted.