Aqueous Shunts for the Treatment of Glaucoma

Jeffrey A. Tice, MD

Disclosures

CTAF 

In This Article

Background

Glaucoma

Glaucoma is the second most common cause of blindness in the United States and worldwide.[1,2] It is a progressive optic neuropathy characterized by nerve atrophy and loss of retinal ganglion cells. In its most common form, patients have progressive loss of peripheral vision that is usually asymptomatic even when it is very advanced. As many as half of people with glaucoma are unaware that they have the disease. Signs of disease include an elevated intraocular pressure (IOP), increased cup to disc ratio, and loss of vision on formal peripheral field testing. Glaucoma is usually identified during routine eye examinations either by noting an enlarged cup-to-disc ratio on fundoscopic examination, an increased IOP, or visual field loss with perimetry.

There are several forms of glaucoma. The most common is open angle glaucoma, which is thought to be due to resistance to flow through the trabecular network located at the angle formed by the iris and the cornea in the anterior chamber of the eye. This leads to decreased drainage of aqueous humor into Schlemm's canal and a relatively increased IOP. Angle-closure glaucoma often presents acutely with a painful red eye. It is characterized by a narrowed or closed anterior chamber angle, which limits drainage of the aqueous humor. Other causes of glaucoma include vasoproliferative disease, pigment dispersion, trauma, and uveitis.

Glaucoma is more common in older patients, African Americans, and those with a family history of glaucoma.[1–9] The primary modifiable risk factor for glaucoma is IOP, but up to 15% of patients with glaucoma have normal IOP.[4,10,11] Normal IOP ranges from 10 to 21 mm Hg. Lowering IOP is the primary therapy to limit or prevent vision loss. Several randomized trials and meta-analyses have demonstrated that interventions to lower IOP lead to decreased vision loss over time.[12–16] Loss of vision from glaucoma can have a significant impact on patients' quality of life. Patients with glaucoma are at increased risk for falls and motor vehicle accidents and it negatively impacts other activities of daily life.[17–23]

There is no single threshold pressure for beginning treatment.[24] Patients with normal IOP, but evidence of optic disc cupping and visual field loss are usually recommended for treatment.[25] Treatment is usually started for patients with IOP greater than 25 mm Hg, even in the absence of signs of early damage to the optic nerve. Pharmacological therapy with eye drops is first line therapy. There are several classes of drops including those intended to increase the outflow of aqueous humor (prostaglandins, alpha adrenergic agonists, and cholinergic agonists) and those intended to decrease aqueous humor production (alpha adrenergic agonists, beta blockers, and carbonic anhydrase inhibitors).[26] There appears to be consensus that the first class to be tried are the prostaglandins because they have fewer systemic side effects and are at least as effective as the other classes of medications.[27,28]

When drugs fail, laser therapy (trabeculoplasty) is usually the next step. Either continuous wave or pulsed lasers are directed at the trabecular network. Laser trabeculoplasty increases aqueous outflow and reduces intraocular pressure, but its effectiveness decreases over time and repeated treatments are needed every few years. It is a very safe procedure, with fewer side effects than either medications or surgery. It also has been shown in one large randomized trial to be at least as effective as eye drops when used as the initial therapy.[29] However, it becomes less effective with repeated treatments and many specialists do not use laser trabeculoplasty more than two or three times on an individual eye.

Surgery is considered for patients who are inadequately controlled or intolerant of medical and laser therapy. The history of glaucoma surgery dates back to the mid-19th century.[30] The standard surgical therapy today is trabeculectomy, also known as filtering surgery, in which part of the sclera is removed to allow aqueous humor to drain in a controlled manner from the anterior chamber into the subconjunctival space. The drainage region over the sclera is called a bleb. The surgery may fail over time due to excessive healing (scar formation) at the drainage site, which increases resistance to the outflow of aqueous humor. Anti-metabolite medications, such as mitomycin C and 5-fluorouracil, have been shown in randomized trials to improve outcomes following surgical trabeculectomy, but they increase the risk for infection of the bleb leading to endophthalmitis and also increase the risk for chronic hypotony and a large, uncomfortable bleb. Both beta-irradiation and post-operative steroids have been used to control the healing process, but they have been abandoned in favor of the anti-metabolite therapies. Trabeculectomy surgery is associated with uncommon, but severe complications, including endophthalmitis, cataract formation, and permanent blindness, so it has traditionally been reserved for patients with glaucoma refractory to less invasive treatments. The Collaborative Initial Glaucoma Treatment Study was a randomized trial that compared trabeculectomy to medical therapy as the initial treatment for open angle glaucoma.[31–34] They found similar visual field outcomes through five years of follow-up, but the surgical arm had higher early rates of visual acuity deterioration. Subgroup analyses suggested that surgery may be preferred as the first line treatment in patients presenting with advanced visual field defects, but that surgery led to worse outcomes in patients with diabetes.[33]

Aqueous Shunts

Aqueous shunts are devices that, like surgical trabeculectomy, create an alternate path for aqueous humor to leave the anterior chamber of the eye and thus lower IOP. They were initially used for patients who failed medical and laser therapy and had an underlying diagnosis that increased the risk that surgical trabeculectomy would fail. These conditions include neovascular glaucoma, uveitic glaucoma, corneal transplant, and iridocorneal endothelial syndrome. However, recent trends suggest that they are being used more often for patients at lower risk for trabeculectomy failure.[35–38]

There are a number of synonyms for aqueous shunts including glaucoma drainage devices, tube implants, and tube shunts. Devices available in the United States include the Ahmed (12 models), Baerveldt (3 models), Krupin, Molteno (6 models), Optimed, or Schocket shunts. New devices include the SOLX Gold shunt and the Ex-PRESS mini-shunt. The general approach of the shunts is to place a tube into the anterior chamber of the eye that drains through a plate or multiple plates attached to the sclera. These shunts vary in the materials used (silicone, silastic, polypropylene, gold, stainless steel), the use of valves in the tube, and the size and number of plates. The tubes provide a conduit to allow the controlled flow of aqueous humor from the anterior chamber to a space between the conjunctiva and the sclera (the bleb) where it is absorbed into the blood.

The initial shunts were not valved and required a suture to be tightened around the drainage tube until healing around the plate caused an increased resistance to flow. Without the suture, the IOP pressure would drop too low (5 mm Hg or less) causing flattening of the anterior chamber, accelerated corneal damage, and cataract formation, a condition known as hypotony. The suture is usually removed four to six weeks after the initial surgery. Some newer shunts include a one-way valve or flow restrictor that limits flow through the device when the pressure in the eye becomes low. This valved approach is intended to decrease the likelihood of post-operative hypotony and avoid the need for a second procedure to remove the ligating suture, but has the potential to limit the long-term effectiveness of the shunt.

The Ex-PRESS shunt is a stainless steel device designed to have more reproducible results with less dependency on surgical skills than other aqueous shunts. The initial subconjunctival implantation of the device had a high complication rate, including erosion of the conjunctiva. This led to a modification of the technique in which the shunt is implanted under a scleral flap.[39]

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