Description of the condition
Glaucoma is a chronic progressive optic neuropathy associated with structural damage to the optic nerve and associated visual field loss, which can lead to vision loss and blindness if left undiagnosed and untreated (Foster 2002). Glaucoma is a heterogeneous group of conditions with multiple etiologies. The two main types are open angle glaucoma (OAG) and angle-closure glaucoma (ACG). Neovascular glaucoma is a severe form of secondary glaucoma that results from occlusion of the trabecular meshwork and secondary closure of the angle by fibrovascular tissue proliferation.
Known risk factors that contribute to damage to the optic nerve include elevated intraocular pressure (IOP), older age, positive family history of glaucoma, African racial background, high myopia, high cup-to-optic disk ratios, exfoliation syndrome, and decreased central corneal thickness (Coleman 2008; Gordon 2002; Landers 2002; Medeiros 2003; Quigley 2011).
However, IOP remains the only modifiable risk factor for glaucoma and prognostic factor for glaucoma outcomes. Thus, therapies for glaucoma, regardless of disease mechanism, target IOP reduction (Coleman 2012). Normally, the rate of aqueous humor production by the ciliary body equals the rate of its outflow. IOP increases when there is excess aqueous humor production or when part or all of the aqueous humor drainage system is blocked (Pan 2011).
Randomized clinical trials (RCTs) of participants with glaucoma have shown that there is a clear benefit to lowering IOP with medications, laser procedures, and incisional surgery to prevent further optic nerve damage and visual field deterioration (Burr 2012; CNTGS 1998; Leske 2003; Lichter 2001; VanVeldhuisen 2000; Vass 2007).
Glaucoma is an increasingly critical public health problem due to the aging world population. Glaucoma is the leading cause of irreversible blindness and the second leading cause of blindness worldwide (Quigley 2011). In 2006, Quigley projected that 60.5 million people worldwide would be diagnosed with glaucoma, and 8.4 million would be bilaterally blind from primary glaucoma in 2010 (Quigley 2006). Quigley 2006 also projected that by 2020 these would increase to 79.6 million and 11.2 million, respectively. OAG is the most common type of glaucoma and accounts for 74% of cases worldwide (Quigley 1996). Worldwide, of those over 40 years old, 2% are estimated to have OAG, 0.7% to have ACG, and 0.4% to have neovascular glaucoma (Quigley 1996; Quigley 2006). In OAG, incidence by gender is similar; however, black people have almost three times the age-adjusted prevalence than white people (Friedman 2004). In ACG, there are considerable difference in prevalence by ethnicity. The highest rates are reported in Chinese, Inuit, and other Asian populations (He 2006; Van Rens 1988).
Symptoms and diagnosis
OAG is often called the “silent thief of sight” because it progresses slowly and may cause irreversible damage before a person with glaucoma notices any vision loss. People with glaucoma typically do not notice visual field loss until central vision is affected in a late stage of the disease; 50% to 90% of people with glaucoma are unaware that they have glaucoma (Weinreb 2014).
ACG can be categorized into acute and chronic cases. Acute ACG requires immediate management to avoid blindness. People with acute ACG present with a painful red eye, blurred vision, headache, nausea, and vomiting (Weinreb 2014). People with chronic ACG present with similar symptoms as those with OAG. Glaucoma is diagnosed using tonometry, gonioscopy, optic nerve imaging, visual acuity, and visual field assessment.
Description of the intervention
Cyclodestructive procedures, first introduced by Vogt in the 1930s (Vogt 1936), are traditionally used in eyes with refractory glaucoma for whom filtration procedures have failed to lower IOP or slow progression, eyes with elevated IOP and limited useful vision on maximal medical therapy, and eyes with no visual potential in need of pain relief (Ansari 2007; Bloom 1997; Lin 2008; Pastor 2001). The goal of treatment is to reduce aqueous humor formation through ablation or destruction of the ciliary body epithelium. A number of different modalities have been used to achieve this aim including diathermy, cryotherapy, laser, ultrasound, and surgical excision (Beckman 1972; Bietti 1950; Coleman 1985; Shields 1985; Vogt 1936). Laser cyclophotocoagulation (CPC), first introduced in the 1970s by Beckman (Beckman 1973), has become the most common surgical method for reducing aqueous inflow. CPC can be performed using a neodymium:yttrium-aluminum-garnet (Nd:YAG) or diode laser (Lin 2004; Martin 2001) and laser energy can be delivered by either the contact or non-contact method (Lin 2002). Although CPC has been used to treat refractory glaucoma successfully, significant postoperative complications and discomfort have been reported by people undergoing this procedure (Lin 2004). Endoscopic cyclophotocoagulation (ECP), a newer method that specifically targets the ciliary epithelium under direct viewing, has become an increasingly popular treatment for refractory glaucoma. Since ECP can selectively ablate ciliary body tissue, ECP has a lower incidence of vision threatening complications (Lin 2002). However, ECP is an intraocular procedure and thus potential risks in order of frequency of occurrence post-treatment include fibrin exudate, hyphaema, cystoid macular edema, vision loss, and choroidal detachment (Lin 2002). Other potential complications that may occur but were not seen in Lin 2002 include endophthalmitis, choroidal hemorrhage, and retinal detachment.
Other glaucoma surgeries include trabeculectomy and aqueous shunts. Trabeculectomy is generally used in eyes where medications and laser therapy are insufficient to control disease (Prum 2016). Trabeculectomy is a filtering surgery where a full-thickness block of eye filtration tissue is removed to decrease resistance to the outflow filtration of aqueous. While trabeculectomy lowers eye pressure and is considered by many ophthalmologists to be the gold-standard glaucoma operation, it is associated with significant postoperative complications. Complications include hyphaema, shallow or flat anterior chamber, hypotony, choroidal detachment, and hypotony maculopathy (Eldaly 2014). Aqueous shunts are used to manage medically uncontrolled glaucoma when trabeculectomy has failed to control IOP or is unlikely to succeed (Prum 2016). Aqueous shunts consist of a tube that diverts aqueous humor to an end plate (Minckler 2006).
How the intervention might work
When there is excess aqueous humor production or when part or all of the aqueous humor drainage system is blocked, IOP increases (Pan 2011; Turkoski 2012). Aqueous humor is produced by the ciliary body epithelium. The laser energy that targets the ciliary body epithelium induces coagulative necrosis of these tissues and results in reduction of aqueous humor production and, thus, reduced IOP (Liu 1994). To achieve long term IOP reduction, multiple treatments are sometimes necessary because the ciliary epithelium can regenerate (Lin 2008).
Why it is important to do this review
While cyclodestructive procedures are not new, there is debate regarding which cyclodestructive method is best and how they compare to other glaucoma surgeries. Additionally, much of the literature reports only small non-comparative case series; such study designs cannot demonstrate a benefit of one therapy over another. A systematic review, ideally incorporating meta-analysis, would be beneficial to consolidate information across studies and to estimate the relative effects of different procedures on IOP control.