Glaucoma characterizes a group of similar diseases defined by progressive damage to the optic nerve (optic neuropathy) that occurs in a characteristic pattern with associated changes in appearance and visual field (Foster 2002). High intraocular pressure (IOP) is associated with glaucomatous optic nerve damage. IOP can elevate when aqueous humor, a clear fluid that continuously flows in and out of the anterior chamber to nourish the eye, does not drain properly (Mapstone 1968; EGS 2014; AAO 2015). When impairment of aqueous drainage occurs at the trabecular meshwork by the iris, this is referred to as angle closure (Emanuel 2014). Optic nerve damage resulting from angle closure commonly is described as primary angle-closure glaucoma (PACG), while optic nerve damage without the angle closure is known as primary open-angle glaucoma (POAG). Both open-angle and angle-closure glaucoma also can be classified as secondary when the condition is traced to an identifiable concomitant cause such as an eye injury, eye inflammation, or other eye illnesses (Law 2013). A further manifestation of angle closure is an acute 'attack' or crisis, in which sudden blockage to the drainage is associated with very high intraocular pressures and symptoms including headache, blurred vision and a severe dull eye pain.
Currently, there is increasing interest in examining the efficacy of interventions for preventing PACG (Yu 2015). These include medical or surgical treatments that aim to equalize the pressures across the anterior and posterior chambers of the eye by allowing the iris to fall back, away from the trabecular meshwork, in an attempt to open the angles and lower IOP. Two common techniques used to accomplish this objective are iridectomy—which involves surgical removal of parts of the iris—and iridotomy—which involves the use of a laser to create a hole in the iris. Early studies examining angle-closure disease in the 1950s documented that when the contralateral eye of a patient with angle-closure was given no treatment or pilocarpine (once or twice daily), there was a 50% chance over a period of 25 year sthat the patient would develop acute attacks or a sudden rise in IOP (Lowe 1962; Lowe 1966). Conversely, only 1 out of 54 patients treated with surgical iridectomy during this same period of time developed an acute attack in the untreated contralateral eye (Ang 2000; Edwards 1982; Snow 1977). Iridotomy is a less invasive and more common procedure than iridectomy (Ramulu 2007). While both achieve the same purpose, there are approximately 51 iridotomies for every iridectomy performed.
For patients and their providers, a better understanding of an intervention that addresses angle-closure glaucoma would assist in deciding the most appropriate prevention modality; for researchers and decision makers, this information facilitates the design and implementation of global screening programs and may be useful for identifying persons at risk.
Glaucoma is among the leading causes of blindness and, particularly due to the irreversible nature of the disease, a pressing public health challenge (Kingman 2004; Resnikoff 2004; Bourne 2013). The World Health Organization characterizes glaucoma as one of its priority eye diseases, and researchers have approximated that about five million people today are blind as a consequence of glaucoma (Osborne 2003; Quigley 2006). A recent systematic review found a global prevalence of glaucoma in the 40 to 80 years age group of 3.54%, and estimated that prevalence will reach 76 million by 2020 and 111.8 million by 2040 (Tham 2014). The prevalence of angle-closure glaucoma in particular is estimated to rise to 21 million by 2020 (Quigley 2006).
PACG is less common among Caucasians, with pooled prevalence of PACG for people aged 40 years or older from European ancestry estimated to be 0.4% (Day 2012). PACG, however, is more prevalent in Asians of Mongolian descent (Bonomi 2002; Tham 2014). Among the 64.26 million people with glaucoma aged 40 to 80 years, 20.17 million are estimated to have PACG in 2013; among this sub-population, 14.47 million are estimated to be living in Asia (Quigley 2006; Tham 2014). For example, in China, 91% of the 1.7 million cases of bilateral blindness are attributable to PACG (Foster 2001; Ng 2012). The prevalence also appears to be greater among older women, compared with their male and younger counterparts in all ethnic populations (Bonomi 2002; Day 2012). There is substantial variation in the data on the incidence rates for PACG, ranging from 4 per 100,000 person-years to 58.7 per 100,000 person-years (Erie 1997; Lai 2001; Ivanisevic 2002).
Description of the condition
The consensus view regarding the mechanism of elevation in IOP in angle-closure glaucoma has identified pupillary block as the major mechanism, which limits the flow of aqueous fluid from the posterior to the anterior chamber of the eye (Friedman 2001; Foster 2002; AAO 2015). The pressure differential created by the build-up of fluid causes the iris to bulge forward and come into iridotrabecular contact (ITC) with the trabecular meshwork and peripheral cornea (Mapstone 1968; AAO 2015). This contact causes a rise in the IOP by reducing the outflow of aqueous fluids.
Another important mechanism that is often associated with the development of angle closure is a plateau iris configuration. A plateau iris is the result of narrowing of the anterior chamber, pushing peripheral parts of the iris forward by displacement of the ciliary body anteriorly, leading to continuation of ITC (AAO 2015).
For this review, we follow a recently proposed classification of angle-closure glaucoma (Foster 2000; Aung 2001; Foster 2002; Ng 2012; AAO 2015). This definition rests on the idea of describing an 'occludable' angle, using terms such as 'narrow' to specify the anatomical predisposition to angle closure, further qualified by degrees of ITC and whether or not the patient has peripheral anterior synechiae (PAS). The drainage angle is easily and painlessly assessable through a gonioscopy during an eye exam.
Primary angle-closure suspects (PACS) are patients described as having narrow angles, where there is appositional or synechial contact 180 degrees or more, as observed on gonioscopy, between the peripheral iris and the posterior trabecular meshwork; however, there is no evidence of permanent aqueous outflow obstruction or damage to the angle. In other words, there is neither elevated IOP nor PAS. Accordingly, there are also no signs of elevated IOP or glaucomatous optic disc neuropathy.
Patients with primary angle-closure (PAC) are those showing signs of chronic angle damage beyond narrow angles with iridotrabecular contact in three or more quadrants at least 180 degrees, therefore obstruction by the peripheral iris has occurred and there is elevated IOP and/or PAS but no signs of glaucomatous optic disc neuropathy.
Primary angle-closure glaucoma (PACG) patients are those with 180 degrees of angle or greater in which the posterior (usually pigmented) and presumed functional trabecular meshwork is not visible and there is the presence of glaucomatous optic nerve damage in addition to elevated eye pressure and/or PAS as described for PAC.
The American Academy of Ophthalmology in its 2015 Preferred Practice Patterns for Primary Angle Closure summarizes clinical findings defining patients seen with angle-closure disease (AAO 2015) (Table 1). This classification for the progression from PACS to PAC to PACG suggests potential for preventing the consequences of narrow angles and angle closure through early detection and treatment. More traditional definitions of PACG and angle-closure disease were based on whether nor not symptoms occur acutely or chronically (e.g., having acute angle-closure crisis or AACC). For this review, we are treating acute angle-crisis or attacks as separate clinical conditions, despite similar mechanisms, and will not consider AACC for this review.
|Primary angle-closure suspect (PACS)||Primary angle closure (PAC)||Primary angle-closure glaucoma (PACG)|
|ITC greater than or equal to 180 degrees||X||X||X|
|Elevated intraocular pressure OR peripheral anterior synechiae||X||X|
|Optic nerve damage||X|
Description of the intervention
Iridotomy is a laser-assisted surgical procedure aimed at creating an opening in the peripheral part of the iris and is conducted as an outpatient procedure involving the use of a laser (e.g., neodymium-doped yttrium aluminium garnet or Nd:YAG laser, argon laser) and slit lamp biomicroscope (AAO 2015; Nolan 2000).
While iridotomy is also the standard of care for treating PAC and PACG (AAO 2015), there are some limitations and risks to using this procedure. By disrupting the natural flow of aqueous fluids in the eye, which may in turn result in significant increase in contact between the lens and the iris, there is a theoretical risk of more rapid development of cataracts (Caronia 1996). Other potential risks include rare occurrence of corneal endothelial damage localized to the surgery site, stray light symptoms, and the development of posterior synechiae (Pollack 1981; Quigley 1981; Robin 1984). Posterior synechiae potentially can limit vision in dimly-lit environments and complicate cataract surgery or pan retinal photocoagulation.
How the intervention might work
Iridotomy removes pupillary block by making an opening in the peripheral iris; this hole—created through the use of a laser—provides for free circulation of aqueous from posterior to anterior chambers even if the pupil becomes blocked (Fleck 1997; Friedman 2001; Ng 2012). This opening prevents IOP from rising further, which in theory should minimize subsequent optic nerve damage and progression of visual field loss.
Why it is important to do this review
Understanding the preventative effects of laser iridotomy in primary angle-closure suspects and patients with iridotrabecular contact are high-priority clinical questions that reflect the American Academy of Ophthalmology's Preferred Practice Patterns recommendations for management of primary angle-closure (AAO 2015). Epidemiologists have pointed out this topic area as an evidence gap for the management of PAC (Yu 2015).
While there is some evidence that a prophylactic iridotomy significantly reduces the risk of angle closure (i.e., PAC or PACG) in the contralateral eye of an individual who has previously been diagnosed with angle closure or PACG, there is much confusion regarding the need for a prophylactic iridotomy in eyes of patients with asymptomatic narrow angles noted through gonioscopy, i.e., PAC suspects (Snow 1977; Edwards 1982; Ang 2000). Similarly, some research findings suggest that iridotomy may be insufficient for long-term control of IOP (See 2011). Lastly, while complications of laser-assisted iridotomy seem minor relative to the risk and burden of angle closure, they are of significant concern for patients considering these procedures, particularly in East Asia and India (Dandona 2000; Foster 2000; Ramakrishnan 2003). For instance, if iridotomy hastens the progression of cataracts significantly, these procedures may cause more cataract-related blindness with widespread screening and usage of iridotomy treatment, especially in an environment such as a low- and middle-income country where cataract services are not universally available.