Glaucoma, Angle Closure, Chronic
Chronic angle-closure glaucoma (CACG) refers to an eye in which portions of the anterior chamber angle are closed permanently by peripheral anterior synechiae (PAS).1,2 Variable and sometimes conflicting terminology has been used to describe different appearing forms. The problem arose from the fact that the terminology developed prior to the advent of indentation gonioscopy and laser iridotomy when the mechanisms of angle-closure glaucoma were poorly understood.
In the era of surgical iridectomy, an attack of acute angle-closure glaucoma (AACG) could arise in an eye that had developed PAS because of gradual angle-closure prior to the development of the attack. Conversely, a prolonged acute attack or a series of subacute attacks could lead to progressive PAS formation. Patients undergoing surgical iridectomy were dilated routinely after surgery. Shallow anterior chambers were not uncommon. Patients undergoing surgical iridectomy for AACG who were dilated postoperatively and had shallow anterior chambers not infrequently formed PAS.4,5
All of these conditions were lumped under the term CACG. The diagnosis was made because of the presence of PAS. The term CACG is used to denote eyes in which chronic appositional closure without PAS has led to elevated intraocular pressure (IOP) or in which appositional closure with the formation of PAS has occurred in the presence of normal IOP. A patient who is noted after iridotomy for AACG to have PAS also is considered to have had CACG prior to developing AACG. Prolonged apposition or repeated subacute attacks lead to gradual PAS formation. These usually begin in the superior angle, which is narrower than the inferior angle, as pinpoint synechiae, reaching to the midtrabecular meshwork, and gradually expanding in width.6 In early cases, in which appositional closure is present and IOP is normal, but in which PAS has not yet formed, the term chronic appositional closure is preferred. This condition can lead to elevated IOP and glaucomatous disc and visual field damage without PAS formation.
Eyes with progressive PAS formation eventually may develop AACG when pupillary block results in closure of the remaining portions of the angle unaffected by PAS. However, many patients develop gradual angle-closure, elevated IOP, and glaucomatous damage in the absence of symptoms. The presentation is similar to that of chronic open-angle glaucoma, with progression of glaucomatous cupping and visual field loss.7
PAS also may form during an acute attack, remaining after iridotomy has opened the unaffected portions of the angle. These PAS usually are high and broad. When first observed at this stage, it is impossible to determine whether the PAS formed before or during the attack, or at both times.
In eyes with darker irides, a second mechanism of progressive angle-closure is more common. The closure is circumferential and begins in the deepest portion of the angle. Closure occurs more evenly in all quadrants, so that the angle progressively becomes more shallow. The appearance over time is of a progressively more anterior iris insertion. Lowe has termed this creeping angle-closure.8 The PAS gradually creep up the ciliary face to the scleral spur and then to the trabecular meshwork.
Combined mechanism glaucoma
Combined mechanism glaucoma refers to situations in which both open-angle and angle-closure components are present. Most commonly, angle-closure glaucoma is treated successfully with iridotomy, eliminating all appositional closure, and IOP still remains elevated, with or without the presence of PAS of any extent.
Conversely, an eye with open-angle glaucoma may later develop angle-closure, either because of the natural development of pupillary block or because of exacerbation by miotic therapy.
Exfoliation syndrome commonly predisposes to combined mechanism glaucoma.9 In this case, open-angle glaucoma can develop independently years after iridotomy for angle-closure, with progressive blockage of the trabecular network. In all of these cases, the residual open-angle component is treated as open-angle glaucoma.
Mixed mechanism glaucoma
This term often is used interchangeably with combined mechanism glaucoma, but should not be, because it creates additional confusion. It is better to reserve this term to describe an eye with angle-closure due to more than one contributory mechanism. When pupillary block is eliminated by iridotomy and the angle opens to a greater degree than before the iridotomy, an appositional closure remains on the basis of plateau iris, phacomorphic glaucoma, or malignant glaucoma, a mixed mechanism may be present.
Plateau iris refers to an anatomic configuration in which the iris root angulates forward and then centrally.10,11,12The iris root often is short and inserted anteriorly on the ciliary face, so that the angle is shallow and narrow, with a sharp drop-off of the peripheral iris at the inner aspect of the angle. The iris surface is relatively flat and the anterior chamber is not unusually shallow.
When appositional angle-closure develops in the presence of a patent iridotomy or iridectomy, either spontaneously or after pupillary dilation, in an eye with this anatomic configuration, plateau iris syndrome is present.13 Some patients may develop AACG. The risk of postoperative pupillary dilation after iridectomy or iridotomy frequently is realized.
Until recently, plateau iris syndrome was considered rare. Two subtypes have been differentiated. In the complete syndrome, which is rare, IOP rises when the angle closes with pupillary dilation. In the incomplete syndrome, IOP does not rise. The differentiating factor is the height of the plateau with respect to the angle structures. If the angle closes to the upper meshwork or Schwalbe line, IOP rises because aqueous outflow is blocked completely, whereas, if the angle closes partially, leaving the upper portion of the filtering meshwork open, aqueous humor can still exit the eye. This condition is far more common, and its detection is important; these patients can develop PAS up to years after a successful iridotomy produces what appears as a well-positioned angle.
Plateau iris occurs because large and/or anteriorly positioned ciliary processes hold the peripheral iris up against the trabecular meshwork.14,15 Iris cysts also may cause a situation equivalent to plateau iris. When dynamic gonioscopy is performed in such an eye, the ciliary processes prevent posterior movement of the peripheral iris. As a result, a sinuous configuration results (ie, double hump sign), in which the iris follows the curvature of the lens, reaches its deepest point at the lens equator, and then rises again over the ciliary processes before dropping peripherally. Much more force is needed during gonioscopy to open the angle than in pupillary block because the ciliary processes must be displaced, and the angle does not open as widely. In a morphometric study of the ciliary sulcus, Orgul et al proposed that the displacement of the pars plicata from the peripheral iris to the iris root during embryogenesis may be incomplete in eyes of shorter axial length.16
Patients with plateau iris tend to be female, younger (30s-50s), and less hyperopic than those with relative pupillary block. They often have a family history of angle-closure glaucoma. Except in the rare younger patients (20s-30s), some element of pupillary block also is present. If plateau iris was not diagnosed before iridotomy and IOP is elevated postlaser, careful gonioscopy should be performed. If the angle is open, secondary damage to the trabecular meshwork or pigment liberation with dilation are the most likely causes. If the angle is closed, the differential diagnosis, besides plateau iris, should include malignant glaucoma, in which the anterior chamber is extremely shallow; PAS, which can be ruled out by dynamic gonioscopy; or incomplete iridectomy.
Miotic-induced angle-closure glaucoma
Prolonged miotic treatment in eyes with open-angle glaucoma and narrow angles may lead to pupillary block and angle-closure glaucoma.17 CACG has been seen to develop after several years of miotic therapy in eyes that initially had wide-open angles. In some eyes, zonular relaxation occurs more readily than in other eyes, so that anterior lens movement and an increase in axial lens thickness may facilitate pupillary block and angle-closure.
In other eyes, little change in the lens occurs, but progressively increasing pressure in the posterior chamber gradually pushes the peripheral iris against the trabecular meshwork. It is believed that eyes with exfoliation syndrome are particularly prone to develop miotic-induced angle-closure. In these eyes, the iris is thicker and stiffer than normal because of deposition of exfoliation material within the stroma. In addition, zonular weakness allows the lens to move forward, leading to pupillary block.
Less commonly, miotic therapy can have a pronounced effect on lens position and trigger malignant glaucoma.18,19,20 Unequal anterior chamber depths, a progressive increase in myopia, or progressive shallowing of the anterior chamber are clues to the correct diagnosis.
No exact figures are known for CACG.
The worldwide prevalence of CACG in those aged 40 years and older is estimated to be 0.69% in 2010, as compared to 1.96% for primary open-angle glaucoma (POAG), with China having the highest prevalence of CACG at 1.26%. The number of people with CACG in the world is estimated to be over 15 million in 2010 (as compared to over 44 million for POAG), with 47.5% of these patients in China. By 2020, the number of patients with CACG in the world may increase to over 21 million.21
If intraocular pressure is not controlled, glaucomatous optic neuropathy and visual field loss may progress.
The prevalence of CACG appears to be highest among Chinese (1.26% among those aged 40 years and older), according to a review of worldwide statistics. The prevalence of CACG in whites of European ancestry is around 0.25%. The prevalence of CACG appears to be lowest among Africans and people of Middle Eastern ancestry (0.16%).21
Among the subtypes of angle closure, creeping angle closure is uncommon in whites, but it is much more prevalent in Asians, in whom it ranks high as a cause of blindness. Black patients with angle-closure also tend to have this form.
Patients with chronic angle-closure glaucoma and plateau iris tend to be female.
Patients with chronic angle-closure glaucoma tend to be elderly, often with coexisting cataract or at least lens thickening.
Patients with plateau iris tend to be in their 30s-50s.
CACG usually is asymptomatic due to the slow onset of the disease, as opposed to AACG that presents with pain and nausea due to the rapid increase of IOP.
- Insertion of the iris at or anterior to the scleral spur is rare in young individuals, and, in many eyes with angle-closure glaucoma that have such an insertion, creeping angle-closure is the underlying reason. It occurs in eyes with slightly deeper anterior chambers than are found in AACG. Gradual shortening of the angle in the presence of iris bombé brings the peripheral iris close to the external angle wall more anteriorly, narrowing the gap between the iris and the trabecular meshwork. Eventually, AACG may supervene, or PAS may permanently occlude the trabecular meshwork and lead to elevated IOP and glaucomatous damage.
- The IOP in eyes with CACG may be normal or elevated. As PAS formation progresses in the absence of intermittent attacks, IOP rises gradually as less and less functional meshwork becomes available. In eyes with intermittent attacks, IOP rises more rapidly relative to the extent of PAS formation caused by recurrent damage to the trabecular meshwork by the transient angle-closure.
- Other signs
- Blotches of pigment on the meshwork, particularly in the superior angle, or deposits of black pigment in the angle of a lightly pigmented iris are highly suggestive of previous appositional closure. If the angle opens, this deposited line of pigment shows the extent of previous angle closure and sometimes can be a helpful diagnostic feature.
- The anterior chamber is quiet and usually deeper than in eyes with AACG.
- The pupil is normal.
- The gradual elevation of IOP does not result in corneal endothelial decompensation, and corneal edema is rare. The IOP usually is less than 40 mm Hg and does not reach the levels found in AACG. Symptoms are absent until the pressure rises high enough to affect the cornea or until extensive visual field damage has occurred. Although iridotomy will eliminate the pupillary block, IOP often remains elevated, and further medical treatment or surgery is required.
Causes of CACG include PAS formation, plateau iris, combined mechanism glaucoma, mixed mechanism glaucoma, and miotic-induced glaucoma.