Angle Closure Glaucoma Classification and Racial Variation

D Minckler,¹ P Foster,² PT Hung^3
1Doheny Eye Institute, University of Southern California, USA
²Institute of Ophthalmology, University College London, London, UK
³Department of Ophthalmology, National Taiwan University Hospital, Taipei, Taiwan

Glaucoma is the second leading cause of blindness in the world.¹ In 2000, there were an estimated 66.8 million people with glaucoma, 6.7 million of whom had bilateral blindness secondary to glaucoma.² Glaucoma has been defined by Ritch et al as "a progressive optic neuropathy characterized by a specific pattern of optic nerve head and visual field damage", with damage to the visual system in glaucoma being caused by death of the retinal ganglion cells.³ While elevated intraocular pressure (IOP) is the principal risk factor for glaucoma, it is not synonymous with glaucoma.

Glaucoma Classification

The glaucomas are currently classified anatomically as open or closed angle, based on gonioscopy, and further subdivided into 'primary' or 'secondary'. In primary glaucoma, the elevation in IOP is not associated with any other identifiable disorder, whereas secondary glaucomas occur in the presence of a recognisable ocular or systemic cause.

Primary angle closure glaucoma (PACG) is essentially an anatomical disorder of the anterior segment of the eye, characterised by closure of part of the filtration angle as a result of iris apposition to the trabecular meshwork.^4,5 Typically, this occurs in in-dividuals with crowded anterior chamber structures.

Primary angle closure glaucoma may be further classified as intermittent,⁶ acute,^6-8 subacute,⁷ creeping,⁶ chronic,^6-8 or suspect.⁸ However, there is confusion regarding the use of these terms. Classically described signs and symptoms do not always match individual patients⁷ and there is considerable overlap of clinical features both within and between the subgroups of angle closure glaucoma.^7,8 An established system for classification and nomenclature is therefore desirable both for understanding the disorder and for comparing findings between separate research groups. Some recommend that the word 'glaucoma' should be appended, especially with regard to angle closure, and used only when optic nerve or other ocular tissue damage consistent with pressure-induced injury is present. For the purposes of a study to determine the ocular parameters of the subgroups of angle closure glaucoma, Sihota et al defined the subgroups as follows:⁷

• Subacute gonioscopy documented presence of peripheral anterior synechiae or clumping of pigment in an occludable angle (with a steep peripheral iris configuration and an angle recess narrowed to less than 20°). Consecutive IOP measurements are within normal limits (although there may be a history of a rise in IOP), with or without a history of unilateral headaches, blurred vision, or coloured halos occurring periodically and resolving spontaneously.
• Acute occurrence of a congestive episode (severe unilateral headache, diminution of vision with nausea and/or vomiting, and the presence of a markedly raised IOP in an eye with a shallow anterior chamber, vertically oval pupil, marked corneal oedema, and a gonio-scopically closed angle).
• Chronic a chronically elevated IOP, gonioscopically confirmed peripheral
  anterior synechiae (PAS) of more than 180°, and optic nerve head and visual field changes, with or without prior symptoms.

Lowe defined intermittent angle closure glaucoma as repeated episodes of raised IOP caused by pupillary block and angle closure, which clear spontaneously, affecting eyes with shallow anterior chambers and narrow angles with normal function between attacks (Figure 1).⁶ Creeping angle closure implies an ongoing evolution of synechiae in the depths of the angle of the anterior chamber in eyes with shallow anterior chambers and narrow angles, while chronic angle closure is described as a permanent obstruction of the angle by either patchy or total PAS.

Figure 1. Schematic representation of the different types of primary angle closure glaucoma.

Kim and Jung have suggested the following 4 diagnostic categories for angle closure glaucoma:⁸

• angle closure suspect normal IOP with occludable angle without glaucomatous optic disc changes
• angle closure hypertension high IOP with closed angle without optic disc and visual field changes
• chronic angle closure glaucoma high IOP with closed angle with optic disc and visual field changes
• acute angle closure glaucoma high IOP with closed angle, ocular pain, and redness resulting from a sudden marked elevation of IOP.

Mechanisms of Angle Closure

Three different mechanisms have been suggested for angle closure glaucoma pupillary block,^4,8 plateau iris,^4,8 and lens factors^3,8 although angle closure glaucoma is traditionally characterised by the presence or absence of pupillary block.^5,8

Pupillary block is the most common mechanism of angle closure glaucoma.^4,9 Pupillary block is believed to be influenced by various anatomical parameters, including anterior chamber depth and structure, eyeball size, corneal size and curvature, and lens thickness.⁹ The initiating event in pupillary block is believed to be a physio-logical block between the pupillary portion of iris and the anterior lens surface, causing build up of aqueous with increased pressure in the posterior chamber, forward shift of the peripheral iris, and closure of the anterior chamber angle (iris bombé).^4,5 Iris bombé following relative pupillary block causes appositional closure of the angle.⁴ Prolonged apposition leads to gradual PAS formation, probably via interference with nutrition to the angle structures from cessation of normal aqueous flow into the angle.

Plateau iris describes an angle configuration in which the iris root angulates forward and centrally.⁴ Gonioscopically, the iris surface appears flat, but the central anterior chamber is not shallow and iris bombè is not present. Plateau iris results from large or anteriorly positioned ciliary processes pushing the peripheral iris forward with apposition of the iris root to the trabecular meshwork.

Plateau iris configuration is defined as a flat iris plane associated with a deep central anterior chamber.^5,10,11 Gonioscopic-ally, the iris appears to sweep directly to the scleral spur or posterior meshwork, with a difficult to discern recess. An anterior position of the ciliary processes results in an abnormally anterior position of the peripheral iris.¹⁰ Wand et al referred to plateau iris configuration as "preoperative findings of a normal depth anterior chamber with a flat iris plane by direct examination with a slit lamp, but an extremely narrow or closed angle by gonioscopic examination."¹² Only gonioscopy or ultrasound biomicroscopy can confirm or rule out the diagnosis.

'Plateau iris syndrome' describes acute angle closure occurring with pupillary dilatation despite a patent iridotomy.^5,10,11 When the pupil dilates, the peripheral iris 'bunches up' and occludes the trabeculum.¹⁰ It has been observed that the ciliary processes are anteriorly situated in plateau iris syndrome, in contrast to the position in healthy eyes and angle closure caused by pupillary block.¹¹

The concept of plateau iris syndrome from 8 case studies by Wand et al¹² is changing because of the introduction of ultrasound biomicroscopy and more meticulous studies such as those by Ritch and Lowe¹³ have changed the classification of preoperative and postoperative plateau iris syndrome defined by Wand.¹²

Kim and Jung have described lens block as block by the forward movement of the lens, which crowds the anterior segment.⁸ However, angle closure glaucoma mechanisms may be inter-related and age-related increase in anterior-posterior lens thickness may trigger relative pupillary block.

Peripheral anterior synechiae may occur after cataract extraction when the iris or vitreous is incarcerated in the posterior aspect of the corneal incision.¹⁴ Peripheral anterior synechiae formation appears to complicate intraocular lens use by producing progressive endothelial cell loss, fibrous metaplasia of the endothelium, and angle cicatrization with glaucoma. Even in un-complicated cataract extractions, superior PAS have been documented in up to 35% of eyes after intracapsular cataract extraction and in 43% of eyes with extracapsular cataract extraction. The incidence of PAS also increases in the presence of posterior wound gape.

Identification of Angle Structures

Schwalbe's line is the most anterior structure in the angle, appearing as an opaque, meandering, frequently irregularly pig-mented line.¹⁰ Anatomically, it represents the peripheral termination of Descemet's membrane and the anterior limit of the trabeculum (Figure 2). The trabeculum extends from Schwalbe's line to the scleral spur and has an average width of 600 µm. The scleral spur defines the posterior edge of the trabecular meshwork and is the most anterior projection of the sclera and the site of attachment of the longitudinal muscle of the ciliary body. The ciliary body band stands out just behind the scleral spur. Its width depends on the position of iris insertion and it tends to be narrower in hypermetropic eyes and wider in myopic eyes. The angle recess represents the posterior sag of the iris as it inserts into the ciliary body. Ciliary processes are small extensions of the anterior portion of the ciliary body (pars plicata) which lie just behind the iris and are usually visible through a peripheral iridectomy. Iris processes most prominent during childhood and in brown eyes, are minute extensions of peripheral iris, some containing small vessels, that appear to vault across the spur from the iris towards the anterior meshwork. Blood vessels can often be observed running in a radial pattern at the base of the angle recess in normal eyes pathological blood vessels run randomly in various directions or, as in rubeosis, cross the spur to arborize on the meshwork.

Figure 2. Anatomy of angle structures.

Eyes with a tendency to angle closure glaucoma have a smaller than average anterior chamber. The angle between the cornea and iris plane is approximately 45° in the average eye. If the angle is narrowed, the iris is situated closer to the trabecular meshwork. Since the lens routinely enlarges with ageing, it is common for the angle to narrow with increasing age.

Sihota et al found that eyes with acute angle closure expressed an extreme shift of anatomical features away from normal, in particular, smaller corneal diameters, large mobile lens, and a crowded anterior chamber.⁷ These features predisposed patients to relative pupillary block and to a ciliary block glaucoma. Chronic angle closure eyes were less divergent from normal than acute angle closure eyes, suggesting a milder form of the same kind of angle closure, but expressed over a prolonged period. Subacute angle closure eyes deviated least from the normal anatomy. All the eyes with angle closure glaucoma had similarly short eyeballs, a steeper corneal curvature, and thicker lenses than control eyes. Corneal diameters and anterior chamber depths were decreased in acute and chronic angle closure glaucoma eyes, compared with subacute angle closure and controls (p < 0.001).

Gonioscopy

There is clearly a correlation between the structure of the anterior chamber angle and the likelihood of development of primary angle closure glaucoma.^10,15-17 Factors such as anterior chamber depth, corneal
diameter and curvature, anterior chamber angle width, limbal chamber depth, relative position of the lens, and ratio of lens thickness to axial length play significant roles in the pathogenesis of angle closure glaucoma.^10,16 Assessment of the drainage angle is therefore important for diagnosing angle closure glaucoma.^10,16,18,19

Gonioscopy, although subjective and difficult to quantify, is still considered the most practical method for assessing the characteristics of the angle.¹⁸ Ultrasound biomicroscopy may supercede gonioscopy for angle assessment, at least for research and classification purposes. The main aims of gonioscopy are to evaluate the functional status of the angle, the degree of closure, and the risk of future closure. The following features should be noted:

• the shape and contour of the peripheral iris (flat, convex, concave)
• the deepest structure seen (Schwalbe's line, trabecular meshwork, scleral spur, ciliary body band)
• amount of trabecular pigmentation (grade 0 to IV)
• presence of peripheral anterior synechiae (number of clock hours involved).

Shaffer Grading System

Shaffer's grading system provides a means of comparing the widths of different cham-ber angles. The system provides a record of the angle in degrees of arc subtended by the inner surface of the trabecular mesh-work and the anterior surface of the iris approximately one-third of the distance from its periphery (Figure 3a).¹⁰ The examiner grades the angle according to the visibility of the various angle structures (Table 1).

Table 1. Shaffer grading system.

Spaeth Grading System

Although the Shaffer grading system is extremely useful, the configuration of the angle recess is frequently too complex and varied to permit valid description by a single characteristic (Table 2, Figure 3).^10,20 The classifications are neither elaborate or precise enough to allow a fully acceptable characterisation of the angle structures.²⁰ The crucial concept behind the Spaeth grading system is that at least 3 different aspects of the configuration are noted (Table 2, Figure 3):

• the angular approach of the iris to the iris recess, expressed in 10° increments (Figure 3a)
• the configuration of the peripheral curvature of the iris (Figure 3b)
• the point where the iris contacts the globe or where it springs from the ciliary body (Figure 3c).

Table 2. Spaeth grading system.

Figure 3a. The angular width of the anterior chamber recess is estimated by constructing a tangent to the anterior surface of the iris about one-third of the distance from the most peripheral portion of the iris.

Figure 3b. The curvature of the peripheral iris is graded as: Q = concave contour of the peripheral iris; R = regular curvature; S = steep, anterior bowing of the peripheral portion of the iris.

Figure 3c. Schematic drawing of locations where the iris may contact the inner side of the globe. A = iris insertion into or anterior to Schwalbe's line; B = just anterior to the posterior trabecular meshwork; C= scleral spur; D = anterior portion of the ciliary body; E = posterior portion of the ciliary body.

The presence and type of iris processes, the amount of pigmentation in the posterior trabecular meshwork and the nature of any adhesions should also be noted.

Studies have validated the Spaeth gonioscopy grading system as an accurate method of characterising the anatomic appearance of the anterior chamber angle (Table 3).²¹ Kim and Jung recommend the Spaeth grading system because of the detail with which the angle structures are graded.⁸

Table 3. Comparison of grading between ultrasound biomicroscopy and the Spaeth gonioscopy grading system (n = 21).