Gonioscopy

examination. Ritch’s trabeculoplasty four-mirror lens was developed to magnify the view for laser treatment, and contains convex lenses over two mirrors. Zeiss8 and Posner four-mirror lenses are inclined at 62 degrees and 64 degrees, respectively, allowing visualization of the entire circumference of the anterior chamber angle without rotation. Several gonioprisms with a small contact lens diameter, such as the Zeiss lens8, the Sussmann lens9 and the Iwata lens,10 can be used for indentation gonioscopy, which is an essential technique to detect peripheral anterior synechia in eyes with an extremely narrow angle.11 The methods and principle of viewing the anterior chamber angle are shown below.

Direct gonioscopy

Direct gonioscopy is performed with a 50-diopter concave goniolens placed on the cornea of a patient in the supine position. The space between the inner surface of the lens and the cornea is filled with a solution, such as saline or methylcellulose. To view the fine structure of the anterior chamber angle, the optical problem ‘critical angle’ should be solved. When the light ray coming from the chamber angle approaches the cornea-air interface, the total light ray shall be reflected back into the anterior chamber, if the oblique angle exceeds the critical angle, about 46 degrees, for the cornea-air interface. The solution to this problem is to use a contact lens with similar refraction index, to neutralize the corneal refractive power, and eliminate the optical effect of the front corneal surface. The anterior curve of goniolenses is designed such that the critical angle is not reached, so the light ray is refracted at the contact lens-air interface, instead of being internally reflected (Fig. 1). The examiner usually holds the gonioscope in one hand, and a portable slit-lamp in the other. In the case of ocular surgery, an assistant may be needed to move the goniolens to the desired position, but coordination of the direction of the patient’s gaze, the lens position and the surgeon’s vantage point requires considerable practice.

Indirect gonioscopy

Indirect gonioscopy is performed at the slit-lamp microscope using a mirrored contact lens. The lens of gonioprism neutralizes the corneal refractive power, and the mirror allows indirect visualization of angle structures. A contact lens with a similar refraction index can eliminate the cornea optically, and allow the light ray from the chamber angle to pass through the cornea-contact lens as well as contact lens-air interfaces, in the same manner as described for direct gonioscopy (Fig. 1). A mirror positioned within the gonioprisms may reflect these light rays at nearly a right angle to the contact lens-air interface and allows a straight-ahead vantage point for viewing the angle structures with the slit-lamp biomicroscope. With indirect gonioscopy, the anterior chamber angle appears inverted as it is reflected by the gonioprism mirror. The examiner’s view is a reflection of structures 180 degrees away, i.e., a gonioprism mirror positioned superiorly views the inferior angle, a gonioprism mirror positioned temporally views the nasal angle, and so on. Two different types of gonioprisms, the Goldmann7 and the Zeiss,8 are generally used in indirect gonioscopy. Goldmann-type gonioprisms are easier to use, because they keep the globe stationary and afford better control during examination. This

Fig. 1. Principle of indirect gonioscopy. A: Diagram of light ray which originates from the anterior chamber angle, exceeds the critical angle at the cornea-air interface, and is totally internally reflected. B: Diagram of light ray which originates from the anterior chamber angle, passes through the cornea-goniolens interface, and is reflected by a goniomirror. (From: see Reference 3)

is an advantage over the Zeiss lens, which does not create a suction effect and therefore requires continued effort to correct for drift. However, the suction effect is not altogether beneficial, since it also distorts the anatomic relationships of the iridocorneal angle and can open a closed angle, causing the examiner to miss the diagnosis of angle-closure glaucoma. Occasionally, the view of the iridocorneal angle is obscured by the convexity of iris root. In such cases, one can see ‘over the hill’ to angle structures with patient’s help to gaze toward the direction of mirror.

One of the major advantages of Zeiss8 and Iwata10 gonioprisms is that they can be used for indentation gonioscopy,11 by oppressing the central cornea and artificially widen the narrow anterior chamber angle. A smaller diameter of these lenses enables direct contact with the anterior corneal surface, depression of the central part of the cornea, and displacement of aqueous humor peripherally and therefore the iris root posteriorly. When the iridocorneal angle is optically closed, this maneuver enables differentiation of reversible appositional closure from irreversible peripheral anterior synechia (PAS). Folds in Descemet’s membrane may be often present during indentation and may distort, but not obscure, the observation of iridocorneal angle structure. In these cases, the intensity of indentation may be too much, or the gonioprisms may be positioned too close toward the corneal limbus along the ocular surface. One should not press the cornea too much, and by asking the patient to gaze toward the direction of mirror, one can observe the iridocorneal angle with less indentation.

Normal gonioscopic anatomy

In order to identify the gonioscopic findings responsible for glaucoma, one should understand the anatomy of anterior segments corresponding to the gonioscopic view (Fig. 2), and normal structural variations. Starting from the iris root and progressing anteriorly toward the cornea, the following structures can be identified by gonioscopy in a normal anterior chamber angle.

Fig. 2. Normal adult anterior chamber angle showing gonioscopic appearance (right), and cross section of corresponding structures (left). 1. Ciliary body band; 2. Scleral spur; 3. Functional trabecular meshwork (degree of pigmentation varies); 4. Schwalbe’s line. (From: see Reference 4)

Ciliary body band

This structure is the part of the ciliary body that is visible in the anterior chamber as a result of the iris insertion into the ciliary body. The width of the band depends on the level of iris insertion, and tends to be wider in myopia and narrower in hyperopia. The color of the band is usually gray or dark brown. Occasionally, strands or lacy cords of uveal tissues are extending from the iris root towards trabecular meshwork, and are called iris processes. They appear predominantly in the nasal part, and mostly attach to the level not exceeding Schlemm’s canal. If the processes are highly inserted, and dendritic or membranous appearance, one should consider developmental disorder of iridocorneal angle.

Scleral spur

This is the posterior lip of the scleral sulcus, which is attached to the ciliary body posteriorly and the corneoscleral meshwork anteriorly. It is usually seen as a prominent white line between the ciliary body band and functional trabecular meshwork, unless it is obscured by dense uveal meshwork or excessive pigment deposition. Variable numbers of iris processes may frequently be seen crossing the scleral spur from the iris to the functional meshwork. The full width of the scleral spur can be identified, except when a highly inserted iris root covers this structure in developmental glaucoma.

Functional trabecular meshwork

This is seen as a pigmented band just anterior to the scleral spur. Although the trabecular meshwork actually extends from the iris root to Schwalbe’s line, it may

be considered in two portions: (a) the anterior part, between Schwalbe’s line and the anterior edge of Schlemm’s canal, which is involved to a lesser degree in aqueous outflow; and (b) the posterior (or functional) part, which is the remainder of the meshwork and is the primary site of aqueous outflow, especially the part immediately adjacent to Schlemm’s canal. The appearance of the functional meshwork varies considerably depending on the amount and distribution of pigment deposition. It has no pigment at birth, but color develops with age from faint tan to dark brown, depending on the degree of pigment dispersion in the anterior chamber. The distribution of pigment may be homogenous for 360 degrees in some eyes and irregular in others. In the functional part of the meshwork, especially when lightly pigmented, blood reflux in Schlemm’s canal may sometimes be seen as a red band.

Schwalbe’s line

This is the junction between the anterior chamber angle structures and the cornea, and is located where Descemet’s membrane ends in a circumferential ring of collagenous fibers. It is a fine ridge just anterior to the meshwork and is often identified by a small accumulation of pigment, especially inferiorly. If there is no pigmentation, the transition between the transparent cornea and translucent trabeculum can be better appreciated when viewed gonioscopically with a thin slit-beam projected into the iridocorneal angle at an oblique angle. The slit of light penetrates the transparent corneal tissues, appearing above Schwalbe’s line as a three-dimensional parallelepiped of light. At Schwalbe’s line, the figure of light collapses to a two-dimensional stripe of light on the trabecular surface.

Grading anterior chamber depth

Shaffer’s classification

Shaffer’s classification12 is based on an estimate of the geometric angle formed by the iris and the corneoscleral wall at the approach to the trabecular meshwork. The iridocorneal angle is graded on a scale of 0 (closed), I (about 10 degrees), II (20 degrees), III (30 degrees) and IV (40 degrees or more) (Fig. 3). It has been found that Shaffer grade-I angles are at considerable risk of closure, either spontaneously or with provocative testing. Smudges of iris pigment on or above the trabeculum or small tentorial PAS provide strong evidence that intermittent appositional closure has taken place. Grade-II angles need cautious follow-up, because they may become shallower in the future, and grade III and IV angles are probably not at risk of angle closure.

Scheie’s classification

Grading in Scheie’s classification system13 is opposite to that of Shaffer’s system,12 so special care is required to document the numeric grading of angular width. This system is based on the extent of iridocorneal angle structures visualized by the examiner. A grade-I angle is widely open, a grade-II angle allows one to see just

Fig. 3. Shaffer’s classification system for grading anterior chamber angle depth. (From: see Reference 12)

to the scleral spur but not to the ciliary body. A grade-III angle allows one to see only to the anterior part of trabecular meshwork, and a grade-IV angle is closed. Interpretation of angular width is difficult in those eyes in which the iris root is narrow but open due to plateau iris configuration, and in those eyes in which the iris insertion is abnormally located at or even anterior to the scleral spur.

Spaeth’s classification

Spaeth has developed a classification system that expands on the Shaffer grading of angles to include a specification of the shape of the peripheral angle and the site of the iris insertion.14 A concave peripheral iris is denoted q, a regularly straight iris r, and a steeply convex iris s. The implication is that an s configuration of the iris brings iris tissue into closer proximity with the functional trabecular meshwork and thereby increases the risk of angle closure. Thus even an iridocorneal angle with a grade-I to -II approach may be at imminent risk of angle closure when a plateau iris configuration is present. The site of iris insertion in this system is specified as A if the insertion is anterior to the trabecular meshwork (e.g., synechial closure to Schwalbe’s line), B if the insertion is just behind Schwalbe’s line, C if the insertion is at the scleral spur, D if the angle is deep with a visible face of the ciliary body, and E if the angle is extremely deep (Fig. 4)

Fig. 4. Spaeth’s classification system for grading anterior chamber angle depth. (From: see Reference 14)

Grading the degree of pigmentation

Scheie’s classification of pigmentation

In childhood, pigment deposition is rarely found in the trabecular meshwork, but the intensity of pigmentation tends to increase in accordance with age. In some pathologic conditions or after undergoing ocular surgery, remarkably intense pigmentation is observed, which is helpful for diagnosing the cause of IOP elevation. Physiological pigmentation is generally denser in the lower portion, so the grade and location of the pigment deposition should be documented in gonio-chart. The color scheme of Scheie’s grading system of pigmentation13 is shown in Figure 5.

Fig. 5. Scheie’s classification system for grading pigmentation of iridocorneal angle. (From: see Reference 13)

Gonioscopic findings of ocular disorders

Besides grading angle depth in angle-closure glaucoma (ACG), the essential aim of gonioscopy is to identify the pathological findings and estimate the causative mechanism in secondary glaucoma. (1) PAS are abnormal adhesions of the peripheral iris to the corneoscleral wall in various shapes and degrees. They are not only

caused by relative pupillary block and plateau iris configuration in ACG, but also formed as a result of iridocyclitis, iris bombe, neovascularization of iridocorneal angle, flattening of the anterior chamber, endothelial cell migration in ICE syndrome, trauma, intraocular surgery, laser trabeculoplasty and so on. Each condition presents PAS of its characteristic configuration and location, so careful gonioscopy offers essential information of the etiological mechanism. (2) Pigment deposition is not always pathological, but extremely dense pigmentation of the entire trabeculum is a characteristic finding of pigmentary glaucoma and pigment dispersion syndrome. In the eyes receiving intraocular surgery or laser iridotomy, the anterior chamber angle shows intense pigmentation. Highly pigmented trabecular meshwork with irregular PAS is observed in the eyes of patients with chronic iridocyclitis. Wavy lines of pigment deposition lying beyond Schwalbe’s line in the lower portion of iridocorneal angle are seen in the eyes of patients with exfoliation glaucoma, which is named Sampaolesi’s line. (3) Neovascularization (NV) of the chamber angle should be distinguished from angle vessels, which are typically broad in character, appear in short segments, do not extend anterior to the scleral spur, and do not arborize in the trabecular meshwork. NV occurs in various ischemic conditions, including diabetic retinopathy, central retinal vein occlusion, occluded internal carotid artery, extremely high IOP in secondary glaucoma, retinopathy of prematurity, tumor, and so on. Small NV seen in the eyes of Fuchs’ heterochromic iridocyclitis is a valuable sign for diagnosis. (4) Nodules in the anterior chamber angle accompanied by tentorial PAS are observed in the eyes of glaucomatous uveitis such as ocular sarcoidosis. According to the origin, stage and complication, gonioscopic findings in uveitis are diversified such as nodules, diffuse pigmentation, pigment pellet, PAS, NV, iris bombe, and so on. (5) Blunt ocular trauma may produce partial or broad posterior displacement of the iris root, named traumatic angle recession. On the other hand, angle recess implies a wide ciliary body band, which is generally observed uniformly in both eyes. Injury also results in cyclodialysis cleft, which allows aqueous humor to infiltrate into the suprachoroidal space, causing hypotony.

(6) The most common feature of gonio-dysgenesis is a high insertion of a flat iris root, or a wrap-around type insertion, observed in the eyes of developmental glaucoma. Another type of gonio-dysgenetic configuration is a persistent uveal cord/membrane with or without prominent Schwalbe’s line, observed in the eyes of Axenfeld-Rieger’s syndrome and other congenital anomalies.

References

1.Palmberg P. Gonioscopy. In: Ritch R, Shields MB, Krupin T (eds) The glaucomas, 2nd ed. St. Louis: Mosby 1996, pp 455-469.

2.Alward WLM. Color Atlas of Gonioscopy. London: Mosby 1994.

3.Zalta AH. Gonioscopy. In: Kaufman PL, Mittag TW (eds) Glaucoma, Vol 7, Textbook of Ophthalmology. London: Mosby 1991.

4.Shields MB. Textbook of glaucoma, 5th ed. Philadelphia: Lippincott Williams & Wilkins 2005, pp 59-72.

5.Kitazawa Y, Nose H. Gonioscopy as a routine test for outpatients. Ganka 1972;14:139-144.

6.Hetherington J Jr. Koeppe lens gonioscopy. In: Brockhurst FJ, Boruchoff SA, Hutchinson BT, et al (eds) Controversy in Ophthalmology. Philadelphia: WB Saunders; 1977.

7.Goldmann H. Augendruck and Glaukom. Die Kammerwasservenen und das Poiseuille’sche Gesetz. Ophthalmologica 1949;118:496-519.

8.Kaufman PL, Neider MW, Pankonin WH. Slitlamp mount for Zeiss gonioscopy lens. Arch Ophthalmol 1981;99:1455.

9.Sussman W. Ophthalmoscopic gonioscopy. Am J Ophthalmol 1968;66:549.

10.Iwata K. Newly designed angle mirror for pressure gonioscopy. Ganka 1968;10:560-565.

11.Forbes M. Gonioscopy with corneal indentation: a method for distinguishing between appositional closure and synechial closure. Arch Ophthalmol 1966;76:488-92.

12.Shaffer RN, Schwartz A. Gonioscopy, ophthalmoscopy and perimetry. Trans Am Acad Ophthalmol Otolaryngol 1960;64:112-25.

13.Scheie HG. Width and pigmentation of the angle of the anterior chamber: a system of grading by gonioscopy. Arch Ophthalmol 1957;58:510-12.

14.Spaeth GL. The normal development of the human chamber angle: a new system of descriptive grading. Trans Ophthalmol Soc UK 1971;91:709-39.