GRADING OF CHAMBER ANGLE

There has been an evolution in the clinical methods used to describe the anterior chamber angle. An earlier scheme described by Scheie emphasized the most posteriorly visible structures within the angle recess on gonioscopy; its scale from 0 to 4 was the reverse of what is now commonly used, with the Scheie grade 4 being the most occludable angle. For over a generation the Shaffer system has enjoyed widespread popularity, with its simplified emphasis on assessing the geometric angle width in four basic grades and its attention to the angle’s potential for occlusion. Recently a more complex classification has been elaborated by Spaeth that notates several three-dimensional aspects of the angle’s configuration. These latter two schemes merit this chapter’s detailed discussion.

The widest angles are characteristically seen in myopia, aphakia, and pigmentary dispersion syndrome. Such eyes have a deep chamber and a flat iris plane that makes an angle of about 45° with the trabecular surface. As the chamber depth shallows, the angle narrows. With increasing relative pupillary block there is increasing danger of angle closure, especially as the angle becomes smaller than 20°.

To compare different angles, it is convenient to have a grading system ( Fig. 7-1 ). The most widely used scheme is the Shaffer classification, which includes numeric grades of angles that can be recorded conveniently on office charts. The widest open angle is a grade 4, and a closed angle is grade 0. There are very narrow angles in which it is impossible to decide whether an opening exists between the iris root and the trabecular surface, despite performing indentation gonioscopy. Such angles are labeled ‘slit.’ Table 7-1 shows the incidence difference in two ethnic populations, of grades 1 to 4 among white and Japanese patients. Table 7-2 indicates angle grades and their clinical significance.

(A–D) Grading of angles by estimated angulation.

Angle width can be estimated during a slit-lamp examination by directing the slit-lamp beam adjacent to the limbus; this is called the van Herick technique. The examiner can use the peripheral anterior chamber depth to indicate angle width ( Fig. 7-2 and Table 7-3 ). This is a useful screening method under various circumstances – for example, when corneal clouding reduces visualization of the anterior chamber. The van Herick method has proven particularly helpful in evaluating large numbers of patients in population studies for angle-closure glaucoma; likewise, it shows its best specificity and sensitivity rate in the presence of narrow angles. When used in combination with other measurements, such as the tangential flashlight screening for shallow chamber ( Fig. 7-3 ), this estimation can be a valuable predictor of eventual angle-closure disease. This is not, however, recommended as a substitute for gonioscopy.

Slit-lamp estimation of angle width: example of a grade 2 angle. Angle width can be estimated with the slit lamp by comparing the anterior chamber depth to the corneal thickness.

From van Herick W, Shaffer RN, Schwartz A: Estimation of width of angle of anterior chamber: incidence and significance of the narrow angle, Am J Ophthalmol 68:626, 1969.

Illumination from temporal side casts shadow on iris if there is considerable bombé.

It is obvious that the various angle grades merge into one another, so that the usefulness of any classification system depends on the skill of the observer in judging which angles are potentially or actually occluded. Though simplified to a single grade, the experienced clinician’s assessment of the angle’s risk for angle closure takes into account the three-dimensional aspects of angle anatomy, such as the level of the iris insertion and peripheral iris configuration.

DIAGRAMMING ANGLE WIDTH, SYNECHIAE, AND PIGMENTATION

Figure 7-4 shows a valuable method of diagramming the angles and recording the visible structures in each quadrant, as well as the position of peripheral anterior synechiae (PAS). The density of the trabecular pigment band can also be recorded. In the diagram the cornea is opened out to place Schwalbe’s line outside the angle recess so that synechiae can be diagrammed as continuous with the peripheral iris stroma ( Fig. 7-4A ). Synechiae as high as the scleral spur do not directly block outflow. In the presence of normal outflow channels there is an inverse relationship between the extent of synechia formation and the facility of outflow.

(A) Diagramming gonioscopy with concentric circles. The inner circle represents the posterior limit of the scleral spur (SS), and the outer circle represents Schwalbe’s line (SL). The trabecular meshwork (TM) occupies the same location as Schlemm’s canal (SC). Here a peripheral anterior synechia (PAS) is drawn up to Schwalbe’s line. (B) Hand-drawn Xs can suffice for charting purposes to indicate gonioscopic quadrants. (C) In this hand-drawn example, the superior angle shows the trabecular meshwork (TM) with equivocal viewing of the scleral spur (SS), which is elsewhere seen by itself or with the ciliary body (CB); a peripheral anterior synechia (PAS) is also seen.

Blood may be seen in Schlemm’s canal under special circumstances: if the patient is supine; if the eye has increased episcleral venous pressure (e.g., Sturge-Weber syndrome), hypotonia, or active uveitis or scleritis; or if the flange of the gonioscopy lens compresses the limbal vessels. Rarely blood will reflux into the anterior chamber, causing elevated intraocular pressure (IOP).

TRABECULAR PIGMENT BAND

In the young normal eye, it is unusual to see any trabecular pigment band. This is because insufficient pigment has been filtered out by the trabecular meshwork to form a visible line. The presence of such a band denotes either aging or disease. The pigment band is usually most prominent in the lower chamber angle. For convenience in recording, the dense dark band of pigmentary glaucoma is recorded as grade 4, and minimal pigmentation is recorded as grade 1. When grading pigment bands, some allowance must be made for the color of the iris. Thus a brown eye generally has a denser pigment band than a blue eye.

The two most common conditions in which the pigment band is prominent are pigmentary glaucoma (in which the angle is usually deep) and exfoliative syndrome (in which the angle may be narrow) (see Figs. 7A-8 and 7A-9 at end of chapter). Lesser amounts of pigment are seen in many situations, including open-angle glaucoma, trauma, iritis, and diabetes, or after laser iridotomy or anterior segment surgery.

SPAETH CLASSIFICATION

In an attempt to reduce a great deal of gonioscopic observational data into a concise format, Spaeth elaborated a complex grading system that captures detailed three-dimensional information in coded form. It was designed for indirect goniolenses (e.g., Zeiss four mirror) that allow for indentation (compression) gonioscopy. Interobserver variability has been found to be minimal. Moreover, this gonioscopic assessment shows high correlation with information obtained during ultrasonic biomicroscopy (UBM) of the angle, as well as with ‘biometric gonioscopy’ using a reticule to facilitate interobserver consistency in clinically grading the angle. As such, (UBM) may be used to delineate gonioscopic findings when gonioscopy itself is difficult, as in cases of congenital glaucoma.

The Spaeth grading system uses an intricate alphanumeric scale, attempting to provide three-dimensional specificity to gonioscopic description. It addresses each of the following items in sequential order: (1) the site of insertion of the iris root in the eyewall; (2) the width or geometric angle of the iris insertion; (3) the contour of the peripheral iris near the angle; (4) the intensity of the trabecular pigmentation, and (5) the presence or absence of abnormalities such as mid-iris bowing, peripheral synechiae, and so on. If there is a discrepancy between the apparent and the actual site of iris insertion, as determined by indentation gonioscopy, this is so noted. Usually the grading is made for the four cardinal points of the angle, but especially in the normal angle’s most narrow (superior) and open (inferior) aspects.

STEP 1: SITE OF IRIS INSERTION ( FIG. 7-5A )

The first grading decision assesses the site of the iris insertion, both as it appears functionally (without pressure on the cornea) and as it actually inserts anatomically (after indentation). The capital letters A through E have simple alphabetic correspondences for the site of iris insertion:

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  A = Anterior to trabecular meshwork (i.e., Schwalbe’s line)

  
  
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  B = Behind Schwalbe’s line (i.e., at level of trabecular meshwork)

  
  
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  C = Centered at the level of the scleral spur

  
  
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  D = Deep to the scleral spur (i.e., anterior ciliary body)

  
  
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  E = Extremely deewp in the ciliary body.

Spaeth’s gonioscopic classification of anterior chamber angle. (A) Site of iris insertion. This iris appears to insert at one of five levels. A = Anterior to trabecular meshwork, at Schwalbe’s line; B = behind Schwalbe’s line; C = centered at the scleral spur; D = deep to the scleral spur, at the anterior ciliary body; E = extremely deep, revealing most of the ciliary body. (B) Angle width. Four approximate geometries of the peripheral iris with respect to the angle. (C) Configuration of the peripheral iris. Four configurations are characterized: b = bowing anteriorly (1 to 4 +); p = plateau; f = flat; c = concave.

When the iris appears to be cramping the angle, with or without appositional touch (grade A or B), indentation gonioscopy may reveal an actual posterior site of insertion – the apparent level is placed in parentheses preceding the actual site. For example, a ‘(B)D’ notation means the iris appeared to insert at the level of the upper trabecular meshwork, but on indentation the actual insertion was revealed to be posterior to the scleral spur.