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.
|Angle depth||White (%)||Japanese (%)|
|Grade 3||60.0||92 *|
|Grade 4||38.6 *||*|
|Angle grade *||Degrees||Numeric grade||Clinical interpretation|
|Wide-open angle||30–40||3–4||Closure impossible|
|Narrow angle (moderate)||20||2||Closure possible|
|Narrow angle (extreme)||10||1||Eventual closure probable|
|Slit angle||<10||S||Portions appear closed|
|Closed angle||–||0||Closure present|
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.
|Grade 4 angle||Anterior chamber depth = Corneal thickness|
|Grade 3 angle||Anterior chamber depth = 1⁄4 to 1⁄2 corneal thickness|
|Grade 2 angle||Anterior chamber depth = 1⁄4 corneal thickness|
|Grade 1 angle||Anterior chamber depth = Less than 1⁄4 corneal thickness|
|Slit angle||Anterior chamber depth = Slit-like (extremely shallow)|
|Closed angle||Absent peripheral anterior chamber|
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.
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.
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:
A = Anterior to trabecular meshwork (i.e., Schwalbe’s line)
B = Behind Schwalbe’s line (i.e., at level of trabecular meshwork)
C = Centered at the level of the scleral spur
D = Deep to the scleral spur (i.e., anterior ciliary body)
E = Extremely deewp in the ciliary body.
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.
STEP 2: ANGLE WIDTH ( FIG. 7-5B )
Exactly as with the Shaffer system, the geometric angle is estimated at the perceived intersection of the imaginary tangents formed by the peripheral third of the iris and the inner wall of the corneoscleral junction. Though some examiners prefer increments of 10°, as with the Shaffer system, and others use increments of 15°, these clinical assessments tend to overestimate by 5° the actual angle, as measured by the ultrasonic biomicroscope.
STEP 3: CONFIGURATION OF PERIPHERAL IRIS ( FIG. 7-5C )
Originally Spaeth discriminated three contour configurations of the peripheral iris, designated in reverse alphabetical order:
s = ‘ s teep’ or convexly configured (e.g., plateau iris)
r = ‘ r egular’ or flat (the most common contour seen)
q = ‘ q uixotic’ or ‘ q ueer’ for deeply concave (e.g., pigment dispersion syndrome).
The newer system describes four iris configurations, indicated by the first letter of their description:
b = ‘ b ows 1 to 4 plus’ (usually indicative of optically-appearing closure, altering with indentation)
p = ‘ p lateau’ (comparable to older ‘s’ designation)
f = ‘ f lat approach’: the commonest iris appearance (comparable to the older ‘r’ designation)
c = ‘ c oncave’ as in posteriorly bowed iris (comparable to the older ‘q’ designation).
STEP 4: TRABECULAR MESHWORK PIGMENTATION
As with the Shaffer scheme, the degree of trabecular meshwork pigmentation (TMP) is labeled from 1 to 4: minimal or no pigment is graded 1, and dense pigment deposition is indicated as grade 4, with lesser degrees between.
E40c, 4+ TMP = An extremely deeply inserting iris root, in a 40° angle recess, with posterior bowing of the peripheral iris and extensive TMP (as might be seen in a myopic eye with pigment dispersion syndrome).
(B)C10f, tr TMP = An iris initially appearing (indicated in parentheses as ‘B’) to be anteriorly in apposition to the trabecular meshwork, but which on indentation reveals a true insertion at the scleral spur (c), creating a 10° angle recess with a flat contour, and with minimal TMP (as might be seen in a hyperopic eye at risk of pupillary block).
(A)B20b3+ < 2+ TMP = An apparent obscuration (indicated in parentheses as ‘A’) of all angle detail by a convex-appearing peripheral iris, until indentation shows the bowed iris inserting above the scleral spur (B), with mild TMP (as might be seen in a plateau iris following iridotomy).
Mastering the Spaeth classification has several merits. The observational precision required by discriminating the various parameters for notation will, with practice, become easier, and hence more useful during the long-term care of a patient. By the same token, mastery of this tool in training programs and multi-partner clinical settings allows for inter-observer consistency in describing angle changes over time. And lastly, this scheme precisely correlates with findings of contemporary imaging devices, such as UBM and anterior segment optical coherent tomography (AS-OCT), which are becoming ever more clinically available. (See Chapter 15 ‘Primary Angle-Closure Glaucoma’ for illustrative examples.)