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The Angle of the Anterior Chamber

Joel S. Schuman, MD, FACS; Kris-Ann Humphrey, MD; Deval D. Joshi, MD; and Lisa S. Gamell, MD

The angle of the anterior chamber normally provides the main outflow system for aqueous humor. In most of the glaucomas, it is here that aqueous outflow is obstructed. Gonioscopy is our clinical means for looking into the angle and identifying the cause of obstruction in many cases. This chapter offers details regarding the examination of the drainage angle while maintaining much of the historical perspective from the original writings of Dr. W. Morton Grant.

GONIOSCOPY (PROCEDURE)

Gonioscopy is one of the most important tools in the management of glaucoma. Historically, many cases of glaucoma were treated successfully without gonioscopy, but in many other cases, gonioscopy may be the sole means of arriving at the correct diagnosis, leading to the correct treatment. Inaccurate gonioscopy may not only be unhelpful, it may actually be misleading. Whereas accurate gonioscopy can give the correct diagnosis and point to the most appropriate treatment, an incorrect interpretation of the gonioscopic findings may lead the ophthalmologist far astray. It is certainly worth the time and effort to learn gonioscopy well and to make the best use of it in every case of glaucoma.

In this chapter, we will describe methods that, in our hands, have been most satisfactory for learning gonioscopic anatomy and for performing gonioscopy on patients. Throughout the text, frequent consideration will be given to gonioscopic findings. Peculiarities of the angle in infants and children, as well as congenital abnormalities of the angle, are discussed in Chapter 69.

LEARNING GONIOSCOPY

The most effective and valuable step in learning gonioscopy makes use of enucleated human eyes, such as those used as donors in corneal transplanting procedures. By studying the angle of the anterior chamber with the binocular microscope and by performing simple dissections, one can obtain the best understanding of the identity and anatomical relationship of the structures seen in the eyes of patients.

To obtain the most instructive view of the anatomy of the angle, one makes a cut with a razor blade perpendicularly through the center of the cornea, bisecting the anterior segment, cutting smoothly through the angle structures and the anterior 2 or 3 mm of sclera on both sides. Then, with scissors, half of the bisected anterior segment is removed by cutting the sclera circumferentially, parallel to the limbus. This leaves half the circumference of angle open to direct viewing as in clinical gonioscopy and, at the same time, provides a cross-sectional display of the angle where the structures have been cut by the razor blade at both sides of the hemisphere.

The eyes prepared in this manner can be examined with a slit-lamp microscope or a dissecting microscope; or one can use the handheld gonioscopy microscope and a focal illuminator, as in clinical Koeppe gonioscopy. In this arrangement, there is, of course, no need for a gonioscopic contact lens, as there is no cornea in the line of view.

One should first identify the structures of the cross-sectioned angles, because this will be the most readily recognizable aspect, because it is similar to the cross-sectional type of view that is so familiar in microscopic histology slides used in the study of ocular pathology. One should have no trouble identifying ciliary muscle, ciliary processes, and iris, as well as sclera and cornea. Finer details may require considerable study to recognize, and it is helpful to have a stained microscopic section to refer to for comparison.

Modern systems of gonioscopy employ mirrored contact lenses, such as the Goldmann lens (Ocular Instruments) and the slit-lamp biomicroscope, and require the patient to be sitting-up. These lenses provide a reversed image, but the image is not crossed. In other words, if one were looking at the inferior angle through a Goldmann lens, with the goniomirror centered at 12 o’clock, the 7 o’clock angle would be seen at the 11 o’clock position, the 6 o’clock angle at the 12 o’clock position, and the 5 o’clock angle at the 1 o’clock position (Figure 7-1).

The lens and light source must be manipulated to alter the angle of view over peripheral convex iris or to examine successive portions of the circumference. The need for these manipulations may result in a false interpretation of angle depth. For example, in eyes having convex irides, the angle may appear to be partially closed with a mirrored contact lens due to the inability to vary the angle of view so as to look down into the crack between iris and angle wall. We have seen numerous examples in which such angles that were judged to be closed by use of a mirrored lens have in fact been observed to be open to the full width of TM, scleral spur, and, occasionally, even narrow ciliary body band with the use of a Koeppe lens system (Ocular Instruments).

On the other hand, corneal distortion and indentation with the use of a mirrored contact lens can actually open an angle that is appositionally occluded. The angle is opened by posterior displacement of aqueous humor against the iris, consequent to the corneal indentation. This phenomenon is actually put to good use with the Zeiss indentation gonioscopy lens by Max Forbes,¹ in which such displacement can be used to identify peripheral anterior synechiae (PAS). Because differentiating open-angle from angle-closure glaucoma is perhaps the most important question asked during clinical gonioscopy, we routinely employ the Koeppe system as the first method of gonioscopy, particularly in new patients, because it allows a direct view into the angle over a convex iris without corneal distortion.2 Additionally, it permits bilateral, simultaneous gonioscopy, as well as examination of the angle by multiple observers without removal and replacement of contact lenses. In cases of suspected angle closure, this can then be followed by indentation gonioscopy with the Zeiss lens to evaluate the angle for the presence of PAS.

We recommend that the beginner learn to perform gonioscopy with the single, simple, universally applicable system of direct examination with the Koeppe lens and then to use the various mirrored indirect lenses (Figure 7-2) For example, the Goldmann gonioscopic lens (Ocular Instruments) provides an excellent high-magnification view of certain angle details, such as new blood vessels, and inflammatory deposits. Use of this gonioscopic lens is required when performing laser treatment to the angle, and it needs to be mastered by the ophthalmologist. The Zeiss indentation gonioscopy lens is invaluable in identifying PAS during indentation and can be used to rapidly view an angle at the slit lamp in a postoperative patient.

It is recommended that gonioscopy lenses are cleaned with either steam or chemical sterilization techniques after each use. Due to the time and cost required for proper sterilization technique, it may be more efficient to use a disposable single-use gonioscopy lens. When compared to the Volk G-1 standard gonioscopy lens, the quality of the image when viewing the angle was comparable with the disposable lens. The single-use lenses decrease the risk of infection and may be a suitable alternative in certain circumstances.³

In certain conditions, such as in suspected traumatic recession of the angle and in congenital glaucoma, comparison of one eye with the other is particularly helpful. This is conveniently accomplished with Koeppe lenses on both eyes, alternating the view with the handheld microscope easily back and forth from one eye to the other.

Modifications that have occasionally been suggested in the direct gonioscopy method consist of suspension of the microscope to lessen its weight and fixation of the light to the microscope to make it a one-handed affair. A simple suspension system consisting of a cord from the handheld binocular microscope going up over a couple of pulleys to a counter weight is simple, practical, and greatly favored by some expert gonioscopists but is not essential, because one can readily learn to rest the elbows on the examining table, the back of a chair, or on one’s knees for support and stability. It is best not to have the light attached to the microscope, in the interests of flexibility and variability of illumination. It is preferable to hold the microscope in one hand and the light in the other with the 2 hands partially interlocked and the elbows resting on some sup port. While looking at the angle, it is useful to be able to vary the path of illumination, particularly when the cornea is not clear, and to be able to change from a sharp focal line in the angle to various degrees of indirect illumination by simple small changes in the position of the handheld light.

Early observation of the angle structures utilized direct gonioscopy, and is still performed largely in the operating room. Currently, the standard clinical approach is to use indirect gonioscopy using mirror or prisms to view the angle. The techniques used by using the gonioscopy lenses can create the appearance of an open angle if too much indentation is unknowingly applied or if there is increased illumination at the slit lamp. This can give an altered configuration of the angle depth. An alternative approach and a more objective way of viewing the angle can be done with anterior segment imaging using anterior segment optical coherence tomography (AS-OCT).4

AS-OCT uses low coherence light to image tissue depth, resulting in a high-resolution image of the angle structures. AS-OCT can rapidly image the anterior segment angle structures without physical contact with the eye. When viewing the angle using imaging, the scleral spur can be readily identified where there is a change in curvature of the angle wall and can appear highly reflective. This is a landmark this is customarily used as the TM is located 250 to 500 μm anterior to the scleral spur along the angle wall. The definition of closed angle on AS-OCT is any contact between peripheral iris structures and angle wall anterior to the scleral spur. Studies show that AS-OCT has a high sensitivity for identifying angles that are closed, even those characterized as open with gonioscopy. Limitations of AS-OCT are the need for clear corneal media and you cannot determine if there is synechial or appositional closure. AS-OCT can be used as an adjunct clinically or as a screening tool for patients, but gonioscopy remains the gold standard for identifying the angle structures. It may be particularly useful in patients who have difficulty tolerating or positioning at the slit lamp for prolonged gonioscopy.⁵

GONIOSCOPY (FINDINGS)

In this section, we will discuss variations and abnormalities that are found by gonioscopy.

Iris Contour and Character

At the start of the gonioscopic examination, one can decide whether the contour of the iris is to be characterized as slightly convex, very convex, flat, or concave (Table 7-1).

A slight convexity of the iris is seen in the majority of normal adult eyes, due to a pushing forward of the pupillary portion of the iris by the anterior surface of the lens and a slight, physiologic ballooning forward of the periphery of the iris caused by a physiologic difference in pressure be tween the posterior chamber and anterior chamber. The pressure in the posterior chamber ordinarily is slightly greater than that in the anterior chamber because of a small resistance that the aqueous encounters in flowing forward through the area of contact of iris and lens en route to the pupil and anterior chamber. We have often noted that the peripheral convexity of the iris in the average eye is lost when a hole is made in the periphery of the iris. This allows aqueous to go freely from posterior to anterior chamber. The convexity also is lost when the pupil is widely dilated or when the crystalline lens is removed. A convex iris contour is seen typically in hypermetropic eyes that have a small anterior segment (Figure 7-3). If the lens is relatively large and positioned forward, shallowing the anterior chamber axially, the resistance to flow of aqueous forward from the posterior to anterior chamber is greater than normal, and this may cause the periphery of the iris to bulge forward so as to narrow or even close the angle (Figure 7-4). Beyond the central convex portion of the iris, in many eyes, a peripheral furrow or sulcus is found. The farther back the root of the iris is attached on the ciliary muscle and the shallower the axial depth of the anterior chamber, the more likely there is to be a peripheral sulcus. The peripheral sulcus develops gradually during childhood and later life, as the lens grows. No sulcus is found in infants’ eyes.

A flat iris contour and, in exceptional cases, even a slight concavity are most characteristically observed in my opia and aphakia. When the iris is flat, one should note where the plane of the iris lies in relation to the structures in the angle. In myopic and aphakic eyes, this plane generally lies well back of the level of the scleral spur. However, in other types of eyes, the plane of the iris is sometimes quite differently situated. Particularly, in the peculiar condition known as plateau iris, the root of the iris is attached close behind the scleral spur, and the plane of the iris is even further forward, sometimes at the level of the anterior TM or Schwalbe’s line. This brings the peripheral iris almost against the filtration portion of the TM with little or no assistance from excess pressure of aqueous in the posterior chamber. This is the rare type of eye in which, even after an iridectomy has been performed, dilation of the pupil can cause closure of the angle and glaucoma. In such eyes, the ciliary processes can be abnormally rotated forward, in contact with the back of the iris, seemingly holding the iris forward in the periphery (Figure 7-5). In normal eyes, the ciliary processes are not in contact with the back surface of the iris.

Most emmetropic eyes demonstrate a slight iris convexity (forward) due to slight relative pupillary block. When a high degree of relative pupillary block is present, angle-closure glaucoma can ensue from the resulting forward movement of the iris into the angle.

Concavity in the iris contour may be associated with pigment dispersion syndrome and pigmentary glaucoma (Figure 7-6); old posterior uveitis, from contraction of inflammatory tissue behind the iris; or PAS in aphakic eyes.

Irregularity of iris contour in moderate degree is not necessarily abnormal. Eyes having a coarse or lacy stromal structure are particularly likely to have wave-like billows or prominences at various places in the angle, without pathologic significance or threat of angle closure. However, a large, rounded, mound-like elevation of one portion of the iris can signify a dislocation and forward tilting of the lens in that area or the presence of cyst or benign or malignant tumor of ciliary body or posterior layer of the iris. A cyst may be solitary and present in only one eye, but more characteristically, cysts are multiple and present in both eyes. If a single isolated cyst is present, it is usually situated temporally. Often, the spaces in the iris stroma over a cyst are slightly spread apart by the stretching over the bulge, so that some of the front surface of the pigment layer can be seen.

Segmental atrophy of the iris with localized flattening, contrasting with a general convexity of the rest of the iris, is seen most typically in eyes that have suffered an attack of acute glaucoma with high intraocular pressure (IOP). In this condition, the atrophic, thinned area of iris has a grayish appearance. Previous herpes zoster with iritis can also cause atrophic areas in the iris but with less characteristic segmental character and gray appearance.

After noting the contour and character of the iris, one may estimate the width of the angle by examining the distance between Schwalbe’s line and the nearest part of the iris. The angle may be described as wide, intermediate, narrow, excessively narrow, or closed (Table 7-2). Simple descriptive words are more explicitly and more widely understood than arbitrary classifications by numbers. The angle tends to be wide in normal, myopic, and aphakic eyes, but narrower in hypermetropic eyes.

The width of the angle normally varies about the circumference, anatomically usually narrowest at the 12 o’clock portion. An uncommon unevenness in width of the angle may be caused by cysts of the posterior layer of the iris or of the ciliary body, by dislocation of the lens, by tumors of the ciliary body (Figure 7-7), or by pupillary block accompanied by extensive posterior adhesions to the lens, to the vitreous, or to membranes in aphakic eyes. One sees excessive narrowing of the angle in eyes subject to angle-closure glaucoma. The angle is uniformly narrowed in eyes subject to severe acute attacks of angle-closure glaucoma, whereas it is narrowed less uniformly in eyes subject to subacute or chronic angle-closure glaucoma. The angle is seen to be closed in the whole circumference in an acute attack and in a part of the circumference in a subacute episode.

After the examiner has formed his or her opinion of the contour of the iris and the width of the angle, he or she should attempt to identify the scleral spur, because this is helpful for orietation in an open angle and also valuable in narrow angles to determine whether the angle is open back beyond the filtration portion of the TM (Table 7-3). In closed or extremely narrow angles, the scleral spur is hidden by the peripheral iris. Even in an open angle, the scleral spur is not always readily visible, for it is often covered and obscured by uveal meshwork. In a wide and open angle, the scleral spur is obscured in this way, most commonly in the nasal quadrant, where the uveal meshwork is usually most abundant. Sometimes, the uveal meshwork is abundant in the whole circumference, but there is almost always some area or portion of the angle in which uveal meshwork is thin enough and the scleral spur is sufficiently distinct so that it can be identified. Once identified, the spur can usually be traced around even in the areas in which uveal meshwork is thickest. When the scleral spur can be seen, it is recognizable as the posterior edge of the corneoscleral TM. It is of the same whiteness as the TM and whiter than the ciliary band that extends posteriorly from the scleral spur.

When the ciliary muscle can be seen, its color has some racial variations dependent upon the presence or absence of pigment among the muscle fibers (Table 7-4). Characteristically, in White races, with either blue or brown eyes, the ciliary muscle itself is unpigmented and appears only slightly less white than the corneoscleral TM. In darkly pigmented races, the ciliary muscle usually contains an appreciable amount of melanin and has a gray appearance or may even show traces of brown deep within the tissue.

An abnormal amount of pigment in the ciliary muscle gives the ciliary band a slate gray or dark brown appearance that may be a clue to malignant melanoma in the ciliary body. When present in only part of the circumference, the discoloration contrasts with the color of the normal portions.

An abnormally light gray appearance with a fine, loose, cobwebby texture, lacking the customary structure of uveal meshwork, is characteristic of a tear into the ciliary muscle. The intact muscle is nearly colorless or very slightly gray in White races, and when the muscle has been torn, the lighter appearance is presumably due to the white of sclera showing through the thinned layer of tissue.

A scleral whiteness of the ciliary band results when the full thickness of ciliary muscle has been torn off the scleral spur, exposing the sclera to gonioscopic view. Sclera may be identified not only by its color but sometimes by the presence of penetrating branches of short anterior ciliary arteries on its inner surface. These vessels emerge from the sclera a variably short distance behind the scleral spur and run directly posteriorly out of sight behind the ciliary body.

Defects in the ciliary band after blunt trauma to the eye (Figure 7-8) often consist of a tearing apart of the ciliary muscle and the overlying uveal meshwork, sometimes associated with tearing or indirect damage to the TM, leading to glaucoma. A defect in the ciliary band is also found after surgical cyclodialysis that leaves a cleft just behind the scleral spur. The cleft following successful cyclodialysis is a bottomless tunnel going back as far as one can see, but when the operation has failed, the cleft can usually be seen to have a definite bottom where the tissues have healed together.

The level of attachment of the periphery of the iris on the ciliary muscle has a wide range of anatomic variation, and this influences the width of the ciliary band. Unusual narrowness of the ciliary band is common in congenital glaucoma. Abnormal synechial attachment of the iris to scleral spur or to corneoscleral TM can narrow or completely hide the ciliary band. Abnormal widening of the ciliary band is usually caused by tearing of the ciliary muscle by blunt trauma or by surgical cyclodialysis. Irregularity or unevenness in width of the ciliary band that is noted as one scans the whole circumference is often a clue to abnormality, such as a tear, in the ciliary muscle, which can be discerned by close inspection.

The uveal meshwork lines the angle from peripheral iris over ciliary band, scleral spur, and part of the TM in all eyes, but it varies greatly in thickness, pigmentation, texture, and extent from patient to patient. It also varies remarkably with age, developing from an inconspicuous and colorless, fine, homogeneous structure in the infant to a coarse, lacy forest of brown strands in some adult eyes or to an unpigmented, fine fibrillar layer in others (Table 7-5).

In addition to the uveal meshwork, which essentially lines the angle, in some eyes, there are occasional separate, distinctive iris processes. The iris processes are long, slender, isolated strands that stand out away from the uveal meshwork and bridge the angle from the periphery of the iris to the anterior half of the corneoscleral TM. These strands may have a remote relationship to the pectinate ligaments of animal eyes. In adults, the iris processes are pigmented or unpigmented, corresponding to the stroma of the iris. In infants, the iris processes may be present at a time when the uveal mesh work is a homogeneous undifferentiated sheet (by gonioscopy), but iris processes are usually colorless and inconspicuous at that stage. The distribution of iris processes seems to be haphazard from eye to eye and from one part of the circumference to another. The iris processes seem to have little diagnostic or pathologic significance, except that one may note that they are ruptured in association with traumatic recession of the angle, and one might speculate that they could provide the initial bridge for development of the strange iridocorneal adhesions of essential atrophy of the iris. We will say no more about iris processes and, in the rest of this section, will discuss only the uveal meshwork proper.

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