Anterior Segment Imaging
Sung Chul (Sean) Park
Syril Dorairaj
Jeffrey M. Liebmann
Robert Ritch
INTRODUCTION
Anterior segment ultrasound biomicroscopy ultrasound biomicroscopy (UBM) uses high-frequency transducers (35 to 75 MHz) to provide in vivo imaging of the anterior segment with an axial resolution of 30 to 70 µm and penetration depth of 2 to 7 mm. The structures surrounding the posterior chamber, hidden from clinical observation, can be imaged and their anatomic relationships assessed. UBM has been used to investigate both the normal structure and disease mechanisms and pathophysiology in many areas of ophthalmology, including glaucoma, cornea, lens, congenital abnormalities, effects and complications of surgical procedures, anterior segment trauma, cysts and tumors, and uveitis. Studies using UBM were initially primarily qualitative, but quantitative studies have become increasingly common.1 Three-dimensional analysis of UBM images is still in its infancy.
Anterior segment optical coherence tomography (AS-OCT) uses infrared light instead of ultrasound and transmits signals of shorter wavelength (820 to 1,310 nm), which produce images of higher resolution (axial resolution of 5 to 15 µm) than UBM.2 It enables non-contact imaging because the refractive index between air and tissue is much less than the acoustic impedance between them. Dynamic relationships between the iris, angle wall, and lens can be assessed through real-time limbus-to-limbus cross-sectional images because of its faster scan speed, reducing eye movement artifacts. AS-OCT also provides software for automatic measurement of various cornea and anterior chamber parameters (Fig. 5-1). However, the ciliary body is rarely visualized owing to the pigmented posterior layer of the iris, which blocks light penetration. AS-OCT has been used similarly to UBM, but, because of its characteristics, has been more useful in quantitative analysis of the anterior chamber and in corneal disease or surgery, such as keratoplasty.
These imaging devices do not replace conventional slit-lamp biomicroscopy or gonioscopy, but supplement and augment clinical practice and provide invaluable research tools. Characteristics of UBM and AS-OCT are compared in Table 5-1.
 FIGURE 5-1. Measurement of cornea and anterior chamber parameters using anterior segment optical coherence tomography (AS-OCT). Corneal thickness, corneal radius of curvature, anterior chamber depth and volume, pupil diameter, and distance between scleral spurs (A), as well as anterior chamber angle parameters, such as AOD500 (angle-opening distance at 500 µm from the scleral spur), TISA500 (trabecular-iris space area at 500 µm from the scleral spur), and TIA500 (trabecular-iris angle at 500 µm from the scleral spur) (B), can be measured using AS-OCT. |
TABLE 5-1. Characteristics of UBM and AS-OCT | |||||||||||||||||||||||||||
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ANGLE-CLOSURE GLAUCOMA
Because of its ability to image the ciliary body, posterior chamber, iris-lens relationships, and angle structures simultaneously, UBM is ideally suited to the study of angle closure. Significant correlations have been found between angle measurements by AS-OCT, UBM, and gonioscopy.3,4 When assessing a narrow angle for occludability, gonioscopy in a completely darkened room, using the smallest square of light for a slit beam to avoid stimulating the pupillary light reflex, is of utmost importance. The effect of ambient light on the angle configuration is well illustrated by performing UBM under illuminated and darkened conditions (Fig. 5-2).
Because most of the important anterior chamber angle parameters for quantitative measurement are based on the identification of the scleral spur, reliable documentation of the angle dimensions using UBM or AS-OCT is therefore dependent on its precise and repeatable localization. In a UBM or AS-OCT image, the scleral spur can be seen as the innermost point of the line separating the ciliary body and the sclera at its point of contact with the anterior chamber. Although it cannot be visualized with UBM or AS-OCT, the trabecular meshwork is located directly anterior to this structure and posterior to Schwalbe line (Fig. 5-3).
Cornea and angle structures are less distorted during AS-OCT because of its noncontact nature, avoiding artifacts induced by inadvertent pressure on the cornea during gonioscopy or on limbal tissues with the eye cup during UBM. Differentiation of appositional and synechial angle closure in eyes with iridotrabecular contact by indention AS-OCT adds to its clinical utility in the evaluation of patients with angle closure.5 Anterior chamber depth and volume measured using AS-OCT may be useful parameters for detecting individuals at risk of developing primary angle closure.
Angle closure can be classified by the site of the anatomic structure or force causing iris apposition to the trabecular meshwork. These are defined as blocks originating at the level of the iris (pupillary block), ciliary body (plateau iris), lens (phacomorphic glaucoma), and forces posterior to the lens (malignant glaucoma).
Relative Pupillary Block
Relative pupillary block is responsible for over 90% of the angle closure in Caucasian populations. In pupillary block, resistance to aqueous flow from the posterior to the anterior chamber through the pupil and the resulting increased aqueous pressure in the posterior chamber forces the iris anteriorly (Fig. 5-4A), causing anterior iris bowing and angle narrowing. An anteriorly convex configuration of the entire iris can be imaged using AS-OCT (Fig. 5-5).
Pupillary block may be absolute, if the iris is completely bound to the lens by posterior synechiae, but most often is a functional block, termed relative pupillary block. Relative pupillary block usually causes no symptoms. However, if it is sufficient to cause appositional closure of a portion of the angle without elevating intraocular pressure (IOP), peripheral anterior synechiae may gradually form and lead to chronic angle closure (Fig. 5-6). If the pupillary block becomes absolute, the pressure in the posterior chamber increases and pushes the peripheral iris farther forward to cover the trabecular meshwork and close the angle with an ensuing rise of IOP (acute angle closure) (Fig. 5-7).
Laser iridotomy eliminates the pressure differential between the anterior and posterior chambers and relieves the iris convexity. This results in several changes in anterior segment anatomy. The iris assumes a flat or planar configuration (Fig. 5-4B), and the iridocorneal angle widens. The region of iridolenticular contact actually increases, as aqueous flows through the iridotomy rather than the pupillary space.
Plateau Iris
In plateau iris, the ciliary processes are either large or anteriorly situated, or both, so that the ciliary sulcus is obliterated and the ciliary body supports the iris against the trabecular meshwork. The anterior chamber is usually of medium depth and the iris surface only slightly convex. Argon laser peripheral iridoplasty contracts and compresses the peripheral iris, pulling it away from the trabecular meshwork (Fig. 5-8).6 Although large or anteriorly situated ciliary processes are rarely visualized by AS-OCT, it can be used to confirm a clinical suspicion of plateau iris configuration (Fig. 5-9).7
Phacomorphic Glaucoma
Lens enlargement may cause shallowing of the anterior chamber and precipitate acute angle closure by forcing the iris and ciliary body anteriorly. Miotic therapy increases the lens axial length and causes it to move anteriorly, which further shallows the anterior chamber, and may paradoxically worsen the situation (Fig. 5-10). AS-OCT is useful in this condition, because anterior chamber depth, iris configuration, and angle structures can be evaluated at a glance.
Malignant Glaucoma
Malignant (ciliary block) glaucoma is a multifactorial disease in which the following components may play varying roles: (1) previous acute or chronic angle closure, (2) shallow anterior chamber, (3) forward lens movement, (4) pupillary block by the lens or vitreous, (5) zonular laxity, (6) anterior rotation or swelling of the ciliary body, or both, (7) thickening of the anterior hyaloid membrane, (8) vitreous expansion, and (9) posterior aqueous displacement into or behind the vitreous.
UBM reveals a shallow supraciliary detachment, not evident on routine B-scan or clinical examination. This effusion appears to be the cause of the anterior rotation of the ciliary body. Aqueous humor is secreted posterior to the lens (posterior aqueous displacement), increasing vitreous pressure, pushing the lens-iris diaphragm forward, and causing angle closure and shallowing of the anterior chamber (Fig. 5-11). Although changes in the shape or position of the ciliary body cannot be accurately assessed, an anteriorly displaced iris-lens diaphragm and shallow anterior chamber are well demonstrated using AS-OCT (Fig. 5-12).
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