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Imaging Devices for Angle Assessment

Mina Pantcheva, MD and Malik Y. Kahook, MD

During the past 20 years, several advances in technology have enabled the clinician to assess the angle with methods other than traditional direct or indirect visualization. Many of these technologies are evolutionary upgrades to commonly used devices for biometry or imaging of the anterior/posterior segment. Although this chapter is not meant to be exhaustive, it will hopefully show the ability of imaging devices to complement traditional methods of examination.

The 2 major categories that devices fall under are ultrasound (contact) and optical (noncontact). Although the range of ultrasound technology falls into a single category of ultrasound biomicroscopy (UBM), optical devices fall under a broader range of optical coherence tomography (OCT), Scheimpflug imaging, and scanning slit topography (Orbscan, Bausch & Lomb).

ULTRASOUND BIOMICROSCOPY

Background

Mechanical waves and vibrations are the basic constituents of ultrasound, and the range can be measured by vibrations per second (Hz). The audible spectrum ranges from 10 to 20 kHz. Traditional B-scan ultrasound is in the range of 10 MHz and allows for resolution of 0.2 mm axially and 0.5 mm transversely. The approximate penetration for a traditional 10-MHz ultrasound is 50 mm, which is adequate enough for gross examination of the anterior and posterior segments. Increasing the frequency of the transducer can increase the resolution obtained by ultrasound. In the early 1990s, following the development of higher-frequency transducers for blood vessel imaging, came the development of high-frequency ultrasound for the eye. UBM is performed in a frequency range between 40 and 100 MHz. The increased frequency leads to decreased depth of penetration (eg, a 60-MHz transducer can penetrate approximately 5 mm). The combination of higher resolution and decreased depth of penetration is ideal for evaluation of the anterior segment. Detailed images of the anterior chamber, drainage angle, and ciliary body can be obtained with an axial resolution of about 25 μm and lateral resolution of about 50 μm (Figure 10-1).^1,2 Polymers such as polyvinylidene difluoride and polyvinylidene difluoride-trifluorethylene have been essential advancements in transducer technology to allow for these higher frequencies. Steps for performing UBM to image the anterior segment are similar to performing an immersion B-scan.

Quantitative Measurements

A commonly described benefit of most new imaging modalities is the ability to quantitatively describe the angle. This brings the hope that interobserver bias becomes reduced and that progression as well as prevention of disease can be reliably monitored. As the utility of these devices continues to be investigated, many new quantitative parameters are described. We will describe a few that are commonplace in UBM and have been translated over to optical devices (Figure 10-2). The principal point of reference in UBM is the identification of the scleral spur. The trabecular meshwork (TM) consistently falls 250 μm anterior from the scleral spur. The anterior aspect of the TM generally falls within 500 μm from the scleral spur. If a line is drawn from either point perpendicular to the TM toward the opposing iris, the length would be described as a very commonly used parameter known as angle opening distance (AOD250 or AOD500). The trabecular-iris angle (TIA) is defined as the angle measured with the apex in the iris recess and the arms of the angle passing through a point on the TM 500 μm from the scleral spur and the point on the iris perpendicularly opposite to it. The trabecular-ciliary process distance (TCPD) is the length of the line extending from the corneal endothelium 500 μm from the scleral spur perpendicularly through the iris to the ciliary process. The angle recess area (ARA) is the area between the AOD and angle recess. If the area posterior to the scleral spur is excluded, the parameter known as trabecular iris space area (TISA) is used.3–5

Clinical Applications

Primary Angle-Closure Glaucoma

Quantitative measurements have enabled researchers to investigate the mechanisms of primary angle-closure glaucoma and offer theories on why some eyes with narrow angles progress to pupillary block whereas others do not. Classical theories for pupillary block and primary angle-closure glaucoma include the idea that an area of iridolenticular touch causes increased posterior chamber pressure. Dilation is thought to exaggerate this to a point where increased posterior chamber pressure results in pushing the peripheral iris forward to occlude the angle. With UBM, it has been shown that iridolenticular contact is small in pupillary block and actually decreases with dilation. UBM has revealed that angle narrowing or closure occurs quickly (no time was required for aqueous pressure build-up) on pupillary dilation and is thought to be caused by a combination of increased iris thickening and increased anterior bowing as the iris tip moves toward the iris root (light dark paper). Thus, UBM can be used to perform provocative testing in narrow-angle eyes. Similarly, a thicker peripheral iris (as well as a narrower angle or anterior position of the ciliary body) was associated with progressive angle closure in fellow eyes of those with primary angle-closure glaucoma attack.

Plateau Iris

Plateau iris configuration has been defined as a normal-depth anterior chamber, a flat iris plane, and an extremely narrow angle. Plateau iris syndrome is defined when a patient with a plateau iris configuration remains capable of angle closure despite the presence of a patent peripheral iridotomy site (Figure 10-3). This is thought to be caused by anterior ciliary process positioning and ciliary sulcus closure. UBM has become the definitive method in the diagnosis of plateau iris configuration and syndrome. Although diagnostic criteria vary, plateau iris may be defined by UBM when all of the following criteria are present in 2 or more quadrants:

• Anteriorly directed ciliary processes, supporting the peripheral iris such that it is parallel to the TM
  
• An iris root with a steep rise from its point of insertion, followed by a downward angulation from the corneoscleral wall
  
• The presence of a central flat iris plane
  
• An absent ciliary sulcus
  
• Irido-angle contact (above the level of the scleral spur) in the same quadrant⁶

UBM can help evaluate tumors of the anterior segment (Figure 10-4). Because of the penetration of UBM, it has the ability to differentiate between cystic structures and solid structures. It allows for delineation of the location, size, and extent of the tumor. Ciliary body cysts appear as clear, echo-free, thin-walled bodies. Reflectivity can help differentiate the nature of solid tumors, such as ciliary body melanoma (low reflectivity), as well as a finding that melanomas show irregularity and convex bowing of the posterior iris plane not seen in nevi.⁷

Cyclodialysis Clefts

Cyclodialysis clefts are readily visible with UBM as well as anterior suprachoroidal effusions, either of which may not be easily detected with conventional exam. Nolan described the advantage of UBM for the detection of cyclodialysis clefts and the value of anterior segment optical coherence tomography (AS-OCT) as a noncontact examination technique for rapid follow-up after treatment.⁸

OPTICAL

Optical Coherence Tomography

In 1995, OCT became available for retinal imaging. Low coherence interferometry measures the delay and intensity of backscattered light by comparing it to light that has traveled a known reference path length using a Michelson-type interferometer.⁹ It is analogous to ultrasound, but uses light instead of sound. Recent changes in clinically utilized OCT technology principally involve increases in speed and wavelength. A traditional OCT device uses a wavelength of light of ~830 nm, which is close to the visible wavelength and therefore cannot visualize the angle due to scatter of light near the limbus.^10,11 With a scan speed of 100 to 400 axial (A) scans/s, an image of the anterior segment would appear coarse and grainy with possible motion artifact when captured over a full second. AS-OCT uses a wavelength of 1310 nm and a scan speed up to 4000 scans/s, which allows for better penetration and resolution by providing images of the cornea, iris, angle, anterior lens, retro-iris lens, ciliary body, and ciliary sulcus. At 1310 nm, 90% of the signal is lost before reaching the retina and currently cannot be used for retinal imaging.^12,13

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