High-frequency ultrasound biomicroscopy (UBM) is the most established anterior segment imaging device, providing objective, high-resolution images of angle structures. UBM allows for visualization of structures in the posterior chamber that are otherwise hidden from clinical observation and can augment gonioscopy in the qualitative and quantitative evaluation of pathologic changes leading to angle closure. Most commercially available instruments use a 50- to 80-MHz transducer with a lateral and axial physical resolution of approximately 50 μm and 25 μm, respectively.¹ UBM can be used to demonstrate a wide variety of anterior segment pathology, including congenital glaucoma, and is also useful for presurgical and postsurgical evaluation. Newer UBM models on the market, with 100-MHz transducers, have been exclusively designed for real-time morphological assessment of the anterior chamber angle and for the evaluation of aqueous drainage passageways, including Schlemm’s canal (iScience Surgical, www.iscienceinterventional.com/US/iultrasound.htm, accessed March 2007).

Anterior segment optical coherence tomography (AS-OCT; Carl Zeiss Meditec, Dublin, California) and slit-lamp–adapted optical coherence tomography (SL-OCT; Heidelberg Engineering, Dossenheim, Germany) are recently developed methods that allow for objective and quantitative imaging of anterior segment structures and angle configuration.² Advantages over UBM include noncontact methodology, with consequent reduction of patient discomfort and risk of corneal injury, and the ability to image the eye in the sitting position. AS-OCT, with an axial and transverse optical resolution of 18 μm and 60 μm, respectively, has better resolution than SL-OCT, which has an axial and transverse resolution of 9 μm and 15 μm, respectively. Both modalities rely on infrared light of 1310 nm wavelength to provide images of the anterior segment; however, AS-OCT and SL-OCT cannot image structures posterior to the pigment epithelium of the iris and ciliary body owing to absorption of light by this layer.

UBM software quantifies distance and area by counting the number of pixels along the measured line or inside the designated area and multiplying the pixel counts by the theoretical size of the pixel. Two factors, physical resolution and measurement precision, are important in image quantification. Resolution refers to how close together two objects can be placed and still appear distinct. As previously mentioned, most commercially available instruments have a lateral and axial physical resolution of approximately 50 μm and 25 μm, respectively. The measurement precision refers to the width and height of a single pixel on the screen identifiable by the operator using the screen cursor.² The standard Humphrey and Paradigm UBM monitors (864 × 432 pixels) have lateral and axial measurement precision of approximately 6 and 12 μm, respectively.¹ Using UBM software, measurement precision can be better than physical resolution by oversampling the signal.³

Limitations of both SL-OCT and AS-OCT include distortion from off-axis measurements, requiring a software correction for optical distortion and for changes in beam angulation due to passage of light through media with different refractive qualities. Sector scanning UBM devices, for example OTI and Sonomed models, also require correction for image distortion. In OCT, the scanning system is a fixed point on the mirror of the slit lamp. The center of the image is the only section scanned vertically, whereas all scans adjacent to the center are influenced by the so-called scanning angle. Using the fan correction, the software recalculates the data to eliminate the influence of the scanning angle with a resulting B-scan in the shape of a fan (Figure 1). Correction for the various refractive indexes of the cornea is also done with OCT software. The image must first undergo segmentation to identify the areas of different tissue. This step is essential for location of the exact position of the anterior and posterior cornea. Recalculating the image for both scanning angle and refractive error leads to a B-scan in a butterfly shape (Figure 2).

Several other imaging devices have been used to evaluate the anterior chamber. The Pentacam-Scheimpflug, a noncontact optical imaging system, has two camera components with a software program used to construct a 3-dimensional image. Important information about the cornea, anterior chamber, and lens may be obtained from these images.⁴ Several studies have shown good correlation of corneal thickness and anterior chamber depth measurements with “gold standard” devices.^5–7 The Pentacam-Scheimpflug has also been used to estimate the anterior chamber angle⁸; however, anterior chamber angle was not found to be significantly correlated with anterior chamber depth or with other quantitative angle parameters. Also, direct visualization of the angle is not possible with this device, preventing morphological assessment of the anterior chamber in cases of plateau iris or lesions of the ciliary body.

Orbscan scanning–slit topography is an alternative noncontact optical system that has primarily been used to view the cornea, although it may also have applications in angle estimation.⁴ Similar to Pentacam-Scheimpflug, Orbscan cannot directly visualize the angle, preventing an assessment of the structural aspects of the anterior chamber. Orbscan has high reproducibility and significant correlation with other clinical parameters⁹ (ie, anterior chamber depth, axial length, corneal diameter). To the best of our knowledge, the quantitative angle parameters obtained with this device have not yet been correlated to those of more established modalities, for example, UBM. Although very promising, more information is needed regarding reproducibility of parameters and agreement with established devices, in order to routinely utilize Pentacam-Scheimpflug or Orbscan for quantitative assessment of the anterior chamber angle.

The technique of performing UBM is similar to traditional immersion B-scan ultrasonography. The probe is suspended from an articulated arm to diminish motion artifacts. The Humphrey and Paradigm UBM models utilize a linear scan format that minimizes lateral distortion. Scanning is performed in the supine position. An eyecup, measuring 18, 20, or 22 mm in diameter, is inserted between the eyelids and holds methylcellulose or normal saline solution as the coupling agent. After insertion of the probe into the coupling medium, the real-time image is displayed on a video monitor and can be stored on videotape for later analysis. Inexperienced examiners with minimal practice can typically obtain good qualitative information; however, acquisition of highly reproducible distance measurements is strongly dependent on examiner technique and experience. The distance from the center of the anterior chamber, plane of section, and the orientation of the probe with respect to the perpendicular may affect the apparent structural configuration of the anterior segment.

There is some indication that use of the UBM eyecup may cause corneal indentation and a resultant widening of the iridocorneal angle. The most recent version of UBM, the Paradigm P60 model, does not require an eyecup due to modifications to the transducer head; however, the parameters obtained with this device have not yet been established as reproducible. The OTI and Sonomed units require a soft silicone eyecup, unlike older models made by Paradigm and Humphrey that utilize a stiffer eyecup. There have not been any studies with these new models regarding the effect of corneal indentation by the new transducer head.

Both SL-OCT and AS-OCT are relatively new, and software for these devices is still being modified and improved. More studies are needed to evaluate the reliability and reproducibility of measurements taken using these modalities. In a comparison of SL-OCT and UBM, AOD as measured by UBM was found to be significantly less than AOD measured using SL-OCT in both the superior and inferior angles (P = .0001) (S. Dorairaj, unpublished data, 2007). In the superior angle, a highly significant difference was seen between AODs in light (P = .002) but not in the dark (P = .153). Also, there was good agreement between SL-OCT and UBM measurements of ITC, with kappa values ranging from 0.65 to 0.47. S. F. Sandler evaluated the intraobserver and interobserver reliability and reproducibility of SL-OCT for evaluation of anterior chamber depth and central corneal thickness (unpublished data, 2007). Although SL-OCT provided operator-independent, highly reproducible measures for both parameters, there was a discrepancy between the measurements as taken by SL-OCT, axial OCT biometry, and ultrasonic pachymetry, highlighting a potential need for calibration of measurement algorithms.

Müller and colleagues²⁶ found low intraobserver and interobserver variability for AS-OCT measurements of anterior chamber angle and opening width, supporting its use for quantitative angle assessment. Nemeth and colleagues²⁷ measured anterior chamber depth with both AS-OCT and a standard A-scan device using an immersion technique and assessed the repeatability, reproducibility, and correlations of the measurements. They reported significantly deeper anterior chamber measurements with AS-OCT than with immersion A-scan. They also found better repeatability of anterior chamber depth measurements than with immersion ultrasound, with equal reproducibility of both methods.

With respect to quantitative evaluation of anterior chamber angle, several factors may affect measurement, including lighting conditions, patient positioning, and variations in software algorithms. As previously described, quantitative parameters of UBM and SL-OCT vary with regard to the patient posture during examination. With UBM, the patient is in the supine position, which may cause posterior movement of the lens and other structures, thus showing fewer instances of apposition on UBM testing.²⁸ However, UBM measurements have actually been found to be smaller in some comparative studies.² It is possible that AOD, as measured in different sectors of the eye, will have varying measurements due to the varying effect of gravity on these different locations. In pigment dispersion syndrome, disruption of the posterior pigment epithelium of the iris is facilitated by a concave iris configuration, which can be reversed by inhibition of blinking, among other factors.²⁹ It is thought that this iris concavity results from reverse pupillary block, and it has been hypothesized that blinking results in transfer of a bolus of aqueous humor into the anterior chamber.³⁰ Prior studies have demonstrated that increased iris concavity, which can be quantified on UBM imaging, in pigment dispersion syndrome appears to be related to increased iridolenticular contact, creating an anatomic configuration that predisposes to reverse pupillary block. When blinking is prevented, accumulated aqueous humor in the posterior chamber alters iris position in pigment dispersion syndrome and increases iridozonular and iridociliary-process distances while minimizing iridolenticular contact. Normal blinking can create transient vector forces to promote aqueous humor flow from posterior to anterior chamber.³¹

AS-OCT was used to analyze 37 normal subjects with open angles and 18 subjects with narrow angles, and AOD 500 measurements taken in light were significantly higher than those measured in the dark for both the open-angle and narrow-angle groups (P = .001) (C. K. Leung, unpublished data, 2007). After adjusting for age, regression analysis identified a positive correlation between anterior chamber depth and AOD difference, and the majority of eyes (n = 47) showed a high correlation between AOD and pupil size. The differences of the angle width measured in the dark and in the light among individuals demonstrate the importance of standardized background light intensity during angle evaluation.

After examination of 17 normal eyes and 14 narrow-angle eyes using UBM, anterior segment OCT, and gonioscopy, Radhakrishnan and colleagues² found that ARA and the TISA had similar discriminative abilities for detection of narrow angles. The areas under the receiver operating characteristic curve using UBM and SL-OCT for ARA 750 were 0.97 and 0.96, respectively, and for TISA 750 it was 0.96 for both devices. A limitation of TISA is that it uses a smaller sample of the AOD in the linear regression analysis. This may add variability to the description of the angle shape when this smaller area measurement is combined with acceleration and the y-intercept of the linear regression analysis. The TICL had a 100% and 79.3% specificity using SL-OCT and UBM, respectively, for ruling out angle closure. Previous UBM studies, however, have demonstrated variations in trabecular height among individuals with glaucoma, therefore making the use of a fixed distance from scleral spur to measure TICL somewhat inaccurate, as it may incorrectly include structures anterior to Schwalbe’s line.³²

TCPD has been used to evaluate the eyes of patients with angle closure. Several studies have shown that a shorter TCPD is associated with anterior placement of the ciliary body.^12,33 Based on this information, the TCPD may prove useful for evaluation of patients with plateau iris, who generally have a more anteriorly displaced ciliary body. Gohdo and colleagues²¹ report an association between eyes with narrow angles and the presence of a thinner ciliary body.

In cases of occludable angles, gonioscopy does not always guarantee a diagnosis. Provocative testing has been described for the detection of potential appositional angle closure in patients with asymptomatic narrow angles. Previous UBM and SL-OCT studies evaluating quantitative parameters by provocative testing from light to dark have concluded that there is increased probability of identifying occludable angles if the imaging tests are done in dark (S. Dorairaj, unpublished data, 2007).

The scleral ciliary process angle (SCPA), measured between the axis of the ciliary process and the line tangential to the scleral surface, has been suggested as a useful parameter for evaluation of anterior placement of the ciliary body, particularly in plateau iris.³⁴ It is possible, however, that the incorrect placement of the reference point from which the tangential line is drawn may result in widely variant SCPA measurements. Until a repeatable and reliable method of reference point placement is established, the use of this parameter should be approached cautiously.

Quantitative measurements may also be useful for the diagnosis and classification of open-angle glaucomas. Potash and colleagues¹⁹ have used the iris concavity parameter to demonstrate a midperipheral iris concavity in 56% of eyes examined, supporting the hypothesis of reverse pupillary block in pigment dispersion syndrome. This information is not only useful for diagnostic purposes but provides the basis for subsequent treatment of iris concavity with miotic therapy and laser iridotomy. Kanadani and colleagues³⁵ studied both eyes of patients with clinically asymmetric pigment dispersion syndrome using UBM and determined that the severely affected eye had a more concave iris configuration than the less affected eye (−0.28 vs 0.08 mm). These measurements confirm the predisposition of patients with a more posterior iris insertion to a phenotypic expression of pigment dispersion syndrome. Sokol and colleagues¹⁸ also measured the SS-to-IR distance to determine the effect of iris insertion on expression of pigment dispersion syndrome. They found that SS-to-IR distance was significantly greater in eyes with pigment dispersion syndrome (0.04 mm ± 0.04 mm) than in control eyes (0.28 mm ± 0.04 mm) (P = .01), as well as the overall distance from Schwalbe’s line to the iris root (0.98 ± 0.04 mm vs ± 0.04 mm). These findings support a more posterior insertion of the iris into the ciliary body in eyes with pigment dispersion syndrome than control eyes.

Quantitative measurement of static and dynamic anterior segment parameters, both normal and abnormal, provides a broad range of parameters for analysis of the numerous aspects of the pathophysiology of the anterior segment of the eye. Table 1 categorizes a comprehensive schematic of reproducible anterior segment parameters by UBM and AS-OCT/SL-OCT. SL-OCT and AS-OCT parameters are limited to the measurement of those structures anterior to the iris, and therefore are not particularly useful for the evaluation of the ciliary body and zonules.

Quantitative parameters have greatly improved our understanding of the anatomical relationships of various anterior segment structures and have also given us useful tools for monitoring patient prognosis. Improvements in software, for example, the development of 3-dimensional UBM, may provide a better understanding of the spatial relationships of anterior segment.³⁶ Modifications to the technology will also increase the image resolution, providing clinicians more precise measurements for clinical decision making. The quantitative information provided by modern anterior segment imaging devices, although useful by itself, should not overshadow the contribution of qualitative information gathered from the images acquired through these devices.

In the routine management of patients by the general ophthalmologist, anterior segment imaging devices can serve as an important screening tool for angle-closure glaucoma and may provide good images of structural defects of the anterior segment. The SL-OCT and AS-OCT are best suited for screening because they can image eyes quickly and in a noncontact manner. By routinely monitoring certain parameters, including anterior chamber depth, AOD, and anterior chamber diameter, physicians can observe any structural changes in anatomy and obtain important information for the diagnosis and treatment of a variety of anterior segment diseases, in particular, angle-closure glaucoma and pigment dispersion syndrome.

UBM is indispensable for imaging of pathology posterior to the iris, as anterior segment OCT is incapable of penetrating beyond this point. In cases of plateau iris, initially diagnosed with gonioscopy, immediate imaging with UBM is warranted for confirmation. Although anterior segment OCT may be able, in certain cases, to image cysts and tumors of the iris, UBM provides the most consistent and reliable method for viewing these structures. Likewise, anterior segment OCT is capable of demonstrating cases of angle closure by providing good views of the anterior chamber; however, for cases of angle closure originating posterior to the iris, for example, phacomorphic glaucoma or malignant glaucoma, UBM provides the best documentation of these findings. The combination of both these devices provides a comprehensive, qualititative picture of trabeculectomy blebs and glaucoma implants (Table 2).

In terms of patient positioning, contact, and comfort, anterior segment OCT is most useful in postsurgical patients, children, and those who are uncooperative with the UBM examination. Advantages of UBM include penetration beyond the posterior iris, established reliability of measurements, and good correlation of measurements with histological samples. Anterior segment OCT, however, provides better resolution of images, is noncontact, and can be done in the sitting position (Table 3).