The reliability of UBM measurements has been previously confirmed. Tello and colleagues24
have studied the reproducibility of Pavlin’s UBM parameters and found good intraobserver reproducibility for all parameters except AOD. Interobserver reproducibility was poor, and this finding was confirmed by Urbak and colleagues,25
who also reported poor interobserver reproducibility for all measurements except central corneal thickness. Most UBM software requires an element of observer input; for example, localization of the scleral spur by an operator is necessary for calculation of most angle parameters. Identification of this important landmark is also dependent on the presence of image quality criteria. It is possible that the combination of objective perception and variations in image quality can lead to low interobserver reproducibility, further highlighting the need for an automated software capable of detecting the scleral spur.
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be significantly less than AOD measured using SL-OCT in both the superior and inferior angles (P = .0001) (S. Dorairaj, unpublished data, 2007).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reliability and reproducibility of SL-OCT for evaluation of anterior chamber depth and central corneal thickness (unpublished data, 2007).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. 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 OBTAINED WITH ULTRASOUND BIOMICROSCOPY (UBM), SLIT-LAMP–ADAPTED OPTICAL COHEREHNCE TOMOGRAPHY (SL-OCT) AND ANTERIOR SEGMENT OCT (AS-OCT)
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 ().
QUANTITATIVE AND QUALITATIVE USES OF ULTRASOUND BIOMICROSCOPY AND SLIT-LAMP–ADAPTED OPTICAL COHEREHNCE TOMOGRAPHY (SL-OCT) AND ANTERIOR SEGMENT OCT (AS-OCT)
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 ().
ADVANTAGES AND DISADVANTAGES OF ULTRASOUND BIOMICROSCOPY AND SLIT-LAMP–ADAPTED OPTICAL COHEREHNCE TOMOGRAPHY (SL-OCT) AND ANTERIOR SEGMENT OCT (AS-OCT)
In summary, several tools are available for imaging the anterior segment. In clinical practice, the modality chosen is dependent on a number of factors, including clinical findings, differential diagnosis, patient comfort, and structures to be imaged. Taking into account these considerations, a number of quantitative parameters are available for use in diagnosis and treatment of many ocular diseases. With continued improvements to the technology, fewer limitations will exist for the routine use of these modalities in the clinical setting.