The Role of UBM and AS-OCT in Glaucoma Imaging

For glaucoma specialists, ultrasound biomicroscopy (UBM) has been the gold standard for the objective and quantitative assessment of the anterior chamber angle.^1,2 The recent introduction of anterior
segment optical coherence tomography (AS-OCT) offers another imaging option. As a glaucoma specialist and researcher of ocular imaging, I am frequently asked which technology is better for imaging the anterior segment. This article discusses the pros and cons of both systems.

Technological Basics
UBM uses high-frequency ultrasound (50 MHz) to provide images that have a relatively high axial resolution (approximately 50 µm) with a reasonable depth of penetration. This technique can produce eight frames of cross-sectional images per second and, because it uses sound waves, can capture images through opaque media.

Ocular coherence tomography uses light waves to produce high-resolution cross-sectional images of the eye. Originally designed to visualize the posterior segment,^3,4 this technology has now been adapted to image the anterior segment. AS-OCT's 13-nm superluminescent LED light source provides high axial resolution (approximately 18 µm) but has a penetration depth of only 6 mm. Like UBM, AS-OCT generates eight frames of cross-sectional images per second, but the limitation of its optical principle makes it suitable for seeing only through clear media. (For details, see “Anterior Segment Optical Coherence Tomography” by Sunita Radhakrishnan, MD, and Scott D. Smith, MD, MPH, in the March/April 2006 issue of Glaucoma Today at http://www.glaucomatoday.com/pages/0306/10.html.)

AS-OCT is becoming an appealing option for imaging glaucoma patients because it is less invasive than UBM. The latter requires a water-bath setup in which an eyecup containing a liquid medium is inserted between the patient's eyelids. In addition to being messy and time consuming, UBM is usually performed with the patient in a supine position. Given a choice, many individuals would prefer to undergo testing with AS-OCT, because it is a completely noncontact method that can be used while they sit upright. Patients simply position their heads on the chin rest and look inside the machine. Overall, AS-OCT is easier and less stressful for them.

Indications for UBM
Although UBM and AS-OCT both provide a clear, objective view of the anterior segment, AS-OCT's utility is limited by its inability to image through the iris.⁵ This limitation is not a problem for routine evaluations when I am trying to assess whether an angle is open or closed, but it can be difficult if I am trying to determine if a patient has a plateau iris configuration.

Plateau iris configuration is characterized by closure of the angle at the peripheral recess that is associated with a relatively wide opening of the angle centrally. In most cases, this condition is caused by the anterior rotation of the ciliary process. To confirm this configuration, however, one needs to determine if the ciliary sulcus is absent. Moreover, plateau iris is sometimes caused by iridociliary cysts and/or iridociliary tumors, neither of which can be visualized with AS-OCT. Because UBM can penetrate the iris, this imaging modality can show the etiology of plateau iris configuration^6,7 (Figure 1).

To date, no study has clearly demonstrated the prevalence of plateau iris configuration among patients with angle-closure glaucoma due to significant variability among populations and ethnic groups. My own clinical experience suggests that the condition is more common than previously thought. Forty percent of the angle-closure patients I see at the UPMC Eye Center in Pittsburgh show some degree of plateau iris configuration.

Indications for AS-OCT
As much as I rely on UBM, I recognize that this imaging modality has limitations, especially for the evaluation of patients who have had previous filtering surgery. Many articles reported that UBM is useful for assessing the bleb's status,^8-10 but this approach can be risky. I have not encountered any complications while using UBM for this purpose, but I have found that performing this type of evaluation can be very stressful and not 100% safe. I therefore do not hesitate to use AS-OCT to assess the anterior chamber angle and the bleb's status in eyes that have sustained trauma or undergone recent surgery. In these cases, AS-OCT's noncontact modality eliminates the risk of mechanical abrasion that can accompany assessment with UBM.

In a recent, prospective, cross-sectional study,¹¹ investigators examined the utility of a prototypic AS-OCT device for examining the internal structure of filtering blebs. They found that the conjunctival/scleral walls of 87% (50 of 57) of the successful blebs appeared to be thickened, whereas only 28.6% (six of 21) of the failed blebs exhibited this characteristic. The investigators also suggested that clinicians may be able to use AS-OCT to determine the exact location of the occlusion in cases of failed blebs and use this information to guide early intervention and optimize aqueous flow through the bleb.

Conclusion
As this article discusses, UBM and AS-OCT have their pros and cons. Ideally, both options should be used in glaucoma evaluation. In reality, however, clinicians do not always have the luxury of performing both tests and, thus, must make an insightful decision.

The quality of images captured by UBM, especially of the peripheral angle recess, is slightly better than with AS-OCT. UBM also provides a detailed view of the structures behind the iris that cannot be viewed with AS-OCT. On the other hand, the noncontact nature of AS-OCT provides a patient-friendly environment and allows clinicians to image vulnerable eyes.

The two modalities also differ in their ease of use. Any technician who undergoes a short training session can operate AS-OCT. To acquire presentable images with the UBM, an operator must undergo longer training and have a good understanding of the anterior segment's anatomy.

Ultimately, both modalities are useful for quantitatively assessing the anterior chamber and the status of the angle, so it is up to individual physicians to choose the platform that best suits their preferences and needs.

Hiroshi Ishikawa, MD, is Assistant Professor, Departments of Ophthalmology and Bioengineering, University of Pittsburgh Schools of Medicine and Engineering, and Director, Ocular Imaging Center, UPMC Eye Center. He is a glaucoma specialist as well as a software engineer and has been active in research utilizing various ocular imaging modalities.
Dr. Ishikawa may be reached at ishikawah@upmc.edu.

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