Anterior segment imaging for glaucoma: OCT or UBM?

Short abstract

Ultrasound biomicroscopy and anterior segment optical coherence tomography both have their own pros and cons in imaging

Ultrasound biomicroscopy (UBM) had played the dominant role in objective imaging of the anterior chamber angle until anterior segment optical coherence tomography (ASOCT) was introduced in 2003. Although ASOCT provides a higher axial resolution (18 μm in commercial ASOCT device vs. 25 μm in 50 MHz UBM), this is not ASOCT's major advantage. The true advantage of ASOCT is its non‐contact scanning method that is performed in the sitting position, whereas UBM requires a plastic or silicone eyecup to hold a coupling medium (methyl cellulose or saline solution), which requires supine position scanning. Therefore, when it comes to patients' compliance and comfort in daily clinical settings, ASOCT is a clear winner. However, from the clinicians' viewpoint, the first and most important question is whether ASOCT provides clinically useful information like that which made UBM famous and popular (eg, angle opening distance, angle recess area, trabecular‐iris angle, etc).

Li et al's¹ study to investigate the repeatability and reproducibility of anterior chamber angle measurements using ASOCT answers part of this question (see page 1490). Their study showed that ASOCT could measure the anterior chamber angle with a high intra‐ and inter‐session repeatability/reproducibility (intraclass correlation coefficient 0.95–0.98 for intra‐session and 0.89–0.95 for inter‐session). In comparison with the modest UBM angle‐measurement reproducibility,²,³,⁴,⁵ these results were impressive.

However, Li et al chose only two angle parameters for their study: the angle opening distance and the trabecular‐iris angle introduced by Pavlin et al.⁶ Both parameters are notorious for their dependency upon variable iris surface undulation. Ishikawa et al proposed angle recess area or ARA to overcome these parameters' shortcomings,⁷ and later Radhakrishnan et al introduced a modified version, the trabecular‐iris space area (TISA), specifically for ASOCT images.⁸ The basic idea behind these parameters is to measure an area where the aqueous contacts the trabecular meshwork, which is less influenced by variable iris surface curvature. Since many reports dealing with quantitative anterior chamber angle assessment using either ASOCT or UBM used both angle opening distance and ARA/TISA, ⁹,¹⁰,¹¹,¹²,¹³,¹⁴ the inclusion of ARA or TISA in the study would have been more desirable. In fact, Radhakrishnan et al reported the ASOCT reproducibility of ARA and TISA as being comparable with those using UBM.⁸ They used a prototype ASOCT unit, whereas Li et al used a commercially available unit.

A unique aspect of Li et al's study is that they assessed the measurement reproducibility under both light and dark conditions. In glaucoma clinics, their good reproducibility results assure us that dark‐room provocative imaging can be performed without compromising the measurement reproducibility. One limitation is that they enrolled only healthy subjects. It would be helpful to know the reproducibility of eyes with a narrow angle or angle closure.

Li et al also pointed out the general limitation of both ASOCT and UBM, which is the lack of easy‐to‐use but reliable software to perform quantitative measurements. Only simple caliper tools are provided, so the development of custom software is necessary for semiautomation and improved operation precision.

Other factors have to be considered for evaluating these technologies in terms of clinical usefulness. What they are able to visualise is the most fundamental question. Both modalities provide the same scanning speed (8 frames/s) and comparable field of view (sulcus to sulcus in one scan). As Li et al pointed out in their discussion, one of the major limitations of the ASOCT is its inability to visualise ciliary body or structures behind the iris. Using UBM, many studies have reported that plateau iris and iridociliary cysts are more common than had previously been thought. ¹⁵,¹⁶,¹⁷,¹⁸ However, due to the aforementioned limitation, ASOCT is not capable of visualising these conditions.

On the other hand, the non‐contact scanning of ASOCT provides us a near‐perfect tool for evaluating filtering blebs. Although there are many studies regarding filtering blebs using UBM, the procedure has some risk of bleb wall damage and infection, owing to the use of eyecups. ¹⁹,²⁰,²¹ We can perform ASOCT immediately after surgery because nothing touches the eye for scanning. This advantage is reflected in the recent increase in the number of published studies about filtering blebs observed with ASOCT.²²,²³

To summarise the pros and cons, UBM has clear advantages in visualising the structures behind the iris, which is important for ruling out plateau iris and angle closure caused by iridociliary cysts. ASOCT, on the other hand, is much easier on patients, owing to its non‐contact scanning, and it is safe to scan eyes with filtering blebs. Both are equally capable of detecting angle closure under variable lighting conditions. In terms of measurement reproducibility, ASOCT may have a slight edge over UBM, but further investigations of the various anterior segment measurement parameters are required.

So, which imaging modality we should have in our glaucoma clinics? The current ideal solution is to have both so that they complement each other.

Abbreviations

ASOCT - anterior segment optical coherence tomography

TISA - trabecular‐iris space area

UBM - ultrasound biomicroscopy