Ultrasound biomicroscopy (UBM) is used to evaluate the anterior chamber angle and a variety of anterior segment structures and implanted devices that cannot be directly visualized or fully assessed with slit-lamp biomicroscopy. Similarly, UBM can be helpful in evaluating the anterior chamber angle of eyes with corneal opacities that prevent gonioscopic examination. UBM has also been used to assess the iris contour in pigment dispersion syndrome, helping to clarify the under lying mechanism of this condition (see Chapter 8).
Structures and conditions that can be evaluated by UBM include
Figure 4-11 Ultrasound bio microscopy b-scan demonstrating an area of angle closure as a result of a peripheral iris pigment epithelial cyst (arrow).
(Courtesy of Angelo P. Tanna, MD.)
Figure 4-12 Schematic diagrams depicting anterior segment parameters captured by ultrasound biomicroscopy and anterior segment OCT. ACA = anterior chamber area; ACD = anterior chamber depth; ACW = anterior chamber width; AOD = angle opening distance; ARA = angle recess area; ICPD = iris–ciliary process distance; ILCD = iris–lens contact distance; LV = lens vault; TCPD = trabecular–ciliary process distance; TIA = trabecular iris angle; TISA = trabecular iris space.
(Courtesy of Chansangpetch S, Rojanapongpun P, Lin SC. Anterior segment imaging for angle closure. Am J Ophthalmol. 2018;188:xvi–xxix.)
Conventional ocular ultrasonography is typically performed using a transducer that operates at 10–20 MHz. Higher frequencies, however, are required to image anteriorly located structures and to provide sufficient resolution for meaningful evaluation of the anterior segment. Thus, anterior segment UBM is performed using 35–60 MHz (or higher frequency for imaging Schlemm canal) linear probes that must be positioned very close to the eye with a fluid interface (often contained within a disposable probe cover) for acoustic coupling. The high ultrasound frequency results in limited penetration—approximately 5 mm for a probe operating at 50 MHz.
The information provided by UBM is complementary to that obtained though gonioscopy and can help elucidate the underlying mechanism in some cases of angle closure (see Chapters 9 and 10). Assessment of UBM images for angle closure begins with identification of the scleral spur and determination of the degree of angle crowding. The mechanism(s) of angle closure can be inferred based on a qualitative assessment of the iris contour, peripheral iris thickness, ciliary body anatomy (size, position, and degree of rotation), anterior chamber depth, lens thickness, and lens vault (Fig 12-4; see also Chapter 9). Various quantitative parameters that characterize some of these anatomic features on UBM are also being investigated with OCT (see the following section). The most important ones are summarized in Table 4-2 and illustrated in Figure 4-12. Automated quantitative analysis is available on some commercially available UBM platforms; however, the operator must usually identify the scleral spur.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.