Ultrasound biomicroscopy

Fig. 1. Left: UD-1000/6000 ultrasound machine and UD-6010 probe. A protective cap covers the probe tip. Right: Model 840 conventional UBM machine and probe.

Fig. 2. Left: Testing using UD-6010 and an eye cup. Right: Testing using Model 840 and an eye cup.

for testing (Fig. 2). Ultrasonic frequency is 50 MHz for Model 840 and 40 MHz for UD-6010. Model 840 uses mechanical linear scanning while UD-6010 uses magnetic linear scanning, and while no marked difference is apparent, the scanning range is 5 mm wide × 5 mm deep for Model 840 and 9 mm wide × 6 mm deep for UD6010. The UD-6010 is thus capable of capturing a larger area from the pupillary

margin to the ciliary body, and allows easy analysis of the posterior pole past the ciliary body. With UD-1000/6000(probe:UD-6010), static images can be stored using compact flash cards, facilitating subsequent imaging processing.

UBM Images

Quality of UBM images

If a probe is positioned perpendicular to the cornea, UBM images show the anterior and posterior surfaces of the cornea as clear reflective lines, along with the scleral spur of the angle (Fig. 3). If the probe is not positioned well, these structures cannot be clearly seen. This is an important point in assessing image quality.

Fig. 3. UBM image clarity and probe angle. Left (UBM image): With proper probe angle, clear reflective lines are seen for the anterior and posterior surfaces of the cornea (arrows). Schwalbe's line is apparent (black triangle). Right (UBM image): Improper probe angle.

Comparison of gonioscopy and UBM images

An UBM-image of the angle merely represents a cross-section, and at this point of time, only captures a single cross-section of 360˚ of the angle at a specific point in time. UBM does not replace gonioscopy, and comparison of results with slit-lamp microscopy or gonioscopy remains important. Figure 4 compares images from a normal eye between UBM and gonioscopy. With UBM, the scleral spur is the only reference point seen on all images, and represents the cross point between inner surface of the cornea and the boundary between ciliary body and sclera. If the probe angle is appropriate, Schwalbe's line can also be seen nearest to the angle recess of the high-intensity reflective line for the inner surface of the cornea. On normal UBM images, the iris is mildly convex in the anterior direction or flat, and the ciliary sulcus is seen between ciliary processes and iris.

UBM findings of plateau iris configuration

Figure 6 shows plateau iris configuration (PIC), and while the anterior segment was not shallow, unlike pupillary block, the iris was flat and displayed a thick root. Due to the ciliary process that rotated anteriorly, the iris was pushed up and the ciliary sulcus was lost. In this patient, gonioscopy was difficult to perform, and because the shape of the ciliary body was unique, UBM was extremely useful as a diagnostic tool.4

Fig. 6. UBM image of plateau iris configuration in a patient with primary angle-closure glaucoma. The ciliary sulcus is unclear (arrow). The ciliary body can be seen (asterisk).

UBM images before and after laser iridotomy in relative pupillary block

Figure 7 compares UBM images before and after laser iridotomy (LI). LI widened the angle recess, and the iris that initially arched anteriorly became flattened. In this manner, changes in the iris and angle due to surgery can be seen on cross sections.5

Indentation UBM

Indentation UBM is an imaging technique to observe the angle by compressing the contralateral cornea using the gonioscopy probe to move the aqueous humor and push down on the iris. With this technique, apposition closure can be differentiated from adhesive closure. The same effect can be achieved by observing the eye by applying slight pressure using a small eye cup, and we have thus performed pressure UBM using an eye cup with a built-in compressor to safely and easily compress the cornea.6 When angle closure is detected by UBM with lighting,, use of this eye cup allows differentiation of appositional angle closure (Fig. 8) from synechial angle closure (Fig. 9). In other words, compression cannot push and

Fig. 7. UBM images before and after laser iridotomy (LI) in a patient with relative pupillary block.

Fig. 8. Indentation UBM in a patient with appositional angle closure. Appositional closure (triangles). Direction of aqueous humor pressure (arrows).

widen iridocorneal adhesion during synechial angle closure. In the case of PIC, compression does not change the shape of the iris root, but state of the angle recess can be observed (Fig. 10).7

Iris observation

Angle closure caused by umbrella iris can increase ocular pressure. Whether umbrella iris is caused by a cyst or solid tumor can be assessed on UBM cross-sections of the iris (Fig. 11).8 Reverse pupillary block can be seen in patients with pigment dispersion syndrome, where the iris is flexed posteriorly (Fig. 11).9

Fig. 9. Indentation UBM in a patient with synechial angle closure. Synechial closure (triangles). Direction of aqueous humor pressure (arrows).

Fig. 10. Indentation UBM in a patient with plateau iris configuration. Direction of aqueous humor pressure is shown (arrows).

UBM images following filtering surgery

Following trabeculectomy, the outflow tract under a scleral flap, inside the filtering bleb, the ciliary body and back surface of the iris cannot be observed optically, but these structures can be assessed by UBM (Figs. 12-14) . This is useful for postoperative management, reoperation and transconjunctival needling procedure under a scleral flap.10 In addition, following trabeculectomy with MMC, filtering blebs are usually assessed by slit lamp microscopy, but this is sometimes difficult. With UBM, patients can be classified into the following four types: low-reflective type where intensity inside a filtering bleb is low, and ocular pressure control is favorable; high-reflective type where intensity of a filtering bleb is high, and the bleb

is clearly seen; flattened type where a filtering bleb is not seen, and ocular pressure control is poor; and encapsulated type where tissue under the scleral flap is scarred and surrounded by Tenon's capsule, blocking function as a filtering bleb11). This type of information is important for planning therapy for patients with poor ocular pressure control.

UBM images of the ciliary body

As the ciliary body is difficult to observe through the pupil, cross-sectional imaging by UBM is useful for pathological clarification and therapy planning. Figure 15 shows a patient with malignant glaucoma12 following filtering surgery. The anterior segment was shallow, and the ciliary process rotated anteriorly and then compressed against the iris. Supraciliary effusion was confirmed. Figure 16 shows a patient with uveal effusion complicated by narrow angle and supraciliary effusion. UBM is also useful for assessing ciliary body dissection or rupture caused by other blunt traumas, supraciliary effusion at low ocular pressure following filtering surgery,13 or dissection of the ciliary body or choroid in Harada's disease14 or following laser photocoagulation in diabetic retinopathy.

Fig. 15. A UBM image of malignant glaucoma. Supraciliary effusion (white arrow), anterior segment loss (black arrow) and ciliary body (asterisk) are shown.

Quantitative UBM

Measurement of the degree of anterior chamber angle opening is significant in the therapy of closed angle glaucoma. Estimating an angle opening by gonioscopy is subjective and generally gross. But UBM is much better suited for objective measurements and evaluation of the anatomy and pathophysiology of the anterior segment. Pavlin et al.3 proposed several parameters to measure the anterior segment. Angle opening distance (AOD500) is defined as the length of the line drawn from the point

Fig. 16. Shallow anterior segment and narrow angle in a patient with uveal effusion. Supraciliary effusion (arrow) and ciliary body (asterisk) are shown.

Fig. 17. Parameters to measure the angle. Left: Angle opening distance (AOD500) and angle (θ1). Right: Angle recess area (ARA).

on the endothelial surface 500 μm anterior to the scleral spur to the iris surface perpendicular to the corneal endothelial surface. The trabecular/iris angle (θ1) is measured with the apex in the iris recess and the arms passing through the point 500 μm from the scleral spur and the point perpendicularly opposite on the iris. In addition to these, Pavlin et al.3 presented other parameters to measure the iris and ciliary body configuration, such as the iris thickness, trabecular-ciliary process distance (TCPD), the angle at which the iris leaves the lens surface (θ2), etc.

Ishikawa et al.15,16 introduced angle recess area (ARA) to estimate the anterior chamber angle width. ARA is defined as a triangular area bordered by the anterior iris surface, corneal endothelium, and a line perpendicular to the corneal endothelium drawn from a point 750 μm anterior to the scleral spur to the iris surface. This method faithfully reflects irregularities of iris contour and curvature.

Summary

UBM is capable of generating high-resolution cross-sectional images of the anterior segment and depicting areas that cannot be detected optically. This technique can also be performed without lighting, which is important for testing narrow angle glaucoma. UBM is thus useful for the diagnosis and treatment of glaucoma, but should be combined with techniques such as gonioscopy to maximize usefulness.

References

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