Ultrasound Biomicroscopy (UBM) is an imaging device used for diagnosing the anterior segment of the eye. It was first introduced in the early 1990s by Foster and Pavlin as a method to obtain cross-sectional images of the eye with microscopic resolution.
Compared to conventional A-mode and B-mode ultrasound (10 MHz), UBM uses much higher frequency transducers (35–100 MHz). This provides axial resolution up to 20 μm and lateral resolution up to 50 μm, with tissue penetration depth of 4–5 mm.
According to the Japanese Ophthalmological Society’s guidelines for ophthalmic examinations, the purpose of UBM is “to capture tomographic images and videos of the anterior segment (cornea, conjunctiva, iris, angle, lens, ciliary body, choroid, and peripheral retina) for clinical diagnosis, particularly for diagnosing angle closure and differentiating the mechanisms of angle closure.”
Conventional B-mode ultrasound uses 10 MHz to observe the entire eye (including axial length, retina, and choroid), whereas UBM uses high frequencies of 35–100 MHz to obtain high-resolution images specialized for the anterior segment. However, due to the high frequency, the penetration depth is limited to 4–5 mm, so conventional ultrasound is needed for observing the posterior vitreous and retina.
Ultrasound biomicroscopy is primarily indicated for the diagnosis and evaluation of the following conditions.
The cornea, sclera, ciliary body, and iris are normal and clearly visible. Specifically, the anterior and posterior surfaces of the cornea, the scleral surface, and the anterior and posterior surfaces of the iris are depicted as high brightness, while the corneal stroma, iris stroma, and ciliary body are depicted as low brightness. In a normal eye, the iris is slightly convex anteriorly or flat, and the ciliary sulcus can be seen between the iris and the ciliary processes.
For observation of the anterior chamber angle, identification of the scleral spur and Schwalbe’s line is essential. The scleral spur is a part of the sclera that protrudes into the anterior chamber, with the trabecular meshwork attached anteriorly, and is an important landmark that can always be identified.
Ultrasound biomicroscopy enables quantitative measurement of the anterior chamber angle, and the following parameters are commonly used.
| Parameter | Definition |
|---|---|
| Angle opening distance (AOD) | Vertical distance between the trabecular meshwork and iris at 500 μm anterior to the scleral spur |
| Angle recess area (ARA) | Area of the triangle bounded by the AOD line and the angle recess |
| Anterior chamber depth (ACD) | Distance from the central corneal endothelium to the anterior lens surface |
| Lens vault | Distance of the lens anterior to the vertical line connecting the scleral spurs |
In primary angle-closure glaucoma (PACG), the angle opening distance and anterior chamber depth are significantly reduced, aiding diagnosis.
Membrane-type ultrasound biomicroscopes (e.g., UD-8060, Tomey Corporation) do not require an eye cup; apply Scopisol® to the membrane tip and place it on the examination site. Examination in sitting or supine positions is possible.
Pupillary Block Angle Closure
Iris bombé: A morphology where the posterior chamber pressure increases and pushes the iris forward.
Generalized angle narrowing: The iris is pressed against the cornea from the Schwalbe’s line region.
Angle closure worsens in dark conditions: The deterioration due to pupillary dilation can be captured.
Plateau Iris
No iris bowing: The central iris is flat and there is no associated pupillary block.
Anterior ciliary body rotation and ciliary sulcus obliteration: Characteristic findings. The ciliary body is rotated anteriorly, mechanically pushing the iris root upward.
Iris root occludes the angle upon dilation: Occlusion can be confirmed under dark-room mydriasis.
In plateau iris, the central anterior chamber is relatively deep, the central iris is flat, the iris root is thick and bends anteriorly, and the angle recess is slit-like and narrow. Anterior rotation of the ciliary body and obliteration of the ciliary sulcus are characteristic findings.
Observation with ultrasound biomicroscopy is extremely useful for confirming the diagnosis of plateau iris that is not relieved even after laser iridotomy. If intraocular pressure does not decrease after laser iridotomy, or if angle closure similar to the preoperative state is confirmed upon dilation, plateau iris is confirmed. However, performing laser iridotomy solely for diagnosis should be avoided due to risks such as bullous keratopathy, and ultrasound biomicroscopy is recommended.
Sudden increase in intraocular pressure due to external force can cause angle recession, iridodialysis, trabecular meshwork damage, and cyclodialysis. In cyclodialysis, accumulation of aqueous humor in the suprachoroidal space is clearly visualized by ultrasound biomicroscopy.
Yeilta et al. reported a case in which ultrasound biomicroscopy visualized a 5×3×2 mm iridociliary melanocytoma (identified as a relatively well-defined lesion) and was used for clinical diagnosis and management. 1) In that report, UBM was used to assess the size and extent of the iridociliary lesion.
Plateau iris does not have a shallow anterior chamber (central anterior chamber depth is normal), and the iris does not bulge forward on slit-lamp examination, making it difficult to differentiate from pupillary block angle closure. The key to diagnosis is confirming anterior rotation of the ciliary body and obliteration of the ciliary sulcus on ultrasound biomicroscopy under dark-room mydriasis.
Ultrasound biomicroscopy itself is an examination device and does not perform treatment. The treatments for diseases diagnosed with ultrasound biomicroscopy are described below.
For cyclodialysis diagnosed by ultrasound biomicroscopy, conservative treatment or surgical resuturing/cyclopexy is generally selected.
Ultrasound biomicroscopy and anterior segment optical coherence tomography (AS-OCT) are both used complementarily as anterior segment imaging devices.
| Feature | Ultrasound Biomicroscopy | AS-OCT |
|---|---|---|
| Principle | Ultrasound (35–100 MHz) | Near-infrared light |
| Resolution (axial) | 20 μm | 5–10 μm |
| Depth of penetration | 4–5 mm | 3–6 mm |
| Observation behind iris and ciliary body | Possible | Difficult (unclear) |
| Contact required | Required (eyecup/membrane) | Not required (non-contact) |
| Dark room imaging | Possible | Possible |
| Need for skilled operator | High | Low |
The greatest strength of ultrasound biomicroscopy is the visualization of structures behind the iris and including the ciliary body. Disadvantages compared to AS-OCT include the need for eye contact via water immersion, longer image acquisition time, and the requirement for a skilled operator.
Differences in tumor evaluation: For ocular surface squamous neoplasia (OSSN), AS-OCT has an advantage in providing diagnostic information by showing detailed internal structure of the lesion. On the other hand, for amelanotic iris tumors, ultrasound biomicroscopy is superior in identifying the posterior border of the lesion and has higher reproducibility.
There are two main mechanisms for the development of angle-closure glaucoma.
By differentiating these two mechanisms preoperatively using ultrasound biomicroscopy, treatment strategies (laser iridotomy alone vs. laser iridotomy plus laser iridoplasty) can be optimized. Approximately 33% of patients who undergo laser iridotomy for primary angle-closure glaucoma have plateau iris, and these patients are at higher risk for peripheral anterior synechiae formation and further angle closure, necessitating detailed ultrasound biomicroscopy examination and close follow-up.
Automated quantitative analysis software for ultrasound biomicroscopy images has been developed, enabling automatic measurement of parameters such as angle opening distance, anterior chamber depth, and lens vault. This is expected to reduce inter- and intra-observer variability and improve diagnostic accuracy.
In a case report by Yeilta et al., for pigment dispersion glaucoma secondary to necrotic iris melanocytoma, ultrasound biomicroscopy was used to evaluate lesion size (5×3×2 mm), and surgical management combining iridocyclectomy and glaucoma shunt surgery was shown to be effective. 1) For iris tumors, including differentiation between melanocytoma and melanoma, comprehensive assessment of clinical findings, imaging findings, and course is necessary.
