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Ultrasound biomicroscopy in glaucoma
Medical expert of the article
Last reviewed: 06.07.2025
Ultrasound biomicroscopy (UBM) of the anterior segment uses high-frequency transducers (50 MHz) to obtain high-resolution images (approximately 50 μm), allowing in vivo imaging of the anterior segment of the eye (penetration depth of 5 mm). In addition, the anatomical relationships of structures surrounding the posterior chamber, which are hidden during clinical examination, can be visualized and assessed.
Ultrasound biomicroscopy is used to study normal ocular structures and the pathophysiology of ocular diseases, including the cornea, lens, glaucoma, congenital anomalies, effects and complications of anterior segment surgery, trauma, cysts and tumors, and uveitis. The method is important for understanding the mechanisms of development and pathophysiology of angle closure, malignant glaucoma, pigment dispersion syndrome, and filter pads. Studies using ultrasound biomicroscopy are qualitative. Quantitative and three-dimensional image analysis of ultrasound biomicroscopy is still in its early stages of development.
Closed-angle glaucoma
Ultrasound biomicroscopy is ideal for studying angle closure because it can simultaneously image the ciliary body, posterior chamber, iridocrystalline relationship, and angle structures.
It is important in the clinical evaluation of possible narrow angle closure to perform gonioscopy in a completely darkened room using a very small light source for the slit lamp beam to avoid the pupillary light reflex. The effect of external light on the shape of the angle is well demonstrated by performing ultrasound biomicroscopy under illuminated and darkened conditions.
The trabecular meshwork is not visible on ultrasound biomicroscopy, but the examination reveals a posteriorly located scleral spur. On ultrasound biomicroscopy, the scleral spur is visible as the deepest point on the line dividing the ciliary body and sclera where they meet the anterior chamber. The trabecular meshwork is anterior to this structure and posterior to Schwalbe's line.
Closed-angle glaucomas are classified based on the location of the anatomical structures or forces that cause the iris to close the trabecular meshwork. They are defined as a block originating at the iris (pupillary block), the ciliary body (flat iris), the lens (phacomorphic glaucoma), and forces located posterior to the lens (malignant glaucoma).
[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ], [ 8 ], [ 9 ], [ 10 ]
Relative pupillary block
Pupillary block is the most common cause of closed-angle glaucoma, accounting for more than 90% of cases. With pupillary block, the outflow of intraocular fluid is limited due to resistance to the passage of aqueous humor through the pupil from the posterior chamber to the anterior chamber. Increased intraocular fluid pressure in the posterior chamber displaces the iris forward, causing it to bend forward, which leads to narrowing of the angle and the development of acute or chronic closed-angle glaucoma.
If the iris is completely soldered to the lens by posterior synechiae, such a pupillary block is absolute. More often, a functional block develops - a relative pupillary block. Relative pupillary block is usually asymptomatic, but this is enough for appositional closure of part of the angle without signs of an increase in intraocular pressure. Then, anterior synechiae gradually form and chronic closure of the angle develops. If the pupillary block is absolute (complete), the pressure in the posterior chamber increases and shifts the peripheral part of the iris further and further forward until the trabecular meshwork closes and the angle is blocked, followed by an increase in intraocular pressure (acute angle-closure glaucoma).
Laser iridotomy eliminates the pressure difference between the anterior and posterior chambers and reduces iris deflection, which leads to changes in the anterior segment anatomy. The iris takes on a flat or flattened shape, the iridocorneal angle widens. In fact, the plane of the iridolenticular contact widens, since most of the intraocular fluid drains through the iridotomy opening, not through the pupil.
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Flat iris
In a flat iris, the ciliary processes are large and/or rotate anteriorly so that the ciliary groove is obliterated and the ciliary body presses the iris against the trabecular meshwork. The anterior chamber is usually of medium depth, and the iris surface is only slightly deflected. Argon laser peripheral iridoplasty causes contraction of the iris tissue and presses its peripheral portion, moving it away from the trabecular meshwork.
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Phacomorphic glaucoma
The swelling of the lens causes a noticeable decrease in the depth of the anterior chamber and leads to the development of acute closed-angle glaucoma due to the pressure of the lens on the iris and ciliary body and their anterior displacement. With miotic treatment, the axial length of the lens increases, inducing its anterior displacement with a subsequent decrease in the anterior chamber, which paradoxically worsens the situation.
Malignant glaucoma
Malignant glaucoma (ciliary block) is a multifactorial disease in which the following components play different roles: previous acute or chronic closed-angle glaucoma, shallow anterior chamber, anterior displacement of the lens, pupillary block by the lens or vitreous, weakness of the zonules, anterior rotation of the ciliary body and/or its edema, thickening of the anterior hyaloid membrane, enlargement of the vitreous body, and displacement of intraocular fluid into or posterior to the vitreous. Ultrasound biomicroscopy reveals a small supraciliary detachment, not visible on routine B-scans or clinical examination. This detachment is likely to be the cause of the anterior rotation of the ciliary body. The intraocular fluid secreted behind the lens (during the posterior displacement of the aqueous humor) increases the pressure of the vitreous body, which displaces the iris-lens diaphragm forward, causing the angle to close and the anterior chamber to become shallower.
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Pupillary block in pseudophakia
Inflammatory process in the anterior chamber after cataract extraction can lead to the appearance of posterior synechiae between the iris and the posterior chamber intraocular lens with the development of absolute pupillary block and angle closure. In addition, anterior chamber lenses can also lead to the development of pupillary block.
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Malignant glaucoma in pseudophakia
Malignant glaucoma may develop after surgical cataract extraction with implantation of a posterior chamber intraocular lens. It is believed that thickening of the anterior hyaloid membrane leads to a posterior deviation of the aqueous outflow with anterior displacement of the vitreous body and superposition of the iris and ciliary body. Ultrasound biomicroscopy reveals a noticeable forward displacement of the intraocular lens. Treatment consists of neodymium YAG laser dissection of the vitreous body.
Pigment dispersion syndrome and pigment glaucoma
Ultrasound biomicroscopy reveals a wide open angle. The midperipheral portion of the iris is convex (reverse pupillary block), presumably creating contact between the iris and the anterior zonules, with contact between the iris and the lens being greater than in a healthy eye. This contact is thought to prevent uniform distribution of intraocular fluid between the two chambers, leading to increased pressure in the anterior chamber. With accommodation, the convexity of the iris increases.
When blinking is suppressed, the iris takes on a convex shape, which returns to its original state when blinking, indicating that the act of blinking acts as a mechanical pump to push intraocular fluid from the posterior chamber to the anterior chamber. After laser iridotomy, the pressure difference between the posterior and anterior chambers disappears, reducing the convexity of the iris. The iris takes on a flat or flattened shape.
Exfoliative syndrome
In the earliest stages, exfoliated material is found on the ciliary processes and the zonule of Zinn. Ultrasound biomicroscopy reveals a granular image reflecting clearly visible ligaments covered with exfoliative material.
Multiple iridociliary cysts
Often a picture similar to a flat iris is observed, functioning cysts are similarly enlarged, the anterior location of the ciliary processes. Such changes are easily determined in UBM.
Tumors of the ciliary body
Ultrasound biomicroscopy is used to differentiate solid and cystic formations of the iris and ciliary body. The size of the tumor is measured and, if there is invasion, its spread to the root of the iris and the surface of the ciliary body is determined.
Iridoschisis
Iridoschisis is the closure of the anterior chamber angle separation of the anterior and posterior stromal layers of the iris. Anterior chamber angle closure is possible.