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Ultrasound biomicroscopy for glaucoma
Last reviewed: 17.10.2021
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With ultrasound biomicroscopy (UBM) of the anterior segment, high-frequency sensors (50 MHz) are used to obtain high-resolution images (approximately 50 μm), which allow you to see the anterior segment of the eye in vivo (penetrating depth - 5 mm). In addition, the anatomical relationships of the structures surrounding the posterior chamber, which are hidden during clinical examination, can be visualized and evaluated.
Ultrasound biomicroscopy is used to study the normal structures of the eye and the pathophysiology of eye diseases, including the cornea, lens, glaucoma, congenital anomalies, effects and complications of anterior segment surgeries, in injuries, cysts and tumors, as well as uveitis. The method is important for understanding the developmental mechanisms and pathophysiology of angle closure, malignant glaucoma, pigment dispersion syndrome and filtration pads. Research using ultrasound biomicroscopy quality. Quantitative and three-dimensional image analysis of ultrasound biomicroscopy is still at an early stage of development.
Angle-closure glaucoma
Ultrasound biomicroscopy is ideal for studying the angle closure, since it is possible to simultaneously obtain an image of the ciliary body, the posterior chamber, the iriodal lens relationship and the angle structures.
It is important in the clinical evaluation of the possible closure of a narrow angle of the eye to conduct gonioscopy in a completely darkened room using a very small light source for the slit lamp beam in order to avoid pupillary light reflex. The effect of external light on the shape of the angle is well shown when conducting ultrasound biomicroscopy in light and dark conditions.
The trabecular network is not visible with ultrasound biomicroscopy, but during the study, the scleral spur, located posteriorly, is determined. In the ultrasound biomicroscopy image, the scleral spur is visible as the deepest point on the line separating the ciliary body and the sclera at the place of their contact with the anterior chamber. The trabecular network is anterior to this structure and posterior to the Schwalbe line.
Angle-closure glaucomas are classified based on the placement of anatomical structures or forces causing the iris to close the trabecular network. They are defined as a block originating at the level of the iris (pupillary block), the ciliary body (flat iris), the lens (phacomorphic glaucoma), and the forces located posterior to the lens (malignant glaucoma).
[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]
Relative pupillary block
The pupillary block is the most common cause of closed angle glaucoma, more than 90% of cases. In the case of a pupil block, the outflow of intraocular fluid is limited due to the resistance to the passage of aqueous humor through the pupil from the posterior chamber to the anterior chamber. An increase in intraocular fluid pressure in the posterior chamber displaces the iris anteriorly, causing it to deflect forward, which leads to a narrowing of the angle and the development of acute or chronic angle-closure glaucoma.
If the iris is completely soldered to the lens by the posterior synechiae, such an pupillary unit is absolute. The functional block, the relative pupillary block, develops more often. The relative pupillary block is usually asymptomatic, but this is sufficient for the approximate closure of a part of the angle without signs of a rise in intraocular pressure. Then the front synechiae gradually form and the chronic closure of the corner develops. If the pupillary block is absolute (full), the pressure in the posterior chamber increases and shifts the peripheral part of the iris further towards the front until the trabecular meshwork is closed and the angle is blocked and the intraocular pressure is raised (acute angle-closure glaucoma).
Laser iridotomy eliminates the pressure difference between the anterior and posterior chambers and reduces the deflection of the iris, which leads to changes in the anatomy of the anterior segment. The iris takes a flat or smoothed form, and the iridocorneal angle widens. In fact, the plane of the iridolentikulyarnogo contact expands. Since most of the intraocular fluid swells through the iridotomy opening and not through the pupil.
[14], [15], [16], [17], [18], [19], [20], [21], [22], [23]
Flat iris
In the case of a flat iris, the ciliary processes are large and / or deployed anteriorly in such a way that the ciliary sulcus is obliterated, and the ciliary body presses the iris to the trabecular network. The anterior chamber is usually of medium depth, the surface of the iris only slightly bends. Argon laser peripheral iridoplasty causes contraction of the iris tissue and squeezes its peripheral part, moving it away from the trabecular network.
[24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]
Fakomorphic glaucoma
Swelling of the lens causes a noticeable decrease in the depth of the anterior chamber and leads to the development of acute angle-closure glaucoma due to the pressure of the lens on the iris and ciliary body and their displacement anteriorly. When treating with miotics, the axial length of the lens increases, inducing its displacement anteriorly, followed by a 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 a different role: anterior acute or chronic angle-closure glaucoma, shallow anterior chamber, anterior crystalline lens displacement, pupillary block of the crystalline lens or vitreous body, weakness of the ligamentum of the ciliary, ciliary body rotation, and anemia. Or edema, thickening of the anterior hyaloid membrane, an increase in the volume of the vitreous body and movement of the intraocular fluid into or away from the vitreous body. Using ultrasound biomicroscopy, a small supraciliary detachment is revealed, which is invisible during routine B-scanners or clinical examination. This detachment, apparently, is the cause of the anterior rotation of the ciliary body. The intraocular fluid secreted behind the lens (when the backward movement of aqueous humor) increases the pressure of the vitreous body, which moves the iridocrystal diaphragm forward, causing the angle to close and the anterior chamber to shatter.
[35], [36], [37], [38], [39], [40], [41], [42], [43]
Pupillary block in pseudophakia
The inflammatory process in the anterior chamber after cataract extraction can lead to the appearance of posterior synechia between the iris and the posterior chamber intraocular lens with the development of an absolute pupillary block and closure of the angle. In addition, anterior chamber lenses can also lead to the development of a pupillary block.
[44], [45], [46], [47], [48], [49], [50], [51]
Malignant glaucoma with pseudophakia
Malignant glaucoma can develop after surgical cataract extraction with implantation of a posterior chamber intraocular lens. It is assumed that the thickening of the anterior hyaloid membrane leads to a deviation of the outflow of aqueous humor posteriorly with the displacement of the vitreous body anteriorly and the imposition of the iris and ciliary body. When ultrasound biomicroscopy determines a noticeable shift of the intraocular lens forward. Treatment consists of conducting neodymium YAG-laser dissection of the vitreous body.
Syndrome pigment dispersion and pigment glaucoma
With ultrasound biomicroscopy, a wide open angle is determined. The middle-peripheral part of the iris has a convex shape (reverse pupillary block) supposedly creating contact between the iris and the anterior cinnamon ligaments, while the contact between the iris and the lens is greater than in the healthy eye. It is believed that this contact prevents the even distribution of intraocular fluid between the two chambers, leading to an increase in pressure in the anterior chamber. When accommodating the bulge of the iris increases.
When blinking is suppressed, the iris assumes a convex shape, which when blinking returns to its original state, which indicates the participation of the blink act as a mechanical pump for pushing intraocular fluid from the back chamber to the front one. After laser iridotomy, the pressure difference between the posterior and anterior chambers disappears, reducing the bulge of the iris. The iris takes a flat or flattened shape.
Exfoliative syndrome
At the earliest stages, the exfoliated material is found on the ciliary processes and the zinn bundle. Ultrasound biomicroscopy reveals a grainy image that reflects well visible ligaments coated with exfoliative substance.
Multiple iridociliary cysts
Often, a picture similar to a flat iris is observed; functioning cysts, the anterior arrangement of the ciliary processes, increase in a similar way. Such changes are easily determined with UBM.
Tumors of the ciliary body
Ultrasound biomicroscopy is used to differentiate between solid and racemose formations of the iris and ciliary body. Measure the size of the tumor and in the presence of invasion, determine its prevalence in the root of the iris and the surface of the ciliary body.
Irishizisis
Iridoshysis is the closure of the anterior chamber angle, the separation of the anterior and posterior stromal layers of the iris. Possible to close the angle of the front camera.