Identifying Structures on the UBM and Their Significance

The anterior segment can be imaged beautifully with the UBM. Especially, in eyes where the cornea is not clear, it has become an indispensable tool for qualitative and quantitative assessment of the anterior segment. In this section, we shall show what the normal eye looks like on UBM imaging. A thorough knowledge of the normal appearance is crucial to determining lesions and their correct interpretation. We shall start with the cornea and go on to describe the appearance of the angle, the anterior chamber, the iris, the lens, and the sulcus.

The normal cornea is a multilayered structure. It’s the first structure to be visualized, as we start the UBM examination. Going from anterior to posterior, the first echogenic line structure is the corneal epithelium, followed by another linear structure, the Bowman’s membrane. The stroma looks like a minimally echogenic band, which is regular. And posterior to this lies another bright echogenic line, the Descemet’s membrane.

Abnormalities in the cornea could be in terms of corneal edema. You can see that the epithelium appears a little loose, and there is a thick cornea with hazy stroma. Sometimes, abnormalities in the cornea may show up with localized corneal opacity with a distinct central clearing. This is a typical Descemet’s defect in Peters’ anomaly where you can see the defect of the Descemet’s membrane manifesting as the clear area that was seen clinically.

The anterior chamber angle is the junction between the cornea and the sclera where the iris root inserts into the ciliary body. Understanding the anatomy of the angle and the structure surrounding it is crucial to a good interpretation of the UBM of the angle. The relatively equilucent cornea and the highly reflective sclera meet at the anterior Schwalbe’s line. This is the termination of the Descemet’s membrane.

The sclera and the underlying ciliary body have distinctly different echogenicity. The anterior termination of the line separating the two is the point of the scleral spur. The accurate identification of the scleral spur is crucial to qualitative measurement of angle parameters. It is the only constant landmark allowing one to interpret ultrasound biomicroscopy images in terms of the angle width.

Localizing the scleral spur accurately is the key for analyzing any angle pathology. The trabecular meshwork area is the structure between the anterior Schwalbe’s line and the scleral spur. Judging the angle open or closed depends upon how much of the trabecular meshwork is available for aqueous outflow. So coming to the scleral spur and the Schwalbe’s line, you can see this thickened area is the area of the trabecular meshwork.

A constant testing environment is essential for angle assessment. We must remember that morphology of the anterior segment structures alter in response to a variety of physiological stimuli such as accommodation and lighting. It is important to maintain a constant testing environment, therefore, if we want to longitudinal compare our angle parameters.

The ciliary body is the area between the peripheral iris and above the ciliary processes. This area is the ciliary sulcus. Clinically, ciliary body effusion manifests as fluid between the sclera and the ciliary body.

The lens is a spindle-shaped structure situated behind the iris. It can usually be imaged right up to the posterior pole. The zonules are the reflective linear structure between the ciliary process and the iris. Here we can see broken zonules in pseudoexfoliation glaucoma. You can also note the cataractous lens, whereas the internal structure is not homogenously echogenic.

The angle can be quantified by the UBM, depending upon angle parameters that we look for. There is a special caliper included in the instrument software package. It needs to be manipulated by the examiner.

You have the information of the angle opening distance 250, which is at 250 microns from the scleral spur. You have AOD 500 or Angle Opening Distance 500 500 microns from the scleral spur. And you have a trabecular meshwork ciliary process distance, which is a perpendicular line drawn 500 microns anterior to the scleral spur right down to the ciliary process.

It depends the port through which the iris must traverse and relates to the potential maximal angle opening. So the angle opening distance AOD, in a relatively pupillary block, there’s an anteriorly bowed iris with a corresponding decrease in the angle opening. The AOD reflects the amount of relative pupillary block, which is possible in patients with occludable angle.

The TCPD or Trabecular Ciliary Process Distance defines the space available between the trabecular meshwork and the ciliary processes. It’s a typical feature in an individual eye. And it’s sum of three segments– the AOD 500 from the scleral spur, the thickness of the iris at that point, and the width of the ciliary sulcus. An anteriorly placed ciliary process can reduce the peripheral anterior chamber depth and make it susceptible to occlusion.

We looked at angle width, and we looked at 124 eyes of 64 patients across the width of the angle. And we saw that, for just the angle width, you don’t really require a UBM. So identification and delineating the normal anatomy is essential before interpreting any abnormality. Familiarity with the machine and its parameters is important to ensure correct interpretation.