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Quantitative techniques are important for researching changes to the anatomy of the anterior chamber angle in primary angle closure spectrum eyes. The authors evaluate the strengths and weaknesses of various such techniques: gonioscopy, ultrasound biomicroscopy, Scheimpflug imaging and anterior segment optical coherence tomography.
In short: Quantitative techniques are important for researching changes to the anatomy of the anterior chamber angle in primary angle closure spectrum eyes. The authors evaluate the strengths and weaknesses of various such techniques: gonioscopy, ultrasound biomicroscopy, Scheimpflug imaging and anterior segment optical coherence tomography.
By Dr Robert M. Feldman, MD
Primary angle closure glaucoma (PACG) affects an estimated 20 million people worldwide, with that number predicted to increase to 32 million by 2020.1 Primary angle closure spectrum disease encompasses patients classified as primary angle closure suspects, primary angle closure (PAC) and PACG.2,3 A current active area of research is how the anatomy of the anterior chamber angle changes with treatment. Gonioscopy is the current gold standard method used to evaluate the angle. However, other methods, such as ultrasound biomicroscopy (UBM), Scheimpflug imaging and anterior segment optical coherence tomography (ASOCT) are being used to quantitatively analyse anatomic changes before and after treatment. With a combination of these techniques, the examiner can get a full picture of the angle anatomy for clinical decision-making and research.
The gold standard for viewing the anterior chamber angle, gonioscopy allows for a circumferential, dynamic view. The technique allows the angle to be viewed under compression and noncompression states in order to differentiate between peripheral anterior synechiae (PAS) and appositional iridotrabecular contact (ITC). The extent of PAS may affect treatment decisions.2
While gonioscopy has some advantages over other techniques, including being quick and inexpensive, it is difficult to perform and does not provide quantitative information. These disadvantages limit its usefulness in research and clinical situations where quantitative data is required.
Another technique used to analyse changes in the anterior chamber is Scheimpflug imaging, which uses light scattering4,5 to produce a high-resolution image of the angle (4 µm laterally and 1 µm axially for the Galilei, Ziemer USA, Inc., Alton, Illinois, USA), from which quantitative measurements can be taken. However, the Scheimpflug technique often cannot adequately image the angle recess, limiting its use in PAC patients.
Unlike the other imaging techniques discussed, UBM uses sound waves to produce an image of the anterior chamber, allowing imaging behind the iris to the ciliary body and sulcus.6 Therefore, UBM is able to visualise anterior rotation of the ciliary body and iris displacement, markers of plateau iris.
While UBM is very useful for diagnosing plateau iris, it is a low-resolution technique (50 µm laterally and 25 µm axially),4 which makes it difficult to provide reproducible measurements, although quantitative studies have been published.7,8 UBM is also operator dependent; because contact is made with the eye, a skilled technician is required, as eye structures can be distorted depending on the angle of the probe and the amount of pressure on the eye.
ASOCT is able to obtain high-resolution images of the anterior chamber angle, including the angle recess, that can be used for quantitative measurements. Unlike posterior optical coherence tomography, which uses light with a wavelength of 820 nm, dedicated ASOCTs use 1310 nm light to image the anterior chamber. This wavelength allows for greater penetration through tissues that scatter light (such as the sclera and limbus) and better visualisation of the cornea, iris, angle and lens.4 Many high-resolution images (10 µm axially and 30 µm transversally with the CASIA SS-1000 from Tomey Corporation, Nagoya, Japan) can be obtained in one session (30,000 A-scans/s, with horizontal and vertical planes scanning simultaneously), allowing for 360° reconstruction of the angle for three-dimensional viewing of the anterior chamber.
ASOCT has many important advantages but also distinct disadvantages that are crucial clinically. Unlike gonioscopy, ASOCT cannot differentiate between PAS and ITC, which can be important for treatment decisions; unlike UBM, ASOCT cannot image beyond the iris to visualise the ciliary body and confirm a diagnosis of plateau iris. This is a limitation of the 1310 nm wavelength used, which cannot penetrate the iris tissues.
Because of its ability to provide reproducible, quantitative measurement of angle parameters, ASOCT is one of the most commonly used techniques to examine the anatomy of the angle and how it changes with treatment in PAC patients.9-12 Common angle parameters measured in the literature include angle opening distance (AOD) and trabecular iris surface area (TISA). In order to measure these parameters, a landmark, usually the scleral spur, must be manually identified on the image. Previous publications from my group have shown that a landmark approximating the scleral spur (“scleral spur landmark”) can be reproducibly identified using predefined criteria, resulting in reproducible measurements for AOD and TISA.13
Building upon this work, we developed a new parameter, trabecular-iris circumference (TICV), which integrates TISA over the entire peripheral angle, and published a database of values for open angle eyes.14 This was possible because of the capability of the CASIA SS-1000 ASOCT to provide a 360° degree, three-dimensional reconstruction of the angle.15 We are now able to assess the state of the peripheral angle, which may be a key to understanding PAC. 3D Three-dimensional reconstruction of the angle also allows clinicians to analyse the anterior chamber as a whole and assess irregular structures in the eye such as irregular PAS or large cyclodialysis clefts, which may guide treatment decisions.
We recently published a study that used ASOCT prospectively to analyse anterior chamber angle changes after treating PAC spectrum eyes with laser peripheral iridotomy (LPI).16 LPI, which is typically the first-line treatment for PAC spectrum eyes, eliminates one of the angle closure mechanisms, pupillary block, by allowing the iris to flatten and drainage through the angle.2 In our study, 42 PAC spectrum eyes were imaged before and 3 months after laser peripheral iridotomy treatment. We found that TISA at 500 µm and 750 µm from the scleral spur landmark increased significantly after LPI, as did TICV at 500 µm and 750 µm from the scleral spur landmark. Across all the parameters, the nasal angle deepened the most while the superior angle deepened the least. Our study was the first to use the novel parameter TICV to determine volume changes in the peripheral angle. We concluded that LPI was effective in deepening the anterior chamber angle in PAC spectrum eyes in the short term.16
Each evaluation technique for the anterior chamber angle has strengths and weaknesses and provides different information. Quantitative techniques are important for researching anatomical changes. However, in order to get a full picture of the angle, information from each of these techniques is crucial.
Dr Robert M. Feldman, MD
Dr Feldman chairs the Department of Ophthalmology and holds the Richard S. Ruiz Distinguished University Chair at the Cizik Eye Clinic of the University of Texas Health Science Center at Houston (UTHealth) Medical School.
Dr Feldman has been loaned a CASIA SS-1000 tomographer by the Tomey Corporation.