The following section will provide you with a systematic framework and background for interpreting anterior segment OCT
Demystifying the Angle with Anterior Segment OCT
Anterior segment OCT (AS-OCT) provides high resolution images of anterior segment structures in a non-contact fashion. Given its ability to image the chamber angle, AS-OCT has been increasingly used in the evaluation and management of anterior segment pathology. This tutorial will provide you with an overview of how to assess the anterior segment with this imaging modality. A Q&A format will be utilized to make the content more palatable and interactive.
1. To start things off, why do we need to use anterior segment OCT to evaluate narrow angles? After all, we are all trained and proficient in gonioscopy, right?
Gonioscopy remains the gold standard in evaluating the chamber angle. However, gonioscopy can be a difficult skill to master with some inter-examiner variability in examination findings. Variation in gonioscopy findings can be due to the misidentification of the trabecular meshwork if it is pale or if the Schwalbe’s line is pigmented. The angle appearance can also change depending on the degree of indentation, or pupil dilation, which is particularly sensitive to the amount of light during the exam.
Remember, AS-OCT as with other OCT, is a non-contact imaging modality meaning that it does not require direct contact with the eye. This is in contrast with gonioscopy, where there is direct indentation of the eye.
2. Which of the following is NOT an advantage of AS-OCT?
AS-OCT may also be useful in the following clinical situations:
If the angle appears equivocal on gonioscopy
For patients who cannot tolerate gonioscopy
In cases where there is no clear view of the angle (ex. Hazy cornea)
An AS-OCT scan can also provide information regarding the mechanism for angle closure. Examples below:
This scan demonstrates angle closure secondary to lens rise. Note the prominent lens vault pushing the iris diaphragm forward.
This scan demonstrates posterior anterior synechiae (red circle) causing synechial angle closure.
Now Let's Go Over an Approach to AS-OCT
There are two approaches to evaluate the angle:
1) Qualitative assessment
2) Quantitative assessment using the programming from your AS-OCT machine
Practically speaking, the qualitative approach is what most glaucoma specialists use to evaluate their patients so that will be our focus in this module.
When assessing an AS-OCT scan, you should first look at the scan as a whole.
Look at the volume of the anterior chamber and look at the insertion angle of the iris. Once you have seen enough scans, you can often determine whether the angle/chamber status just from eyeballing the scan. Two examples are shown below. Just from looking at the scans, you can already determine that the angle/chamber of scan B is narrower than scan A.
Now if you have not seen enough scans, it is important to have a step-by-step approach to assess the chamber angle. We outline an approach below. To assess the angle, you need to first identify the pertinent angle structures.
Question: What is the first structure you should identify on AS-OCT when evaluating the angle?
Step 1 - Identify the scleral spur. We will go over four ways to identify the scleral spur:
Tip #1: It is the point where there is a change in curvature of the inner surface of the angle wall, often appearing as an inward protrusion of the sclera. The blue arrow is pointing towards the sudden change in curvature, which is where the scleral spur is located:
Tip #2: The scleral spur is the prominent extension of the sclera at its thickest part. As you can see from the figure below, the blue line denotes the thickest part of the sclera. When traced towards the inner surface of the eye, that corresponds to location of the scleral spur.
Tip #3: Look for the point of colour transition (change in reflectivity). This is the point where the less reflective ciliary muscle (dark grey) meets the more reflective sclera ((light grey). At the location indicated by the blue arrow, notice the change in hyper-reflectivity.
Tip #4: The scleral spur can be identified by first identifying the Schwalbe Line (SL). Identify the U-shaped interface (yellow line), which is comprised of the darker corneal stroma and the lighter sclera at the limbus. The SL is formed by the intersection (blue arrow) of the U-shaped junction and inner corneal border. The SS is located 1 mm (redline) posterior to the SL along the posterior corneal surface.
Step 2 - Identify the trabecular meshwork
The decision to perform laser peripheral iridotomy is often dependent on whether the trabecular meshwork can be visualized on gonioscopy. Thus, it is crucial to identify the trabecular meshwork on AS-OCT.
Once you have identified the scleral spur, you can use it as a reference point to identify the trabecular meshwork. The trabecular meshwork is located approximately 250 – 500μm anterior to the scleral spur. Remember that the central corneal thickness is approximately 500μm, so you can use this as a reference to determine the trabecular meshwork in reference to the scleral spur.
In the figure below, the red line denotes the central corneal thickness, which is approximately 500 microns. Since we know that the trabecular meshwork is located 250-500 microns anterior to the scleral spur, we can approximate the location of the trabecular meshwork.
Step 3 - Look at the position of the iris relative to the trabecular meshwork
When the trabecular meshwork is identified, look at the position of the iris relative to the trabecular meshwork. If the iris is completely apposed to the trabecular meshwork, the angle is closed. Another way of assessing this is by looking at the angle between the iris and the trabecular meshwork. If the angle is less than 20 degrees, the angle is likely narrow/closed. However, if the angle is greater than 20 degrees, the angle is more likely to be open.
We will work through two examples:
In this case, the red line denotes the approximate position of the trabecular meshwork, which is approximately 250-500 microns anterior to the scleral spur. The scleral spur can be identified based on the change in the inner curvature and reflectivity. Note that the iris is NOT apposed to the trabecular meshwork, and the angle of the iris relative to the trabecular meshwork is approximately 45 degrees. Putting this together, this angle is likely to be open.
In this second case, the red line denotes the approximate position of the trabecular meshwork. Note that the iris is completely apposed to the trabecular meshwork. This is a narrow/closed angle.
In summary, a step-by-step approach to assessing the chamber angle on AS-OCT is as follows:
Identify the scleral spur
Identify the trabecular meshwork (250 – 500 μm anterior to the scleral spur)
Look at the position of the iris relative to the trabecular meshwork
While this approach works most of the time, sometimes you cannot use it because the scleral spur cannot be readily identified. Several studies show that the scleral spur is NOT visible in 25-30% of AS-OCT. However, even when the scleral spur cannot be visualized, you can often make a qualitative judgement based on the angle in which the iris inserts.
Another important point to keep in mind is that a single AS-OCT scan only provides information through a single cut of the iris and chamber angle. The image above illustrates this concept with the horizontal blue line denoting the information a single AS-OCT scan can provide. The rest of the chamber angle is not assessed. As a result, it is important to take multiple scans/cuts to provide more information about the chamber angle and iris insertion.
Lastly, there are some quantitative tools available to assess the chamber angle, which are usually built into the OCT machine. Some measurements that you may encounter include angle opening distance (AOD) or trabecular space area (TISA). From a practical standpoint, most experts agree that it is more clinically relevant to qualitatively assess the angle as opposed to relying on this quantitative tools to guide management.
1. Gonioscopy remains the gold standard of assessing the chamber angle
2. AS-OCT is useful if the angle appears equivocal on gonioscopy, for patients who cannot tolerate gonioscopy, and in cases where there is no clear view of the angle on gonioscopy (ex. Hazy cornea)
3. In AS-OCT, find the scleral spur using the following tips: 1) Look for the change in curvature of the inner surface 2) Look for the thickest part 3) Look for change in reflectivity 4) Schwalbe line method
4. The scleral spur cannot be identified in up to 30% of cases in AS-OCT.