Anterior Segment Optical Coherence Tomography (AS-OCT)

Key Points at a Glance

Key Points of This Examination

• Anterior segment optical coherence tomography (AS-OCT) is a non-contact imaging device that obtains cross-sectional images of the anterior segment.
• In glaucoma practice, it is used as an auxiliary test for angle evaluation.
• Its resolution is superior to ultrasound biomicroscopy (UBM), but it cannot visualize the ciliary body.
• There are two types: SS-OCT (1310 nm) and SD-OCT (840 nm); SS-OCT is more suitable for the anterior segment.
• Angle imaging cannot replace gonioscopy and is considered supplementary ⁶⁾.
• Recently, automated detection of angle closure using deep learning has become a research frontier ²⁾.

1. What is Anterior Segment Optical Coherence Tomography (AS-OCT)?

Anterior segment optical coherence tomography (AS-OCT) is a non-contact optical coherence tomography device specialized for the anterior segment. It acquires cross-sectional images of the tear film, cornea, iris, anterior lens surface, angle, and sclera, and is used for understanding the pathology of anterior segment diseases and various biometric measurements.

History and Development

AS-OCT imaging was first reported by Izatt et al. in 1994. Initially, it used the same 830 nm wavelength as retinal OCT, but its penetration into scattering tissues such as the sclera was poor, making it unsuitable for angle imaging. Subsequently, devices using a longer wavelength of 1310 nm were developed, greatly improving scleral penetration and imaging speed.

Currently, Fourier-domain OCT (FD-OCT) is mainstream, offering superior measurement speed, resolution, and three-dimensional analysis capability compared to time-domain OCT (TD-OCT). FD-OCT includes two types: swept-source OCT (SS-OCT) and spectral-domain OCT (SD-OCT).

SS-OCT

Wavelength: 1310 nm (long wavelength)

Penetration depth: High (images the entire anterior segment in one frame)

Resolution: Lower than SD-OCT but sufficient for clinical use

Representative device: CASIA (Tomey Corporation)

SD-OCT

Wavelength: 840 nm (short wavelength)

Penetration depth: Shallow (difficult to image the entire anterior segment)

Resolution: Higher than SS-OCT

Use: Suitable for detailed observation of the cornea and conjunctiva

AS-OCT is a diagnostic device that allows non-contact observation of the angle. Its resolution is superior to ultrasound biomicroscopy, but the ciliary body cannot be observed ³⁾. Its usefulness as an auxiliary diagnostic tool in glaucoma care is widely recognized ³⁾.

Q What is the difference between AS-OCT and conventional fundus OCT?

A

Fundus OCT is a device that acquires cross-sectional images of the retina, using a light source with a wavelength of 840–870 nm. AS-OCT is specialized for observing the anterior segment (cornea, angle, iris, etc.), and the SS-OCT method uses a long wavelength of 1310 nm to enhance penetration into deep tissues. The observation target and wavelength used are different.

4. Examination Methods and Evaluation Parameters

Angle measurement evaluated with anterior segment OCT

Cureus. 2024;16(4):e58703. Figure 1. PMCID: PMC11110096. License: CC BY.

This image compares preoperative and postoperative anterior segment OCT. The measurement points of TISA and AOD and the widening of the angle can be directly confirmed, depicting the actual angle measurement using AS-OCT.

Examination Procedure

AS-OCT examination is performed in a seated position. The patient looks at a fixation point while the examiner aligns the scan position and captures the image. It is non-invasive and does not require an eyecup or water immersion method. Imaging can be performed in the dark, allowing angle assessment under physiological pupillary dilation. The main features of AS-OCT are listed below.

• Non-invasiveness: No contact with the eye
• High-speed measurement: Enables real-time imaging
• No photophobia: Long-wavelength light causes no glare
• High resolution: 15 μm (higher than UBM’s 50 μm)
• Reproducibility: Good, suitable for longitudinal comparison
• Dark room imaging: Less affected by lighting conditions

For Patients Undergoing the Examination

AS-OCT is a non-contact test with no pain or discomfort. The examination takes only a few minutes. Eye drops to dilate the pupils (mydriatics) are usually unnecessary; the test can be performed in a dark room with the pupils in their natural state.

Qualitative Assessment

The most important landmark for interpreting AS-OCT images is the scleral spur. The scleral spur is the junction between the inner surface of the sclera and the curvature of the cornea, visible as an inward projection of the sclera. By evaluating the apposition between the iris and the inner wall of the corneosclera, angle closure can be detected.

However, it has been reported that the scleral spur is not visible in approximately 25% of cases when using scan protocols without image averaging.

Quantitative Parameters

The main parameters used for quantitative measurement of the anterior chamber angle are listed below.

Parameter	Abbreviation	Definition
Angle opening distance	AOD	Distance between the iris and a point 500/750 μm anterior to the scleral spur
Angle recess area	ARA	Area bounded by AOD, iris, and corneoscleral inner wall
Trabecular-iris space area	TISA	Trapezoidal area from scleral spur to AOD line

Other parameters such as iris thickness, anterior chamber width, and lens vault have also been reported.

Comparison of AS-OCT and Ultrasound Biomicroscopy

Ultrasound biomicroscopy (UBM) is also used for anterior segment tomographic imaging. The characteristics of both are compared.

Item	AS-OCT	Ultrasound Biomicroscopy
Principle	Optical	Ultrasound
Resolution	15 μm	50 μm
Maximum scan range	16 × 6 mm	5 × 5 mm

• Eye contact: AS-OCT is non-contact. Ultrasound biomicroscopy does not directly contact the eye but requires a water immersion method using an eyecup.
• Visualization behind the iris: AS-OCT cannot visualize behind the iris. Ultrasound biomicroscopy can visualize posterior structures including the ciliary body and zonules.
• Clinical use: With the widespread use of AS-OCT, the frequency of ultrasound biomicroscopy has decreased. However, ultrasound biomicroscopy is valuable in situations requiring observation of the ciliary body, such as diagnosis of malignant glaucoma and evaluation of plateau iris.

Gonioscopy is indispensable in glaucoma practice ³⁾, and in Japan, the Shaffer classification and Scheie classification are commonly used to describe angle findings ³⁾.

Q Does AS-OCT examination hurt?

A

AS-OCT is a non-contact examination; the instrument does not touch the eye. There is no pain or discomfort. Anesthetic eye drops are not required, and the examination takes only a few minutes.

5. Clinical Applications in Glaucoma

Angle Assessment

In glaucoma clinical practice, AS-OCT is useful as an adjunct to gonioscopy, or as an alternative when gonioscopy is difficult due to corneal disease or lack of patient cooperation. Because it is non-contact and can be performed in the dark, angle assessment under physiological mydriasis is possible.

Based on iris morphology and the position of the lens relative to anterior segment structures, mechanisms of angle closure such as pupillary block and anterior lens displacement can be identified ⁴⁾. It has become indispensable for observing morphological changes of the iris such as shallow anterior chamber, narrow angle, and plateau iris.

It is also useful as a patient education tool when recommending laser iridotomy.

Limitations of Angle Imaging

Angle imaging devices cannot replace gonioscopy ⁶⁾. Gonioscopy should be performed in all patients with suspected glaucoma ⁶⁾.

AS-OCT is useful for identifying iris morphology in narrow angles, assessing lens effects, and triaging eyes where gonioscopy is difficult ⁶⁾. However, peripheral anterior synechiae (PAS), pigment deposition, and other secondary causes of trabecular dysfunction may be missed, so evaluation solely by angle imaging should be avoided ⁶⁾.

Points to Note in Angle Assessment

AS-OCT is not a substitute for gonioscopy. Findings such as peripheral anterior synechiae (PAS), pigment deposition, and neovascularization that can be confirmed by gonioscopy may be difficult to detect with AS-OCT. If glaucoma is suspected, be sure to undergo gonioscopy.

Evaluation of Glaucoma Surgery

AS-OCT is also applied in preoperative and postoperative evaluation of glaucoma surgery. It is used to assess the morphology of filtering blebs after trabeculectomy and to confirm the position of intraocular drainage devices.

Tanito et al. (2024) clearly visualized the stent condition, which was difficult to evaluate with conventional 2D cross-sectional images, using raster scanning and 3D AS-OCT imaging in cases 2 years after PreserFlo MicroShunt (PFM) implantation. In the right eye, a C-shaped deformation was confirmed, suggesting that the fin may have prolapsed from the scleral pocket ¹⁾.

This C-shaped deformation has rarely been reported in the literature and is thought to be caused by compression from surrounding scar tissue ¹⁾. Adding 3D images to 2D images significantly improved the accuracy of stent evaluation ¹⁾.

Use for Structural Progression Assessment

Measurement of peripapillary retinal nerve fiber layer thickness (RNFL) and macular inner retinal layer thickness using OCT can be used to assess structural progression in glaucoma ³⁾. Each OCT device is equipped with a program to detect changes over time.

However, imaging conditions (e.g., measurement position shift, image quality) affect the measured values, so caution is needed not to take the values at face value ³⁾. In advanced glaucoma eyes, a floor effect occurs where further thinning becomes difficult to detect, making OCT progression assessment more suitable for relatively early cases ³⁾.

Glaucoma diagnosis should not be based solely on OCT, as results indicating “outside normal limits” may be false positives ⁶⁾. Comprehensive judgment integrating clinical findings and visual field testing is essential ⁶⁾.

Computer-aided diagnostic devices, including OCT, are used for detecting glaucoma and identifying progressive optic neuropathy ⁵⁾. With advances in device technology (e.g., high-resolution SD-OCT), improvements in diagnostic performance are expected ⁵⁾.

Q Can AS-OCT completely replace gonioscopy?

A

No, it cannot. AS-OCT has the advantage of non-contact imaging in dark conditions, but findings such as peripheral anterior synechiae, pigmentation, and neovascularization in the angle may be difficult to detect with AS-OCT ⁶⁾. Gonioscopy should be performed in all patients suspected of having glaucoma ⁶⁾.

7. Latest Research and Future Perspectives

For Patients: Please Read Carefully

The following content is currently under research or development and is not a standard test or technology available at general hospitals. It is reference information for professionals regarding future medical developments.

Overview of Research Trends

Huang et al. (2024) conducted a bibliometric analysis of 20 years (2004–2023) of literature on AS-OCT applications in glaucoma, analyzing 931 reports. The United States had the most publications with 288, followed by China with 231, and Singapore with 124. By author, Aung Tin had the most publications (80) and citations (3595) ²⁾.

The number of papers has increased rapidly since 2012, and since 2015, more than 60 papers have been published stably each year²⁾. Since 2018, advances in artificial intelligence (AI) have led to a notable shift from manual measurement to automated detection and recognition in research²⁾.

AI and Deep Learning for Angle Closure Detection

A recent research frontier is the automated detection of angle closure using deep learning²⁾. Conventional AS-OCT image evaluation relies on manual measurement of various parameters, which is time-consuming, subjective, and has low reproducibility.

Deep learning algorithms learn directly from image data and demonstrate the ability to classify open, narrow, and closed angles with high accuracy. A 3D deep learning-based digital gonioscopy system (DGS) has shown diagnostic accuracy comparable to ophthalmologists in detecting narrow iridocorneal angles and peripheral anterior synechiae²⁾.

3D AS-OCT Imaging

FD-mode AS-OCT operating at a wavelength of 1310 nm is enabling rapid three-dimensional cube scanning of the anterior segment. This is expected to allow the following evaluations:

• 360-degree angle assessment: Evaluation of the entire circumference of the angle at once
• Volume parameters: Measurement of iris volume and anterior chamber volume
• Dynamic factor evaluation: Analysis of dynamic changes in iris area and volume with changes in pupil diameter

3D AS-OCT has also demonstrated utility in postoperative evaluation of glaucoma surgical devices, clearly visualizing the overall shape of stent deformation or displacement, which is difficult with 2D images¹⁾.

OCT Angiography (OCTA)

Optical coherence tomography angiography (OCTA) is a rapidly developing technology. It is considered less susceptible to floor effects than retinal nerve fiber layer measurements and may be advantageous over OCT for progression assessment in advanced glaucoma eyes, but standardized clinical application methods have not been established³⁾.

Q Is AI-based AS-OCT image analysis in practical use?

A

It is still at the research stage. Although automated detection of angle closure using deep learning algorithms has shown high accuracy²⁾, it has not yet been widely implemented in clinical practice. Challenges such as insufficient data and lack of unified diagnostic criteria remain.

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8. References

1. Tanito M, Omura T, Iida M, et al. Anterior Segment Optical Coherence Tomography (AS-OCT) 3D Observation of PreserFlo MicroShunt. Cureus. 2024;16(10):e72511.
2. Huang Y, Gong D, Dang K, et al. The applications of anterior segment optical coherence tomography in glaucoma: a 20-year bibliometric analysis. PeerJ. 2024;12:e18611.
3. 日本緑内障学会. 緑内障診療ガイドライン（第5版）. 日眼会誌. 2022;126:85-177.
4. American Academy of Ophthalmology. Primary Angle-Closure Disease Preferred Practice Pattern. San Francisco: AAO; 2020.
5. American Academy of Ophthalmology. Primary Open-Angle Glaucoma Suspect Preferred Practice Pattern. San Francisco: AAO; 2020.
6. European Glaucoma Society. Terminology and Guidelines for Glaucoma, 5th Edition. Br J Ophthalmol. 2025;109:bjo-2025-egsguidelines.