Anterior Segment OCT (AS-OCT)

Key Points of This Disease

• Anterior segment OCT (AS-OCT) is a diagnostic imaging device that uses near-infrared light interference to non-invasively obtain tomographic images of the cornea, anterior chamber, lens, and angle.
• It allows non-contact, non-mydriatic examination and can evaluate the angle in dark conditions.
• There are two methods: SS-OCT (1310 nm) and SD-OCT (840 nm); SS-OCT is suitable for evaluating the entire anterior segment.
• Typical quantitative parameters include AOD (angle opening distance), ARA (angle recess area), and TISA (trabecular-iris space area).
• It enables observation and quantitative evaluation of areas that cannot be examined with a slit lamp microscope.
• AS-OCT cannot detect neovascularization or pigmentation of the angle and cannot replace gonioscopy ⁶⁾.
• In recent years, automatic detection of angle closure using deep learning has been at the forefront of research ²⁾.

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

Anterior segment OCT (AS-OCT: Anterior Segment Optical Coherence Tomography) is a non-invasive imaging device that uses near-infrared light interference to obtain cross-sectional images of the cornea, anterior chamber, lens, and angle. It enables observation and quantitative evaluation of areas that cannot be examined with a slit lamp microscope, and is widely used in screening for angle closure, corneal diseases, before and after refractive surgery, and angle evaluation in glaucoma.

History and Development

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

Currently, Fourier-domain OCT (FD-OCT) is mainstream. Compared to time-domain OCT (TD-OCT), it offers superior measurement speed, resolution, and three-dimensional analysis capabilities. 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 practical use

Representative model: CASIA2 (Tomey)

SD-OCT

Wavelength: 840 nm (short wavelength)

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

Resolution: Higher resolution 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 (UBM), but it cannot visualize the ciliary body ³⁾. Its usefulness as an auxiliary diagnostic tool in glaucoma care is widely recognized ³⁾.

Representative Models

• CASIA2 (Tomey): Equipped with SS-OCT, enables narrow-angle screening with 360° automatic angle analysis (AOD500, narrow angle index).
• Cirrus HD-OCT 5000/6000 Anterior Segment Mode (Zeiss): Anterior segment mode added to existing posterior segment OCT.
• SPECTRALIS Anterior Segment Mode (Heidelberg): Supports high-resolution SD-OCT for detailed corneal evaluation.

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

A

Fundus OCT is a device that captures tomographic 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 longer wavelength of 1310 nm to enhance penetration into deep tissues. The observation target and wavelength used differ.

2. Comparison of AS-OCT and UBM

There are two types of anterior segment tomographic imaging: AS-OCT and ultrasound biomicroscopy (UBM). Both have commonalities and clear differences.

Commonalities

1. Cross-sectional images of the anterior segment can be obtained.
2. Useful information for determining glaucoma treatment strategies can be obtained from cross-sectional images of the angle.
3. Quantitative evaluation of images is possible (angle analysis software).
4. Observation of changes under light and dark conditions is possible.

Comparison Table

Item	Anterior Segment OCT (AS-OCT)	Ultrasound Biomicroscopy (UBM)
Principle	Light (wavelength 0.7–1.3 μm)	Ultrasound (30–50 MHz)
Contact	Non-contact	Contact (requires water immersion)
Position	Sitting (some supine possible)	Supine
Resolution	15 μm	50 μm
Maximum scan range	16 × 6 mm	5 × 5 mm
Ciliary body observation	Unclear	Possible
Posterior iris surface	Unclear	Possible
Corneal surface / tear meniscus	Useful	Unsuitable
Image analysis software	Comprehensive	Limited
Immediately after surgery	Possible (no infection risk)	Difficult

Clinical Differentiation

AS-OCT is the first choice in daily practice due to its advantages of being non-contact, fast, and high-resolution. On the other hand, UBM excels in observing the ciliary body, zonules, and posterior iris surface, which are difficult to visualize with AS-OCT. UBM is particularly useful when observation of the ciliary body is needed, such as in the diagnosis of malignant glaucoma or detailed evaluation of plateau iris.

3. Examination Techniques and Precautions

Basic Procedure

AS-OCT is performed according to the following procedure.

1. Non-contact, no anesthetic eye drops needed: No contact with the eye, no anesthetic eye drops required
2. No dilation needed: Angle evaluation is performed without dilation. For evaluation of changes in dark conditions, images are taken in both light and dark conditions.
3. Seated position, looking straight ahead: The patient gazes at a fixation light.
4. Automatic evaluation with angle analysis software: CASIA2 automatically analyzes AOD500 over 360°.

Narrow-Angle Screening Using CASIA2

CASIA2 (equipped with SS-OCT) performs automatic 360° angle analysis to calculate AOD500 around the entire circumference and quantifies narrow-angle risk with a narrow angle index. Combined with gonioscopy results, it can be used for staff education and patient explanations.

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 not needed; the test can be performed with natural pupils in a dark room.

Limitations of the Examination

AS-OCT cannot detect neovascularization or pigmentation of the angle. Secondary causes such as peripheral anterior synechiae (PAS), pigmentation, and trabecular meshwork dysfunction may be missed if evaluated only with AS-OCT⁶⁾.

Cautions in Angle Evaluation

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.

Q Can AS-OCT completely replace gonioscopy?

A

No, it cannot. AS-OCT has the advantage of being non-contact and allowing imaging in dark conditions, but findings such as peripheral anterior synechiae, pigment deposition, and neovascularization may be difficult to detect with AS-OCT⁶⁾. Gonioscopy should be performed in all patients with suspected glaucoma⁶⁾.

4. Evaluation Parameters and Interpretation of Results

Identification of the Scleral Spur

The most important landmark when interpreting AS-OCT images is the scleral spur. The scleral spur is visible as a structure where the inner surface of the sclera and the curvature of the cornea meet, with the sclera protruding inward. 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 shown 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 enclosed by AOD, iris, and corneoscleral wall
Trabecular-iris space area	TISA	Trapezoidal area from scleral spur to AOD line

Other measurable parameters include iris thickness, anterior chamber width, and lens vault.

Corneal evaluation parameters

AS-OCT is useful not only for angle assessment but also for detailed evaluation of corneal cross-sections.

• Corneal cross-sectional image: Structures of each corneal layer (epithelium, Bowman’s layer, stroma, Descemet’s membrane, endothelium) are clearly visualized.
• Central corneal thickness (CCT) measurement: Enables accurate quantitative assessment of corneal thickness.
• Corneal shape analysis: Models with topography function can also analyze corneal shape.

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

5. Clinical applications

Angle assessment in glaucoma

Cross-sectional image of the anterior chamber angle by SD-OCT (anatomical structures: scleral spur, trabecular meshwork, iris)

Wies6014. SD OCT - Anterior Chamber Angle Cross-Section. Wikimedia Commons. 2013. Source: https://commons.wikimedia.org/wiki/File:SD_OCT_-_Anterior_Chamber_Angle_Cross-Section.png. License: CC BY-SA 4.0.

Cross-sectional image of the anterior chamber angle of a 57-year-old male captured by spectral-domain OCT (SD-OCT), clearly depicting the scleral spur, trabecular meshwork, and iris. This corresponds to the angle evaluation by AS-OCT discussed in the section “Angle evaluation in glaucoma”.

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 poor patient cooperation. Because it is non-contact and can be performed in dark conditions, 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 lens-induced angle closure can be differentiated ⁴⁾. As a diagnostic aid for angle closure (PAC/PACS), it helps in decisions regarding laser iridotomy (LPI) and cataract surgery ⁴⁾.

It is also useful as a patient education tool when recommending laser iridotomy ⁵⁾. It has become indispensable for observing morphological changes of the iris such as shallow anterior chamber, narrow angle, and plateau iris.

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 status of the stent, which was difficult to assess with conventional 2D cross-sectional images, using raster scanning and 3D AS-OCT imaging in a case 2 years after PreserFlo MicroShunt (PFM) implantation. In the right eye, C-shaped deformation was confirmed, suggesting possible fin extrusion from the scleral pocket ¹⁾. Adding 3D images to 2D images significantly improved the accuracy of stent evaluation ¹⁾.

Applications to corneal diseases

Cross-sectional image of a normal cornea by SD-OCT (clear layered structure of corneal epithelium, stroma, and endothelium)

Wies6014. SD-OCT Corneal Cross-Section. Wikimedia Commons. 2013. Source: https://commons.wikimedia.org/wiki/File:SD-OCT_Corneal_Cross-Section.png. License: CC BY-SA 4.0.

Cross-sectional image of a normal cornea of a 24-year-old male captured by spectral-domain OCT (SD-OCT), showing high-resolution depiction of the three-layer structure: corneal epithelium, stroma, and endothelium. This corresponds to the corneal cross-sectional evaluation discussed in the section “Applications to corneal diseases”.

AS-OCT evaluates the depth of corneal opacities in cross-section and helps in selecting the surgical technique for corneal transplantation.

• Superficial lesions (epithelium to Bowman’s layer): Useful for considering DALK (deep anterior lamellar keratoplasty)
• Deep stromal to Descemet’s membrane lesions: Contributes to differentiation from DSAEK/DMEK (Descemet’s membrane endothelial keratoplasty)
• Full-thickness lesions: Used for determining the indication for PK (penetrating keratoplasty)
• Keratoconus: Quantitative evaluation of corneal thinning and consideration of corneal cross-linking (CXL) indications

Preoperative Assessment for Cataract Surgery

Before cataract surgery, AS-OCT is used for quantitative evaluation of the anterior segment.

• Anterior chamber depth: Aids in IOL power calculation
• Lens vault: Assessment of risk of mechanical contact with the iris
• Presence of angle narrowing: Assessment of risk of postoperative angle-closure glaucoma

Applications in Refractive Surgery

• Preoperative: Evaluation of corneal thickness and cross-sectional shape. Anterior chamber depth assessment before ICL (implantable collamer lens) surgery
• Postoperative: Confirmation of flap morphology (after LASIK) and residual corneal stromal thickness

Q Does AS-OCT examination hurt?

A

AS-OCT is a non-contact examination; no instrument touches the eye. There is no pain or discomfort. No anesthetic eye drops are needed, and the examination takes only a few minutes.

6. Detailed Measurement Principles

Basic Principles of OCT

AS-OCT utilizes the principle of the Michelson interferometer. Light from the source is split into a “reference arm” and a “sample arm” (irradiation to the eye), and the reflected light from each is interfered to obtain the reflection intensity from each depth in the tissue as an A-scan signal. The A-scan signal is converted into a depth-wise luminance distribution by Fourier transform, and a tomographic image is generated by two-dimensional scanning.

Comparison of SD-OCT and SS-OCT

FD-OCT (Fourier-domain OCT) has two implementation methods.

• SD-OCT (Spectral-domain OCT): Uses a broadband light source near 840 nm wavelength. The reflected light is split into spectral components by a spectrometer and acquired simultaneously. It offers high resolution but limited visualization of deep anterior segment structures.
• SS-OCT (Swept-source OCT): Sweeps a single wavelength rapidly near 1310 nm. The 1310 nm wavelength has high penetration through light-scattering tissues of the anterior segment (sclera, scleral spur), allowing coverage of the entire anterior segment in a single image.

SS-OCT at 1310 nm has a penetration depth that reaches the posterior lens surface and ciliary body, and has become the de facto standard for AS-OCT applications.

Development of 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 more advantageous than OCT for progression assessment in advanced glaucoma, but standardized clinical applications have not yet been established ³⁾.

7. Latest Research and Future Perspectives

For patients: Please read this

The following content is currently at the research or development stage 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 AS-OCT applications in glaucoma over 20 years (2004–2023), analyzing 931 reports. The United States had the most publications (288), followed by China (231) and Singapore (124). Aung Tin had the most publications (80) and citations (3595) ²⁾.

The number of papers increased sharply after 2012, and since 2015, more than 60 papers have been published annually ²⁾. Since 2018, advances in artificial intelligence (AI) have led to a notable shift from manual measurement to automated detection and recognition ²⁾.

AI and Deep Learning for Angle Closure Detection

A recent research frontier is the automatic detection of angle closure using deep learning ²⁾. 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) showed diagnostic accuracy comparable to ophthalmologists in detecting narrow iridocorneal angles and peripheral anterior synechiae ²⁾.

3D AS-OCT Imaging

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

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

3D AS-OCT has also demonstrated utility in postoperative evaluation of glaucoma surgical devices, clearly visualizing the overall deformation and displacement of stents that were difficult to assess with 2D images¹⁾.

Q Has AI-based AS-OCT image analysis been put into practical use?

A

It is still at the research stage. Deep learning algorithms for automatic detection of angle closure have shown high accuracy²⁾, but have not yet been widely implemented in clinical practice. Challenges remain, such as insufficient data and lack of unified diagnostic criteria.

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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.