Anterior Segment OCT

Q: Can AS-OCT replace gonioscopy?

Not completely. AS- OCT provides quantitative angle measurements that complement gonioscopy , but gonioscopy remains the gold-standard clinical angle assessment, particularly for detecting peripheral anterior synechiae, pigmentation, and abnormal tissue. The two are often used together.

A non-contact light-based imaging method that produces detailed cross-sections of the cornea, anterior chamber, and angle. Often used in glaucoma and corneal evaluation.

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Anterior segment OCT (AS-OCT) is a variant of optical coherence tomography optimized for imaging the front of the eye - the cornea, anterior chamber, drainage angle, and lens. Unlike standard OCT (which targets the retina), AS-OCT uses different scan parameters to image structures up to the iris and anterior lens. It is non-contact, fast, and a common imaging adjunct for angle assessment in glaucoma, corneal pathology, and refractive surgery planning.

Key Takeaways

AS-OCT uses light to image the front of the eye - non-contact, painless, takes seconds
It cannot see through the iris - for ciliary body and posterior iris evaluation, ultrasound biomicroscopy is needed
Common uses include angle assessment for angle-closure glaucoma, corneal pathology including keratoconus, refractive surgery planning, and post-LASIK flap evaluation
Quantitative measurements include central corneal thickness, anterior chamber depth, angle opening distance, and detailed corneal mapping
AS-OCT vs. UBM: AS-OCT is faster and contact-free; UBM gets through the iris

What AS-OCT Shows

A single AS-OCT scan provides:

Anterior segment OCT diagram labeling cornea, anterior chamber, iris, drainage angle, lens, and angle width — AS-OCT images the cornea, anterior chamber, iris, drainage angle, and anterior lens without touching the eye.

Cornea - full thickness imaging with epithelial, stromal, and endothelial detail
Anterior chamber - depth measurement and lens position
Drainage angle - width measurements, open vs. closed configuration
Iris contour - flat, bowed forward (pupillary block), or plateau pattern
Lens position (visible portion)

What AS-OCT does not show:

Behind the iris (ciliary body, sulcus, zonules) - near-infrared light is largely absorbed and scattered by iris pigment; newer swept-source systems penetrate better, but UBM remains the reference imaging test for structures behind the iris
Posterior segment (retina, optic nerve) - separate OCT scan needed
Peripheral angle in some patient configurations

Common Uses

Angle Assessment in Glaucoma

Suspicion of narrow angles or acute angle-closure glaucoma
Pre- and post-laser peripheral iridotomy evaluation
Following the angle in pigmentary glaucoma, exfoliation syndrome
Surveillance of patients at angle-closure risk

Corneal Pathology

Keratoconus - thinning maps, posterior corneal elevation
Fuchs dystrophy and other endothelial disease
Post-refractive-surgery evaluation (LASIK, PRK) - flap thickness, residual stromal bed, ectasia screening
Corneal scarring depth and extent
Bullous keratopathy documentation
Pre-operative planning for endothelial keratoplasty

Surgical Planning

Phakic IOL sizing using angle-to-angle (white-to-white) measurements as a surrogate for sulcus diameter; some surgeons prefer UBM-based sulcus-to-sulcus measurement for ICL sizing
Pre-cataract planning in difficult anatomy
Trabeculectomy bleb evaluation

Lens and Refractive Surgery Planning

Anterior chamber depth for IOL calculations
Angle anatomy for ICL planning

How It Differs From Ultrasound Biomicroscopy (UBM)

Feature	AS-OCT	UBM
Modality	Light	Sound (high-frequency)
Contact	Non-contact	Contact (saline cup or gel)
Resolution	Excellent for cornea/iris surface	Excellent for ciliary body
Sees through iris	No	Yes
Speed	Seconds per scan	Several minutes
Patient comfort	High	Tolerable, requires anesthetic
Setting	Routine clinic	Specialty / glaucoma clinic

For many angle-screening questions, AS-OCT is a preferred first-line imaging test. UBM is reserved for cases where structures behind the iris need evaluation.

What to Expect

No special preparation
Patient sits at the machine and looks at a fixation target
Scans take seconds per eye
No drops needed in most cases (though dilation may be requested for specific assessments)
No vision restrictions afterward

Frequently Asked Questions

How is this different from "regular" OCT?

Regular OCT of the retina uses settings optimized for posterior segment imaging. Dedicated AS-OCT systems often use longer-wavelength light (around 1310 nm) and deeper scan ranges for the anterior segment, while some retinal OCT machines use anterior-segment attachments or scan modes at their usual wavelength. Some machines can do both with different scan modes or attachments.

Can AS-OCT replace gonioscopy?

Not completely. AS-OCT provides quantitative angle measurements that complement gonioscopy, but gonioscopy remains the gold-standard clinical angle assessment, particularly for detecting peripheral anterior synechiae, pigmentation, and abnormal tissue. The two are often used together.

Will I need any drops?

Typically not. Some specific evaluations (e.g., pre- and post-dilation angle scans for provocative angle-closure assessment) require dilation, but routine AS-OCT does not.

References

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