Why can't you see the anterior chamber angle without a special lens?

Light reflecting from the anterior chamber angle undergoes total internal reflection at the corneal-air interface, preventing direct visualization. A goniolens placed on the cornea eliminates this interface, allowing the reflected light to pass through the lens and into the examiner's view. This is similar to how you cannot see the bottom of a glass at a steep angle, but a lens placed on top would allow you to see it.

What is the normal central corneal thickness?

The average central corneal thickness is approximately 540-550 micrometers. Corneas significantly thicker than this (over 600 um) may cause IOP overestimation on tonometry, while thinner corneas (under 500 um) may cause underestimation. Thin CCT is also an independent risk factor for developing glaucoma, beyond its effect on IOP measurement.

How does AS-OCT compare to gonioscopy?

AS-OCT is non-contact, comfortable, and provides objective, reproducible images of the anterior chamber angle. Gonioscopy requires a contact lens and topical anesthesia but provides direct visualization of angle structures including pigmentation, peripheral anterior synechiae, and neovascularization that AS-OCT may miss. Gonioscopy remains the gold standard for angle assessment, while AS-OCT is a valuable complementary tool.

Anterior Segment Imaging: Slit Lamp Photography, Gonioscopy, AS-OCT, and Pachymetry

Overview of Anterior Segment Imaging

Anterior segment imaging encompasses techniques that document and evaluate the structures at the front of the eye: the cornea, iris, anterior chamber, crystalline lens, and the anterior chamber angle. These methods complement posterior segment imaging (fundus photography, OCT) to provide a comprehensive ocular assessment.

Slit Lamp Photography

The slit lamp biomicroscope provides magnified, illuminated views of anterior segment structures. With an attached camera system, these views can be captured as permanent photographic records.

Common uses include documenting:

Corneal pathology: Abrasions, ulcers, scars, dystrophies, and foreign bodies
Anterior chamber: Cells, flare (protein in the aqueous), hypopyon (layered white blood cells), hyphema (blood)
Iris abnormalities: Nevi, synechiae, rubeosis, atrophy
Cataract: Type, density, and location of lens opacity
Conjunctival lesions: Pterygium, pinguecula, masses

Slit lamp photographs provide a baseline for comparison at future visits and serve as valuable communication tools when referring patients to specialists.

Gonioscopy

Gonioscopy is the technique of visualizing the anterior chamber angle, the junction where the iris meets the cornea. This area cannot be seen directly because of total internal reflection at the corneal surface, so a special mirrored goniolens is required.

Types of Goniolenses

Indirect gonioscopy (Zeiss, Posner, Sussman): Small lenses held on the cornea that provide a mirror image of the opposite angle. The patient sits at the slit lamp.
Direct gonioscopy (Koeppe): Larger lens placed on the supine patient, providing a direct view of the angle. Used primarily in the operating room.

What You See

From posterior to anterior in the angle, the visible structures are:

Ciliary body band (most posterior)
Scleral spur
Trabecular meshwork (the drainage structure; appears as a pigmented band)
Schwalbe's line (the most anterior structure, marking the end of Descemet's membrane)

Gonioscopy determines whether an angle is open or narrow, which is essential for classifying glaucoma type and assessing the risk of angle-closure.

A common grading system is the Shaffer classification (Grade 0 to 4). Grade 4 is wide open (35-45 degrees), Grade 3 is open, Grade 2 is moderately narrow, Grade 1 is very narrow (10 degrees), and Grade 0 is closed. Grades 1 and 2 may be at risk for angle closure.

Anterior Segment OCT (AS-OCT)

Anterior segment OCT uses the same principle as retinal OCT but with wavelengths optimized for imaging anterior structures. It produces cross-sectional images of:

Corneal layers: Epithelium, Bowman's layer, stroma, Descemet's membrane, endothelium
Anterior chamber depth
Anterior chamber angle: Provides a non-contact alternative to gonioscopy for angle assessment
Iris configuration: Plateau iris, bombe configuration

AS-OCT is non-contact and does not require topical anesthesia or a coupling lens, making it more comfortable than gonioscopy. However, it cannot see through the pigmented posterior iris and does not provide the same level of angle detail as gonioscopy.

Pachymetry

Pachymetry measures central corneal thickness (CCT), typically in micrometers. The two main methods are:

Ultrasound pachymetry: A small probe contacts the anesthetized cornea and measures thickness using sound wave travel time. The gold standard for CCT measurement.
Optical pachymetry: Non-contact methods using OCT or specialized devices to measure corneal thickness without touching the eye.

CCT is clinically important because:

It affects the accuracy of IOP measurements (thick corneas overestimate, thin corneas underestimate)
Thin CCT is an independent risk factor for glaucoma
It is essential for refractive surgery screening (sufficient corneal thickness is required for safe LASIK)
It monitors corneal edema in conditions like Fuchs endothelial dystrophy

Not measuring CCT in glaucoma patients or suspects. Without knowing the corneal thickness, IOP readings cannot be properly interpreted. A patient with thin corneas and a seemingly "normal" IOP may actually have elevated true IOP that is being underestimated.

Key Takeaways

Slit lamp photography documents anterior segment pathology for baseline comparison and referral communication
Gonioscopy uses mirrored lenses to visualize the anterior chamber angle, essential for glaucoma classification
AS-OCT provides non-contact cross-sectional imaging of corneal layers and anterior chamber structure
Pachymetry measures corneal thickness, which affects IOP interpretation and is critical for glaucoma and refractive surgery evaluation
These techniques complement each other for a comprehensive anterior segment assessment