Why is gonioscopy necessary if OCT can image the angle?

While AS-OCT provides excellent angle imaging without contact, gonioscopy remains the gold standard for angle assessment because it allows dynamic manipulation (indentation or compression gonioscopy) to distinguish appositional from synechial angle closure. Gonioscopy also allows direct visualization of angle pigmentation, trabecular meshwork details, and any neovessels that may be subtle on OCT images.

What is the clinical significance of corneal pachymetry in glaucoma?

Goldmann applanation tonometry is calibrated for an average corneal thickness. Patients with thin corneas (less than approximately 500 micrometers) have their IOP underestimated by Goldmann, while those with thick corneas have IOP overestimated. In glaucoma suspects, thin corneas are an independent risk factor for progression regardless of the measured IOP, making pachymetry an essential part of glaucoma risk stratification.

When is AS-OCT preferred over ultrasound pachymetry?

AS-OCT (or Scheimpflug imaging) is preferred when a full corneal thickness map is needed rather than a single central point measurement. This is particularly valuable for keratoconus screening (where the location and degree of thinning across the entire cornea matters) and for comprehensive pre-LASIK evaluation. Ultrasound pachymetry remains useful when portability matters or when a quick single-point measurement is sufficient.

What does a Shaffer grade describe in gonioscopy?

The Shaffer grading system classifies the angular width of the anterior chamber angle as seen on gonioscopy. Grade 4 (widest, about 35-45 degrees) is a fully open angle with minimal closure risk. Grade 3 (25-35 degrees) is open. Grade 2 (20 degrees) is potentially closable. Grade 1 (10 degrees) is very narrow with high closure risk. Grade 0 (0 degrees) indicates a closed angle, often with no visible angle structures.

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

What Is Anterior Segment Imaging?

Anterior segment imaging refers to a set of specialized techniques for visualizing, measuring, and documenting the structures at the front of the eye: the cornea, iris, anterior chamber, anterior chamber angle, and crystalline lens. While the posterior segment (retina, optic nerve) often gets attention for imaging, the anterior segment harbors many pathologies that require careful documentation and quantitative measurement.

As a CPO, you may operate several of these instruments, prepare patients for procedures, or capture images for the supervising optometrist to review. Understanding what each modality shows and when it is used is essential knowledge for both the exam and daily practice.

Slit Lamp Photography

Slit lamp photography uses a camera mounted on the slit lamp biomicroscope to capture high-magnification images of anterior segment structures. The slit lamp's variable illumination options (diffuse, direct focal, specular reflection, retroillumination) allow documentation of findings from the eyelid surface all the way through to the posterior lens capsule.

Common uses include:

Documenting a corneal ulcer's size, depth, and margin at each visit to monitor healing or progression.
Photographing an iris nevus or lesion for longitudinal comparison.
Capturing a lens opacity to stage cataract severity over time.
Recording corneal staining patterns (with fluorescein or lissamine green) before and after treatment for dry eye.

Good slit lamp photos require precise focus and appropriate illumination. For corneal pathology, use a narrow slit beam and direct focal illumination. For retroillumination of the lens, use an indirect beam reflecting off the fundus. Spending a moment to select the right illumination makes a significant difference in image quality.

Gonioscopy

Gonioscopy is the clinical examination of the anterior chamber angle, the recess where the trabecular meshwork, Schlemm's canal, and drainage structures are located. The angle cannot be seen directly because light from that region undergoes total internal reflection at the cornea. Gonioscopy overcomes this by placing a special contact lens (the goniolens) on the eye, which eliminates the total internal reflection and allows direct or mirrored visualization.

There are two main systems:

Type	Examples	View
Direct gonioscopy	Koeppe lens	Direct view, typically with patient supine
Indirect gonioscopy	Goldmann 3-mirror, Zeiss 4-mirror	Mirror view, patient at slit lamp

Gonioscopy is essential for:

Assessing angle width in glaucoma suspects (open-angle vs. narrow-angle vs. angle-closure risk).
Grading the angle using the Shaffer or Spaeth classification systems.
Identifying peripheral anterior synechiae (adhesions between the iris and angle).
Ruling out angle neovascularization in conditions like diabetic retinopathy or central retinal vein occlusion.

Patients sometimes mistake gonioscopy for tonometry because both involve a lens touching the eye. Gonioscopy is a diagnostic procedure to look at the drainage angle, not to measure pressure.

Anterior Segment OCT (AS-OCT)

Anterior segment OCT applies the same interferometry principle as posterior OCT to image the cornea, anterior chamber angle, and iris in cross-section. It is non-contact (no lens touches the eye) and provides quantitative measurements that are impossible to obtain by clinical examination alone.

Key applications include:

Corneal thickness mapping: Identifying patterns consistent with keratoconus (inferior thinning and steepening) or post-LASIK corneal changes.
Angle assessment: Measuring anterior chamber angle width and area, detecting appositional or synechial angle closure without direct contact.
Post-surgical evaluation: Examining corneal flap position after LASIK, evaluating Descemet membrane following DSAEK or DMEK.
Fitting specialty contact lenses: Mapping corneal topography and elevation to design scleral lens fits.

AS-OCT angle assessment can be performed in both light and dark conditions. Images in dim illumination reveal angle narrowing that may not be apparent in bright conditions, which is clinically important for detecting angle-closure risk.

Pachymetry

Pachymetry is the measurement of corneal thickness. The cornea averages approximately 520 to 555 micrometers in a healthy adult at its thinnest central point. Pachymetry is performed using:

Ultrasound pachymetry: A small probe directly contacts the cornea (after anesthetic drops) and uses sound wave transit time to calculate thickness. Fast and portable.
Optical pachymetry: Performed as part of AS-OCT, Scheimpflug imaging (e.g., Pentacam), or some topographers. Non-contact, and provides a full corneal thickness map rather than just a central point.

Clinical indications for pachymetry include:

Adjusting IOP interpretation (thin corneas cause Goldmann to underestimate true IOP).
Pre-operative screening for LASIK (minimum safe residual stromal bed after ablation requires knowing starting thickness).
Diagnosing and monitoring keratoconus (progressive corneal thinning at the cone apex).
Assessing corneal edema (corneal swelling increases thickness).

Key Takeaways

Slit lamp photography documents corneal, iris, and lens pathology for longitudinal comparison.
Gonioscopy uses a contact lens to directly visualize the anterior chamber angle for glaucoma assessment.
AS-OCT provides non-contact cross-sectional images of the cornea, angle, and iris with quantitative measurements.
Pachymetry measures corneal thickness, essential for IOP interpretation, refractive surgery screening, and keratoconus monitoring.