Corneal diagnostic tests aid in the evaluation and treatment of disease. Thanks to recent advances in technology, we have many valuable tools to enhance the outcome of surgical intervention and help to determine proper course of action when dealing with complex cases. Further understanding of these modalities and their applications enhances corneal work-ups and assists physicians in their diagnoses.

This guide into the world of corneal diagnostic testing will discuss the five most commonly used modalities as they relate to corneal pathology.

1. Corneal topography

Corneal topography is the most commonly used instrument for corneal measurement and evaluation. The imaging is based on Placido-based reflective image analysis. This technique measures the anterior corneal surface by using the reflected images of several concentric rings projected on the cornea. Corneal topography typically measures the abnormality of reflected rings and calculates the curvature of the corneal surface points in an axial direction. Therefore proper lubrication prior to imaging is essential for quality measurements.

Clinical applications of corneal topography include:

• Keratoconus and pellucid marginal degeneration
• Screening before refractive surgeries
• Evaluation of irregular astigmatism
• Effects and stability of all refractive procedures
• Contact lens fitting, especially for difficult cases
• Pre- and post-refractive cataract surgery.

2. Slit-scan imaging

Frequently used slit-scan devices include:

• Orbscan. The Orbscan is used particularly for pre- and post-refractive patients. The Orbscan not only analyzes the anterior cornea, it also give us corneal thickness and evaluates the posterior surface of the cornea. The current version of the Orbscan uses a combination of reflective corneal topography and optical slit design for its analysis. This exam is recommended for both pre- and post-refractive patients.
• Pentacam. The Pentacam images the anterior segment with a rotating Scheimpflug camera. This system integrates two cameras. One is located in the center for two purposes: orientation of the pupil and assessment of fixation. The second camera rotates on a wheel to capture 50 images of the anterior segment in about two seconds. The Pentacam images from anterior surface of the cornea to the posterior surface of the lens. The three-dimensional images show us depth and allow us to measure corneal thickness and lens density and evaluate IOL positioning. The Pentacam creates a 360-degree image of the anterior chamber with the use of software which builds images and can analyze the anterior segment. The Pentacam has similar clinical applications as corneal topography with added benefits, providing the following:
  • High-resolution of the entire cornea
  • Ability to measure corneas with severe irregularities
  • Calculate pachymetry from limbus to limbus
  • Corneal wavefront analysis to detect higher-order aberrations.

3. Specular microscopy

Specular microscopy is the most accurate way to examine the corneal endothelium. Patients with conditions such as glaucoma, uveitis, and Fuchs’ endothelial dystrophy, can demonstrate changes in the structure of the corneal endothelium. These changes can result in corneal edema, which can impair vision. Furthermore, contact lens wear and intraocular surgery may also compromise the endothelium and can cause corneal edema.

Specular microscopy can help physicians to accurately diagnose endothelial disease. Specular microscopes analyze the size, shape, and amount of endothelial cells.

This instrument uses light that projects on the cornea and captures the image. The image is then displayed as a specular photomicrograph. Keep this test in mind when following the endothelium for possible edema or tracking corneal dystrophies.

4. Slit lamp photography (biomicrography)

Photographic imaging of corneal pathology is imperative for documentation and follow-up, particularly when multiple providers see the patient during treatment. Slit lamp photography is commonly ordered to document corneal pathology, including ulcers, dystrophies, lens abnormalities, iris lesions, and any other pathology within the anterior chamber of the eye.

Slit lamp photography is a great tool that can display progression of disease and can also document results post-surgical intervention.

5. Anterior-segment optical coherence tomography

AS-OCT is used to document pathology, but it also has measurement capabilities. This style of imaging is non-contact and can document corneal scars, LASIK flaps, IOL placement, capsule abnormalities, evaluation of narrow angles, and other anterior chamber pathology. The images can be viewed as individual lines or as 3-D images, which allow us to see depth. It is sometimes used to follow a patient who underwent a corneal surgical procedure such as Descemet’s stripping automated endothelial keratoplasty (DSAEK) and Descemet’s membrane endothelial keratoplasty (DMEK).

Conclusion

Corneal ophthalmic imaging has had many valuable advances in the past decade. New technologies allow us to provide patients with the most accurate assessment of pathology. In addition, they aid in the delivery of the most appropriate and effective treatments available today. We are very fortunate to have the ability to take our ophthalmic exam to the next level with all of these wonderful tools. By incorporating them correctly into our cornea evaluations, we provide the physician the best information available for evaluation and treatment.

Adding corneal testing where indicated is essential for the high level of care expected by today’s patients. Furthermore, understanding how we, as technicians, can integrate these important tools into our work-ups can enhance the patient experience and enable consistent high-level care, especially when dealing with extreme or complex cases. OP

Rosa M. Long, CRA, is a senior imaging technician with Bascom Palmer Eye Institute in Plantation, FL. As an ophthalmic photographer, she performs imaging of all structures of the eye using highly specialized equipment. She will complete her bachelor’s degree at The University of Miami in 2018. Email Ms. Long at r.long@med.miami.edu.