Choose the content to read

• What is Corneal topography?
• Utilization of corneal topography
• Candidates for tomography-based corneal topography
• Preparing before using corneal topography techniques
• Results of corneal topography techniques
• A note from MedPark's doctor

Corneal topography

Corneal topography provides highly detailed information about corneal curvature/refractive power, shape, and thickness. This information is invaluable for keratoconus treatment and planning for laser refractive surgery - PRK, LASIK, SMILE Pro, and cataract surgery with an IOL implant.

What is Corneal topography?

The cornea is the transparent anterior part of the eye, responsible for about 70 percent of the eye's refractive power. Irregularly shaped cornea will cause blurred or distorted vision.

A corneal topography is a detailed representation of the corneal surface, like how one would depict a mountainous terrain with peaks and valleys with a topographic map. Corneal topography could only capture the shape of the anterior surface of the cornea. Devices now combine technology such as scanning-slit technology, Scheimpflug-based imaging, and anterior segment OCT to take slit images of the cornea and convey data on both the anterior and posterior corneal surfaces, rendering a three-dimensional corneal map. As a result, tomography-based corneal topography gives overall corneal shape profiles, including curvature, thickness, and elevation. In addition, computed topographic and tomographic systems may display other data: pupil size and location, indices estimating regular and irregular corneal astigmatism, estimates of the probability of having keratoconus, simulated keratometry, and corneal asphericity. Other topography systems may integrate wavefront aberrometry data as well.

Utilization of corneal topography

• Screening for Keratorefractive surgery
  Corneal topography is necessary to screen candidates for keratorefractive surgery by analyzing corneal shapes and patterns of astigmatism and ruling out suspicious or keratoconic patterns. Keratoconus is an established contraindication to refractive surgery. The ability to differentiate regular from irregular astigmatism is clinically significant in keratorefractive surgery - PRK, LASIK, SMILE pro.
• Early identification & monitoring of ectatic corneal disorders
  Early keratoconus is not detectable by slit lamp examination. Because of this, corneal topography and tomography have become standard for screening keratoconus suspects. In cases with established keratoconus, tomography-based corneal topography plays a role in monitoring progression.
• Cataract surgery planning
  Keratometry provides information for selecting an intraocular lens. Tomography-based corneal topography can evaluate corneal curvature more thoroughly with higher precision, yielding information on corneal astigmatism that helps guide the accurate selection of the toric intraocular lens model and improve postoperative outcomes.
  Tomography-based corneal topography can identify cataract patients who previously had LASIK/PRK. It is crucial to select appropriate vision correction methods.
• Diagnosis of cornea and ocular surface disorders
  Disorders such as pterygium, limbal dermoid, corneal scar, or degenerations can cause corneal curvature distortions and irregular astigmatism.
• Other uses
  For hard/ rigid gas permeable (RGP)/ scleral contact lens fittings and managing post-corneal transplant astigmatism.

Candidates for tomography-based corneal topography

• Individuals intended to undergo laser refractive surgery - PRK, LASIK, and SMILE Pro: This test is integral to the pre-operative evaluations to qualify candidates for refractive surgery.
• Individuals with rapid change of spectacle prescription: Rapid increase of diopter value can be a sign of keratoconus.
• Prospective cataract surgery patients: Topography helps quantify corneal astigmatism, cylinder, and axis and identify patients with corneal abnormalities that preclude multifocal IOLs. 
• For patients with corneal diseases -- keratoconus, corneal scar, corneal dystrophy, and Salzman nodules -- corneal topography can evaluate the refractive effects of these conditions and aid in disease monitoring and surgical planning.
• Hard contact lens fitting: This test guides contact lens fitting, especially RGP (Rigid Gas Permeable Lenses) and scleral contact lenses.

Preparing before using corneal topography techniques

There is nothing special to do before having a corneal topography exam. If you wear contact lenses, the ophthalmologist may ask you to stop wearing them for at least three days.

During the procedure

You sit facing the equipment and position your forehead and chin on the designated spots. The device automatically begins scanning when you focus on a target light, transforming the data into color-coded topography maps on a computer screen in just a few minutes. Your eyes remain untouched during the entire procedure.

What are the risks of corneal topography?

The corneal topography procedure is akin to a photo shoot for your eyes. Therefore, it is safe and carries no risk.

Results of corneal topography techniques

Common types of maps produced by tomography-based corneal topography include:

• Axial curvature map: This map depicts the curvature of the anterior corneal surface, displayed with a color scale representing the refractive index in diopter value at each point. Warmer colors (red, orange, and yellow) represent steeper corneal curvature, while cool colors (blue and purple) represent flatter curvature.
• Elevation map: The elevation maps display the shape of the cornea by comparing it to a computer-generated best-fit sphere (BFS). Warmer colors denote regions where the cornea elevates above the BFS and cool colors where it dips below the BFS.
• Pachymetry map: A pachymetry map is a color map that displays the thickness of the whole cornea; cool colors depict thicker and warm colors thinner (labeled with numerical values) areas.

Furthermore, the device may have built-in keratoconus screening software, corneal densitometer, wavefront aberrometer, and cataract analysis.

After the corneal topography test

Your ophthalmologist will discuss and interpret the results of the tests with you on the same day or at the next follow-up appointment.

A note from MedPark's doctor

Corneal topography and tomography are quick and painless imaging tests that provide crucial information about the structure and function of the cornea for diagnosing and treating a wide variety of ocular diseases. Should you have any questions regarding your eye health or corneal topography, consult our ophthalmologists at the hospital.