How is corneal topography different from keratometry?

Keratometry measures corneal curvature at only four paracentral points in two meridians, providing a simple average K reading for each meridian. Corneal topography maps thousands of data points across the entire corneal surface, revealing the full shape including peripheral curvature, irregularities, and subtle patterns like early keratoconus that keratometry may miss.

What does a keratoconus pattern look like on topography?

Keratoconus typically appears as an area of focal inferior steepening (warm-colored, high-power region inferiorly) with superior-inferior asymmetry. The steepest point may be displaced below center, and there is often skewing of the flat and steep meridians so they are not perpendicular. Between eyes, the two topography maps often show asymmetry in severity.

Why is topography required before LASIK surgery?

LASIK removes corneal tissue to reshape the surface and correct refractive error. If the patient has keratoconus or suspected keratoconus (forme fruste keratoconus), removing tissue from an already abnormal and potentially progressive cornea can trigger post-LASIK ectasia, where the cornea continues to steepen and thin, causing irreversible vision loss. Topography screening identifies these contraindications before surgery.

Corneal Topography: CPO Exam Guide

What Is Corneal Topography?

Corneal topography is a non-invasive imaging technique that maps the entire anterior corneal surface in detail. While keratometry measures curvature at only four central points, topography analyzes thousands of data points across the full corneal surface, providing a comprehensive picture of corneal shape. It is an essential tool for detecting irregular corneal conditions and planning refractive surgery or specialty contact lens fitting.

How Topography Works

Most clinical topographers use a Placido disc system, which projects a series of concentric rings (similar to a bull's eye target) onto the corneal surface and captures their reflection. A camera records the reflected image, and software analyzes the distortion of the rings to calculate curvature at thousands of points across the cornea.

Closely spaced rings reflect from a steep (more curved) cornea
Widely spaced rings reflect from a flat (less curved) cornea
Distorted or irregular rings indicate irregular corneal curvature

Corneal topography must be performed before dilating the patient, as dilation changes the tear film and can affect measurements. Also ask patients to blink immediately before capturing the image to ensure a fresh tear film for accurate ring reflection.

Reading Topography Maps

Topography software displays results as color-coded maps:

Warm colors (red, orange): steeper areas with higher power
Cool colors (blue, green): flatter areas with lower power
Axial (sagittal) map: shows overall corneal shape; most commonly used for initial assessment
Tangential (instantaneous) map: shows local curvature changes; better for detecting irregular astigmatism and peripheral changes
Elevation maps: show how the corneal surface deviates from a reference sphere; used for surgical planning

Clinical Applications

Keratoconus Detection

Keratoconus is a progressive corneal condition where the cornea thins and steepens into a cone shape. Topography reveals characteristic findings:

Inferior steepening (inferior-superior asymmetry)
Skewed radial axes between the flattest and steepest meridians
Classic "cone" pattern of focal steepening
Asymmetry between the two eyes

Keratoconus is a contraindication for standard LASIK surgery. Detecting it with topography before refractive surgery is critical because operating on a keratoconic cornea can accelerate ectasia and cause devastating vision loss. Topography screening is mandatory for all refractive surgery candidates.

Contact Lens Fitting

Topography guides specialty contact lens fitting for patients with irregular corneas:

Orthokeratology (Ortho-K): uses topography to design lenses that temporarily reshape the cornea overnight
Scleral lenses: topography maps help design lenses that vault over an irregular cornea
GP lens fitting: topography-derived simulated K readings guide base curve selection

Surgical Planning

Topography is used before cataract surgery to assess corneal astigmatism for toric IOL planning, and before refractive surgery to map the treatment zone and screen for contraindications.

Assuming keratometry and topography provide the same information. Keratometry measures only four paracentral points; topography maps thousands of points across the entire cornea. Keratometry may appear normal in early keratoconus while topography reveals the characteristic inferior steepening that indicates the diagnosis.

Key Takeaways

Topography maps the entire corneal surface at thousands of points; keratometry measures only four
Warm colors (red/orange) = steeper; cool colors (blue) = flatter
Inferior steepening on topography is the hallmark of keratoconus
Keratoconus is a contraindication for LASIK; topography screening is mandatory pre-operatively
Perform topography before dilation and with a fresh tear film