Prescribing for Astigmatism
Detecting Keratoconus with Topography and Tomography
BY HEIDI G. MILLER, OD; TIFFANY GATES, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Although placido disk-based corneal topography is commonly used in diagnosing keratoconus, it generally will not identify patients who have subclinical keratoconus. Instead, computerized corneal tomography measurements tend to be more sensitive in detecting subtle corneal changes. Tomographers assess the front and back corneal surfaces, while also providing information on corneal thickness.
Placido disk-based topographers had been the most sensitive method to detect keratoconus using reflected images from the anterior corneal surface to produce a corneal topography map. Newer instruments using either slit scanning or a rotating Scheimpflug camera provide access to additional data to diagnose subclinical keratoconus. Anterior segment tomography uses an optical slit scan, which projects onto the anterior cornea, posterior cornea, anterior crystalline lens, posterior crystalline lens, and anterior iris (Figure 1) (Belin et al, 2014). The measurements from these surfaces are then used to create a 3D topographic color map (Agarwal, 2007). This scanning technique provides more information about the anterior segment of the eye, including anterior and posterior corneal curvature, elevation maps of the anterior and posterior surface (floats), and corneal thickness (Figure 2) (Agarwal, 2007).
Figure 1. Scheimpflug optical cross-sectional analysis.
Figure 2. OD corneal curvature, elevation, and thickness map.
According to some, tomographers are vital due to early posterior corneal changes in keratoconus that are missed by Placido disk-type topographers. But, how reliable are these indirect measures in diagnosing keratoconus?
In a study examining the precision of one tomographer in analyzing subclinical keratoconus with normal eyes, it demonstrated considerable variability in specificity for some indices, resulting in up to 10% of missed diagnoses of early keratoconus (Ruiseñor Vázquez et al, 2014). This is in contrast to previous studies that reported corneal thickness distribution indices. It also illustrates that even though Scheimpflug tomography can detect most early ectatic changes, subclinical keratoconus cases can also go undetected with this method.
Anterior or Posterior?
There is debate in the literature as to whether keratoconus-associated changes first occur in the anterior or the posterior portion of the cornea. Although the pathophysiology of keratoconus is not entirely understood, there is diagnostic value in utilizing computerized corneal topography data. CLS
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Dr. Miller and Dr. Gates are the cornea and contact lens residents at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). Dr. Edrington is the cornea and contact lens residency coordinator at SCCO. He is also a Fellow of the American Academy of Optometry and a Diplomate in their Cornea, Contact Lens, and Refractive Technologies Section.