GP lens eye Rounds

KERATOCONUS: Diagnosis and Testing

Emily Gottschalk OD, FAAO

How is Keratoconus detected clinically?

• Symptoms: progressive decreased vision, monocular diplopia, photophobia, glare, and distorted vision.¹
• Clinical signs of early keratoconus: scissor reflex on retinoscopy, irregular mires on keratometry, Fleischer’s ring (iron ring at the base of the cone), and Charleaux sign (irregular shadow on retroillumination through a dilated pupil resembling an oil droplet).¹
• Clinical signs in advanced keratoconus:
  • Vogt’s striae (vertical lines within the stroma that disappear when pressure is applied).¹
  • Munson’s sign (lower lid protrusion on downgaze).¹
  • Rizzuti sign (conical beam of light at the nasal limbus formed by temporal illumination of the cornea).¹
  • Stromal paracentral inferior thinning.¹
  • Scarring at the apex of the cone.¹

How is Keratoconus diagnosed and monitored with imaging?

• Placido disk topography:
  • Projected concentric rings create topographical maps of the anterior corneal surface (Figure 1).
  • Identify asymmetrical or skewed astigmatism and inferior steepening characteristic of keratoconus.²
  • Indices for keratoconus:
    1. High corneal curvature > 47.2D
    2. Asymmetry > 1.4D within the 3mm radius in the upper and lower regions (IS value)
    3. Axis skew of 20o ³

Figure 1: Axial power map using placido disc topography of a keratoconic cornea.

• Corneal tomography:
  • Posterior corneal surface information including elevation maps and full pachymetric map.⁴ The posterior elevation map/float shows increase in elevation before the anterior elevation map in keratoconus.⁵ (Figure 2)
  • Provides analysis of corneal layers where the epithelium will be thinner over the cone surrounded by thickened epithelium.⁶

Figure 2: Corneal tomography of the same keratoconic cornea as Figure 1.

Sources

1. 1. Krachmer JH, Feder RS, Belin MW. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol 1984;28:293–322.
   2. Wilson SE, Lin DTC, Klyce SD. Corneal Topography of Keratoconus. Cornea 1991;10(1):2-8.
   3. Rabinowitz YS. Videokeratographic indices to aid in screening for Keratoconus. J Refract Surg 1995;11:371–406.
   4. Belin M, Ambrósio R. Scheimpflug imaging for keratoconus and ectatic disease. Indian J Ophthalmol 2013;61:401.
   5. Belin MW, Khachikian SS. An introduction to understanding elevation-based topography: how elevation data are displayed – a review. Clin Exp Ophthalmol 2009;37:14–29.
   6. Salomão MQ, Hofling-Lima AL, Lopes BT, et al. Role of the corneal epithelium measurements in keratorefractive surgery. Curr Opin Ophthalmol 2017;28:326-336.