Early stage. Computerized corneal topography (CCT) techniques using curvature-based analysis and newer forms of elevation-based tomography appear to be the most sensitive methods for detecting early keratoconus.
Prior to these, the most sensitive methods included 1) apical decentration on the keratometer using the Soper topogometer, 2) subtle changes in the retinoscopic reflex that progress to early scissoring and 3) changes in the ability to visualize the endothelial specular reflection relative to the epithelial reflection during slit-lamp biomicroscopy.
Elements of earlier techniques- keratometry and Placido disk–based keratoscopy-have now been integrated into sophisticated, computerized systems capable of generating color-coded curvature and elevation maps. Even with these technological advances, it is increasingly recognized that elevation data using scanning slit or Scheimpflug imaging provides data that are diagnostically very helpful when screening patients with possible corneal ectasia.
A number of diagnostic algorithms can help diagnose early keratoconus; however, there remains no universally agreed-upon diagnostic criterion that, by itself, conclusively diagnoses the earliest cases. Typically, changes in elevation relative to a best-fit sphere occur on the posterior corneal surface more rapidly than similar changes are seen on the anterior surface. Other findings include apical decentration, corneal thinning, increasing irregular corneal astigmatism, and Rizzutti’s sign, which is a sharply focused conical reflection obtained on the nasal cornea after a penlight is shone on the temporal side.
Intermediate stage. Even in patients with intermediate-stage disease, corneal changes may still be subtle and not readily apparent by slit-lamp biomicroscopy alone. The retinoscopic streak usually demonstrates scissoring, and it is hard to neutralize the streak reflection during refraction. Computerized corneal topography and elevation-based tomography-Orbscan IIz (Bausch & Lomb) and Pentacam (Oculus) are probably the most widely used-typically demonstrate findings that are usually more obvious with regard to both diagnosis and monitoring disease progression.
Advanced stage. In more advanced cases, keratoconus is readily diagnosed by characteristic slit-lamp findings:
- Stromal thinning-usually inferiorly.
- Apical decentration and conical protrusion.
- A Fleischer ring-an iron line within the deep epithelium surrounding the cone at the base.
- Vogt lines-fine vertical lines in the deep stroma and Descemet’s membrane that disappear transiently with gentle digital pressure.
In addition to these corneal signs, external ocular manifestations include:
Munson’s sign. The lower lid protrudes when a patient with advanced corneal disease looks downward.
Corneal hydrops. Occasionally, patients can progress to acute corneal hydrops, a stromal edema caused by aqueous penetration through breaks in Descemet’s membrane.
The onset of corneal hydrops is classically associated with a sudden decrease in both uncorrected and corrected vision as well as redness, pain and photophobia. The corneal edema may persist unpredictably for weeks or months, with gradual resolution as the posterior break is replaced with a posterior collagenous scar. Topical osmotic agents such as hypertonic saline may accelerate the recovery of vision; however, this treatment is most helpful to alleviate edema within the epithelium and anterior stroma.