Keratoconus Workup

Updated: Oct 17, 2018
  • Author: Karen K Yeung, OD, FAAO; Chief Editor: Hampton Roy, Sr, MD  more...
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Workup

Laboratory Studies

No laboratory workup is necessary in keratoconus.

Careful refraction, slit-lamp biomicroscopy, corneal topography, and corneal tomography allow the clinician to observe evidence and progression of keratoconus.

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Imaging Studies

Corneal topography and corneal tomography are the standard diagnostic imaging tools used for the detection of keratoconus. These are especially useful when the typical biomicroscopy signs of Vogt striae and Fleischer ring are absent.

Corneal topography  and tomography  [25]

This commonly shows inferior corneal steepening in keratoconus, although a small percentage of patients with keratoconus show central astigmatic changes.

An even smaller number show superior peripheral steepening. Pellucid marginal degeneration typically shows an inferior lobster claw like map due to against-the-rule astigmatism.

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Procedures

Pachymetry shows a thinner cornea paraxially.

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Histologic Findings

All layers of the cornea are affected by keratoconus.

Superficial epithelial cells located at the nodule are elongated and arranged in a whorl-like fashion.

Iron deposition in the basal corneal epithelial cells form the characteristic Fleischer ring.

Localized breaks are present in the basement membrane.

A decrease in the number of stromal collagen lamellae is noted, as well as a loss of the fibular arrangement within the lamellae. [26]

Folds and ruptures occur in the Descemet membrane. Some studies have reported endothelial cell loss in association with Descemet rupture.

Cornea thinning is caused by the splitting of the lamellae into multiple bundles of collagen fibrils. [26]

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Grading

Many grading are systems available to quantify the severity of keratoconus. Most systems consider the amount of corneal irregularity through higher-order aberrations and corneal thickness. Various nomograms have been proposed to quantify keratoconus, although no standard protocol is used for treatment.

Findings are as follows:

  • Normal: No clinical signs of keratoconus, no scissoring on retinoscopy, and symmetrical astigmatism
  • Keratoconus suspect: The fellow eye of an individual with keratoconus, with mild inferior steepening on topography and no clinical signs of keratoconus; the average K reading is less than 47 D
  • Forme fruste keratoconus: The fellow eye of an individual with keratoconus, with steeping seen on corneal topography and some corneal thinning seen on corneal tomography
  • Early keratoconus: No findings of keratoconus on slit-lamp examination; scissoring on retinoscopy; average K reading of less than 47 D
  • Keratoconus: Stromal corneal thinning on slit-lamp evaluation accompanied by one or more of the following clinical signs: Vogt striae, iron ring, Munson sign, scissoring on retinoscopy
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