Keratoconus Workup

  • Author: Barry A Weissman, OD, PhD, FAAO; Chief Editor: Hampton Roy Sr, MD   more...
 
Updated: Nov 23, 2011
 

Laboratory Studies

No laboratory workup is necessary in keratoconus (KC).

Careful refraction, keratometry, videokeratography, and slit lamp biomicroscopy and rigid gas permeable (GP) contact lens application allows the clinician to observe evidence of keratoconus.

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Imaging Studies

Computer-assisted videokeratography and diagnostic use of rigid contact lenses (CLs) are sometimes required, especially when the typical biomicroscopy signs of Vogt striae and Fleischer ring are absent. Several quantitative indices are available using computer-assisted videokeratography information to screen for keratoconic corneal shape factors. The two most commonly known indices are those of Rabinowitz and Maeda/Klyce.

The Rabinowitz diagnostic criteria consists of 3 videokeratography derived indices, which, when abnormal in value, should alert the clinician to consider a diagnosis of keratoconus.[24] These indices are as follows:

  • K value quantifies the central steepening of the cornea that occurs in keratoconus. A value of 47.20 D or greater is suggestive of keratoconus.
  • I-S value quantifies the inferior versus superior corneal dioptric asymmetry that occurs in keratoconus. A value of 1.4 D or greater is suggestive of keratoconus.
  • KISA% incorporates the K and I-S values with a measure quantifying regular and irregular astigmatism into one index. This index is highly sensitive and specific in separating normal from keratoconic corneas. A value of greater than 100% is highly suggestive of frank keratoconus, and the range from 60-100% represents keratoconus suspects.

On the other hand, Maeda et al designed an alternative computer expert program, based on linear discriminant analysis of 8 indices drawn from the corneal map and a binary decision tree.[25] The program assigns the topographical map a quantitative percentage score of the severity of keratoconus called the KCI%. A value of greater than zero is believed to be suggestive of keratoconus.

Studies of corneal optics by wavefront analysis also suggest keratoconus induces higher order aberrations (root mean square [RMS]), especially coma.[26, 27, 28]

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Procedures

  • Keratometry
    • Images of the keratometry mires are commonly steep, highly astigmatic, irregular, and often appear egg-shaped (rather than circular or oval) in keratoconus.
    • Some patients with keratoconus do not exhibit these signs.
  • Videokeratography[29]
    • 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 (PMD) typically shows an inferior lobster claw like map due to against-the-rule astigmatism.
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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.
  • Folds and ruptures occur in the Descemet membrane. Some studies have reported endothelial cell loss in association with Descemet rupture.
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Contributor Information and Disclosures
Author

Barry A Weissman, OD, PhD, FAAO  Chief of Contact Lens Service, Professor, Department of Ophthalmology, Jules Stein Eye Institute, University of California, Los Angeles, David Geffen School of Medicine; Adjunct Professor of Optometry, Southern California College of Optometry

Barry A Weissman, OD, PhD, FAAO is a member of the following medical societies: American Academy of Optometry, American Optometric Association, California Optometric Society, and International Society for Contact Lens Research

Disclosure: VSP None Speaking and teaching; Alcon None Speaking and teaching; Vistakon/The Vision Care Institute Grant/research funds support of Fellowship program

Coauthor(s)

Karen K Yeung, OD, FAAO  Director of Optometry, Arthur Ashe Student Health and Wellness Center, University of California, Los Angeles, David Geffen School of Medicine

Karen K Yeung, OD, FAAO is a member of the following medical societies: American Academy of Optometry

Disclosure: Nothing to disclose.

Specialty Editor Board

Fernando H Murillo-Lopez, MD  Senior Surgeon, Unidad Privada de Oftalmologia CEMES

Fernando H Murillo-Lopez, MD is a member of the following medical societies: American Academy of Ophthalmology

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Christopher J Rapuano, MD  Professor, Department of Ophthalmology, Jefferson Medical College of Thomas Jefferson University; Director of the Cornea Service, Co-Director of Refractive Surgery Department, Wills Eye Institute

Christopher J Rapuano, MD is a member of the following medical societies: American Academy of Ophthalmology, American Society of Cataract and Refractive Surgery, Contact Lens Association of Ophthalmologists, Cornea Society, Eye Bank Association of America, International Society of Refractive Surgery, and Pan-American Association of Ophthalmology

Disclosure: Allergan Honoraria Speaking and teaching; Allergan Consulting fee Consulting; Alcon Honoraria Speaking and teaching; RPS Ownership interest Other; EyeGate Pharma Consulting fee Consulting; Bausch & Lomb Honoraria Speaking and teaching; Bausch & Lomb Consulting; Merck Honoraria Speaking and teaching

Lance L Brown, OD, MD  Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri

Disclosure: Nothing to disclose.

Chief Editor

Hampton Roy Sr, MD  Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences

Hampton Roy Sr, MD is a member of the following medical societies: American Academy of Ophthalmology, American College of Surgeons, and Pan-American Association of Ophthalmology

Disclosure: Nothing to disclose.

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An optic section of a keratoconic cornea shows corneal thinning. Vogt striae and some scarring can also be seen centrally; superiorly, a small (brown) section of the Fleischer ring is noted.
The fluorescein pattern of a rather flat-fitted rigid contact lens on an advanced keratoconic cornea.
 
 
 
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