eMedicine Specialties > Ophthalmology > Cornea

Keratoconus: Differential Diagnoses & Workup

Author: Barry A Weissman, OD, PhD, FAAO, Chief of Contact Lens Service, Professor, Department of Ophthalmology, Jules Stein Eye Institute, University of California at Los Angeles
Coauthor(s): Karen K Yeung, OD, FAAO, Director of Optometry, Arthur Ashe Student Health and Wellness Center, University of California at Los Angeles
Contributor Information and Disclosures

Updated: Jan 8, 2007

Differential Diagnoses

Contact Lens Complications
Diplopia
Keratitis, Interstitial
Pellucid Marginal Degeneration

Other Problems to Be Considered

Consider other causes of irregular oblique astigmatism, imprecise variable subjective refractions and noninflammatory central corneal scarring, such as corneal surgery, healed infections, and trauma.

Stromal striations similar to Vogt striae occur when corneas swell (rather than thin) from either hypoxia (associated with wear of contact lenses) or with early Fuchs corneal dystrophy.

Iron depositions in the basal epithelium (similar to a Fleischer ring) occur in any area of relative depression in the corneal surface in association with pterygium, surgical scars, and central depressions from refractive surgery.

Pellucid marginal degeneration (PMD) is often considered a variant of KC, in which corneal thinning occurs about a millimeter above the inferior limbus, resulting in advanced against-the-rule corneal astigmatism that may be observed by keratometry or videokeratography.

Terrien marginal corneal degeneration is a mildly inflammatory disease, usually of the superior limbus, which also can induce irregular against-the-rule astigmatism and corneal thinning (often with vascularization and lipid deposits).

Keratoglobus (KG) is an extremely rare corneal disease in which the entire cornea, from limbus to limbus, thins, sometimes to the point where spontaneous perforation becomes possible. KG probably is unrelated to KC.

Posterior keratoconus is another extremely rare disease in which the posterior corneal surface suffers a loss of substance. This condition is probably unrelated to KC.

Workup

Laboratory Studies

  • No laboratory workup is necessary. Careful refraction, keratometry, corneal topography, and slit lamp biomicroscopy allow the clinician to observe evidence of KC.

Imaging Studies

  • Corneal topography and diagnostic use of rigid CLs are sometimes required, especially when the typical biomicroscopy signs of Vogt striae and Fleischer ring are absent. Several quantitative indices are available using corneal topography information to screen for keratoconic corneal shape factors. The two most commonly known indices are those of Rabinowitz and Maeda and Klyce.
  • The Rabinowitz diagnostic criteria consist of 3 corneal topography derived indices, which, when abnormal in value, should alert the clinician to consider a diagnosis of keratoconus. 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 and Klyce designed an alternative computer expert program, based on linear discriminant analysis of 8 indices drawn from the corneal map and a binary decision tree. 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.

Procedures

  • Keratometry
    • Images of the keratometry mires commonly will be steep, highly astigmatic, irregular, and often appear egg-shaped rather than circular or oval in keratoconus.
    • Some patients with keratoconus will not show these signs.
  • Videokeratography commonly shows inferior corneal steepening in KC, although a small percentage of patients with keratoconus show central astigmatic changes. An even smaller number shows superior peripheral steepening. PMD typically shows an inferior lobster claw–like map because of the against-the-rule astigmatism.

Histologic Findings

All layers of the cornea are affected by KC. 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 present, 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 the rupture of the Descemet membrane.

More on Keratoconus

Overview: Keratoconus
Differential Diagnoses & Workup: Keratoconus
Treatment & Medication: Keratoconus
Follow-up: Keratoconus
Multimedia: Keratoconus
References
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References

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Further Reading

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Keywords

KC, corneal disease, stromal thinning, corneal scarring, irregular astigmatism, myopia, vision loss, Fleischer ring, Descemet membrane, intracorneal plastic rings, Intacs, corneal transplantation, penetrating keratoplasty, PKP, contact lens, contact lenses, CL, CL wear, contact lens wear, eye allergies, ocular allergies, eye rubbing

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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 at Los Angeles
Barry A Weissman, OD, PhD, FAAO is a member of the following medical societies: American Academy of Optometry and Phi Beta Kappa
Disclosure: Nothing to disclose.

Coauthor(s)

Karen K Yeung, OD, FAAO, Director of Optometry, Arthur Ashe Student Health and Wellness Center, University of California at Los Angeles
Karen K Yeung, OD, FAAO is a member of the following medical societies: American Academy of Optometry
Disclosure: Nothing to disclose.

Medical Editor

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.

Pharmacy Editor

Simon K Law, MD, PharmD, Assistant Professor of Ophthalmology, Jules Stein Eye Institute; Chief of Section of Ophthalmology Surgical Services, Department of Veterans Affairs Healthcare Center, West Los Angeles
Simon K Law, MD, PharmD is a member of the following medical societies: American Academy of Ophthalmology, American Glaucoma Society, and Association for Research in Vision and Ophthalmology
Disclosure: Nothing to disclose.

Managing Editor

Christopher J Rapuano, MD, Professor, Department of Ophthalmology, Jefferson Medical College; Co-Chairman of the Cornea Service, Co-Chairman of Refractive Surgery Department, Wills Eye Hospital
Christopher J Rapuano, MD is a member of the following medical societies: American Academy of Ophthalmology, American Society of Cataract and Refractive Surgery, Eye Bank Association of America, Pennsylvania Medical Society, and Philadelphia County Medical Society
Disclosure: Allergan Honoraria Speaking and teaching; Allergan Consulting fee Consulting; Alcon Honoraria Speaking and teaching; Inspire Honoraria Speaking and teaching; RPS Ownership interest Other

CME Editor

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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