Keratoconus 

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

Background

Keratoconus (KC) is a progressive, noninflammatory, bilateral (but usually asymmetrical) ectatic corneal disease, characterized by paraxial stromal thinning and weakening that leads to corneal surface distortion. Visual loss occurs primarily from irregular astigmatism and myopia, and secondarily from corneal scarring. Protrusion usually but not exclusively affects the axial and inferonasal cornea. Examples of keratoconus are shown in the images below.

An optic section of a keratoconic cornea shows corAn 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 riThe fluorescein pattern of a rather flat-fitted rigid contact lens on an advanced keratoconic cornea.
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Pathophysiology

All layers of the cornea are believed to be affected by keratoconus, although characteristic structural changes include epithelial basement membrane fragmentation and scarring and breaks in the anterior limiting lamina (ie, Bowman membrane), with axial stromal thinning and scarring. Deposition of iron in the basal epithelial cells forms the Fleischer ring. Breaks in and folds close to the Descemet membrane result in acute hydrops rarely and striae commonly, respectively.

The literature on keratoconus is large and contradictory regarding the roles of disruption of collagen fibers, lamellae, and proteoglycans in the disease; however, keratoconic corneas have been shown to have altered antioxidant enzymes, accumulations of cytotoxic reactive oxygen/nitrogen species, activated caspase pathways, and mitochondrial DNA damage. Abnormal oxidative stress-related properties have been found in keratoconic corneal cells. Oxidative stress elements can induce activation of degradative enzymes and degradation of tissue inhibitors of metal-low proteinases. Genomic deletion in the superoxide dismutase 1 (SOD1) gene has also been associated with the disease.[1, 2, 3, 4, 5, 6]

Although usually believed to be noninflammatory, some data suggest an inflammatory component.[7, 8]

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Epidemiology

Frequency

United States

Reported prevalence in the general population varies (50-200 cases per 100,000 population), perhaps with differences in diagnostic criteria. It is commonly an isolated ocular condition but sometimes coexists with other ocular and systemic diseases.[9, 5, 10]

Commonly recognized ocular associations include vernal keratoconjunctivitis, retinitis pigmentosa, and Leber congenital amaurosis; systemic putative associations include many of the connective tissue disorders (eg, Ehlers-Danlos and Marfan syndromes), mitral valve prolapse, atopic dermatitis, and Down syndrome.

Particular risk factors include atopic history, especially ocular allergies, and perhaps either or both rigid contact lens (CL) wear, and vigorous eye rubbing.[9, 11, 10, 12, 13]

Most keratoconus cases appear spontaneously, although approximately 14% of cases present with evidence of genetic transmission.[12]

International

Anecdotal reports suggest an increase in prevalence in some parts of the world, Arabia, the Indian subcontinent, and New Zealand.

Mortality/Morbidity

Because few elderly patients have been noted with keratoconus, many wonder if keratoconus is associated with a fatal disease. However, several recent studies provide compelling evidence that keratoconus does not result in increased mortality.[14]

Advanced keratoconus may rarely progress to corneal hydrops, so-called "acute keratoconus," wherein breaks occur in the Descemet layer that cause aqueous to enter the stroma, leading to central stromal edema and potentially secondary severe corneal scarring. Patients report sudden loss of vision and some ocular discomfort in one eye but usually not much pain or conjunctival injection. Acute treatment of hydrops is palliative; many corneas flatten secondary to hydrops, and both visual acuity and contact lens application may improve following such events. If secondary scarring is severe, corneal transplantation (penetrating keratoplasty [PKP]) may be warranted.

Corneal surgery is otherwise indicated when contact lenses are either no longer tolerated or no longer useful in vision correction. When good contact lens care is available, only 10-20% of patients with keratoconus eventually require PKP.[15] The need for PKP increases when optimal contact lens care is not available, but many patients still require contact lens care for optimum visions following PKP.

Laser-assisted and other penetrating lamellar keratoplasty procedures (eg, FACT, DALK) of various types are gaining popularity as an option.[16, 17] Other surgical treatments include removal of superficial nodular scars, implantation of plastic ring segments in the cornea (Intacts TM), and collagen crosslinking (with riboflavin and ultraviolet light), perhaps combined with other procedures.[18, 19, 20, 21]

Sex

An equal incidence of keratoconus occurs in males and females.

Age

Keratoconus typically presents at puberty and progresses until the third and fourth decades of life, although it can occur or progress at any age. Keratoconus progresses at various rates but tends to progress more rapidly in young patients.

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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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