- Author: Barry A Weissman, OD, PhD, FAAO; Chief Editor: Hampton Roy, Sr, MD more...
Keratoconus (KC) is a progressive, noninflammatory, bilateral (but usually asymmetric) 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. See the image below.
Signs and symptoms
Patients with keratoconus may report the following:
Monocular diplopia or ghost images
Multiple unsatisfactory attempts to obtain optimum spectacle correction
Keratoconus is differentiated into mild, moderate, and advanced cases.
Characteristics of mild keratoconus may include the following:
Absent or minimal external and corneal signs
Oblique astigmatism on refraction; moderate-to-high myopia
Irregular astigmatic keratometry values
Corneal inferior steepening, central corneal astigmatic steepening, or bilateral temporal steepening on computer-assisted videokeratography
Typical nipple pattern with the application of a diagnostic rigid contact lens
Characteristics of moderate keratoconus may include the following:
Presence of one or more corneal signs of keratoconus (eg, enhanced appearance of corneal nerves, Vogt striae, Fleischer ring, corneal scarring)
Superficial corneal scarring (fibular, nebular, or nodular)
Deep stromal scarring
Scarring at the level of the Descemet membrane resembling posterior polymorphous corneal dystrophy
Paraxial stromal thinning as seen on corneal optical coherence tomography (OCT) pachymetry with biomicroscope slit-lamp examination
Keratometry values of 45-52 diopters (D)
“Scissoring” or the oil drop sign
Characteristics of advanced keratoconus may include the following:
Keratometry values greater than 52 D
Enhancement of all corneal signs, symptoms, and visual loss/distortion
Vogt striae; Fleischer ring and/or scarring
Acute corneal hydrops
See Presentation for more detail.
No laboratory workup is necessary.
Diagnostic measures that may yield evidence of keratoconus include the following:
Rigid gas-permeable (GP) contact lens application
Rabinowitz diagnostic criteria for keratoconus include the following 3 videokeratography-derived indices and OCT pachymetry measurements (inferior-superior [I-S] value and minimum pachymetry) which, when abnormal, suggest the diagnosis:
K value - Measures central steepening of the cornea; a value of 47.20 D or greater suggests keratoconus
I-S value - Measures inferior-versus-superior corneal dioptric asymmetry; a value of 1.4 D or greater suggests keratoconus
KISA% - Incorporates K and I-S values with a measure quantifying regular and irregular astigmatism into a single index; a value in the range of 60-100% suggests keratoconus, and a value exceeding 100% strongly suggests frank keratoconus
Pachymetry/asymmetry (PA)/I-S index - The minimum pachymetry value divided by the I-S value ; identifies keratoconus suspects and forme fruste keratoconus when the PA/I-S index is less than 105
See Workup for more detail.
Nonsurgical treatment measures include the following:
Rigid contact lenses or scleral lenses (mainstay of therapy) - Patients with early keratoconus may successfully use spectacles or spherical/toric soft contact lenses; those with moderate-to-advanced keratoconus almost always require rigid contact lenses
Corneal collagen cross-linking
Although no direct pharmacologic management is available, the following agents may be useful aids:
Nonsteroidal anti-inflammatory drugs (NSAIDs)
Mast cell stabilizing topical medications
Surgical options include the following:
Superficial keratectomy with a blade
Excimer laser phototherapeutic keratectomy
Implantation of intrastromal corneal rings (better for moderate than for advanced disease)
Bowman layer transplantation
Deep anterior lamellar keratoplasty (DALK)
Penetrating keratoplasty (PKP)
Keratoconus is a progressive, noninflammatory, bilateral (but usually asymmetric) 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.
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]
Although usually believed to be noninflammatory, some data suggest an inflammatory component.[6, 7]
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.[8, 4, 9]
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, although this was not found in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study.
Particular risk factors include atopic history, especially ocular allergies, and perhaps either or both rigid contact lens wear, and vigorous eye rubbing.[8, 11, 9, 12, 13]
Most keratoconus cases appear spontaneously, although approximately 14% of cases present with evidence of genetic transmission.
The prevalence of keratoconus varies by ethnicity. Keratoconus is more common in blacks and Latinos than in whites, with odds ratios of 1.57 and 1.43, respectively. It is less common in Asians than in whites, with an odds ratio of 0.61.
Personal income/access to health care and education levels have no known correlation with keratoconus.
Anecdotal reports suggest an increase in prevalence in some parts of the world, Arabia, the Indian subcontinent, and New Zealand.
Because few elderly patients have been noted with keratoconus, many wonder if keratoconus is associated with a fatal disease. However, several studies provide compelling evidence that keratoconus does not result in increased mortality.
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 (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. 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.
DALK is gaining popularity as an option.[17, 18] Other surgical treatments include removal of superficial nodular scars, implantation of plastic ring segments in the cornea (intrastromal corneal rings [ICRS]), Bowman layer transplantation, and ultraviolet collagen cross-linking (UV-CXL), which may be combined with other procedures.[19, 20, 21, 22]
Early literature suggested that keratoconus was autoimmune in origin and therefore more likely to affect females. However, recent studies have suggested that keratoconus may be more common in males.
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 and stabilizes approximately 20 years after initial onset.
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