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Ophthalmology > CORNEA
Keratoconus
Article Last Updated: Jan 8, 2007
AUTHOR AND EDITOR INFORMATION
Section 1 of 11  
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 is a member of the following medical societies: American Academy of Optometry and Phi Beta Kappa
Coauthor(s):
Karen K Yeung, OD, FAAO, Director of Optometry, Arthur Ashe Student Health and Wellness Center, University of California at Los Angeles
Editors: Fernando H Murillo-Lopez, MD, Senior Surgeon, Unidad Privada de Oftalmologia CEMES; 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; 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; Lance L Brown, OD, MD, Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri; Hampton Roy Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Author and Editor Disclosure
Synonyms and related 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
Background
Keratoconus (KC) is a progressive, noninflammatory, bilateral (but usually asymmetrical) disease of the cornea, characterized by paraxial stromal thinning that leads to corneal surface distortion. Visual loss occurs primarily from irregular astigmatism and myopia and secondarily from corneal scarring.
Pathophysiology
All layers of the cornea are believed to be affected by KC, although the most notable features are the thinning of the corneal stroma, the ruptures in the Bowman layer, and the deposition of iron in the basal epithelial cells, forming the Fleischer ring. Breaks in and folds close to the Descemet membrane result in acute hydrops and striae, respectively.
Frequency
United States
Reported prevalence in the general population varies (50-200 per 100,000), perhaps with differences in diagnostic criteria. It is commonly an isolated ocular condition but sometimes coexists with other ocular and systemic diseases.
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, rigid contact lens (CL) wear, and vigorous eye rubbing.
Most KC cases appear spontaneously, although approximately 14% of them present with evidence of genetic transmission.
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 KC, many wonder if KC is associated with some fatal disease. However, several recent studies provide compelling evidence that KC does not result in increased mortality.
- Advanced KC rarely may progress to corneal hydrops, the so-called acute keratoconus, wherein breaks occur in the Descemet layer, which lead 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 CL application may improve following such events. If secondary scarring is severe, corneal transplantation (penetrating keratoplasty [PKP]) may be warranted.
- PKP is otherwise indicated when CLs and intrastromal plastic rings (Intacs) are either no longer tolerated or no longer useful in vision correction. When good CL care is available, only 10-20% of patients with KC eventually require PKP. The need for PKP increases when optimal CL care is not available, but many patients still require CL care for optimum visions following PKPs.
Sex
An equal incidence of KC occurs in males and females.
Age
- KC typically presents at puberty and progresses until the third and fourth decades of life, although it can occur or progress at any age.
- KC progresses at various rates but tends to progress more rapidly in young patients.
History
- Patients often report decreasing vision (distortions, glare/flare, and monocular diplopia or ghost images) with multiple unsatisfactory attempts in obtaining optimum spectacle correction.
- Soft contact lenses and spectacles may initially give satisfactory vision, but vision tends to decline over time and requires rigid gas permeable (RGP) contact lenses for correction.
Physical
KC is divided into mild, moderate, and advanced.
- Mild KC
- External/corneal signs - Often none or minimal
- Suspicious signs - History of multiple inadequate spectacle corrections of one or both eyes, which may include oblique astigmatism on refraction as well as moderate-to-high myopia.
- Irregularly astigmatic keratometry values (egg-shaped), not necessarily on the steep side of normal (approximately 45 diopters [D]), are consistent with diagnosis.
- Diagnosis can be confirmed with corneal topography, which may reveal corneal inferior steepening (approximately 80% of KC cases), central corneal astigmatic steepening (approximately 15% of KC cases), or even bilateral temporal steepening (extremely rare).
- Diagnosis also may be aided by applying a diagnostic rigid CL with its base curve equal to the flat keratometry value. One observes a typical nipple pattern by use of sodium fluorescein dye in the underlying tear film.
- Moderate KC
- Often shows one or more corneal signs of KC
- Enhanced appearance of the corneal nerves
- Approximately 40% of eyes with moderate KC develop Vogt striae (fine stress lines) in the deep stroma.
- Approximately 50% of eyes with moderate KC develop the deposition of iron in the basal epithelial cells in a (often partial) ring shape at the base of the conical protrusion called the Fleischer ring.
- Approximately 20% of eyes with moderate KC develop corneal scarring.
- Superficial corneal scarring can be fibular, nebular, or nodular.
- Deep stromal scarring may occur, perhaps representing resolved mini-hydrops events.
- Some patients show scarring at the level of the Descemet membrane, consistent in appearance with posterior polymorphous corneal dystrophy.
- Paraxial (usually inferior to the pupil) stromal thinning may be appreciated.
- Keratometry values typically increase to 45-52 D.
- Distortion of the retinoscopy and direct ophthalmoscope red pupillary reflex may allow observation of "scissoring" or an inferior distortion termed the oil drop sign.
- Munson sign - Upon downgaze, observation of a "V" shape to the cornea's profile against the lower lid margin, an accentuation of the conical shape of the modest to advanced KC cornea
- Advanced KC
- Often results in keratometry values greater than 52 D and enhancement of all corneal signs, symptoms, and visual loss/distortion.
- Vogt striae are seen in approximately 60% of eyes, and both Fleischer ring and/or scarring are seen in approximately 70% of eyes.
- Acute corneal hydrops can occur.
Causes
- Although not definitively identified, genetic inheritance; systemic and ocular associations; eye rubbing; atopy, specifically ocular allergies; and CL wear are proposed risk factors.
- Several reports suggest, perhaps coincidentally, associations with KC and other corneal dystrophies.
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.
Lab 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.
Medical Care
- Rigid CLs are the mainstay of treatment.
- Patients with early KC successfully may use spectacles or spherical/toric soft contact lenses. They may even rarely find that spectacle vision is superior to rigid CLs.
- Patients with modest-to-advanced KC almost always require rigid CLs. When rigid CLs are no longer tolerated, some patients can maintain CL wear and usable visions with hydrogel CLs, piggyback CLs, or scleral (haptic) CLs but usually at a physiological or visual cost.
- CL wear often is complicated by episodes of intolerance, allergic reactions (eg, giant papillary conjunctivitis), corneal abrasions, neovascularization, and other problems, sometimes leading to total intolerance.
Surgical Care
- Surgically removing central nodular scars by shaving the corneal surface (superficial keratectomy with a blade or excimer laser phototherapeutic keratectomy) may improve CL tolerance, decrease the rate of associated corneal abrasions, and preclude the need for corneal transplant.
- Although still controversial, intrastromal corneal rings (Intacs) have been implanted in the eyes of patients who have become intolerant to contact lenses. More traditionally, these patients, including those whose vision is not correctable to better than 20/40, are referred for corneal transplants (or PKPs).
- PKPs are very successful in the treatment of KC, resulting in clear visual axes in greater than 90% of all cases.
- Contact lenses are often still required postgraft for optimum vision.
- PKPs require continuing professional care to watch for rejection, suture-related problems, wound dehiscence, and other difficulties.
- Although extremely rare, KC can recur in a graft.
- A surgical treatment involving riboflavin and UV light has been proposed. With this treatment, the corneal epithelium is first removed, and the corneal stroma is subjected to riboflavin and exposed to UV light. The cornea is then allowed to reepithelialize. This treatment is an effort to induce increased collagen cross-linking to enhance corneal rigidity and to decrease keratoconic corneal steepening. While some act as advocates for this treatment, others have significant concerns that it may be harmful rather than beneficial. Additional human and animal studies are in progress.
Consultations
- Consult with a cornea specialist (a graduate of a cornea fellowship program) and/or CL specialist who provide rigid CL care.
- An ophthalmologist who is a cornea specialist assists in identifying appropriate clinical conditions and timing for surgical intervention, such as superficial keratectomy, PKP, or implantation of plastic corneal ring segments.
- A specialty contact lens practitioner monitors CL care to optimize vision while minimizing complications of CL wear. The practitioner also helps to establish the appropriate clinical conditions and timing of surgical intervention, should this become necessary.
Activity
Patients should avoid (vigorous) eye rubbing.
No direct pharmacological management of KC is available, although nonsteroidal anti-inflammatory (NSAID), antihistamine, or mast cell stabilizing topical medications are occasionally helpful in controlling the often concomitant signs of ocular allergies, especially pruritus, that can lead to eye rubbing.
Episodes of hydrops may require treatment with hyperosmotics to reduce corneal swelling or topical steroid drops to reduce inflammation. Topical antibiotics are used for suspected infection.
Drug Category: Mast cell stabilizers
Used to manage signs and symptoms of long-term ocular allergies, which can lead to patient discomfort and increased vigorous eye rubbing.
| Drug Name | Alamast (Pemirolast) or alocril (Nedocromil) |
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| Description | Mast cell stabilizers are effective to treat long-term ocular allergies, specifically giant papillary conjunctivitis, which can accompany contact lens wear (especially in keratoconus). This is not effective for acute allergies. |
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| Adult Dose | 1 gtt in each eye qid for 1 wk, then bid prn |
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| Pediatric Dose | <3 years: Not established
>3 years: Administer as in adults |
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| Contraindications | Documented hypersensitivity |
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| Interactions | None reported |
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| Pregnancy | B - Usually safe but benefits must outweigh the risks.
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| Precautions | Drops should be used prior to and following contact lens wear; most frequently reported adverse reactions include ocular stinging upon instillation |
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| Drug Name | Olopatadine hydrochloride (Patanol), epinastine (Elestat), azelastine (Optivar) |
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| Description | Inhibitor of histamine release from mast cell and devoid of effects on serotonin, alpha-adrenergic, muscarinic, and dopamine receptors. |
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| Adult Dose | 1 gtt in affected eye(s) bid |
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| Pediatric Dose | <3 years: Not established
>3 years: Administer as in adults |
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| Contraindications | Documented hypersensitivity |
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| Interactions | None reported |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
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| Precautions | Do not use while wearing contact lenses; not for injection |
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Drug Category: Corticosteroids
These agents have anti-inflammatory properties and cause profound and varied metabolic effects. They modify the body's immune response to diverse stimuli.
| Drug Name | Loteprednol (Lotemax) |
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| Description | Site-specific steroid for acute severe allergic reactions. |
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| Adult Dose | 1-2 gtt in affected eye(s) qid |
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| Pediatric Dose | Not established |
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| Contraindications | Documented hypersensitivity; contact lens wear; glaucoma; ocular infections (viral, fungal, bacterial); corneal epithelial defects |
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| Interactions | Ophthalmic NSAIDs combined with corticosteroids may delay or slow ocular wound healing; adverse reactions can include dry eyes, watery eyes, foreign body sensation, itchy eyes, photophobia, headache, rhinitis, and pharyngitis |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
|
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| Precautions | Caution in hypertension; known to cause cataract formation with long-term use; suspect fungal invasion in any persistent corneal ulceration where a corticosteroid has been used or is in use (obtain fungal cultures when appropriate) |
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Drug Category: Antihistamines
Reduce symptoms of itching that can lead to eye rubbing, thereby decreasing eye rubbing both in duration and intensity.
| Drug Name | Emastadine (Emadine) |
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| Description | Relatively selective H1-receptor antagonist. |
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| Adult Dose | 1 gtt in affected eye(s) qid |
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| Pediatric Dose | <3 years: Not established
>3 years: Administer as in adults |
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| Contraindications | Documented hypersensitivity |
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| Interactions | None reported |
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| Pregnancy | C - Safety for use during pregnancy has not been established.
|
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| Precautions | Adverse reactions include headaches. |
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Drug Category: Hyperosmolar diuretics
May reduce inflammation in cornea by creating an osmotic gradient across an intact blood barrier.
| Drug Name | Sodium chloride 2% and 5% (Muro 128, Adsorbonac, Afrin Saline Mist) |
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| Description | Used for temporary relief of corneal edema. |
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| Adult Dose | Solution: 1-2 gtt in affected eye(s) q3-4h or as directed
Ointment: Apply 0.25-inch ribbon of ointment inside lower lid q3-4h or as directed |
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| Pediatric Dose | Not established |
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| Contraindications | Documented hypersensitivity |
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| Interactions | None reported |
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| Pregnancy | A - Safe in pregnancy
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| Precautions | May cause temporary burning and irritation upon use; if pain, change in vision, continued redness or irritation of the eye(s) occurs, or if initial condition/problem worsens or persists, reevaluate therapy; do not use product if it changes color or becomes cloudy |
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Further Outpatient Care
- Frequently observe patients with KC, especially since most wear CLs.
- Even in the absence of signs or symptoms of complications, CL care should be provided at 4- to 6-month intervals to promptly address changes in fit or optics.
- Patients with KC who use CLs often have a corneal abrasion, particularly if corneal steepening has made the current rigid CL fit flat. Addressing such problems may preclude increased scarring, which could necessitate PKP.
In/Out Patient Meds
Deterrence/Prevention
- Patients should avoid eye rubbing.
Complications
- Advanced KC rarely progresses to acute corneal hydrops (acute keratoconus), wherein breaks occur in the Descemet layer that lead to central stromal edema and secondary severe corneal scarring.
- Patients report a 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 CL application rarely may improve following such events.
- If secondary scarring is severe, corneal transplantation (PKP) may be warranted.
- Patients with KC develop all complications of CL wear, especially abrasion and giant papillary conjunctivitis.
Prognosis
- Most patients with KC do well with rigid CL care.
- About 10-20% of patients eventually require corneal transplantation, but this number is believed to increase if good CL care is unavailable.
- Recent data suggest that this disease, although progressive, stabilizes after some time in most patients.
Patient Education
- Patients with KC and their family members probably should receive genetic counseling.
Medical/Legal Pitfalls
- Identification of keratoconus particularly with topography gives earlier options of contact lens or surgery.
- Patients should approach any cosmetic refractive surgery with caution if KC is suspected or has been diagnosed.
| Media file 1:
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. |
 |  View Full Size Image | |
Media type: Photo
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| Media file 2:
The fluorescein pattern of a rather flat-fitted rigid contact lens on an advanced keratoconic cornea. |
 | View Full Size Image | |
Media type: Photo
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Keratoconus excerpt Article Last Updated: Jan 8, 2007
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