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.
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.
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.
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 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.
An equal incidence of KC occurs in males and females.
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.
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.
KC is divided into mild, moderate, and advanced.
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.
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
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.
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.