Keratoconus is a slowly progressive thinning and bulging of the cornea, usually bilateral, beginning between ages 10 and 25. Its cause is unknown.
Risk factors include the following:
Family history of keratoconus
An atopic disorder
Vigorous eye rubbing
Lax eyelids
Certain connective tissue disorders (eg, Ehlers-Danlos syndrome Ehlers-Danlos Syndrome Ehlers-Danlos syndrome is a hereditary collagen disorder characterized by articular hypermobility, dermal hyperelasticity, and widespread tissue fragility. Diagnosis is clinical. Treatment is... read more
, Marfan syndrome Marfan Syndrome Marfan syndrome consists of connective tissue anomalies resulting in ocular, skeletal, and cardiovascular abnormalities (eg, dilation of ascending aorta, which can lead to aortic dissection)... read more 
, osteogenesis imperfecta Osteogenesis Imperfecta Osteogenesis imperfecta is a hereditary collagen disorder causing diffuse abnormal fragility of bone and is sometimes accompanied by sensorineural hearing loss, blue sclerae, dentinogenesis... read more 
)Congenital disorders with poor vision (eg, Leber congenital optic neuropathy Leber hereditary optic neuropathy Hereditary optic neuropathies result from genetic defects that cause vision loss and occasionally cardiac or neurologic abnormalities. There is no effective treatment. Hereditary optic neuropathies... read more , retinopathy of prematurity Retinopathy of Prematurity Retinopathy of prematurity is a bilateral disorder of abnormal retinal vascularization in premature infants, especially those of lowest birth weight. Outcomes range from normal vision to blindness... read more , aniridia)
The distorted cone shape of the cornea causes major changes in the refractive characteristics of the cornea (irregular astigmatism) that cannot be fully corrected with glasses. Progressing keratoconus necessitates frequent change of eyeglasses. Contact lenses may provide better vision correction and should be tried when eyeglasses are not satisfactory. A stepwise approach is applied to choosing contact lenses, typically starting with a toric soft contact lens, then rigid gas-permeable lenses, followed by hybrid rigid gas-permeable lenses with a soft outer ring, and finally by scleral contact lenses. Patients have good visual results with the initial steps of contact lens fitting if their cornea has a mildly distorted curvature. If the patient's cornea has a moderately severe cone-shaped curvature, trials may skip the initial types of contact lenses and start with scleral contact lenses. The further up the contact lens treatment ladder, the more expensive and uncomfortable the contact lenses may be. Corneal transplant surgery Corneal Transplantation Corneal transplantations are done for several reasons: To reconstruct the cornea (eg, replacing a perforated cornea) To relieve intractable pain (eg, severe foreign body sensation due to recurrent... read more 
may be necessary if visual acuity with contact lenses is inadequate, contact lenses are not tolerated, or a visually significant corneal scar (caused by tearing of stromal fibers) is present.
Newer treatments improve visual results by increasing tolerance of contact lenses and thus saving selected patients from transplantation. These include implantation of corneal ring segments, which push up the shoulders of the cone and thereby reduce the relative magnitude of the cone, and corneal collagen cross-linking, an ultraviolet light treatment that stiffens the cornea and thereby prevents further corneal thinning and bulging.
