Keratoconus

Keratoconus is a slowly progressive thinning and bulging of the cornea, usually bilateral, beginning between ages 10 and 25. Its cause is unknown (1).

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, Marfan syndrome, osteogenesis imperfecta)

• Down syndrome

• Congenital disorders with poor vision (eg, Leber congenital optic neuropathy, retinopathy of prematurity, aniridia)

• Obstructive sleep apnea

The distorted cone shape (1) 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 a 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 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 to 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.