Poor vision that cannot be corrected fully with glasses may indicate a condition known as conical cornea or keratoconus. A rare condition, keratoconus primarily affects people in their early 20's.
With keratoconus, the cornea, the "clear window" at the front of the eye, may become thin and bow outwards. It is this irregular distortion of the cornea that makes vision correction with glasses less than optimal. For this reason other means of correcting vision are often necessary.
Vision correction with rigid gas permeable lenses.
Mild to moderate keratoconus is best corrected with rigid gas permeable contact lenses, which provide a smooth surface in front of the cornea, making clear vision possible. Because the lens is rigid, the tears between the lens and the cornea form a 'liquid lens,' which smoothes the irregularities of the cornea and makes clear vision possible again. Soft lenses, which 'wrap' onto the cornea and take up its shape much more closely than rigid lenses, are less successful at correcting keratoconus.
Corneal replacement surgery may be necessary.
As keratoconus progresses, some scarring of the cornea can occur. Eventually, contact lenses may no longer be a successful treatment. Instead, the cornea may need to be replaced surgically with a cornea of more regular shape. The prognosis for corneal replacement surgery is generally very good.