0521779407-18 CUNY1086/Karliner 0 521 77940 7 June 13, 2007 8:1
Refractive Disorders (Ametropias) 1273the lens power or the contour of the retina at its optical center
(fovea).
■Myopia
➣Prevalence – most common, 10–60% of the population; in USA
at 30%
➣Definition – Myopia is characterized by a corneal power that is of
excessive convex power (i.e., too steep), lens power with excessive
convex power and/or an axial length that is too long.
➣Myopic astigmatism – Myopia may be compounded by astigma-
tism, a condition known as myopic astigmatism.
➣Onset in childhood and progression during puberty and early
adult years. Pathologic myopia is uncommon, characterized by
progressive myopia with scleral and retinal thinning that can
cause permanent visual loss.
➣Optical correction – Myopia can be corrected using minus or
concave lenses, either in glasses or contact lenses.
➣> Surgical correction – See below.
■Hyperopia
➣Prevalence – in USA, at 10%
➣Definition – Hyperopia is characterized by inadequate corneal
power (i.e., too flat), a lens power that is of inadequate convex
power, and/or an axial length that is too short
➣Hyperopic astigmatism – Hyperopia compounded by astigma-
tism
➣Causes – most cases are naturally occurring, but can also be
postsurgical, as from extraction of the crystalline lens with-
out replacement (aphakia) or overcorrection following refractive
surgery for myopia
➣optical correction – Hyperopia can be corrected with plus or con-
vex lenses, either in glasses or contact lenses
➣Surgical correction – See belowAstigmatism
■Prevalence – Most cases of myopia and hyperopia are compounded
by the concurrent presence of astigmatism.
■Definition – Astigmatism is a nonspherical refractive error. Com-
monly, astigmatism is corneal in origin; the cornea is toric (i.e.,
football-shaped) rather than spherical (i.e., basketball-shaped). In
the case of irregular astigmatism, the curvature is irregularly dis-
tributed.