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Nervous System
soft contact lens, particularly overnight or without proper
disinfection.
14
It also may occur in non–contact lens wear-
ers after exposure to contaminated water or soil.
Acanthamoeba keratitis is characterized by severe
pain, redness of the eye, and photophobia. Diagnosis is
confirmed by scrapings and culture with specially pre-
pared medium. In the early stages of infection, epithe-
lial debridement may be beneficial. Treatment includes
intensive use of topical antimicrobials. However, the
organism may encyst within the corneal stroma, mak-
ing treatment more difficult. Keratoplasty may be neces-
sary in advanced disease to arrest the progression of the
infection.
Disorders of the Lens
The lens is a remarkable structure, which like that in
a camera, functions to bring images into focus on the
retina. The lens is an avascular, transparent, biconvex
structure, the posterior side of which is more convex
than the anterior side.
2,15,16
It is positioned just posterior
to the iris and is held in place by suspensory ligaments
known as the
zonules
, which are composed of numerous
fibrils that arise from the ciliary body (see Fig. 38-3B).
The pull of the zonular fibers and lens capsule is nor-
mally under tension, causing the lens to have a flat-
tened shape for distant vision. Relaxation of the fibers
allows the lens to assume a more spherical shape for
near vision. There are no pain fibers, blood vessels, or
nerves in the lens.
The lens has three principal components: the lens cap-
sule, a subcapsular epithelium, and lens fibers. The lens
capsule is a transparent elastic structure that envelops
the entire lens. The subcapsular epithelium is a cuboidal
layer of cells that is present on the anterior surface of the
lens. The lens fibers, which constitute the bulk of the lens,
continue to produce new fibers throughout life, with the
older fibers being compressed into the central nucleus of
the lens. This causes the lens to gradually become larger
and less elastic with age.
Refraction refers to deflection or bending of light as it
passes from one transparent medium to another of dif-
ferent density. When parallel light rays pass through the
center of a lens, their direction is not changed; however,
the divergent rays passing peripherally through a lens are
bent (Fig. 38-5A). The refractive power of a lens is usually
described as the distance (in meters) from its surface to
the point at which the rays come into focus on the retina
(i.e., focal length). Usually, this is reported as the recipro-
cal of this distance (i.e., diopters).
16
For example, a lens
that brings an object into focus at 0.5 m has a refractive
power of 2 diopters (1.0/0.5 = 2.0). The closer the object,
the more divergent the peripheral rays, and the stronger
and more precise the focusing system must be.
In the eye, the major refraction of light begins at the
convex corneal surface. Further refraction occurs as
light moves from the posterior corneal surface to the
aqueous humor, from the aqueous humor to the anterior
lens surface, from the anterior lens surface to the poste-
rior lens surface, and from the posterior lens surface to
the vitreous humor.
Disorders of Refraction
A perfectly shaped eyeball and cornea result in opti-
mal visual acuity, producing a sharp image in focus at
all points on the surface of the retina. Unfortunately,
individual differences in eyeball length result in inap-
propriate focal image formation. If the anterior-poste-
rior dimension of the eyeball is too short, the point of
focus will fall behind the retina. This is called
hypero-
pia,
or
farsightedness
16
(see Fig. 38-5B). In such cases,
the accommodative changes of the lens (to be discussed)
can bring distant images into focus, but near images
become blurred. If the anterior-posterior dimension of
the eyeball is too long, the focus point falls in front of
the retina. This condition is called
myopia,
or
nearsight-
edness
16
(see Fig. 38-5C). Persons with myopia can see
close objects without a problem because accommoda-
tive changes in their lens bring near objects into focus,
but distant objects are blurred. A refractive defect of
the corneal surface, or
astigmatism,
is usually the result
A
B
C
FIGURE 38-5.
(A)
Accommodation.The solid lines represent
rays of light from a distant object, and the dotted lines represent
rays from a near object.The lens is flatter for the former and
more convex for the latter. In each case, the rays of light are
brought to a focus on the retina.
(B)
Hyperopia corrected by a
biconvex lens, shown by the dotted lines.
(C)
Myopia corrected
by a biconcave lens, shown by the dotted lines.
