(3) Vision may be corrected with the use of a convex lens, which will cause
the light rays to focus on the retina.
c. Presbyopia. Presbyopia is a refractive error caused by a decrease
in the power of accommodation of the crystalline lens, due to weakening of the ciliary
muscles with aging.
(1) The near point of distinct vision becomes further from the eye. (This is
why you may see someone reading while holding the reading material at arm's length.)
(2) Vision may be improved with the use of corrective lenses such as
bifocals, which have a small lens for near vision placed within the lower half of a larger
lens for distant vision.
d. Astigmatism. This is a refractive error caused by uneven curvature of the
refractive surfaces of the eye.
(1) Light rays are not focused along equal planes because of the curvature,
causing vision to be unfocused.
(2) Vision can be improved with the use of a lens called a cylinder lens,
which compensates for the uneven curvature and allows the light rays to focus sharply.
e. Cataract. A cataract is an opacity of the crystalline lens or its capsule. It is
the leading cause of blindness in the United States.
Vision becomes distorted and/or unfocused. Bright lights produce a
(2) Vision is improved by surgical removal of the opaque lens. The missing
lens is compensated for by the use of special eyeglasses or contact lenses, or by
intraocular implant of an artificial lens.
f. Glaucoma. Glaucoma is a condition in which there is increased pressure
within the eyeball. If uncorrected, glaucoma will lead to blindness.
Increased pressure is caused by faulty drainage of aqueous fluid.
(2) Drug therapy is used to promote drainage of the aqueous fluid, or
decrease the production rate of the aqueous fluid, or both.
(3) When drug therapy is not effective, surgical intervention is indicated.
There are many different procedures used, but the objective of all is to decrease
intraocular pressure by facilitating the filtration and drainage of the aqueous fluid.