Biomedical Engineering Reference
In-Depth Information
B
A
Figure 8-1.
A.
When distant objects are viewed, the anterior surface of the crystalline
lens is at its flattest, thereby minimizing the lens' refractive power. The cornea provides
about two-thirds of the distance refractive power of the eye, with the remainder pro-
vided by the crystalline lens.
B.
When near objects are viewed, the sphincter-like ciliary
muscle constricts; this reduces the tension on the zonules, thereby allowing the anterior
surface of the lens to bulge forward, decreasing its radius of curvature. As a result, the
dioptric power of the lens is increased. (
Reproduced with permission from Schwartz,
SH
. Visual Perception: A Clinical Orientation
, 4th ed.
http://www.accessmedicine.com.
Copyright © 2010 McGraw-Hill Education. All rights reserved.
)
When the loss of accommodation due to aging leads to clinical symptoms—
such as near blur and asthenopia
1
—the condition is referred to as
presbyopia
.
Presbyopia is corrected by adding plus lens power to the patient's distance
correction. This additional plus lens power is often in the form of a
bifocal add
(see Fig. 11-9).
Presbyopia does not develop at the same rate in all patients. Exposure to high
levels of ultraviolet radiation, as occurs with people living close to the equator,
appears to accelerate its development.
TABLE 8-1.
AMPLITUDE OF ACCOMMODATION AS A FUNCTION OF AGE
Age (years)
Typical Amplitude of Accommodation (diopters)*
10
12.50
20
9.75
30
7.25
40
4.00
50
2.50
60
1.25
70
0.50
75
0.00
*Extrapolated from the data of Donders (1864) and Duane (1912).
1.
Asthenopia
is another term for eyestrain.
