Biomedical Engineering Reference
In-Depth Information
the retina, making the astigmatism compound myopic. The vertical meridian of the
eye, which is focused closest to the reduced eye's front surface, requires a correction
of
5.00 D. Since the eye's vertical
meridian has the most converging power (requiring more minus power to correct
it), the patient has with-the-rule compound myopic astigmatism.
−
7.00 D and the horizontal meridian requires
−
Let's take another example. Classify the astigmatism when the following correc-
tive lens is required:
+
2.00
−
3.00
×
090
As can be seen in Figure 9-12B, the correction for the vertical meridian is
+
1.00 D. This is a case of mixed astig-
matism because one meridian (the horizontal meridian) is focused anterior to the
retina and the other (the vertical meridian) posterior to the retina. Since the eye's
horizontal meridian has the greater converging power, the eye has against-the-rule
mixed astigmatism.
It is not uncommon for young human infants to have 0.75 to 2.00 D of astigma-
tism (Howland et al., 1978). The nature of the astigmatism is apparently related to
race, with Chinese infants tending to have with-the-rule astigmatism and Caucasian
infants tending to have against-the-rule (Thorn et al., 1987). This astigmatism gen-
erally decreases by school age (Gwiazda et al., 1984).
In the fifth decade of life and beyond, the astigmatism is apt to become more
against-the-rule. This is apparently due to aging of the crystalline lens.
2.00 D and for the horizontal meridian it is
−
JACKSON CROSSED-CYLINDER TEST
The Jackson crossed-cylinder (JCC) test is the most commonly used subjective
clinical procedure to determine the amount of a patient's astigmatism. The power
in one of the principal meridians of a crossed cylinder is equal and opposite to the
power in the other principal meridian. In the example in Figure 9-13A, one merid-
ian has a power of
0.25 D. The
crossed cylinder is mounted in front of the patient's eye such that it can be quickly
flipped between the position where the vertical meridian has plus power and the
position where it has minus power.
How can a crossed cylinder be used to determine the amount of astigmatism?
Figure 9-13B shows the position of the retinal images formed when an emmetropic
eye views a distant object through a crossed cylinder. The circle of least confusion
is located on the retina. When the crossed cylinder is flipped, the sequence of the
line images reverses but the amount of astigmatism remains the same (i.e., 0.50 D)
and the circle of least confusion remains on the retina. Consequently, the patient
will report that the image is more-or-less equally blurred for both positions of the
crossed cylinder.
+
0.25 D and the other meridian has a power of
−
