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Astigmatism is derived from
greek word: ‘stigma” meaning mark, spot, or puncture
What happens in astigmatism?
an optical system where perpendicular planes or rays do not focus at the same point
toric means
different curvatures in different meridians resulting in different refracting powers in each meridian
Simple astigmatism
one principle meridian focuses on the retina, the other does not
compound astigmatism
neither principle meridian focuses on the retina
What is a simple hyperopic astigmatism
one principle meridian focuses light on the retina, the other (hyperopic meridian) is behind the retina
What is a mixed astigmatism?
one principle meridian focuses light anterior to the retina, the other focuses light posterior to the retina
With the Rule Astigmatism
Flat corneal meridian near axis 180
Refractive minus cylinder near axis 180
What does a meridian focus?
light that is perpendicular to it
When do you perform a fogged astigmatic test?
when acuity after ret is poor and you don’t know why
How do you perform a clock dial test?
Use the clock dial as the target,
TAKE OUT ALL THE CYL
ask the pt what is the darkest, take that # x 30
then bracket the two closest lines to the darkest
What is the patient says that both lines are the same darkness after picking their darkest line, what do you do?
leave the axis at the darkest
If the pt picked 1 o’clock first, then said the 2 o’clock was darker what do you do and what is your final axis
•If it is a lot darker… go with the 10 degrees
•If it is just a little darker…go with the 5 degrees
•Final Axis: 35 to 40 degrees
After picking the correct axis how do you fine tune the power?
•Ask the patient to compare the lines that were initially the darkest and the line that is 90 degrees away – they can ignore all of the other lines.
•Example: If patient reported that the 1’ o clock was darkest, you would ask them to compare this with the 4/10 o’ clock line.
•Begin adding minus cylinder power -0.25 D at a time, asking the patient which is darker.
leave the pt at the first equal
How do you perform the stenopaic slit method?
With the patient viewing a distance target, rotate the slit to the position where the patient reports greatest clarity
At this point, the slit should be aligned with the minus cylinder axis
