Monocular Fogged Astigmatism

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