CYLINDER POWER SEARCH

CYLINDER POWER SEARCH
You now have learned nearly everything you need to know to do a monocular subjective refraction. There is one more thing that needs to be discussed though.
The “cylinder axis refinement” step only works when we already have astigmatism power dialed into our instrument. However, sometimes we start with a prescription that doesn’t have a significant astigmatism component. This is where our little detour, the “cylinder power search” comes in.

In other words, the goal of the ‘cylinder power search’ is to find any significant uncorrected astigmatism power. If we find some, we can then refine it with the rest of the steps in the refracting process.

…

First we need to get the JCC into the right position. We’ll swing it down in front of the eye, and then manipulate the instrument’s axis to 180 degrees…. and , if necessary, rotate the JCC so that the axes are at 90 and 180 degrees, ..like this…

Then, we give Joe the same old question:
“which is better, ..one …”
and then flip the JCC
“…or two.”

Basically what we’re doing here is fishing for astigmatism. We’re asking our patient “do you like astigmatism at 90 or 180 degrees?”

Let’s say that Joe says he likes choice two better. What he’s telling us is that he likes cylinder power at 90 degrees, since that’s where the white dots were when he made his choice. We’ll dial in .50 diopters at 90 degrees.
..and, of course, don’t forget to maintain the spherical equivalent by adding a quarter diopter of minus sphere power to compensate for the cyl you added…

And that’s it.

Of course, if Joe had chosen choice one, the choice where the white dots were at 180 degrees, we would have dialed in .50 diopters of cylinder at that axis.

….
If you show your patient cylinder power at 180 and 90 and he doesn’t feel that one is significantly better than the other, we’ll try one more thing. We’ll turn the JCC axes to 45 and 135 degrees and give Joe one more choice.
“Which is better, one…”
…and flip..
“…or two.”

As before, if Joe likes the white dots at one location or the other we’ll dial .50 diopters of cylinder power at that axis.

If our patient doesn’t accept any astigmatism power at 90 or 180 or 45 or 135 degrees then he doesn’t have any significant astigmatism. If he doesn’t have any significant astigmatism we’re done refracting this eye.

On the other hand, if he does accept astigmatism power at any of these axes we can leave the ‘cylinder power search’ step.
With the ½ diopter of astigmatism we now have in the instrument we can proceed to the “cylinder axis refinement” step and continue our refraction as we normally would.


To summarize, the cylinder power search involves..
…positioning the JCC with the axes at 90 and 180 degrees.
…flipping the JCC and see if the patient prefers an option.
...if one axis is preferred, dial in .50 diopters at that axis and proceed to the cylinder axis refinement step.
…otherwise, try again with the JCC at 45 and 135 degrees.

If the patient doesn’t like any of these choices, they don’t have significant astigmatism and you’re done refracting this eye.