Introduction to Refracting - Minus

Chapter 1: INSTRUMENTATION
It’s a little difficult to describe the controls used for refracting because every instrument is set up differently. If possible, refer to a manual for the specific instrument that you’ll be using.
That being said, most refracting instruments have the same general concepts. We’ll discuss the location of the controls on the most popular refractors.

In our example here we have a large wheel on each side of the instrument that is used to change the sphere power. Moving the sphere wheel up makes the power become more minus or less plus. ..note how the numbers in the sphere power window are red, which traditionally denotes minus power.
On the other hand, moving the sphere wheel down makes the power more plus or less minus. …Traditionally, black numbers represent plus power.

Oftentimes the refracting instrument will also have a way to change the sphere power by larger increments. In our example here, turning this outer dial will change the sphere power in 3 diopter steps.

To change the power and the axis of astigmatism we use the two concentric knobs at the bottom of the instrument. The outer knob turns the axis. Most of the new refractors will change all the parts of the instrument that relate to the axis.
…..
The inner knob is for adding and subtracting cylinder power. Since refractors can be minus-cyl or plus-cyl , make sure that you know what type you’re using. Usually plus cyl instruments will have black numbers in the cylinder power window while minus-cyl instruments will have red numbers.

When refining cylinder axis and power we’re going to use the controls located on this arm that swings in front of the patient’s eye. This assembly includes a special lens called the Jackson Crossed Cylinder, or JCC. This lens has a low amount of minus power at the axis represented by the red dots ..and an equal amount of plus power at the axis represented by the white dots. What makes this lens so useful is that, when you flip it,….the axes switch locations.
When the lens is back in the instrument we can flip it and show our patient what subtle differences in axis look like. We can also turn the whole unit to align with the axis of astigmatism, which allows our patient to see more and less astigmatism power, letting them decide option they prefer. We’ll discuss the details of this later in this tutorial.

There are several other controls of importance. In a monocular refraction we’re going to want to close off one eye. In our instrument, change the dial in the upper left from O (for open) to OC (for occlude) to do this.

And lastly, but very importantly, we have the controls that align the instrument. There’s usually a dial to make sure the refractor is perfectly horizontal, which can be verified using the level.
There’s a dial for adjusting the pupillary distance to ensure that the instrument’s oculars are lined up with your patient’s eyes.
Finally, there’s also a knob for adjusting the vertex distance. This is used to bring the instrument closer too or further from the patient’s forehead so you can make the lenses sit about the same distance from the patient’s eyes as their glasses lenses will.

And those are the basic controls you’ll be using. Of course, there’s a number of other controls that you may find on your refractor. These other devices are a bit beyond the scope of this tutorial, but feel free to read about them in another resource if you’re curious.

Chapter 2: INTRODUCTION
When you watch a good refractionist perform a monocular subjective refraction it looks like a complicated flurry of hand motions and spinning dials. We’re going to try to slow things down a little and show you that refracting isn’t as difficult as it may look. In fact, we can break it down into a simple, step-wise process that will allow you to refract nearly anyone.

As you watch this tutorial, you may find it helpful to stop and rewind some sections. We’ve included pause and play buttons to help you do that. In addition, I’d recommend taking notes. You’ll probably want to refer back to them when you start practicing.

I’d like to start by addressing our goals for doing a monocular subjective refraction. It’s actually pretty simple - we want to make our patients see well. I’ll take this one step further, however. We want to get the best vision through the prescription that has the least amount of minus power (..or the most amount of plus power).

Let’s look at this another way. If we have an eye looking at an image, ..we want to find the spectacle lens that focuses the image on the retina as well as possible. …

You might be wondering why it’s such a big deal to find the most plus or least minus lens. Well, if we add too much minus power or too little plus power the light will begin to focus behind the retina. A young patient, however, may see quite clearly in this situation because they’re able to use their accommodative muscles to change the shape of the physiologic lens and just focus through it. The problem is, if you prescribe glasses with this much power the muscles will need to maintain this tension to maintain stable vision, which will likely result in headaches and eyestrain, especially with reading.


Now that we have established our goals, let’s start thinking about how to accomplish them.
First we’ll need a patient. This is our patient, Joe. Hi Joe! ...We’ll get him behind the instrument.
Before starting a refraction, we’ll need to get everything set up. We’ll want to make sure the patient is comfortable. …We’ll make sure the oculars are centered over the patient’s eyes, that the patient is about 12mm behind the lenses, and that the instrument is level.

We’d rather not have to start from zero power so it’s nice to be able to dial in a place to start from. Typically we use a prescription from the patient’s retinoscopy measurements, …their autorefractor readings, ..or the prescription from their old glasses…well, make that old spectacles..

Now, let’s talk about the steps we’ll take to refract our patient.

At it’s simplest, refracting follows a path with four steps.

The four steps are
Number 1.. Establish the sphere power .
Number 2 .. Refine the Cylinder Axis .
Number 3 .. Refine the Cylinder power .
..and Number 4 .. Refine the sphere power.

There’s one more thing. If our starting point doesn’t include any significant astigmatism, sometimes there is a short detour in the process of refracting called the “cylinder power search”.

The rest of this tutorial is dedicated to discussing each of these steps in detail.

We’ll stop here so you can take a minute to look over these steps. Like I said, you might even want to write them down. When think you’ve got a handle on everything so far, click the ‘next step’ button to start learning about the ‘sphere power establishment’ step

CHAPTER 3: Sphere Establishment
Now we’ll start the first step of refracting, establish the sphere power.

The goal of this step is to establish the sphere power that gives the patient their best vision. If we go back and look at how light is focusing in the eye, our objective is to make an image focus as close to the retina as possible. If our patient doesn’t have any uncorrected astigmatism, when we’re through we should be able to get light to focus the whole image on the plane of the retina, like this…
However, many of our patients will have uncorrected astigmatism. And, of course, if there’s astigmatism that’s not corrected light from one meridian of the eye will focus at a certain point and light 90 degrees away will focus at a different point. In this case, our goal will be to make both meridians of the image focus as close to the retina as possible. When we’re done with the sphere establishment step, both parts of the image should be the same distance away from the retina, like this…

So let’s get to it…
Since this is a monocular refraction, we’re going to occlude the left eye so we can work with the right eye by itself. This is an easy thing to forget. If you do, it can really mess things up, so don’t forget!
Because we want to make sure the patient doesn’t get too much minus power (or too little plus power), it’s preferable to start with the patient slightly fogged. Adding plus .75 diopters by turning the sphere wheel down 3 clicks should do it.

At this point, we’ll ask the patient to read the chart. …

“Joe, please read the chart”

..and our patient reads most of the 20/30 line..




This is what we want. He should be no clearer than 20/30 to 20/40. If he were seeing better than that, we’ll add more plus power by turning the sphere wheel down further.

Now let’s tell Joe what we expect of him. We’ll say something like..
“Joe, I’m going to give you two choices. They may both be a little blurry, but I want you to tell me which is more clear. This is choice one…”

At this point we’ll take away plus or add minus by turning the sphere wheel upward, one step at a time.
“…or two…”

..so, Joe tells us he likes choice two better.

Because he likes the second choice, we’ll keep the sphere power where it is, at least for now. If he’d chosen option one, we would roll back the sphere wheel to its original position.

Even though our patient told us which lens he prefers, we’re going to check his vision.
“Please read the chart”

…and now he’s reading about 20/25.

To avoid giving him too much minus, make sure that each step in power yields an appreciable improvement in the patient’s ability to read the eye chart. In this case, Joe’s vision was originally 20/30ish, now it’s 20/25ish. This is pretty typical: A good rule of thumb is that for each .25 increase in minus power, the patient should improve by about one line on the chart.


If you add more minus but the vision doesn’t improve, you’re probably adding too much power.

As we said in the beginning, adding too much minus or too little plus is a no-no.
The bottom line is, be stingy! Think of minus power as if it were money. The patient can only earn more minus power if they can demonstrate to you that it improves their vision.

We’ll repeat this step until the patient’s vision is no longer objectively improving.
When we get to a point where more minus or less plus doesn’t improve our patient’s ability to read the eye chart, we’re done with this step.

Keep in mind that, since we have not refined the astigmatism at this point, the patient may not refract to a perfectly clear 20/20.


To summarize, the sphere establishment part of refraction starts with fogging the patient about 0.75 D and then adding minus (or removing plus), one step at a time. With each step, make sure the vision has objectively improved on the eye chart.

CHAPTER 4: Cylinder Axis Refinement
The next step is to refine the cylinder axis.

Our goal with this step is to get the axis of astigmatism oriented into the position that gives the maximum visual acuity.

The first step in refining the cylinder axis is to isolate a line of letters. Generally, the 20/30 line is used. If the patient cannot see 20/30 at this point, then choose a row that’s one line above his or her best vision.
Swing the JCC in front of the eyes so that the axes of the lens …straddle… the axis of the instrument’s cylinder.

In other words, at this point we don’t want the axes of the lens to be in line with the instrument’s cylinder, like this…..,
…but instead we want it to straddle the axis like this…..

Next, we’re going to flip the JCC lens and, just like in the last step, we’re going to observe which choice our patient likes better. This time, we’re going to pay particular attention to where the red dots are located when Joe makes his choice.

So lets’ do it
We’ll again ask “which is better, one….”.
And note that the red dots are here…
..and then flip the JCC like this…
“…or two…”
…and note the red dots are now here.

In this case, Joe likes lens two better,
This option is where the red dots were, …. clockwise… to the current cylinder axis.

By choosing choice two Joe is telling us that he wants the cylinder axis to be moved towards the red dots. At this point we’re going to move it about 15 degrees in that direction, like this…

From here, we’re going to keep doing what we call “chasing the red dot” and essentially “narrow in” on the patient’s true axis. Every time the patient makes a choice, we’re going to move the cylinder axis towards where he liked the red dots.


If the patient keeps asking us to move the axis in the same direction, in this case clockwise, we’ll keep changing it in 15 degree increments. If he reverses the direction and wants the axis moved counterclockwise we’ll start to adjust it in 5 degree increments. Every time he switches directions we’ll change the axis by smaller amounts until he isn’t able to make a choice anymore.
If he can’t make a choice between the options we’re giving him, we’re where we want to be and this step is over.

One more thing: Sometimes when you’re showing the patient different options with astigmatism, they comment that the letters look stretched or compressed or lean one way or the other. Ask the patient to ignore these details and to focus on which option has the better contrast or simply looks better.


To summarize, the cylinder axis refinement part of refraction involves
… isolating the 20/30 line on the eye chart (or one line bigger than the best vision)
… positioning the JCC’s axes so they’re straddling the astigmatism axis
… flipping the JCC and noting which choice the patient likes better
…and moving the cylinder axis toward where the red dots were when the patient made their choice.

Chapter 5: Cylinder Power Refinement
The third step is refining the cylinder power.
The purpose of this step is to establish the amount of astigmatism power needed to focus both meridians of an image on the retina. You recall that when we established the sphere power in the first step we created a situation where two images were being formed at an equal distance from the retina. Now, as we refine the cylinder power, each step will move these images closer and closer until they both focus on the retina.

To do this we’ll first turn the JCC so that the axes are no longer straddling the refractor cylinder and are instead aligned with it.

As in the last step, we’ll identify the first choice as one,
..then flip the JCC and say “two”,
…and note which choice the patient prefers.

In a minus cyl refractor, we’re going to continue “chasing the red dot”. If the patient likes the choice where the red dots are in parallel with the cylinder axis, ..like it is now, we’re going to add cylinder power. If, on the other hand, the patient prefers the option where the white dots are aligned with the cylinder axis, ..like this, we’ll take away cylinder power. In a nutshell, red dots ..mean.. increase cyl power, ..white dots ..mean ..take it away.

Let’s go back to our patient.
“Joe, Which one is better, one
….or two?”
…..Joe says two…..
Since he said “two”, and because this is the option where the red dots were aligned with the cylinder axis, we’re going to add -0.50 to the cylinder power.

Now here’s the tricky part. We’re going to do something called “maintaining the spherical equivalent”. The easiest way to explain the purpose of this is to go back and look at the optics inside the eye.

If we simply add astigmatism power, we’re only going to change the power in one of these meridians. As a consequence we’ll move one closer to the retina but the other one will stay where it is.
However, to get the most accurate refraction we want these two images to remain the same distance from the retina.
To fix this, every time we add minus cylinder power we’re going to compensate by adding plus sphere power too.

We’ll do this by changing the sphere power by half the amount we changed the cylinder power, and in the opposite direction.

For example, if we add a half diopter of minus cyl we’ll… compensate by removing a quarter diopter of minus sphere.
Note how both meridians on the image were brought forward and are now an equal distance from the retina.

On the other hand, if we had taken away a half diopter of minus cyl we’d also remove a quarter diopter of plus sphere power.

Note how when we maintain the spherical equivalent we accomplish our goal of keeping both meridians focusing an equal distance from the retina.

Eventually this becomes second nature: Add 2 clicks of cyl, turn the sphere wheel down. …subtract two clicks of cyl, turn the sphere wheel up.


Why don’t we go back and start at the beginning…
Let’s flip the JCC and ask our question again.
“Joe, which is better, One…or …two”
And Joe says he likes one.
Since the patient likes the red dots we’ll add a half diopter of cyl, …remembering to maintain the spherical equivalent by removing a quarter of minus sphere power.
We’ll ask again, “which is better, one ..or ..two”
And Joe says he likes one again.
Since he likes the white dots this time we’ll switch directions and remove .25 diopters of minus cyl. Since we’re only changing the cyl power by .25 diopters we won’t worry about maintaining the spherical equivalent at this point.


Like the cylinder axis refinement step, we’re kind of “narrowing in” on the best cyl power. When the patient keeps asking to change the cylinder power in one direction, we change it in .50 steps. …When the patient switches directions, we start changing the power in .25 steps. When you get to a point where the patient cannot choose between the two options or is vacillating between two cyl powers you’ve reached your endpoint and it’s time to go on to the next step.



To summarize, the cylinder power refinement part of refraction involves
… positioning the JCC’s axes so they’re aligned with the astigmatism axis
… flipping the JCC and noting which choice the patient likes better
…adding minus cyl when the patient likes the red dots, decreasing cyl when the patient likes the white dots.
…And lastly, don’t forget to maintain the spherical equivalent by changing the sphere power by half the amount you changed the cyl power and in the opposite direction.

Chapter 6: Sphere Power Refinement
Ok, we’re almost done. Now we need to simply make sure the sphere power is where we want it to be. If we did a good job of maintaining the spherical equivalent in the last step we should be pretty close to our endpoint.
This step is essentially the same as the very first thing we did, the “establish the sphere”step. We’ll start by fogging the patient by adding a small amount of plus power. Then we’ll check the patient’s vision to see where we are.

“Joe, please read the lowest line you’re able to”

..And he’s reading 20/25 minus.

Now, we’ll progressively click in more minus (or less plus) one step at a time.
“Which is better, one
…or two?”
As we would predict, Joe likes choice two better.
To make sure each increase in power yields about one line of improvement on the chart we’ll continue to monitor our patient’s vision.
“Please read the lowest line you can, Joe”

And now he’s reading about 20/20-, ..so he’s gained the expected one line of vision with a .25 increase in sphere power.

And we’ll just repeat this process of adding minus in .25 steps.
Within one or two steps we should get to a point where adding more minus power does not make an appreciable improvement. When that happens, we’re done with this eye.

After learning the first three steps this one should be pretty easy. In fact, because this step is so simple and so similar to the first step we won’t watch a video of this being done.

To summarize, the sphere refinement step starts with again fogging the patient …and then adding minus (or removing plus), one step at a time. With each step, make sure the vision has objectively improved on the eye chart.

These four steps you’ve learned should work well for many situations, but we still need to learn one more thing, the cylinder power search.

Chapter 7: 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 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 red dots were when he made his choice. We’ll dial in .50 diopters at 90 degrees.
..Don’t forget to maintain the spherical equivalent by removing 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 red 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 red 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.