Subjective Refraction Instruction
Standard Subjective Refraction Technique Instruction Sheets
- Plus Cylinder Subjective Refraction, for Clinicans
- Plus Cylinder Subjective Refraction, for Ophthalmic Technicians
- Minus Cylinder Subjective Refraction, for Clinicans
- Plus and Minus Cylinder Subjective Refraction Techniques for Clinicians
Plus Cylinder Subjective Refraction Instructional Video
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This is Mark Wilkinson from the University of Iowa Department of Ophthalmology & Visual Sciences. In this presentation, I will discuss how to perform a plus cylinder refraction.
We will be discussing how to refract using a plus cylinder phoropter. How to use the Jackson Cross Cylinder for cylinder power and cylinder axis determination. Duochrome testing. Binocular balancing techniques and reading correction determination. Finally, we will discuss testing considerations for exam rooms that are shorter than optical infinity.
The subjective refraction is the part of the eye exam that most people are familiar with. "Which is clearer; 1 or 2?" The big surprise is that there is no absolute right answer to this question. Small variations can be found from day to day and doctor to doctor on any given patient. This is why you should show the patient the prescription change you found, if any, in a trial frame or with loose lenses held over the patient's current glasses, so they can determine if the prescription change you found makes a functional difference in how they see, in the distance, and/or or at near.
The goal of the subjective refraction is the achievement of clear and comfortable vision. If your patient does not see clearly and comfortably; they are not going to be happy, regardless of what else you have done for them.
The Ophthalmic Protractor is found on the phoropter or the trial frame.
I will discuss refracting with a trial frame in another presentation.
The horizontal meridian is the 0 and 180 axis, with 0 degrees on the patient's left side for both eyes. As you move counterclockwise, you go from 0 to 45 degrees, then 90 degrees, 135 degrees and then 180 degrees on the far right. If the astigmatism correction is oriented on the horizontal midline, the axis is recorded as 180, not 000.
There are four main components to the subjective refraction. The subjective refraction starts after retinoscopy, or after autorefraction.
I will discuss how to perform retinoscopy in a separate presentation.
Retinoscopy or autorefraction provides you with an objective assessment of the patient's refractive error. It is possible to start with the patient's previous RX. However, this is the least desirable way to start your refraction, because you have no objective information about the patient's current refractive error.
The first step in your subjective refraction is the Initial Maximum Plus to Maximum Visual Acuity (MPMVA). This is followed by Jackson Cross Cylinder (JCC) testing for cylinder axis and cylinder power refinement.
Next comes a second MPMVA, if cylinder power or cylinder axis has changed significantly from your initial MPMVA. Once both eyes have been refracted to their best individual level, binocular balancing is performed.
In the past, refractions were done with the room lights dimmed or sometimes off. This is no longer necessary, and can in fact result in the patient being over minused, secondary to night and instrument myopia. With this in mind, you will want to do your refractions with the examination room lights on.
You will start the subjective refraction with the net results from your retinoscopy or autorefraction findings in the phoropter. Alternately, you can start with the patient's previous RX, if retinoscopy or autorefraction results are not available.
Set the patient's pupillary distance. Level the phoropter and occlude the left eye. By convention, testing is done on the right eye first.
The initial MPMVA is done to be sure the patient is not over minused by retinoscopy or autorefraction findings, or their current spectacle correction. If starting with retinoscopy or autorefraction results, check the patient's acuity. Next add +0.75D sphere power by dialing down 3 clicks. Check acuity again. If the patient has not lost 1-2 lines of vision, or if they are seeing better, add an additional +0.75D. Be sure your patient is blurred from their initial acuity.
Once you have blurred the patient from their starting acuity, add minus power in -0.25D increments. Each -0.25D click should improve visual acuity by about 1 line. Eventually, the letters will just appear smaller and darker. If the letters just look smaller and darker, back up one click, by add +0.25D back into the phoropter.
Beware, patients often like extra minus power!
The Cardinal Rule for subjective refractions is to treat minus power like money. Do not give it away unless you get something in return for it! That something is improved vision. Remember that for each -0.25D sphere power, you should expect approximately one line of improvement in acuity.
If astigmatism correction was found during retinoscopy or auto refraction, or is in the patient's current glasses, you will next use the Jackson Cross Cylinder to refine cylinder axis and cylinder power.
It is important to remember that like every other refracting technique, the JCC is not fool proof. What I mean by this is that sitting behind the phoropter is unnatural and can result in the patient giving incorrect responses during the subjective portion of the exam. If you have determined by objective measurement that the patient only needs 1.00D of cylinder power, and they are up to 2.00D and asking for more, the JCC is most likely not working for this patient. Remove the JCC and check cylinder power by adjusting the cylinder power knob.
This is why objective assessment of refractive error is so important in the art and science of refraction.
On a historical note, Edward T. Jackson invented the JCC, in 1897. JCC techniques are used to refine the axis and power of the astigmatic correction, after the initial MPMVA has determined the tentative spherical prescription.
To begin your JCC testing, you have two options. One, is to put a single line of letters on the eye chart, a couple of rows larger than the patient could see at the end of their initial MPMVA. This is done because the introduction of the JCC induces a small amount of astigmatism that will slightly blur the patient's vision.
I prefer to use several lines of letters, at and above the patient's best visual acuity. Here you see the 20/40 to 20/15 letters on a visual acuity chart. By showing this range of letters, I do not have to spend time switching from larger to smaller letters on the eye chart while doing the subjective refraction.
For patients with less than 1.00D of cylinder, found by retinoscopy or autorefraction, you will refine the cylinder power before the cylinder axis. This is done because there are times when small amounts of cylinder power are found during retinoscopy or autorefraction that are not part of the patient's refractive error. If you attempt to find a cylinder axis for a patient who needs no cylinder correction, you will find yourself doing a 360-degree search for an axis that does not exist. This is why it is best to make sure there is a cylindrical comment to the patient's refractive error, before searching for the cylinder axis.
Once you have determined an astigmatic correction is needed, refine the cylinder axis, and then recheck the cylinder power.
If 1.00D or greater cylinder correction is found by retinoscopy or autorefraction, you will want to refine the cylinder axis before the cylinder power.
When performing cylinder power refinement, you will position the JCC dots (White or Red) so they lie on the cylinder axis. For cylinder power refinement, the dots are parallel and perpendicular to the cylinder axis.
In this example, the axis is set at 180 degrees and the red and white dots are aligned at 180 degrees and 90 degrees.
When performing cylinder power refinement, inform the patient that you will be showing them two views of the same letters and that both views will be slightly blurred. You want to know, which choice is clearer or sharper. "No difference" is okay. It is important to show the patient two fresh choices by flipping the JCC wheel while asking, which is better, choice one or choice two, choice three or choice four, etc.
When performing cylinder power refinement, if view number 1 is preferred (red dot on axis), remove +0.50D cylinder power and add +0.25D sphere power to maintain spherical equivalent.
A comment about maintaining the spherical equivalent. When you add or remove cylinder power, you must counteract it with sphere power to keep the circle of least confusion on the retina. This is called maintaining the spherical equivalent.
When astigmatism is not corrected, two focal lines occur, corresponding to the two different focal powers of the astigmatic eye. In this diagram, we have an eye that has compound myopic astigmatism. Both focal lines are in front of the retina.
The circle of least confusion is the midpoint between the two astigmatic focal lines.
The Spherical Equivalent is calculated by adding the sphere power + ½ cylinder power.
In this example, a spectacle correction of -1.00 +1.00 x 090 has a spherical equivalent of -1.00D sphere + half of +1.00D of cylinder correction, which equals -1.00D + (+0.50D) = -0.50D.
Using spherical equivalent to maintain the circle of least confusion on the retina, it is important to remember that for every 0.50D of cylinder power added or subtracted, adjust the sphere power by 0.25D in the opposite direction. For example, if you add +0.50D of cylinder power, subtract -0.25D of sphere power. If you subtract +0.50D of cylinder power, add +0.25D of sphere power.
Remember, +0.50D of cylinder power is equal to a spherical equivalent of +0.25D sphere.
If view number 2 is preferred (White dot on axis), add +0.50D cylinder power and remove +0.25D sphere power to maintain spherical equivalent.
Eventually, you will only change the cylinder power by 0.25D, instead of 0.50D. This occurs once the patient reverses; chooses the red dot after previously choosing the white dot or vice versa. When this happens, adjust the cylinder power by 0.25D in the opposite direction of your previous change. When you adjust cylinder power by a 0.25D, no change in the sphere power is needed.
Once again, check the cylinder power with the JCC to see if the patient wants more or less power.
Continue adjusting the cylinder power, until the patient feels both views are nearly the same, or you are going back and forth. This occurs when the patient first wants 0.25D more cylinder power. When given the extra cylinder power, they than want it out; now preferring the red dot. The goal is the least amount of cylinder power that provides the clearest vision.
When performing cylinder axis refinement, you will position the JCC with the axis wheel corresponding to the cylinder axis initially determined by retinoscopy or autorefraction, or the patient's previous RX. In this picture, the wheel is set at axis 180.
For axis refinement, the white and red dots are 45 degrees on either side of the cylinder axis.
When performing cylinder axis refinement, again inform the patient that you will be showing them two views of the same letters and that both views may be slightly blurred. You want to know, which choice is clearer or sharper. "No difference" is okay.
Show the patient two fresh choices by flipping the JCC wheel while asking, which is better, choice one or choice two, choice three or choice four, etc.
When using a plus cylinder phoropter, you will "Chase the white dot" to determine cylinder axis. For cylinder corrections of 2.00D or less, initially rotate the cylinder axis knob 15 degrees towards the white dot. Make smaller changes, initially 5 degrees, when the starting cylinder power is greater than 2.00D. Repeat the test at the new axis and continue chasing the white dot with smaller and smaller increments, after a reversal, until both views are nearly equal.
For example, with 2.00D of cylinder power or less, if the two choices are not the same, move the cylinder axis 15 degrees from its initial orientation, towards the white dot. If the patient reverses, change back 10 degrees. If they reverse again, change back 5 degrees, eventually changing with reversals to 3 degrees and finally only 1 degree at a time.
Remember, you control the examination.
If retinoscopy or autorefraction found no cylinder power to be needed, but you suspect there may be some, based on reduced acuity after initial MPMVA, do a cylinder power search. With your JCC oriented for power at 90 and 180 degrees, ask the patient, which is better, choice one or two. If no preference, repeat at 45 and 135 degrees.
When doing cylinder power searches, if the patient has a preference, add +0.50D cylinder power at the axis of preference, along with -0.25D sphere power.
Using standard JCC technique, refine the cylinder power and cylinder axis using the techniques previously described.
Once JCC testing is completed, remove the JCC and check acuity.
The second MPMVA is performed if the cylinder power has changed by 0.50D or more, and/or if the cylinder axis has change by 10 degrees or more during cylinder power and cylinder axis refinement.
To perform the second MPMVA, add +0.50D sphere power and check acuity again. If unchanged, add +0.50D sphere power more. Check acuity again.
When the acuity is reduced by at least 1 line, add -0.25D steps until the patient achieves their best acuity or until there is no further improvement.
Remember to be sure vision is improving with each -0.25D increase in sphere power.
Stop adding minus power when: The smaller, darker endpoint is reached. The Duochrome test endpoint is reached, and/or the expected best visual acuity is reached.
Here is how the Duochrome or Red-Green Test works.
Red light sits in the visible light spectrum at 700nm. Green light is at 500nm. Because of this difference, green light is refracted more quickly, bent more than red light in the eye. This results in green light focusing in front of the retina, while red light focuses behind the retina for an emmetrope or corrected ametrope.
With the patient viewing letters on a red-green chart, ask the patient to tell you which letters look blacker; the ones on the red side or the ones on the green side? "Looks blacker" is the key question for this test. Not, which is clearer or better. If green is blacker, the green focal line is closer to the retina and you must add +0.25D to the sphere power to pull or converge the focal line forward. If red is blacker, the red focal line is closer to the retina and you must add -0.25D to the sphere power to push or diverge the focal line back. Equal is the desired endpoint.
Like the JCC test, Duochrome testing is not foolproof. Some patients prefer red, and will repeatedly prefer red letters. This is why we ask, which letters look blacker. With this in mind, if you patient is seeing well, yet continues to prefer the red letters, go back to the power found after your second MPMVA.
Remember, to avoid over minusing, make sure the patient is seeing better with each 0.25D sphere power added in the minus direction.
If the letters are just becoming smaller and darker as you add -0.25D sphere, the patient is over minused. Add +0.25D sphere in steps until you are back to where the patient is just able to read the smallest letters.
If the patient's vision is worse than when you started your refraction, or with his or her own glasses, something is wrong. Recheck everything.
Once the second MPMVA is completed, occlude the patient's right eye and repeat the aforementioned procedures on the patient's left eye. Once you have finished the subjective refraction of both eyes, you are now ready for the Binocular Balance.
Binocular balancing is done when the patient's visual acuities are relatively equal between their two eyes. Be sure both eyes are not occluded and the patient has both eyes open. Add +0.75D sphere power to both eyes to fog the patient. Check acuity. If their vision is not about two lines worse than their second MPMVA, add another +0.50D sphere power. Check acuity again.
The patient needs to be slightly blurred for binocular balancing to work. If vision is not blurred, the patient's dominant eye will take over and they will end up with their non-dominant eye being over minused.
There are two ways to perform binocular balancing. The first technique uses the Risley Prism on the phoropter. For this technique, swing the Risley Prisms into place with 3 prism diopters Base Up in front of the right eye and 3 prism diopters Base Down in front of the left eye. This should cause vertical diplopia.
With prisms, the image is shifted away from the base, towards the apex of the prism. This results in the base up right eye seeing the lower image and vice versa. The base down left eye will see the higher image.
Ask the patient which letters appear clearer; the ones on the top or ones on the bottom? If the patient notes that one image is clearer, fog the clearer eye with +0.25D sphere.
I personally don't like to use this test because you have to instruct the patient to ignore the background brightness difference of the two charts they now see. Ignoring the brightness difference can be difficult for some patients to do.
The binocular balance endpoint is reached when either both sets of letters look the same or when the patient's dominant eye appears slightly clearer than their non-dominant eye.
Next, remove the Risley prisms.
Remember, the patient will still be a fogged.
I prefer to use the alternate occlusion technique for binocular balancing. I feel it is a more natural binocular balancing technique.
To perform alternate occlusion binocular balancing, first start by fogging the vision in both eye as noted with the Risley prism technique. Next, alternately occlude one eye and then the other to find out which eye sees clearer. Add +0.25D sphere to the clearer eye.
The endpoint for both forms of binocular balancing is reached when both eyes look the same or when the patient's dominant eye appears slightly clearer than their non-dominant eye.
Remember, the patient will still be a fogged after binocular balancing.
With several lines of letters on the chart, have the patient start reading the smallest letters they can. When they get stuck, add -0.25D sphere power to both eyes to clear the image.
Binocular Duochrome testing can be used as part of the final RX refinement. Remember, if green is clearer, add +0.25D sphere to both eyes. If red is clearer, add -0.25D sphere to both eyes.
You will again, stop adding minus power when: The smaller, darker endpoint is reached. The Duochrome test endpoint is reached, and/or the expected best visual acuity is reached.
The distance subjective refraction is now completed.
Presbyopia is the natural loss of focusing ability that occurs as we age. In fact, accommodation begins to decrease around 10 years of age, but typically does not become a problem until around the mid-40s for many, but not all. Some people will not need a reading correction until well into their 50s, while others will need a reading correction in their mid to later 30s.
For patients with near vision problems, especially those who are 44 years and older, you can expect to prescribe a reading correction. For ease of estimating the reading power a patient will need, expect that around age 44, you will be prescribing about a +1.25D reading correction.
Because accommodative abilities will continue to decrease over time, you will likely need to add +0.25D sphere power to this starting power every 2 years. With this in mind, you can expect that a 50 year old will need around a +2.00D reading correction.
The near point rod with a near acuity card, set at a 40cm (16") working distance, is useful for determining the reading correction. Unless your patient is monocularly aphakic or monocularly pseudophakic, and less than 50 years of age, the reading correction determination is done binocularly.
Remember to adjust the pupillary distance when doing near vision testing. This is accomplished by using the alignment levers on the phoropter as shown in this image.
When determining the reading correction, it is important to find out where the patient likes to hold things when reading or doing other near point activities. Additionally, are they taller or shorter? Use this information to prescribe the appropriate reading correction.
Final reading power is best determined with a trial frame, using normal sized print at the patient's preferred working distance.
Lets discuss the cardinal rule of presbyopia. The cardinal rule states, more plus power in the reading correction is not always better. For every increase in the bifocal power, the patient's working distance gets closer.
This is because the dioptric power of the reading correction is directly related to the patient's working distance, To calculate working distance, use the formula, Working Distance = 100 cm/x(D) or 40"/x(D).
For example, a +3.00D add results in a 33cm/13"working distance.
For many, this is an uncomfortably close working distance.
Computer users normally do not have their monitors positioned at 40cm. Usually, monitors are 50-60cm away, or even farther.
Patients who work on a computer, may need a trifocal. A trifocal lens is half the power of a bifocal lens. A trifocal lens allows for clearer vision at intermediate distances. Even better for many is a Progressive Addition Lens. A PAL provides variable power that will allow the user to easily adjust their focus from far distance to intermediate distances as well as near for reading and other detail tasks.
It is important that you encourage your patients who work on computers to adjust their monitor heights lower and their chair height higher when using a PAL, to avoid neck problems.
Alternately, your patient may do best with a single vision computer RX set to focus clearly at their normal computer distance.
When writing a spectacle prescription, or entering it into an electronic health record, the following components need to be included. The spherical power. The cylinder power. The cylinder axis, if there is an astigmatic component to the patient's prescription.
With astigmatism, the lens powers are always oriented 90° away from each other. The axis specifies the orientation of the cylinder power.
Here we see the various components of a Spectacle Rx
OD = oculus dexter = right eye
OS = oculus sinister = left eye
1st number (-2.75 & -3.25) = spherical power
2nd number (+1.00 & +1.25) = cylinder power
x = "axis"
Final number (078 & 005) = cylinder axis orientation
It is important to always use at least three digits, including zeros for placeholders when necessary, when writing a spectacle correction. For example, use –0.50D versus -.5D and for axis use 012 versus 12. This clarifies the prescription for the optician who will be filling the RX.
Additionally, if there is no astigmatism correction in one or both eyes, write, "sphere" in the cylinder section of the RX, so it is clear to the optician that there is no correction for astigmatism.
I want to make a few comments about what the final prescription should be when refracting in a shorter exam room. A shorter exam room is one that is less than optical infinity, which is 20 feet or 6 meters. As you are probably aware, or will soon learn, shorter exam rooms are now the norm, outside of pediatric practices.
It is important to recognize that when refracting in a shorter exam room, accommodation is in play. For example, when refracting with a 10' (120") chart distance, vergence is a factor that must be taken into account.
To calculate vergence, one uses the formula 1/x (meters), or 100/x (cm) or 40/x (in). Given this, the vergence demand in a 10' exam room is 40/120 = 0.33D. With this in mind, when testing acuity in a 10' lane, the patient is effectively getting an extra -0.33D of refracting power from the shorter room.
For patients refracted in shorter exam rooms to be focused at infinity, you will need to add additional minus power to what is found in the phoropter for every patient, regardless of their refractive error. The amount of additional minus power to be added is based on the patient's distance to the eye chart.
For example, with a 10' chart distance, add an additional -0.25D. For a 6' chart distance, you will need to add an additional -0.50D.
I also want you to consider what happens when you are testing visual acuity in a shorter exam room. In a shorter exam room, the patient is getting at least an extra -0.25D of improvement in their vision on the eye chart. This is why someone can have 20/20+2 entrance acuity and still need an extra -0.50D sphere power in his or her final RX, to see 20/15 at infinity.
It is important to know that the acuity charts in shorter exam rooms are adjusted to the correct letter height for the room's testing distance, so the visual acuity measured in a shorter exam room is the correct acuity.
Finally, with respect to visual acuity testing, when a patient leans in to see the chart better, the testing distance can be 12-20" less. For me, my standard lean is 16 inches. In a 10' exam room, this is equivalent to a one-line improvement in vision. With this in mind, it is important for accuracy to have the patient sit back in the chair.
In summary, subjective refraction techniques allow the clinician to further refine their retinoscopy findings or the findings from autorefraction.
The goal of the subjective refraction is to provide your patient with the most plus or least minus spherical power that provides them with their clearest distance vision. Similarly, the goal is the least amount of cylinder power that provides for clearest distance vision. Finally, reading corrections are prescribed based on the patient's complaint of near vision difficulties, not their age.