Add powers less than 1.00 D

Refraction rule: A bifocal glasses prescription should not have an add power less than 1.00 D. In general, if the add power of an early presbyope measures less than 1.00 D, then that person does not yet need a bifocal. Most people are not anxious to start wearing bifocals, and are usually happy to hold their reading a little farther out, or to use a bright reading light. In my experience, bifocals (particularly PALs) that are less than 1.00 D are commonly thought of by the patient as being of little help. 

A reading glasses correction (not bifocals) for a hyperope is a different situation. For example: Roberta is a 40 year old with a distance refraction of +0.75 D OU. Her add measures to be +0.50 D for reading at 16”. She complains of eyestrain when reading late in the day. She has no distance vision complaints and is not interested in bifocals. We would combine the add and the distance correction for a reading glasses correction of +1.25 OU.

If you were to do a cycloplegic refraction on Roberta you might find her CR for distance to be +1.25 OU. So you are actually giving the latent distance correction so that the patient is comfortable at near.

What if Roberta wants bifocals? In this case you would prescribe an add power of +1.00 instead of the measured add power of +0.50, and encourage her to get progressive lenses. A conventional flat top bifocal may seem a little strong to her. With progressive lenses she can shift her gaze to the point in the power progression that gives comfortable vision for the give focal distance.

Add powers greater than 2.50 D

Refraction rule: Do not indicate an add power greater than +2.50 unless the best corrected vision is less than 20/25, or unless there is a reason why the patient wants to view material closer than the normal reading distance of 16”.

If an add power greater than +2.50 is indicated, then the near point will be closer than the normal reading distance of 16”. For example: An add power of +3.25 will place the near point at the most, at 12 inches. If the patient has significant accommodation, then the near point will be even closer than 12 inches.

These higher add powers are generally reserved for patients with best corrected vision OU of less than 20/25. The closer near point that simple magnification provides gives a type of proximal magnification for patients with low vision. Proximal magnification means the object makes a bigger image on the retina because the patient is viewing the object at a closer distance.

The actual distance of the near point (with no accommodation) can be read from the reading rod, or figured using the F(m) = 1/D formula. You test- takers, of course, remember that F(m) stands for focal distance in meters, and that D stands for diopters.

For example: We want to know how close the patient with a +4.00 add will have to hold his reading material. If you are in an exam room, just look on the reading rod which is calibrated in diopters, centimeters, and inches.

F(m) = 1/D insert 4 for D
F(m) = 1/4 convert 1/4 to a decimal
F(m) = .25 meter (or 25 centimeters)

The conversion factor from centimeters to inches is approximately .39. If you are doing the calculation in your head, just use a factor of 4. Multiply 25 (centimeters) by 4 to get 100. The actual factor is .4, not 4.0, so move the decimal point over one space from 100.0 to 10.0.

A patient with a +4 add will have to hold his reading material at about 10 inches (closer if he accommodates) to focus well on the material.

If you are going to recommend a reading power greater than +2.50, you will need to explain to the patient that she will need to hold reading material at the appropriate distance.

These higher reading powers generally work better in reading glasses, or in a flat top trifocal, than they do in a progressive lens. The patient will need the intermediate seg of the trifocal to see the food on his plate.

As a general rule, the worse the best corrected vision is , the higher the add power needs to be. A patient with BCV less than 20/80 may need an add greater than +4, which would require base-in prism, and perhaps a visit to the low vision clinic.
 

As you perform refraction after refraction in the process of becoming an experienced and skilled refractionist, certain optical properties of the human eye become apparent to you through repetition. Knowledge of these properties can be used to increase the speed and accuracy of the refraction and aid you in the art of “prescription writing”.

The characteristics of astigmatism presented here represent three such properties that become apparent to you as you gain experience. If you have experience, you may have already “discovered” these properties on your own.

Symmetry

The power and axis of astigmatism tends to be symmetrical between eyes. The symmetrical nature of the power component of astigmatism is fairly obvious for anyone who sees very many glasses prescriptions. Although there are many exceptions, people with astigmatism in one eye tend to have astigmatism in the other eye, and often in a similar amount in terms of power.

What is not so obvious is the symmetrical nature of the axis component of astigmatism. Although it soon becomes obvious to the novice refractionist that an axis of 180 degrees in the right eye is often accompanied by an axis of 180, or 05, or 175 in the left eye, it is not so obvious that an axis of 50 degrees in one eye and an axis of 130 degrees in the other eye also represents symmetry between two eyes.

Look at the axis combinations in columns A and B below. If you are an experienced refractionist, think about how many times these combinations (or combinations a few degrees off) have shown up in your refractions.  Also notice that the OD and OS axis numbers always add up to 180.

A

B

Of course not everyone has symmetrical (or near symmetrical) corneas, but the property occurs often enough to be useful. Same axis symmetry (such as 135 OD, 135 OS) occurs also, but not as often as the axis combinations that add up to 180. Also keep in mind that surgical procedures such as cataract surgery and refractive surgery may (will?) alter any pre-existing symmetry, although I have noticed that many eyes find their way back to symmetry even after surgery.

When performing retinoscopy, use your knowledge of corneal symmetry to “look for” a symmetrical axis orientation in the left eye once you know the axis of the right eye. For example: Suppose you find astigmatism of approximately 1.5 D at axis 60 in the right eye. Mentally subtract 60 from 180 to get 120. Look for a similar amount of astigmatism at axis 120 in the left eye.

When performing refractometry, knowledge of corneal symmetry is particularly useful when trying to locate the axis of low powered cylinders. Let’s say you have found .5 D of astigmatism at about 180 on retinoscopy in the right eye, but you are unsure of any astigmatism being present in the left eye.

Your refractometric refinement reveals that there is need for a .5 D cylinder correction at axis 160. When refining the left eye, dial in .5 D cylinder power and give the patient a comparison between axis 20 (180-160=20) and axis 110 (90 degrees away, for a stark contrast). If there is a strong preference for one or the other, proceed with axis refinement. If the choices seem about the same to the patient, you can usually assume that there is no need for a cylinder correction, particularly if the vision is good with a spherical correction.

Your knowledge of astigmatic axis symmetry can give you some comfort with regard to whacky axis changes.  For example, suppose Ferd Berfel's axis readings last year were OD 165 OS 45.  On today's refraction Ferd is telling you that he likes OD 165 OS 15.  Ordinarily we are not crazy about changing an axis 30 degrees, but hey, the way I look at it, Ferd's eyes are just trying to fulfill their destiny (165+15=180).  Now, if that left eye was at 15 and was trying to go to 45, well, that would make me a little uncomfortable.

The near advantage of against-the-rule astigmatism

Astigmatism is sometimes referred to as “with-the-rule” or “against-the-rule.” With-the-rule means that the majority of people with astigmatism have it in this particular axis orientation (or near to it).

With-the-rule astigmatism has greater diopter power at 90 degrees as compared to 180 degrees. An Rx of +1.00-2.00X180 or –1.00+2.00X90 is with-the-rule.
A glasses prescription of –0.50-1.00X75 or –1.50+1.00X165 would be considered to be against-the-rule. The axis does not have to be exactly at 90 or 180, but if the axis strays too far away, perhaps past 75 degrees for example, it may be considered to be oblique.

A patient with a moderate amount of uncorrected against-the-rule astigmatism (minus axis ~90, plus axis ~180) has an advantage when reading. Don’t be surprised by the 59 year old with distance correction OD +0.50-2.00X90, OS plano-2.00X90, who says he can read just fine without glasses.

Try it yourself. Take a 1.50 plus cylinder lens from the trial lens case. Hold the lens so that the axis mark is vertical. Hold it up to your eye and view a near card, moving the card to a position that brings the letters into focus. Now take the lens away without moving the near card. Also compare your near vision with the axis at 90 as compared to the axis at 180.

Assuming your vision is corrected for distance, the 1.50 D plus cylinder lens held with axis at 90 degrees has created an against-the-rule astigmatism in your optical system. If you only have minus cylinder trial lenses, the same effect can be produced by holding up a +1.50 sphere trial lens along with a 1.50 D minus cylinder trials lens with axis aligned at 180.

Practical application

Don’t be too quick to try to convince that patient with against-the-rule astigmatism that she needs a bifocal or reading glasses. It is helpful to check a patient’s vision without correction as well as with correction, particularly a new patient. Presbyopic soft contact lens patients with against-the-rule astigmatism may find an advantage at near in not correcting the astigmatism, particularly in one eye.

The distance advantage of with-the-rule astigmatism

Yes, the person with who has with-the-rule astigmatism is not without a bonus of sorts too.  Remember that with-the-rule astigmatism is characterized by a minus cylinder axis of 180, or a plus cylinder axis of 90.  This person has somewhat of a distance advantage, compared to astigmatism at other axes, if the spherical component of the minus cylinder correction is fairly close to plano.

An example would be plano-200x180.  These eyes can see well in the distance if the patient squints. It is the principle of the stenopaic slit.  The stenopaic slit is that seeming useless item in the trial set that looks like this:

The stenopaic slit can be used refractively to estimate the axis and power of astigmatism, because the slit eliminates light rays from all but one axis.  When an eye squints, it is creating the effect of a stenopaic slit aligned at 180 degrees.  At this position, the solid part (or the lids) is blocking the light rays of the -200x180 part of the prescription, and the slit is allowing the plano rays to be focus on the retina.

You can demonstrate this to yourself by holding a 2 D plus cylinder lens from the trial set so that the axis marks are horizontal.  Look through the lens (assuming you are corrected for distance) at a distant object.  Now hold up the stenopaic slit in front of the lens so that the slit is also horizontal.  The image should clear up when looking through the slit.  You can also try squinting as you look through the 2 D plus cylinder lens.

Practical application:

Don't be surprised by the patient who has a plano-150x180 refractive error who insists he does not need glasses for distance.  Also, don't be surprised by the same patient who comes back a year later asking for distance glasses because he is tired of squinting.

Also, don't be quick to pre-judge the refractive error of the new patient who has 20/25 vision.  You may be saying to yourself that this person probably does not have much of a refractive error, only to be surprised upon retinoscopy that there is 2 D of astigmatism present.  But of course, we will be able to guess at what axis the correction will be.  It will be with-the-rule.

Keratometry and refractometry: The Javal Rule

Because of certain optical peculiarities, astigmatic power as measured by the keratometer may not translate directly to refractive power.

More specifically the Javal Rule states that:

1. For keratometer readings of less than 1.75 D of astigmatism the glasses Rx will probably need about .25 D less correction if the astigmatism is with-the-rule (minus axis 180, plus axis 90).

2. For against-the-rule astigmatism (minus axis 90, plus axis 180) of less than 2.50 D as measured by the keratometer, the glasses Rx will probably need .75 to 1.00 D more correction.

3. When K readings reveal astigmatism greater than 2.5 D, both with-the-rule and against-the-rule types need greater correction. Against-the-rule corrections need the greatest change. For example: If the K readings reveal 4 diopters of astigmatism, a with-the-rule correction may need to be 4.5 D and a glasses correction for against-the-rule astigmatism may need to be 5.5 D.

Practical application

If you are using keratometry to guide you in astigmatic correction, be aware that the refractive correction may need a significantly greater cylinder correction than the K reading if the astigmatism is against-the-rule or if the K readings reveal astigmatism greater than 2.5 D.

Procedure review

For those of you new to refractometry here is a review of the procedure for converting K readings to phoropter settings.

1 Subtract the lower number reading from the higher number reading. This is the cylinder power value.

2 If working in plus cylinder, take the axis of the higher number as the axis of the cylinder.  If working in minus cylinder, take the axis of the lower number as the axis of the cylinder.

Example:
K1: 46.50 x 90  K2:  45.00 x 180
The difference is 1.50 D
minus cylinder = -1.50 X 180
plus cylinder = +1.50 X 90
 

2. The patient must have relatively good vision, and must be relatively “sharp” mentally.

3. The technique is more time consuming than the cross-cylinder, and it is not more accurate.

So, why should you spend your time reading this section? Good question.
If you use minus cylinder, it is an accurate alternative to the cross-cylinder test, and you may be tested on this information if you are taking the COT or the COMT exam. If these reasons don’t apply to you, then you may want to read out of curiosity.
 

Types of astigmatic dials

There are two basic types of dials. The clock dial has 12 spokes labeled with the clock hours. It is easier for the patient to identify the blackest lines with this type, but the axis accuracy is less because of the wide spacing.

You can use a plus cylinder phoropter, but you have to transpose as you go along.  For example, the axis of the plus cylinder would be lined up with the darkest line identified by the patient.  You would then add plus cylinder power, but for each amount of plus cylinder power you have to add an equal amount of minus sphere power.  You would continue until the patient reports all lines are equally black.

There are devices that make astigmatic dials easier to use. One projector slide has a pointer on the dial that is controlled by a knob on the slide. The patient guides the refractometrist to align the pointer with the blackest line.

Some slides also have a separate dial that is coordinated with the pointer. This dial may be a cross or a parabola that is used to further refine the axis and/or the power. The patient is asked to indicate when the two arms of the cross are equally black as the cylinder power is increased.

If you are currently using the cross-cylinder, you are probably not going to switch to an astigmatic dial. Here is what to remember for the test:

1. Astigmatic dials work best with minus cylinder phoropters. 

2. If using a clock type dial, multiply the small hour number by 30 to arrive at the axis of the correcting minus cylinder.

3. If using a degree type dial, the axis of the correcting minus cylinder is 90 degrees from the darkest spoke.

4. The correct minus cylinder power is reached when the lines of the dial or cross look equally black and sharp.
 

I am not a big fan of this test, and I rarely use it.  I find that 9 times out of 10 the benefits do not justify the time spent.  The red-green test usually only works well if the the patient sees well, and I would rather perform binocular balancing (Module 3) on these patients.  The red-green test may have the most benefit for the patient with good vision in only one eye.

Procedure

The fellow eye is occluded and the patient views the 20/30 or 20/40 line with the red/green slide in place in the acuity projector. Half the letters have the red background and the other half have the green background. The patient is asked which letters look darker and sharper, the letters in the green, or the letter in the red. If the patient answers “red”, then .25 D sphere is subtracted. If the patient answers “green”, then .25 D sphere power is added. The procedure is repeated until the letters look equally dark and sharp in the red and the green. Some refractionists prefer to keep the patient barely “in the green” so that the patient has a small amount of accommodative control.

The test is not dependent upon color vision. If the patient is red/green confused, you can just identify "letter on the right side of the line" vs. "letters on the left side of the line" instead of "letters in the red" vs. "letters in the green".

How it works

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