Orientation to Refraction

Ben Freed,OD, 2000

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    The manual determination of the refractive state of the eye is an essential and fundamental skill for the student of clinical ophthalmology to learn. This outline is designed as a first-order initial orientation to be read as you begin your clinical work. It is far from a thorough manual on the subject, and not a source of theory.

    As you learn to refract, it is recommended to adhere to the following sequence in order to arrive at the refractive error accurately: Lensometry, refractive case history, visual acuity, retinoscopy and subjective. Keep in mind that we are making objective and subjective determinations of the optical status of the eye; the subsequent eyeglass prescription is usually a modification of the measurements.

Lensometry: Measuring the dioptric power of the patient’s current eyeglasses first will orient you towards the patient’s refraction. In the following steps, you will be taking two power measurements, the difference between the two being the cylinder (the amount of astigmatic correction). You will also be measuring the axis of the cylinder.

A.    Turn on the instrument and focus the eyepiece(ocular)by focusing the hair-line black reticle. If it is not in focus, the readings will be erroneous. (Many optical instruments have a reticle that needs to be adjusted in this way to accommodate the user’s own refractive error.) fig.1                                          

B.    Place the back vertex (the ocular side) of the unknown lens against the stop of the lensometer. Make sure both eyewires, right and left, are touching the stage. If they are not, then the frame is not level and you will read the axis of the cylinder incorrectly.   fig.2                          

C.    By moving the lens side to side, you can center the lighted lines (the target) at the center of the black reticle. This places the optical center of the lens at the stop (The optical center of the lens is the point on the lens through which light travels ndeviated and therefore where the prismatic power is zero).

D.    Notice that the lighted target lines are a cross composed of two sets of lines oriented 90° apart. Try focusing the lighted target lines. If all the lighted lines are in focus simultaneously, the lens is spherical, not cylindrical. If only one set is in focus (the other set 90° away is blurred), the lens is cylindrical. If this is the case, focus one set of lighted lines with the power drum, while simultaneously turning the axis dial so that the lines are not only clear, but they are also unbroken. Note the power on the drum.  fig. 3.                                        

If the unknown lens is cylindrical, the set of lines 90° away will be out of   focus. The power of the first set of clear lines is called the sphere reading.

E.     Now focus the other set of lighted lines (90° away from the first set) so that they are clear and unbroken. Note the new power on the drum. Also note the axis reading now, which can be read on the outside dial. fig 4.

Here’s the tricky part:  Look at the orientation of this second set of clear lighted lines. If it doesn’t match the axis you just noted on the outside axis dial, turn the dial 90 degrees so that the axis reading on the outside axis dial does match the orientation of the second set of lighted lines. Now that the cylinder axis as read off the protractor on the target matches the axis reading on the outside axis dial, you have a correct axis reading on the outside dial. Make note of it.

      If the second set of lines was focused at a lower plus(more minus)power, you have measured in minus cylinder form (as opposed to plus cylinder form),  which is the convention used here at the Infirmary. Since our convention is minus cylinder form, you should get used to making the less minus measurement your first measurement. That would mean that your second measurement would be more minus, thus allowing you to record the reading in minus cylinder form. In this case (minus cylinder form), the first measurement is the sphere, and the increase, or change in minus power you had to travel to the second, (higher minus power) focus is the cylinder power.  In the record, notate the measurement as “W”, (for “wearing”), in the following order: sphere, cylinder, and axis. For example:                                 OD: +3.50 sph., -2.50cyl. X 25

                                                 OS: +3.25 sph, -1.75cyl. X 175

[Note: If the second set of lines was focused at higher plus amount (less minus), you have measured in plus cylinder form, in which case you should start again by first clearing the higher plus (lower minus) reading. As you get comfortable with the feel of the lensometer, you will automatically begin by focusing on the higher plus (less minus) reading and then making your second measurement the lower plus (more minus) reading.]

Measure the bifocal add: Turn the glasses around so that the front of the lens is up against the lens stop. Take a reading of one set of unbroken, focused lighted lines through the distance portion, then move the stage up until the bifocal add is against the stop. Re-focus the same lighted lines again, but now through the add. The difference between the first and second reading is the add strength.

Refractive History

In order to properly prescribe spectacles, it is critical to determine the patient’s

subjective complaints of blur or asthenopia (eyestrain) with regard to distance and near vision.  Ask the patient about their perception of their vision with and without their current spectacle correction.  For example, many far-sighted individuals may choose to not wear correction for distance, even though one has been prescribed.  Conversely, many

near-sighted patients take their glasses off to read.  Determine whether the patient is satisfied or not with their current prescriptions, both far and near, and the particular fit of the glasses, including the height of the bifocal segment, if applicable.  Consider asking the following questions regarding each individual pair of glasses

1.Are you satisfied with these glasses?(DV and NV)

2.Do you see well with the glasses; do you see better without the glasses?

3.Do your eyes feel comfortable with this pair or do you experience eye strain, blurry vision, fatigue, double vision, a sensation of pulling, or headaches?

4.Are you able to see/read what you need to at a particular required distance; for instance the computer distance (20 inches), the blackboard, driving, small print?

5. Can you read for extended periods of time?

Visual Acuity

It is necessary, before beginning the actual refraction to measure the “entering” visual acuity. If there is no ocular pathology, the acuity will give you an idea of the degree of uncorrected refractive error. This is done by testing each eye individually, (with correction) while occluding the other eye.  A 20-foot test distance is used (or simulated) and the  "Snellen" fraction is noted, with the numerator being the distance at which the eye was tested, and the denominator is the distance at which their smallest perceived letter subtends a visual angle of 5 minutes of arc.  In other words, the 200 size letter actually subtends 5 minutes of arc when presented at a distance of 200 feet. 

     If the patient cannot determine the largest letter projected (in most cases, the large E is a 400 foot letter) then you may consider bringing the chart closer to the patient.  For example, if you have a size 400 letter on a printed chart and a patient is not able to discriminate the letter until it is brought 5 feet to the patient, then record the acuity as 5/400.  If you do not have access to a hand-held chart, then determine whether the patient can count fingers at 20 feet, 10 feet, or closer.  If the patient can count fingers at 3 feet and also count fingers at 10 feet but no farther, record the acuity as "count fingers at 10 feet" as this is a higher level of visual function that "count fingers at 3 feet:".  Remember that Snellen fractions of corrected visual acuity are equivalent.  In other words, a patient who reads 5/100 with their current correction, should, theoretically, be able to discriminate 20/400. Note if the patient uses a particular quadrant of their peripheral vision to discriminate the chart. Here again is the hierarchy of tests with which you should test visual acuity:

      1. 20 foot Snellen chart acuity, up to 20/400

      2. walk hand-held chart up to 1 or 2 feet from the patient, or, if not available, finger counting at a specified distance

3.     hand motion

4.     light projection(can discriminate location of light source)

      5.   light perception

Pinhole acuity:  when looking through a pinhole, the size of the blur circle on the retina created by uncorrected refractive error is reduced, so better visual acuity though the pinhole may indicate that the vision may be improvable with refraction.

Refraction

The refraction consists of two parts: the objective and the subjective: The objective determination of the refractive error is done with a retinoscope and requires no subjective responses from the patient.  As you might guess, the subjective part of the refraction requires subjective responses on the part of the patient. 

Objective refraction...Retinoscopy

Retinoscopy becomes easier with practice, so be patient.  The objective measurement will help in decision-making regarding malingerers, media clarity, children’s and non-verbals’ refractive error, and unknown causes of reduced visual acuity.

There are two types of retinoscopes, a “spot” and a “streak” retinoscope, and the “streak” is the one you will learn to use here. The following is a cookbook method to get you started with retinoscopy(sparing you the theory): Start by turning off all the lights.

     1.     the examiner peers through the retinoscope at a known distance from the patient, while the patient is directed to look through the phoropter at a distance target. Customarily, students learn to do the test by holding the scope at 66 cm. The retinoscope’s light source is directed into the patient’s pupil and the reflex (the resulting reflection of the light source back out through the pupil) is observed. Test the right eye first, and make sure the left eye does not accommodate. This is done by placing adequate plus lens power before the left eye. Direct the patient to look at a distance target. (Note that for patients who cannot sit behind a phoropter, the test can also be done easily by using hand-held lenses.)

     2.     Turn on the retinoscope light and make sure the sleeve of the scope is in the “down” position. Fig.5.                                         

Tilt the retinoscope side to side or up and down to create movement of the light across the pupil. As your light sweeps across the pupil in a particular direction, the reflection of light returning from the patient’s pupil, called the reflex, will have its own direction of motion. As you sweep in a given direction, orient the “streak” light of the retinoscope so that it is 90 degrees away from the direction of its sweeping motion.

3.     Locate the axis of the cylinder using the retinoscope: As you sweep in a given direction and observe the reflex, watch to see if the motion of the reflex is parallel to the direction of the sweeping motion. If you see ”scissors”(non-parallel) motion, you have not located the orientation of the principle meridians; in other words, you are not sweeping along the correct axis. Re-orient the direction of the sweeping motion until the reflex moves parallel, “with” or “against” the direction of your sweeping motion.

If you sweep vertically, you are testing the 90th meridian, and if you want to test the 180^th meridian, sweep side to side, with the light streak image oriented vertically. Observe the speed and direction of motion of the reflex.

When the motion of the reflex is in the same direction as the sweeping motion of the light across the pupil, it is known as with motion; conversely, when the motion of the reflex is in the opposite direction as the sweeping motion of the light across the pupil, it is known as against motion.

      4.     Remember the following rule:    “with” motion………. add plus lenses

                                                          “against” motion……add minus

This rule means that if you see “with” motion, add plus lenses in front of the patient’s eye so that the motion of the reflex becomes “neutral” (neither with or against) motion. If you add too much plus, the motion will become against, so reduce the plus back to “neutrality”. Neutralize one meridian with spherical lenses. Then look at the meridian 90° away. (If you see “with” motion in this second meridian, start over and begin by neutralizing this (second) meridian first.). If you see against motion in the second meridian, neutralize this meridian with minus (remember, against motion…. add minus), but now use minus cylinder. You should orient the cylinder so that its axis is 90° away from the motion of the light movement, meaning that if the “against” motion is left to right as your light sweeps right to left, orient the axis of the minus cylinder at 90°.( The power of a cylinder is maximum at 90° away from its axis.)

     5.     Now you have a gross sphere, cylinder and axis in the phoropter, but it is not yet the final objective reading. At this point, there is too much plus sphere in the phoropter because you have focused the patient’s eye at the location of the retinoscope. You must subtract the dioptric equivalent of the working distance; in this case, it is 1/.66meters= 1.5 diopters. So, if the neutralizing lenses in the phoropter were, for example, +3.75sphere, -1.50cylinder X 95°, your final objective reading would be +2.25sphere, -1.50cylinder X 95°.(Another example: if the phorpter showed –3.00sphere, -0.75 cylinder X 125, you would notate –4.50 sphere, -0.75 cylinder X 125).

Subjective refraction(the manifest): Here is a cookbook method to get you started on the subjective. Hold your nose and take the plunge! After retinoscopy, you presumably have the patient fogged - over-corrected in plus power, in this case by 1.50diopters. This means that they see a very blurred chart. You also may have some cylinder in place. The basic steps in performing the subjective properly are always as follows:

1.     find the best sphere before astigmatic testing

2.     astigmatic testing

3.     final best sphere after astigmatic testing

Start by occluding the other eye, and direct the patient to read the smallest print they can resolve, which at this point is probably going to be about 20/200, since they are fogged.

1.     Find the best sphere before astigmatic testing:  Slowly reduce the amount of sphere in front of the eye, in half-diopter steps(two clicks). The chart will become clearer. As you introduce a choice of two lenses for the patient to discrimainate, ask the patient, “which is clearer, one or two?” As the chart becomes clearer, present the patient with smaller targets and ask the patient to read the chart. You will hopefully ultimately get the patient to read somewhere in the range of 20/30 to 20/20(that is, if they don’t have a visually impairing pathology). Important: don’t continue to reduce plus power(adding minus) unless you are sure they really need it to see clearer. At a certain point adding more minus will not make the chart any clearer, it will only seem to make the letters smaller(minified). So if they can see 20/20, don’t add more minus. In fact, when they read the 20/20 line, you should add a half diopter of plus to check that you haven’t over-minused them. One half diopter of plus should make the chart blurry, so take it away, bringing them back to 20/20, or whatever is the best possible acuity they can achieve without over-minusing them. The goal here is to find the best sphere, meaning the “most plus to best acuity”. When you have found the least minus lens with which they can still see 20/20(or close to it), you have found the best sphere. This is the point at which astigmatism can be tested.

2.     Testing astigmatism: After finding the initial best sphere, you can test for astigmatism. If retinoscopy indicates astigmatism, refine axis first then power using the Jackson cross cylinder(JCC). The JCC on the phoropter has a strength of +,- 0.25(to be explained later).  Fig.6

Refining the cylinder axis.

            The JCC is a single lens that is a combination of a plus and minus cylinder. The one in the phoropter is a +0.25 and a –0.25 cylinder. The red dots are the axis of the minus cylinder, and the white dots are the axis of the plus cylinder. While testing axis, you need to “straddle” the correcting cylinder with the axes of the JCC. In other words, the red dots and the white dots should be 45°  from the correcting cylinder.

Flip the JCC and ask the patient which side of the flip is clearer.

Turn the dial of the correcting cylinder axis in the direction of the red (minus) cylinder axis of the preferred orientation of the JCC. When the correcting cylinder is at the patient’s true axis, the patient will respond that both sides of the JCC flip are equally blurry. Remember to straddle the correcting cylinder with the JCC axes during the axis test. Fig. 7

Refining the cylinder power: Turn the JCC 45° so that the dots line up with the correcting cylinder axis. Now as you flip the JCC, you will either be adding –0.50 cylinder or taking away -.050cyl.    fig.8.                           

Flip the JCC. When the red dots are lined up with the correcting cylinder axis, you are increasing corrective cylinder power. Conversly, when the white dots are lined up, you are taking away corrective cylinder power. Ask the patient, “which is better, one or two?” Their preference will direct you towards increasing or decreasing the corrective cylinder. If their response directs you to add cylinder, add one half of a diopter. If their response directs you to decrease cylinder, decrease by one half of a diopter. When the responses to the JCC flip are equivocal, (equal blur on both sides of the flip) then you have theoretically arrived at the correcting cylinder amount.

NOTE: Always keep the spherical equivalent constant when changing cylinder powers. The purpose is to keep the astigmatic interval straddling the retina, which is necessary for valid astigmatic testing (theoretical explanation to come). This is done by changing the sphere power by half the amount of the cylinder change in the opposite direction. For example:  If you increase the corrective cylinder by -0.50cyl., change the sphere by +0.25sphere. And if you take away –0.50 cylinder, change the sphere by –0.25 sphere.

3.     Final best sphere after astigmatic testing: After the tests for astigmatism, the final step in the subjective is to retest for best sphere:  Start by fogging the eye slightly, by about a diopter. In one quarter diopter steps, decrease the plus in front of the eye (add minus) until the patient reads the smallest print possible(hopefully 20/20 or 20/15). Do not reduce the plus any further, or you will risk “over-minusing” the patient, which is likely to cause symptoms.

Record the subjective test as the “manifest refraction” as follows:

                                       OD: -2.50sph., -1.75 X 82          20/20+

                                                   OS: -1.75sph., -1.00 X 110        20/20-

4. Test the other eye!

 [Note: If retinoscopy indicates a spherical(non-astigmatic) error, test subjectively for the presence of astigmatism after the best sphere has been determined. Check for the  presence of cylinder power, with or against-the-rule by flipping the cross cylinder in front of the best sphere with its handle oriented at 45 degrees(the power meridians will then be at 90 and 180). If the responses indicate astigmatism, place at the appropriate orientation, adjust the sphere to keep the spherical equivalent constant, and then refine axis and power. If the responses indicate equal blur on both sides of the flip, the patient either has no astigmatism, or has cylinder axis 45 or 135, and you should test for the  possible existence of this oblique cylinder by changing the orientation of the JCC by 45 degrees and flipping again. If the responses indicate equal blur on both sides of the flip, you can be confident that there is no cylindrical component. But if the responses indicate a preference for oblique cylinder, place –0.50D cylinder at the appropriate axis, keeping the spherical equivalent constant, and refine axis and power. After refining cylinder, go back and retest for best sphere using the method discussed above.]

 END