Subjective Refraction and Lens Prescription
Subjective refraction begins with the result from retinoscopy, autorefraction, or the patient’s current glasses. When no prior information is available, subjective refraction can begin de novo.
When starting de novo (“from the beginning”) two methods can be used to determine if cylinder is present. (See Sixteen Tips for Accurate Subjective Refraction Results, Tip 9 on page 64.)
The patient’s refinement of the starting point is necessary to arrive at the best correction for the refractive error, even when retinoscopy is performed expertly or a modern autorefractor is used.
The starting prescription is dialed into the refractor, usually referred to as a phoropter or by the identical sounding trade name, Phoroptor (Reichert Technologies). Alternatively, loose lenses or lenses put in a trial frame can be used when a phoropter is unavailable or for a patient unable to cooperate. The phoropter is a collection of these lenses, with excellent design to provide greater ease and convenience for the refractionist (Figure 2-1).
Figure 2-1. Phoropter.
▸ Sphere power dial: Whether one is working in the plus or minus range, pulling down is adding plus power and pushing up is adding minus power.
▸ Jackson cross cylinder (JCC): When lowered into position, it clicks into place. Likewise, when its dots are in proper position (straddling or on axis), they click into place. When changing from the straddling position to the on axis position, turn the JCC clockwise. There are two sets of corresponding dots and arrows; choose one set to work with (the set more easily seen). The JCC has two flip dials; pick the one most comfortable to use, and always turn it in the same direction, even when repeating choices.
▸ Cylinder axis indicators (one large, one small): The degrees are marked from 0 to 180, matching above and below the horizontal. The large and small axis indicators correspond to each other, with the larger marked in 5-degree increments and the smaller in 15-degree increments. When the JCC is clicked into position, it is in front of the smaller axis indicator.
▸ Cylinder axis knob: It has two arrows 180 degrees apart, each indicating the axis on the larger cylinder axis indicator. This knob turns three things: its two arrows, the two arrows of the smaller cylinder axis indicator, and the JCC. All three are synchronized.
The arrows on the small and large cylinder axis indicators correspond and move together.
Note
Hand-held JCCs are also available. They come in .25-diopter steps up to 1 diopter, both in plus and minus cylinder.
The JCC has been known to instill dread in some beginning refractionists. Just the opposite should be the case. This brilliant two-in-one little lens will find both the cylinder axis and power with a few easy-to-learn maneuvers. We owe it our gratitude for what it can do for us. It should be loved, not feared!
Prior to beginning, check these four (in this order):
1. Distance/Near convergence levers
2. Position of pupils
3. Vertex distance
4. Leveling bubble
Prior to positioning the patient, clean the forehead rest and the back of the phoropter with an alcohol pad.
The phoropter will elevate slightly as it is being locked into position. After the phoropter is in place, check these four: distance/near convergence levers, the position of the pupils, the vertex distance, and the leveling bubble.
The forehead rest should be extended to its maximal length, with the patient being told, “I’ll bring the machine to you.” Extending the forehead rest prevents the patient from being too close to the phoropter lenses, possibly rubbing lashes against them. Bringing the phoropter toward the patient, rather than having the patient lean forward at the start, allows you to move the forehead rest (with the patient’s head gently against it) to the proper position without the patient possibly leaning forward excessively.
Center each pupil in the viewing aperture by adjusting the interpupillary distance (PD) knob. Do so after making certain the distance/near convergence levers are in the wide position, with each pushed fully to the side. After the distance refraction is completed, if near vision is to be evaluated to determine the proper Add, the levers are then moved fully inward. This will position the phoropter lenses for the convergence that will occur with close reading.
The patient’s working position should be such that the phoropter lenses are positioned at the anticipated position of the eyeglasses lenses, or the glasses plane. The vertex distance is the interval between the outer surface of the cornea and the inner surface of a correcting lens. This distance plays an important role in precise focusing onto the retina, especially with larger refractive errors.
The patient’s position is best adjusted by glancing around the side of the phoropter and turning the forehead adjustment knob until the patient’s lashes are near, but not touching, the lenses. Alternatively, one can use the mirrors located at the sides of the phoropter. When refraction of the right eye has been completed, it is good to recheck the patient’s position once again before refracting the left eye.
The leveling bubble must be adjusted so that it is centered. It is checked last because the bubble’s position will change if the phoropter is moved. An exception to centering the leveling bubble occurs if the patient has a chronic head tilt, which will result in the glasses frames being tilted. In that situation, the phoropter should have a matching tilt.
Stand on the right side of the patient when refracting each eye. Remember to occlude the left eye when beginning the right eye’s refraction, and conversely for the left eye’s refraction. Prior to beginning the left eye’s refraction, open its viewing aperture before occluding the right eye. If not done in this sequence, both eyes will be occluded momentarily, and some patients find this disconcerting.
Note
It is not necessary to have all of the lights off in the examining room when subjective refraction is being performed. Most modern projection charts have sufficient luminosity to permit some ambient light. The projection chart should be approximately 30 times as luminous as the ambient light.
Having dim illumination in the exam room allows the refractionist to see the numbers while changing power and axis.
It is preferable to have a consistent level of illumination during subjective refraction so that it is not a variable when comparing visual acuity from one visit to the next.
▸ Manifest refraction: A refraction without cycloplegic drops; it is also called a “dry” refraction.
▸ Cycloplegic refraction: A refraction done with cycloplegic drops given to dilate the pupils and prevent accommodation; it is also called a “wet” refraction.
▸ Post-cycloplegic refraction: A dry refraction performed on a visit at least several days after a wet refraction. The purpose is to see how much of the full cycloplegic refraction found on the previous visit can be tolerated.
THE FOUR STEPS OF SUBJECTIVE REFRACTION
Subjective refraction consists of four sequential refinement steps performed in the following order:
- Step 1: Sphere
- Step 2: Cylinder axis
- Step 3: Cylinder power
- Step 4: Sphere
The purpose of each of the four steps is to locate the correct endpoint. This is accomplished by moving first in larger, then smaller, increments in order to hone in on the endpoint for each step. The process is referred to as refinement. This method is similar to how an address can be located on a map, first finding the city, then neighborhood, then street, and then house.
During each of the four steps, the patient is given a series of choices and asked to make a comparison, letting you know which of the two choices you are showing is clearer, or whether they appear to be the same in clarity.
There are two systems used for determining a patient’s refractive error: the plus cylinder method and the minus cylinder method. They are two different ways of measuring the same thing, just like inches and meters. Most refractionists work in one of the methods exclusively, depending on their training. Where the methods differ, they are discussed separately with black tabs designating plus cylinder pages and red tabs designating minus cylinder pages.
Refining cylinder axis always precedes refinement of cylinder power. The correct axis can be located before power is refined, but the correct power can only be determined at the correct axis.
The right eye should always be refracted first and recorded first. This will prevent the prescription being written with the eyes reversed (and for the rest of the eye examination, the right eye should be evaluated first to prevent notation error).
Work with the smallest line on the acuity chart that is successfully read. This allows the patient to make the finest discrimination between the choices presented. As the refraction progresses, the line is lowered periodically. Improvement in visual acuity confirms the refraction is moving in the right direction.
Step 1: Sphere
The large sphere power dial at the side of the phoropter is used to show the patient a series of two choices to compare. The patient is asked to compare the two and tell you which is more clear, or that they appear the same in clarity.
▸ With the sphere power to be refined in place, choices are given in the plus and minus direction, changing the sphere based on the patient’s preferences.
▸ Begin with choices 0.50-diopter apart before further refining in 0.25-diopter increments. If the patient has poor vision, or has difficulty making fine discriminations, give choices in larger increments.
▸ Always checking in the plus direction first is helpful in trying to avoid stimulating accommodation, and it keeps the method consistent. Whether in the plus or minus range, pulling down is adding plus power and pushing up is adding minus power.
Endpoint is the spherical lens that is reported by the patient to yield the clearest view of the letters on the acuity chart. If the patient reports the clarity between two choices to be the same, always favor less minus. (See Over-Minusing on page 60.)
Sphere is refined in Steps 1 and 4:
♦ Step 1 (in an eye with astigmatism) places the circle of least confusion, the blur circle, onto the retina.
♦ Step 4 (after the astigmatism has been corrected in Steps 2 and 3) places the focal point onto the retina.
Note
If the patient has no astigmatism, Step 1 places the focal point onto the retina; it is the same as Step 4.
Step 2: Cylinder Axis—Plus Cylinder Method Locating the cylinder axis precedes determining the cylinder power. The correct axis will be found with the not yet refined (incorrect) cylinder power. Cylinder power will not be correct if measured at the wrong axis. The JCC is used to refine the cylinder axis. It leads and determines how the axis knob is turned. After it is lowered into position, the thumb and forefinger are placed on top of the flip dials. The JCC is then turned clockwise until it clicks into position with the white and red dots on either side of the axis arrow. The JCC is now straddling the axis. The working axis is identified by two arrows 180 degrees apart on the large axis “clock,” which has a mark every 5 degrees. There are corresponding axis arrows 180 degrees apart beneath the JCC with a mark every 15 degrees. To avoid confusion, pick one of the arrows behind the JCC and work only with it and the dots surrounding it. The set most easily viewed should be used. The JCC dial is then flipped, thereby giving a first and second choice to compare. The patient indicates which of the two choices can be seen more clearly. It is most efficient to always flip the dial in the same direction, even when repeating a pair of choices. When the patient indicates which position of the JCC is clearer, the axis is rotated in the direction of the white dot for that choice (i.e., “follow the white”; Figure 2-2). Begin with larger steps, typically 10-degree changes. Then, as the correct axis is approached, make 5-degree shifts. If initially there is significant uncertainty about axis location, you can move in 15-degree steps. Reminder: The JCC—Dr. Jackson—leads and the cylinder axis knob follows its instructions. Figure 2-2. Refining the PLUS CYLINDER AXIS using the JCC. A. Position the JCC so that the red and white dots are “straddling the axis” to be refined (i.e., one dot on each side of the axis arrow). B. Look at one axis arrow and the set of dots surrounding it. Flip the cross cylinder to give the patient two choices, asking which is more clear. C. Rotate the axis in the direction of the white dot: FOLLOW THE WHITE. If choice 1 is clearer, the white dot is on the left; turn the axis to the left. If choice 2 is clearer, the white dot is on the right; turn the axis to the right. The correct cylinder axis has been located when the cross cylinder is flipped and the patient responds that the two choices are “the same” in clarity. The higher the cylinder power, the easier it is to locate the axis. If, in Step 3, cylinder power increases significantly, go back and recheck the axis (and then power again.) Endpoint when refining cylinder axis: The two choices given to the patient are reported to be the same in clarity. At this point, the correct axis is being bracketed. As cylinder axis refinement is concluding, a patient may go back and forth between two 5-degrees–apart choices, not responding “the same” for either choice. This indicates the axis is somewhere between the two choices. In this situation, it is fine to select the choice closer to 90 or 180 degrees as the endpoint (or the one closer to the axis in the patient’s current glasses). Note When positioning the JCC in front of the patient’s eye with the axis straddled, let the patient know that the letters on the acuity chart will become more blurred. Although neither of the choices you are giving for comparison is completely clear, you want to know which is more clear. It may be necessary to move up one line on the acuity chart for the patient to see the letters well enough to compare the choices given. Remember to return to a smaller line for Step 3. Step 2: Cylinder Axis—Minus Cylinder Method Locating the cylinder axis precedes determining the cylinder power. The correct axis will be found with the not yet refined (incorrect) cylinder power. Cylinder power will not be correct if measured at the wrong axis. The JCC is used to refine the cylinder axis. It leads and determines how the axis knob is turned. After it is lowered into position, the thumb and forefinger are placed on top of the flip dials. The JCC is then turned clockwise until it clicks into position with the white and red dots on either side of the axis arrow. The JCC is now straddling the axis. The working axis is identified by two arrows 180 degrees apart on the large axis “clock,” which has a mark every 5 degrees. There are corresponding axis arrows 180 degrees apart beneath the JCC with a mark every 15 degrees. To avoid confusion, pick one of the arrows behind the JCC and work only with it and the dots surrounding it. The set most easily viewed should be used. The JCC dial is then flipped, thereby giving a first and second choice to compare. The patient indicates which of the two choices can be seen more clearly. It is most efficient to always flip the dial in the same direction, even when repeating a pair of choices. When the patient indicates which position of the JCC is clearer, the axis is rotated in the direction of the red dot for that choice (i.e., “follow the red”; Figure 2-3). Begin with larger steps, typically 10-degree changes. Then, as the correct axis is approached, make 5-degree shifts. If initially there is significant uncertainty about axis location, you can move in 15-degree steps. Reminder: The JCC—Dr. Jackson—leads and the cylinder axis knob follows its instructions. Figure 2-3. Refining the MINUS CYLINDER AXIS using the JCC. A. Position the JCC so that the red and white dots are “straddling the axis” to be refined (i.e., one dot on each side of the axis arrow). B. Look at one axis arrow and the set of dots surrounding it. Flip the cross cylinder to give the patient two choices, asking which is more clear. C. Rotate the axis in the direction of the red dot: FOLLOW THE RED. If choice 1 is clearer, the red dot is on the left; turn the axis to the left. If choice 2 is clearer, the red dot is on the right; turn the axis to the right. The correct cylinder axis has been located when the cross cylinder is flipped and the patient responds that the two choices are “the same” in clarity. The higher the cylinder power, the easier it is to locate the axis. If, in Step 3, cylinder power increases significantly, go back and recheck the axis (and then power again.) Endpoint when refining cylinder axis: The two choices given to the patient are reported to be the same in clarity. At this point, the correct axis is being bracketed. As cylinder axis refinement is concluding, a patient may go back and forth between two 5-degrees–apart choices, not responding “the same” for either choice. This indicates the axis is somewhere between the two choices. In this situation, it is fine to select the choice closer to 90 or 180 degrees as the endpoint (or the one closer to the axis in the patient’s current glasses). Note When positioning the JCC in front of the patient’s eye with the axis straddled, let the patient know that the letters on the acuity chart will become more blurred. Although neither of the choices you are giving for comparison is completely clear, you want to know which is more clear. It may be necessary to move up one line on the acuity chart for the patient to see the letters well enough to compare the choices given. Remember to return to a smaller line for Step 3. Step 3: Cylinder Power—Plus Cylinder Method White = Add cylinder power Red = Subtract cylinder power The JCC is used to refine the cylinder power. It leads and determines how the cylinder power knob is turned. Both flip dials are again held to rotate the JCC clockwise until it clicks into place when the red or white dot is aligned with the axis determined in Step 2 (Figure 2-4). The JCC is now on axis; a dot overlies the arrow. Choices are given by flipping the JCC, and the patient indicates which of the two choices you have shown is clearer. It is most efficient to always flip the dial in the same direction, even when repeating a pair of choices. If it was necessary to move up one line on the acuity chart for Step 2, return to the previous line to begin Step 3; acuity is better with the JCC on axis than when it is straddling the axis. If the patient responds that the letters appear to be more clear when the white dot is positioned on axis, then plus cylinder power is added (i.e., white = add). If the letters are more clear when the red dot is positioned on axis, then plus cylinder power is subtracted (i.e., red = subtract). Cylinder power is usually modified by 0.25-diopter changes from the start. Reminder: The JCC—Dr. Jackson—leads and the cylinder axis knob follows its instructions. During the process of refining cylinder power, an adjustment to spherical power must be made whenever the cylinder power has been modified by 0.50 diopter. The adjustment is necessary to reposition the circle of least confusion onto the retina. Endpoint when refining cylinder power: The two choices given the patient are the same in clarity. At this point, the correct cylinder power is being bracketed. As cylinder power refinement is concluding, a patient may go back and forth between two 0.25-diopter–apart choices, not responding “the same” for either choice. This indicates the power is somewhere between the two choices. In this situation, it is fine to select the choice with less cylinder power as the endpoint. Figure 2-4. Refining the PLUS CYLINDER POWER using the JCC. A. Turn the JCC clockwise so that the red or white dot is “on axis” (i.e., aligned with the axis determined by the previous step). B. Flip the cross cylinder to give the patient two choices, asking which is more clear. C. Add or subtract cylinder power: WHITE = ADD; RED = SUTRACT. If choice 1 is clearer, the white dot is chosen; add plus cylinder power. If choice 2 is clearer, the red dot is chosen; subtract plus cylinder power. (See The Adjustment Within Step 3 on page 45.) The correct cylinder power has been determined when the cross cylinder is flipped and the patient responds that the two choices are “the same” in clarity. Step 3: Cylinder Power—Minus Cylinder Method Red = Add cylinder power White = Subtract cylinder power The JCC is used to refine the cylinder power. It leads and determines how the cylinder power knob is turned. Both flip dials are again held to rotate the JCC clockwise until it clicks into place when the red or white dot is aligned with the axis determined in Step 2 (Figure 2-5). The JCC is now on axis; a dot overlies the arrow. Choices are given by flipping the JCC, and the patient indicates which of the two choices you have shown is clearer. It is most efficient to always flip the dial in the same direction, even when repeating a pair of choices. If it was necessary to move up one line on the acuity chart for Step 2, return to the previous line to begin Step 3; acuity is better with the JCC on axis than when it is straddling the axis. If the patient responds that the letters appear to be more clear when the red dot is positioned on axis, then minus cylinder power is added (i.e., red = add). If the letters are more clear when the white dot is positioned on axis, then minus cylinder power is subtracted (i.e., white = subtract). Cylinder power is usually modified by 0.25-diopter changes from the start. Reminder: The JCC—Dr. Jackson—leads and the cylinder axis knob follows its instructions. During the process of refining the cylinder power, an adjustment to spherical power must be made whenever the cylinder power has been modified by 0.50 diopter. The adjustment is necessary to reposition the circle of least confusion onto the retina. Endpoint when refining cylinder power: The two choices given the patient are the same in clarity. At this point, the correct cylinder power is being bracketed. As cylinder power refinement is concluding, a patient may go back and forth between two 0.25-diopter–apart choices, not responding “the same” for either choice. This indicates the power is somewhere between the two choices. In this situation, it is fine to select the choice with less cylinder power as the endpoint. Figure 2-5. Refining the MINUS CYLINDER POWER using the JCC. A. Turn the JCC clockwise so that the red or white dot is “on axis” (i.e., aligned with the axis determined by the previous step). B. Flip the cross cylinder to give the patient two choices, asking which is more clear. C. Add or subtract cylinder power: RED = ADD; WHITE = SUBTRACT. If choice 1 is clearer, the red dot is chosen; add minus cylinder power. If choice 2 is clearer, the white dot is chosen; subtract minus cylinder power. (See The Adjustment Within Step 3 on page 45.) The correct cylinder power has been determined when the cross cylinder is flipped and the patient responds that the two choices are “the same” in clarity. Optically: ♦ A change in cylinder power moves one focal line—the one parallel to the axis. ♦ A change in sphere power moves the conoid of Sturm—both focal lines.
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