Photograph by Iris Johnson
By Palmer R. Cook, OD
As Mr. Adrian Monk, the obsessive-compulsive police detective of the long-running TV series “Monk” says, “It’s a blessing... and a curse.” Of course, he is referring to his own peculiar ability to deal both with crimes and his own personal world. In our ophthalmic world, we may be echoing his words as the number of compensated prescriptions proliferate, unless we understand them, use them and dispense them appropriately.
To unravel the challenge of compensated prescriptions, the best place to start is with a good understanding of what “prescription” really means. One of the witches in Macbeth chanted, “In the cauldron boil and bake; eye of newt and toe of frog, wool of bat and tongue of dog, adder’s fork and blind-worm’s sting; lizard’s leg and owlet’s wing,” as they concocted their own “prescription” or “recipe.” Certainly the witches of Macbeth had an outcome in mind, although it was not clear, single binocular vision.
There is an unspoken meaning in the lens prescriptions that every doctor writes. If you consider a simple, single vision lens prescription such as OD -1.00DS and OS +2.00DS, the meaning is that the doctor wants one diopter of optical power subtracted from the patient’s right eye and two diopters of optical power added to the left eye in order to give optimal images on the patient’s retinas. He does not mutter, “With the refractor I did toil and rake, among all powers for vision’s sake, testing with lenses—cylinder and sphere, and through their centers he did peer, so it’s said with no doubt, this is the ’scrip I wheedled out,” though he or she might be tempted to do so at the end of a long day.
With lens prescriptions, the doctor is not telling you to simply place lenses in front of the patient’s eyes that show the powers by lensometry to match those written on the Rx form. Rather, he wants the lenses you place in front of that patient to have the effect of combining the powers he has written with the optical powers of a patient’s visual system. The doctor is specifying what he wants, and until the advent of digital surfacing, we mostly stuck with using the powers the refractor yielded. Before compensated technology, it was usually only with strong Rxs that we even took the power shift into account that occurs when the fitting vertex is not the same as the refracting vertex.
In the days before computers and digital surfacing, a doctor who prescribed a -5.00DS could have written the Rx -4.16 -0.64 x 077 .05rBO, and he could have included instructions to produce it on a +2.50 base curve and dispense the lens in minus cylinder form with a 14-degree wrap, 8-degree pantoscopic tilt and fit it at a 13.5 mm vertex distance. After placing orders for a few Rxs like that one, he would probably have found a bottle of Eye of Newt and Toe of Frog in his mailbox, compliments of his lab.
Even if a lab in pre-computer days produced such an Rx, the optician would be challenged with finding a frame that both pleased the patient and had the required wrap, panto and vertex distance. Today, the doctor can simply keep the lenses of his refractor properly oriented and positioned as he examines, and then the optician (after fitting the frame of choice) can take the measurements. The result will be a lens that is spot-on power when the patient wears it in the proper orientation and position if the patient is given a lens design that is individualized (i.e., that compensates for vertex, panto and wrap).
Take an uncut -5.00 sphere from its envelope. Center it and clamp it in your lensmeter. You read a solid -5.00DS. Now loosen the clamp and tilt the lens as though it had 20 degrees of pantoscopic tilt and some wrap. Check the power. Your -5.00 sphere will now be a sphero-cylinder with a minus axis somewhere around 180, and it may have a prism component. The bottom line: a lens has various powers depending on how light flows through it. That’s what compensated prescriptions are all about.
Today, when you receive a premium, individualized lens (i.e., one designed to give exactly the effect the doctor wants) or any lens design that compensates for position of wear, it will arrive with a notation of the power that will show when you verify it with light rays passing through the lens at right angles to the lens’ surfaces (e.g., in your lensmeter). You (and your patient) are dealing with not one, but two prescriptions. Both are correct, depending on how you look at it. Your lensmeter measures it and reads a power that varies from the written Rx, but when your patient looks through it (if positioned correctly), it will add or subtract power from the patient’s eyes exactly as the doctor wants.
A compensated prescription is the power your lensmeter must read in order to give the correct Rx when the lens has been tilted for wrap (around a vertical axis) and for panto (around a horizontal axis), and if it is fitted at some vertex distance other than the refracting vertex distance.
The doctor examines the patient with light flowing through the lens at a right angle to the lens plane, and when you fit the lens with pantoscopic tilt and/or wrap, and/or at a different vertex distance than the refracting distance, the power through the optical center of the lens changes. As we all know, when the patient looks away from the optical center (as in looking down to read), pantoscopic tilt is needed for vision and comfort. The result is that when we fit patients with non-compensated lens designs, and if we give the needed panto or wrap, we are no longer fitting them with the prescribed power at the Major Reference Point or Distance Reference Point in the case of PALs.
DON’T SACRIFICE ACCURACY
Older lens technology sacrifices straight-ahead vision because of the power change induced by the tilt. If the frame has some face-form or wrap, the Rx is changed. Adaptation times that range from “slow-to-never” can result, especially if the patient is sensitive and/or the power of the lens is moderate or stronger. To better understand “compensated prescriptions,” keep in mind that the term refers only to performance through the optical center or Distance Reference Point, and curves used to minimize peripheral aberrations are a “whole ’nother thing.”
Your biggest challenge with compensated prescriptions will probably arise when patients experiencing adaptation problems decide the problem might be your incorrect filling of the prescription. This, more often than not, leads them to a “third party.” The patient’s “aha” moment arrives when the third party tells them that the -5.00 lenses their doctor prescribed are not at all what they received. Of course, you can expect them on your doorstep shortly thereafter. You may then be faced with adder’s fork and blind-worm’s sting, and as you try to explain, the misinformed patients may ultimately offer you a serving of lizard’s leg and owlet’s wing. In other words, you could find yourself in a difficult position.
When the lenses are fitted, you can pre-empt problems if you dispense a written cautionary statement to alert anyone who analyzes the lenses that the prescription is compensated (Fig. 1). Also caution the patient that their new high-tech lenses will not work their best unless they are fitted and maintained properly.
You might be tempted to release the “compensated Rx” to your patient, but that’s probably not a good idea. If you dispense the compensated Rx, there is always a possibility that someone would try to put those powers into eyewear with different fitting characteristics or replace the lenses with a different index, and such changes would not work well. It could lead to a lot of confusion for the patient as well as having potential for creating problems for your patient. As an additional service, you might jot down the date, along with the wrap, panto and vertex measurements on your business card or on the dispensing card. The patient should be told to keep that card and show it anytime someone is going to realign his or her eyewear because the realignment should bring the lenses to their original wrap, panto and vertex positions.
THE ACTION IS AT THE SWEET SPOT
For purposes of clarity, it is important to recognize that when you are considering “compensated prescription” lenses, it is all about the power at the point in the lens that the patient uses for his best distance vision. The topic of peripheral aberrations is also important, and today’s ophthalmic technology can do very good things there too, especially with power error and more importantly, with marginal astigmatism. The only aberration that compensated Rxs address is defocus (i.e., failure to correct the refractive error).
BUT WHAT IF . . .
Incorrect measurements can give poor results for your patient, but with just basic, inexpensive tools, you should get very usable and effective results. The simplest tools are an inclinometer, wrap template and a PD rule. More sophisticated tools may increase accuracy.
As Table 1 shows, errors of 1 mm in vertex and 1 degree in wrap can be small compared to the errors introduced by using a non-compensated lens design. If you can take a measurement with no more than a 1-mm or 1-degree error, you can be better off than using average or default measurements. If you feel you need greater accuracy, it’s time to invest in the higher tech instruments for taking measurements. But using the same default or average measurements for all patients is not good practice.
BE MORE THAN A DISPENSING ECP
When you fill an Rx with a “compensated prescription,” you are crossing a threshold. Just as some pharmacies are “dispensing” and sell only already-compounded medications, while others are known as “compounding pharmacies” because they use the doctor’s Rx (i.e., recipe) to make a “custom-designed” formulation, your office can now be “dispensing” for patients whose lenses are not compensated and “compounding” for those who receive compensated lenses.
When you design eyewear with lenses that are compensated, be prepared to shoulder the responsibility of understanding what that means and how it benefits the purchaser. You should also protect your patient or client and yourself by giving full disclosure about how the Rx was filled. In the long run, that will be to everyone’s benefit.
As a “compounding” dispenser, you take appropriate measurements and order lenses that when fitted correctly, give precisely the required powers. Doing this for your patients is a service that allows you to fill prescriptions that perform as the doctor intended. Whether a prescription warrants dispensing in a compensated form, and whether the patient is sensitive enough to appreciate the difference is totally up to your professional judgment unless the doctor has already specified “Compensated” or “Non-Compensated” as part of his or her Rx.
- Compensated prescriptions assure that your patient gets the prescribed power at the center of the lens. It gives a benchmark or starting point to assure that the “digital magic” in the periphery is also going to work as well as the selected design allows.
- Measurements should be taken. In Table 1, all of the patient outcomes are very close to the required Rx, especially when compared with the outcome with a non-compensated design.
- Lens compensations vary with the index of the lens material, base curve, positioning, lens thickness and the power of the prescription.
- A compensated Rx will not work the same if the pantoscopic tilt, wrap and vertex change, so patients should know that their eyewear must be kept properly aligned to the original parameters.
- Compensated lenses that are fit in a frame with different panto, wrap or vertex fitting characteristics will not have the same spot-on power as the prescription calls for.
- If you do not supply the refracting vertex distance, your lab will assume that the refracting vertex distance and the fitting vertex distance are the same for purposes of power compensations.
Would Mr. Monk approve of all this? Possibly not, since the compensated Rx and the written Rx don’t look the same, and the right and left prescriptions are seldom exactly equal, but surely he would appreciate the combination of science, art and experience needed for dispensing the best that technology has to offer.