By Palmer R. Cook, OD

For years the ophthalmic industry rolled along using the Major Reference Point (MRP), a term that refers to the one point in every lens that gave exactly the prescription that the doctor wants for his or her patient. One way to think of this union of refractive power and prism power is that the refractive power was the extroverted, always-in-the-action partner, and the prism power was the shy, retiring spouse that only appeared as part of the prescription when its strength was greater than zero. Prism was an ever-present silent partner.

The term “prescription” meant the amount of refractive power that the doctor wants to add (plus power) or subtract (minus power) from the optical system of his patient’s eye for distance seeing unless specified otherwise. Of course, if the power varied from meridian to meridian, the prescription included the “sphere” power, the “cylinder” power and an “anchor” meridian, which was either the plus cylinder axis or the minus cylinder axis.

The other factor in the lens prescription is prism, which changes the direction of light passing through what would otherwise be the optical center. Prism is prescribed to allow the eyes to work together, even though the attachments of the eye’s aiming muscles are not ideal.

By industry agreement, prism, which is needed relatively infrequently, was not included in lens prescriptions unless a change in the apparent position of whatever the patient might want to view was needed. A prescription to simply subtract one unit of optical power from the patient’s eye was written -1.00 DS, rather than -1.00 DS 0Δ. Only when the direction of light through the MRP needed to be changed, did the amount and direction of the prism appear. This was a practical system for all lenses for many years.

PROGRESSIVE LENSES
Prior to the development of progressive addition lenses (PALs), the appearance of lines in glasses heralded to the world that youth had fled, and “change of life” was pounding at the door. When multifocals without lines were perfected to the point that patients could wear them comfortably, the union of the primary refractive correction and the prism-or-no-prism components suffered a separation, or divorce. For PALs, the joining of refractive power and prism-or-no-prism was put asunder by a distance that is commonly 8 to 10 mm.

This separation allowed a longer corridor and a lens with less thickness and weight. For many patients, this separation causes only minor adaptation problems. There are some patients though, who are not so fortunate, and they endure marginal to severe difficulties.

YOU GOTTA DO SOMETHING...
These patients’ complaints range from vague (“There’s something just not right.”) to blunt (“Nothing is clear.”), and sometimes adjectives (“*!X~@≥”) are included in their descriptive remarks. Dispensers find that many of these complaints are persistent and difficult to resolve.

When these “you-gotta-do-something” patients return, the areas of unwanted cylinder in the lower lateral and nasal portions of the lenses are usually re-explained, and patients are instructed to become better “nose pointers.” For some patients, that is helpful. But for others, resolution of the difficulties remains elusive. For those patients, neither the traditional base curve changes and frame re-alignment, nor tinkering with the cylinder axes and powers help. Ultimately, many of these “problem patients” go back to lenses with lines, accept refunds or just fade away to try their “optical luck” elsewhere.

PAL GEOGRAPHY
When you specify the location of the Fitting Cross as you do for every PAL, you are giving the landmark that determines where the distance reference point (DRP), prism reference point (PRP), near reference point (NRP) and beginning of the corridor are to be positioned. If you place a lens order with no prism for a PAL you are specifying that you want the optical center at the PRP, which often is 2 to 4 mm below the Fitting Cross. You are also specifying that the point that gives the best refractive power for distance seeing, the DRP, which is 8 to 10 mm above the PRP. You do this even though you try to place the Fitting Cross at the center of the pupil, and both your lensmeter and your patient’s line-of-sight must be higher (at the DRP) for measuring the refractive power and for good vision using the prescribed refractive power. In order to know where you are placing these lens landmarks, you must understand the geography of whatever lens design you are fitting (Fig. 1).


WHERE’S THE BEEF?
It is not possible to identify the cause for every difficulty related to lens adaptation, but the separation of the DRP and PRP created a subdivision of fairly easy-to-identify patients with moderate to high potential for experiencing difficulty with PALs. In fact, some of these unfortunate individuals are faced with a dilemma. In some cases, they can lower their chins to look through the DRP and see clearly, but they see two of everything. Or they can raise their chins a bit and see singly, but because they are then at, or near, the PRP (i.e., in the corridor) their distance vision is not clear. In other cases single binocular vision is achievable, but it may be neither comfortable nor clear.

In general, these patients have correctable refractive errors in both eyes, and they often may not have experienced problems with single vision lenses. The one thing that they have in common is a power imbalance between the right and left lenses in the vertical meridians. As long as the patient can look through both lenses with either no prism, or the same amount and direction of prism (e.g., 2ΔBD OU), both eyes can see without fusional blur or diplopia. This is because the prism deviation is the same in both eyes. If the prism is 2ΔBD OU, both eyes will look upward to see an object straight ahead as in Fig. 2.



Because we make converging and diverging movements as we look at objects closer or further from us, we usually have little problem adjusting the direction in which our eyes must point if a bit of unwanted BI or BO prism is encountered. But rarely do we need to move one eye upward and the other downward, to keep both eyes aimed at the same object. As a result, when unwanted BU prism is encountered by one eye, and the other encounters unwanted BD prism, single binocular vision often becomes difficult or impossible. The same would be true if the vertical prism imbalance has the same direction of the bases, but differing amounts, for example, 1ΔBD OD and 2Δ BD OS (See Fig. 3).



DIRECTION OR AMOUNT
The imbalance shown in Fig. 2 is created by the bases of the prisms found at the DRP level being in opposing directions. An imbalance can also be related to simply differing amounts of prism at the DRPs. This is shown in Fig. 3, and it is probably a more common imbalance situation although it may be easier to overlook.

THE BIG QUESTIONS
Are the PALs you are ordering going to give problems that are related to the separation of the DRP and PRP? The key to answering this is to estimate how much prism imbalance is induced at the patient’s primary distance seeing point—the DRP.

The second big question is how to know whether a patient returning with problems is having difficulties related to that same separation. A predictive clue is to check for prismatic imbalance at the DRPs and NRPs as you verify the eyewear upon its return from the lab. This reveals potentially excessive imbalance prior to dispensing. You can also simply occlude one eye and then the other of a patient returning with difficulties. If each eye separately is clear at distance yet the binocular vision is reduced, the problem may well be a prismatic imbalance. This same test can be applied at the reading level.

The third and possibly most important question is how to know whether recently dispensed patients are experiencing problems that they have simply chosen to not report. This can happen for many reasons, but it is only good business and good health care to know about these unwanted outcomes. A simple phone progress report, verifying the resolution of the patient’s presenting complaints is probably the most practical way of finding those patients that are experiencing a disappointing outcome.

If the power difference between the vertical meridians is significant, your choice of lens design and/or the way you place the order with the lab can greatly reduce or eliminate the unwanted effects of the power differences. If there is a minimal imbalance, you may be able to reduce its effect by using a lens design with a shorter drop from the DRP to the PRP. Also, using the shortest available corridor for the design may result in a lesser imbalance for reading. When the imbalance is greater, you may be able to order overall prism that will allow comfortable vision at the DRP. Your lab consultant should be able to calculate the best way to give the “no imbalance” for distance that both the doctor and patient desire. Once you know what prism power(s) are required for distant viewing, your lab will be able to calculate the prism needed for no imbalance for near work.

CAVEATS
It is important to remember that lens designers separated the MRP into a DRP and PRP in order to save on weight and thickness, so when you add prism to reduce the imbalance, you should expect some increase in both weight and thickness. There will also be only limited binocular use of the corridors for intermediate distances. To avoid these problems, there are the alternatives of using lined lenses, distance and near glasses, and the possibility of using contact lenses (alone or in conjunction with glasses).

THE SILVER LINING
The separation of the DRP and the PRP makes the wearing of a PAL more comfortable for many patients. For those patients who need prism to avoid imbalance at the DRP level, the extra calculations to achieve fusion are neither complicated nor particularly difficult. There will be more imbalance at the reading level after compensation is made for distance seeing, the slab-off line for a PAL is cosmetically more acceptable than the line formed by a conventional slab-off. Even better, you can specify the placement of the slab-off line so that it will interfere less on stairs and other “look-down” situations. Usually about one-third to one-half of the way down the corridor works well. If you don’t specify the location of the line, your lab will probably put it at the level of the PRP, and that may be too high.

Even if your patient with a vertical imbalance is wearing lined lenses successfully with neither slab-off nor prism, he may require both if you switch him to a PAL design. By knowing the geography of the lenses you are ordering and doing some simple calculations or by calling your lab for a consult before ordering, you may avoid expensive remakes and professional embarrassment.


Contributing editor Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics in Cincinnati, Ohio.

Helpful Definitions

Clear, Single Binocular Vision—The condition in which the retinal images from the right and left eyes are successfully combined into a single percept.

Corridor—The area of a PAL lens that gives increasing plus power for viewing at intermediate distances. Most manufacturers consider the corridor to start at the Fitting Cross and end at the near reference point (NRP).

Diplopia—Seeing double. If the problem is binocular (or two-eyed diplopia), simply covering one eye will eliminate one of the images.

Fusional Blur—The blur that patients who are having a problem maintaining clear, single, binocular vision sometimes experience as they try to maintain the images from the right and left eyes as a single percept.

Minimum Fitting Height—Most manufacturers consider this to be the vertical distance from the Fitting Cross to the near reference point (NRP).

Prism Deviation—This is the apparent displacement of a visual target when it is viewed through a prism. This displacement is always away from the prism’s base and in the direction of its apex.

Prism Imbalance—The condition in which there is a difference in the power or direction of the base of the prism experienced by one eye compared to what the other eye experiences. Patients vary in their ability to adapt to prism imbalance, but as little as 0.5Δ of imbalance can cause difficulty.

Slab-off—Slab-off is a lens grinding technique that allows a compensation of the vertical prism part of the prescription to be varied for patients who need lined lenses or for PALs. The available slab-off range for lined lenses can range from about 1.5Δ to 6Δ. For PALs it can range from about 1.5Δ to 10Δ.

—PRC