Causes of Non-Tolerance

Dealing with intolerance to spectacles is a common daily aspect for practitioners in optometric practice. Patient expectations are high, as they should be from a caring service profession. Fundamentally patients want to see as well as they are capable of, which is taken by them as a given, whilst looking as great as they can in their new fashionable eyewear. However, given the complex physical, physiological and psychological processes involved with the sense of sight it is not surprising that sometimes it goes wrong. As a consequence, remakes and erosion costs are a recurring concern for the practice accounts. Successfully managing non-tolerance issues requires the practitioner to take a considerate and empathetic approach, in order to resolve all concerns to the patient's complete satisfaction.

The role of the dispensing practitioner is to ensure that the patient is dispensed with an optical correction that provides maximum optical performance, functionality and wearing comfort, whilst also being cosmetically pleasing to the wearer. To achieve all of these aims the dispensing practitioner will be required to have a comprehensive product knowledge, dispensing practical experience and effective communication skills. On occasion this expertise is put to the test after new spectacles have been collected.

When the expected optical performance, or final appearance of the spectacles, is not to the patient’s complete satisfaction then the cause has to be identified and resolved in an empathetic, prompt and professional manner. Otherwise the patient may lose confidence in their practitioner's ability, which could indirectly have a negative public relations impact on the practice. Conversely a happy resolution may win over the long-term loyalty of the patient, as well as their family and circle of friends to whom they recommend you.

In essence a non-tolerance patient is a symptomatic patient, with symptoms resulting from differential causes. At the outset of the concern the practitioner should set apart early adaptive symptoms to new spectacles, that are transient and settle down quickly with wear, from persistent or enduring symptoms that cannot be tolerated by the patient without restorative action. It is necessary to understand nevertheless that enduring symptoms may or may not be due to the new prescription dispensed. In general symptoms can broadly be categorised by way of their duration effect:

• Temporary elimination: the prescription has an initial placebo effect and the primary cause of the symptoms is unlikely to be due to the new spectacles.
• Elimination replacement: the new spectacles may have removed the original patient symptom and replaced this with another e.g. first single-vision reading correction improves the near acuity whilst blurring intermediate-distance.
• Symptom continuation: the spectacles are not likely to be the cause, and other aspects of the patient's health, lifestyle, vocational demands, work environment etc. should be explored.
• Intense continuation: the new prescription has an aggravating effect on the symptom e.g. unwanted induced prism breaks down the fusion system by adversely disrupting a weak ocular motor balance.
• Symptom production: typically occurs when correcting astigmatism for the first time in adults who were previously asymptomatic.

Fortunately, many of the causes for intolerance to spectacles can easily be prevented at the outset by careful management. The real skill in dispensing is to consider the potential optical, functional and cosmetic challenges before they ever develop into real concerns. At the initial dispensing the practitioner should always consider the optical and cosmetic challenges for the given prescription, in concert with the patient's lifestyle/vocational needs. Many of the common causes for non-tolerance can easily be prevented at the initial dispensing through clear and effective communication to establish specifically what the individual wants their spectacles for, making an appropriate recommendation, and accurate ordering. Checking the final order against the record when completed, setting up of the frames before collection, and giving clear instruction on the intended use of the spectacles will all help minimise the possibility of intolerance arising.

When intolerance to spectacles does arise it is important to keep an open mind, as dispensing error is not the only possible cause. The differential causes can easily be classified into one of five key categories; refraction, dispensing, communication, acute ocular disease and psychological.

Practice Data

From a practice based survey undertaken in one location, 110 patients returning for a retest with symptoms were classified into one of the five key categories based on the primary cause. The percentage of those resulting from dispensing errors and communication causes (including dispensing related) was found to be 25% and 30% respectively (Figure 1). The patients were all seen over a six month period by the same practitioner. During this period the retests accounted for 5.22% of all eye examinations undertaken within the practice.

[CaptionComponent="3038"]

Relating to the category of dispensing non-tolerance the causes were as many as they were varied. Nevertheless, the differential causes under this category can be grouped with regard to the most common errors encountered in practice. The following group causes are in no particular order of significance and are often interrelated:

• Vocational Needs (this will be dealt with in a series of CET articles later in the year)
• Frame Fitting
• Optical Centration
• Lens Form and Spectacle Magnification
• Cosmetic
• Communication

Fortunately, the dispensing causes of non-tolerance are not sight threatening. In modern society optometric services are extensively available to meet with patient's busy lifestyles, and patients are often impatient when concerns do arise. New spectacles may need a period of transient adaptation, which requires the patient to be reassured that all is as it should be. Nevertheless, before the cause is put down to adaptation, the differential causes for the non-tolerance should also be considered and ruled out. The best way to approach this is through a logical and organised check list of everything that has been done previously. Always listen actively to what the patient tells you, as indicators to the specific cause are generally there. Remember that a significant number of the causes for non-tolerance can be avoided by attention to detail and consideration of the potential optical and cosmetic challenges at the outset. Let us now consider in more detail the dispensing causes of non-tolerance.

Frame Fitting

A suitably fitting spectacle frame must be ordered at the initial dispensing. Spectacle frames must always be comfortable to ensure they are worn as intended and should fit correctly in order to effect the best possible optical performance from the lenses. The common optical errors resulting from a poorly fitting spectacle frame include;

• changes to lens effectivity
• reduction in the useful field of view
• introduction of oblique distortion
• prismatic error.

It is customary to record the back vertex distance (BVD) of the chosen frame and to compute this against the prescription for powers of 4.00D or more in any one meridian. Failure to take account of any difference between the prescribed and frame BVD will result in over or under correction of the final lens power. The BVD of the frame can vary significantly depending upon the pantoscopic tilt, pad angles, bow of the front, head width, temple width, length to bend and inward or downward angle of drop. If the frame fitting, as a result of poor measurement or adjustment, is incorrect then the effective power of the spectacle lenses will be altered and if significant may give rise to a reduction in expected VA levels and possibly a reduced range of near focus with multifocals. In addition, if the BVD is inadvertently increased then the effective field of vision is reduced. This will be significant in both progressive power lenses (PPLs) and small segment bifocals where an additional reduction in the useful width of vision at near is of genuine concern to the wearer.

In order to ensure maximum optical performance from aspheric lenses, in either single-vision or multifocal designs, the pantoscopic tilt should be carefully set to ensure the optical and visual axis are closely aligned when the patient lowers their eyes from distance to reading. A normal range for pantoscopic tilt is between 5° and 15° and the vertical optical centres (OCs) should be lowered accordingly. As a general rule the vertical OCs should be lowered by 0.5mm for every 1° of pantoscopic tilt. The positioning of the vertical centre in an aspheric lens is extremely important and if incorrect often results in a lens that is optically poorer than the equivalent spherical design. Failure to set the pantoscopic angle and therefore vertical OCs accurately in aspheric designs is a common cause of non-tolerance. An incorrect pantoscopic tilt will lead to oblique aberrational error that will be increased as the patient looks further away from the optical centre.

Another common error relating to poor frame fitting is induced prismatic effects, which may lead to an acute onset of ‘diplopia’ especially in cases of anisometropia. Alternatively, the image ‘jump’ common in bifocals and trifocals may become more apparent and disturbing to the wearer if the back vertex distance is increased as a result of loose fitting frames.

Optical Centration

A well-documented cause of spectacle intolerance is incorrect centration of lenses for the patient’s interpupillary and/or near centration distance. Prismatic error will result in line with Prentice’s rule (P = cF). The greater the decentration from the optical centre (c) and/or focal power (F) the larger will be the induced prismatic error (P) that is experienced by the wearer. The binocular fusion system has a greater tolerance for horizontal errors than vertical. If the differential prism is greater than the patient’s binocular status can cope with, the binocular system becomes stressed and associated symptoms result whilst wearing the new spectacles.

In theory the vertical tolerance is around 1to 2 prism dioptres and in the presence of anisometropia, where the difference in the vertical meridian is 2.00D or more, the patient should therefore be unable to tolerate the differential prismatic error at the near visual point (NVP). However, in practice spectacle lens wearers often tolerate large amounts of vertical differential prism and it is often the effects of smaller amounts of prismatic error, not experienced in the old spectacles, that give rise to non-tolerance symptoms. For this reason, the previous OCs (horizontal and vertical) should always be checked on existing spectacles before dispensing new ones. Nevertheless, accurate centration, to match the patient’s PD in their new spectacles, may not be the best way forward in cases where the patient has clearly adapted to induced prism resulting from centration errors. Furthermore, base-in prismatic error may be of benefit to elderly patients with reduced convergence. For existing contact lens wearers, it should be remembered that prismatic effects are generally not experienced during wear, even in anisometropia, as the lenses move safely on the pre-corneal tear film. This movement allows the patient to maintain the correct optical centration in peripheral gaze directions. However, when changing to spectacles the inherent prismatic error on looking away from the optical centres may cause concern leading to spectacle non-tolerance.

Another error may result from poor optical centration in multifocal (bifocal, trifocal or PPL) designs. Incorrectly setting the horizontal distance OC will normally result in poor alignment of the vertical corridors of vision for both intermediate and near use, thereby reducing the inherent limited useful field of view. It is also important to ensure that the vertical fitting heights are correct for the pantoscopic tilt (as mentioned previously). This should always be measured by the practitioner on the same plane as the patient and with the patient’s head in their habitual posture. It is always a good routine to have this measurement double-checked by another member of the practice staff before completing the order. Remember that small adjustments to the vertical positioning can easily be made at collection through minor frame adjustment e.g. pantoscopic tilt. As previously mentioned the vertical OC’s should always be recorded in aspheric designs to eliminate induced prismatic effects and aberrational error when viewing off-axis.

Spectacle Magnification

When a prescription has changed significantly from that previously worn, following refraction, the spectacle magnification (SM) of the new dispensed lenses is also transformed. In the case of minus lenses, the SM is less than 1 (unit magnification) and the image is reduced in size. The converse is true for positive lenses. If the Rx change is significant then the cognitive system will interpret objects as being either closer or further away. In time the patient will ‘adapt’ to this change, but initially judging distances may present a problem i.e. the myope assumes objects are further away whilst the hyperope believes them to be much closer. As a rule of thumb 4.00D of power difference is approximately equal to a 6% change in image size. In the case of anisometropic patients the relative image size difference (or aniseikonia), between the right and left eye, may lead to a breakdown in the binocular fusion system and spectacle non-tolerance. Care should be taken when dispensing anisometropic contact lens wearers with new spectacles. As the BVD during contact lens wear is negligible (almost zero) then unit magnification exists (SM = 1) regardless of the prescription. In this situation there is no relative image size difference. However, when wearing spectacles, the BVD is always significant and may result in differential image size problems for the contact lens patient.

The formula for spectacle magnification is often summarised as;

SM= Power Factor x Shape Factor

As the ‘shape factor’ is dependant upon the surface powers, lens thickness and refractive index; then theoretically changing any of these factors will effect an alteration in the SM experienced by the lens wearer. Non-tolerance symptoms may therefore arise when the lens form (surface powers), refractive index and/or lens thickness are altered from that previously worn. Similarly, any difference in these parameters between the finished right and left spectacle lenses may also result in non-tolerance. For this reason, care should always be taken when changing patients to aspheric designs, altering the lens refractive index, or changing the centre thickness in plus lenses through use of different blank sizes. The initial effect on perceived image size may result in patient symptoms, which requires reassurance and adaptation time. However, the lens form should always be checked and matched as closely as possible to that of the previously worn lenses.

Communication

A breakdown in communication between the practitioner and patient is often a major cause of patient dissatisfaction in practice. With regard to the spectacle dispensing this may impact directly on several areas including;

• functional use of the spectacles
• reading/intermediate powers
• instruction on the use of multifocal lenses
• care of coated lenses
• computer/order error.

It is essential that the spectacles dispensed are functionally suitable for their intended use. It is not uncommon to establish at a later date that the patient would like to use the spectacles for a task that was not discussed at the initial dispensing. The common example is single-vision reading spectacles that are not suitable for intermediate tasks and vice-versa. At dispensing the practitioner must always establish the occupational needs of the patient, basically this means talking to the patient, in order to identify the most appropriate lens and frame option and advise them of the optical and functional limitations. Ideally the working distance for each task should be measured and the dioptric equivalent determined in order to assess the clear range of near focus available to the patient. If the range required can not be achieved with single vision lenses, then an appropriate multifocal design should be discussed. Be flexible in your approach to the patient’s visual needs i.e. consider the value of bifocal forms with an intermediate and near correction for occupational use, but always check with a colleague that you have calculated the correct powers to use on the order. Always make sure the frame eyesize depth is adequate for the intended multifocals, otherwise the patient may not be able to maintain a comfortable head posture for extended periods of closework. The importance of full and correct instruction on the use of multifocals should not be underestimated, especially in first time wearers, as this often leads to complaint and intolerance. Patients regularly do not adhere to the instructions given regarding proper care of their lenses and this is particularly challenging when the lenses contain MAR coatings. Failure to care for the coatings will reduce the optical performance and may result in an apparent reduction in visual acuity, which is always worrying for the patient.

An automatic cause of intolerance will result from an error in writing, or typing to computer, the prescription to be ordered. The final order will always match the completed spectacles in this case and the cause initially overlooked, prolonging patient-practitioner anxiety, unless the prescription is traced back to the patient record, which will not be directly available for external prescriptions. This is often time consuming, costly and embarrassing for the practitioner. Again it may be useful to have a second member of the practice team double check the prescription details on the order/computer before glazing.

Cosmetic

Although the optical performance and functionality of the finished spectacles are important these are often taken for granted by the patient, whose main concern at dispensing is traditionally cosmesis. With this in mind the finished product may not meet with the patient’s expectations and as a result they become dissatisfied. Professionalism and good communication throughout the dispensing are essential to manage patient expectations, so they will feel pleased with their new spectacles and the final image created. However, even with the best advice available from an experienced dispensing practitioner some patients will still not be happy with the end result. Regular causes for this include lens edge/centre thickness, multifocal visibility, residual hue from anti-reflection coating, weight of the finished spectacles and a dislike of the frame style or colour now that they can see properly through the new prescription!

Conclusion

Fortunately, the dispensing causes of non-tolerance are not sight threatening, but in modern society everything appears to be available instantly and patients are often impatient when problems arise. However, new spectacles may need a period of adaptation, which requires the patient to be reassured that all is as it should be. Nevertheless, before the cause is put down to adaptation, all other possible causes for the non-tolerance should be ruled out first. The best way to do this is through a logical and organised checklist of everything that has been done previously. Always listen to what the patient tells you as the required pointers to cause are there. Remember that a significant number of the causes of non-tolerance can be avoided through attention to detail and consideration of the potential optical and cosmetic challenges at the outset. In the dispensing area prevention is definitely less stressful and more rewarding than cure.

Jim Farrell is an optometrist and Examiner and Lead Assessor for the College of Optometrists

Next month we will begin a series of case studies representing a wide range of problematic dispenses