Dispensing causes of non-tolerance

The role of the dispensing practitioner is to ensure that the patient is dispensed with an optical correction that provides maximum optical performance, functionality, wearing comfort and is always cosmetically pleasing to the wearer. To achieve all these the dispensing practitioner will require comprehensive product knowledge, practical dispensing experience and good communication skills. On occasion this expertise is put to the test after the new spectacles have been collected.
When the expected optical performance or final appearance of the spectacles are not to the patient's complete satisfaction, the cause has to be identified and resolved in a professional manner, otherwise the patient may lose confidence in the practitioner's ability. Fortunately, many of the causes for intolerance to spectacles can easily be prevented at the outset. The real skill in dispensing is to consider the potential optical, functional and cosmetic challenges before they ever develop into real problems. At the initial dispensing the practitioner should always consider the optical and cosmetic challenges for the given prescription and should address these before the patient returns to haunt them.
The causes of dispensing non-tolerance are as many as they are varied and it is not possible to discuss them all in this short article. However, the main causes can easily be categorised with regards to the most common errors encountered in practice. The often inter-related causes discussed are not listed in any particular order of significance. For both single-vision and multifocal lenses the major categories for consideration are:

Frame fitting
Optical centration
Spectacle magnification
Communication
Cosmetic.

FRAME FITTING
Pearls: BVD, useful field of view, oblique distortion, prismatic error and image jump
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 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 effectiveness, a reduction in the useful field of view, introduction of oblique distortion and 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 penses (PPLs) and small segment bifocals where an additional reduction in the useful width of vision at near is of genuine concern to the wearer.
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 degrees 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 degree 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 BVD is increased as a result of loose fitting frames.

OPTICAL CENTRATION
Pearls: Vertical prism effects, multifocal heights, vertical centration in aspherics
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 the induced prismatic error (P) 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 while wearing the new spectacles. In theory the vertical tolerance is around 1-2 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. It is often the effects of smaller amounts of prismatic error, not experienced in their 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, eg pantoscopic tilt. As previously mentioned, the vertical OCs should always be recorded in aspheric designs to eliminate induced prismatic effects and aberrational error when viewing off-axis.

SPECTACLE MAGNIFICATION
Pearls: Prescription Changes, Lens Form, Shape Factor
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, ie the myope assumes objects are further away while the hyperope believes them to be much closer. As a rule of thumb, 4.00D of power difference is approximately equal to a 6 per cent change in image size. In the case of anisometropic patients the relative image size difference, 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 negligible relative image size difference until high levels of anisometropia are corrected. 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 the product of the power factor and 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
Pearls: Intended use, near adds, PPL instruction, coating care, computer error
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 the functional use of the spectacles, reading/intermediate powers, instruction on the use of multifocal lenses, care of coated lenses and also 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. This means talking to the patient 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 dioptre equivalent determined to assess the clear range of near focus available to the patient. If the range required cannot 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, ie 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
Pearls: Centre or edge thickness, multifocal visibility, weight, frame design
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 MAR 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 today 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 the professional programmes manager at Boots Opticians