This article explores the common reasons for patients returning to the practice with a spectacle non-tolerance issue and provides guidance on achieving a successful resolution
All practitioners will be familiar with the scenario where occasionally a patient’s new spectacles do not meet their expectations or visual requirements, resulting in a non-tolerance. Often this is a case of the wrong product dispensed correctly, rather than the right product dispensed incorrectly.
To resolve a non-tolerance case, a back-to-basics approach should be adopted. Evaluation of the patient’s record card will highlight any marked prescription changes and indicate the anticipated level of visual acuity. The spectacles should be checked in accordance with BS EN ISO 21987:2009 standards to ensure they comply with the intended specification.
Common causes of non-tolerance may result from, for example, an increase to the near addition creating increased jump, shortening of the focal range and reduction of the near vision area with progressive power lenses (PPLs). Modification to the cylinder power or axis may also result in adaptation issues for the patient.
Misinterpretation may occur when there is a communication breakdown between patient and practitioner. Often patients feel under pressure when returning to the practice with a problem and have difficulty expressing their visual symptoms or explaining their needs, therefore, effective questioning and listening communication skills on behalf of the practitioner are essential. Adopting a mix of open questions will make the patient feel at ease combined with closed questions to allow the practitioner to identify the specific nature of the problem and decide upon the correct solution. The use of paraphrasing aids the practitioner as a method of confirming the patient’s expectations.
To pre-empt non-tolerance situations, many practitioners use lifestyle questionnaires to identify dispensing solutions to suit the patient’s specific requirements. Frequently more than one pair of spectacles is required; therefore, at the time of dispensing it is crucial to make personalised recommendations. Use of ‘props’ such as relevant frames, lens and tint samples demonstrate optical appliances to match the patient’s needs, thereby reducing the chance of a problem later on.
Dealing with different personalities can frequently lead to a misunderstanding during the patient/practitioner communication cycle; from the silent types who are cautious and will not disclose their visual requirements to the more outgoing personalities who think they know best and pay little attention to the practitioner’s recommendations.
Progressive power lenses
Non-tolerances arising from PPL dispenses are often linked to measurement accuracy when dispensing or glazing, lens design selection, adaptation, and prescription problems associated with high spheres, cylinders and additions, as well as anisometropia and antimetropia.
Recommending the most suitable lens type is paramount to a successful dispensing due to the variety of designs available, including short corridor progressives, bi-aspheric, personalised and occupational/lifestyle lenses, such as restricted distance, room distance and degressives. Standard progressive lens designs are intended to be dispensed within the typical parameters of frame tilt 10°, face form angle 4°, fitting at a vertex distance of 10mm. Therefore, pre-adjustment of the frame is essential.
Personalised progressive designs are individually tailored to meet the patient’s visual needs and precise facial fitting requirements; therefore exact measurements relating to the frame must be supplied to the lens manufacturer. The trial frame vertex distance and frame tilt are critical when dispensing personalised lens designs and should be considered when fitting the frame as it can result in the patient achieving excellent vision in the consulting room but an unsatisfactory result when translated into a pair of spectacles. The selected lens design and frame fitting need to match the patient’s requirements. A range of visual problems associated with progressive lenses can often be corrected by adjusting the frame alone; however, some patients experience problems that require further investigation (see Table 1: Resolving PPL non-tolerances. Adapted from the Varilux Trouble Shooting Guide 2015 with permission of Essilor).
Evaluation of the patient’s use of their lenses assists in solving non-tolerance cases. The practitioner should check the patient’s habitual head position for distance, intermediate or near vision including head movements when viewing laterally placed objects.
Complaints may arise when the inset required for near is atypical resulting in difficulty when reading due to a limited or incorrectly positioned near vision area. This can be checked by remarking the lenses and asking the patient to wear their spectacles, allowing verification of the fitting cross position relative to the patient’s pupil centres for distance vision. Using a mirror held at the patient’s normal reading distance and position, check their reflection by verifying the patient is looking through the reading circles for near vision (see Figure 1: Checking PPL near vision centration using a mirror). If inadequately centred there are two solutions: the lenses can be supplied with the reading centres set at the correct near centration distance; dispense personalised progressives which provide the required accurate inset.
Lens material plays a key factor in a non-tolerance situation. Colour fringing may be noticeable to first time wearers of polycarbonate or high index materials due to the low v-value.
Non-tolerance to prism thinning can occasionally occur; however, reported symptoms are often vague. They include the patient requiring to lower their head for distance but raising their head for reading, a severely limited distance field of vision, a pulling or drawing sensation, a blurred band across the intermediate channel, or the lenses ‘just not feeling right.‘ As the reading addition increases, the curvature towards the top of the lens subsequently increases; therefore, lens manufacturers balance the thickness difference between the upper and lower lens edges by introducing prism thinning.1 Two thirds of the addition is generally surfaced as base down prism, thereby inducing the effect of removing base up prism from the distance portion. An amount of residual base down prism is often present which can be checked at the lens prism reference point. Prism thinning results in lenses that are balanced in thickness at the upper and lower edges, which is of greater benefit to positive powered lenses and is generally not required for low powered negative lenses. Manufacturers can calculate a precise amount of prism thinning, known as ‘intelligent prism thinning.’ Checking non-tolerance to prism thinning is probably the last item to verify when a patient complains about their vision when wearing progressive lenses. This can be confirmed by checking the amount of prism thinning surfaced at the prism reference point by taking equivalent amounts of prism from the trial set, correctly positioning the spectacle frame on the patient and simultaneously placing the trial lenses base up before each spectacle lens. An improvement to the patient’s vision would indicate lenses without prism thinning would improve vision.
Changes in lens curvature arise when dispensing high index aspheric lenses and curved frames such as sports eyewear and prescription sun spectacles, due to an increased face form angle.
Aspheric lenses are surfaced on flatter base curves when compared to standard lens forms, typically by two to three dioptres, providing the cosmetic benefits of thinner lenses and less spectacle magnification. Although aspheric lenses are designed to minimise oblique errors,2 wearers frequently experience problems adapting to the lens design due to a reported increase in distortion. For positive powered lenses the patient experiences pincushion distortion, and will report of objects sloping away, whereas minus powered lenses induce barrel distortion and complaints include objects curving outwards.3 Adaptation time should be advised to new aspheric wearers and lenses should be correctly centred at the time of dispensing and are required to be decentred 1mm downwards for every 2° of frame tilt to ensure the optical axis coincides with the visual axis at the eyes’ centre of rotation.2 However, if this was not considered at the time of dispensing, adjusting the frame tilt by reducing the angle of side, will effectively lower the optical centres and an improvement should be noticed by the wearer.
Hyperopic patients are often sensitive to changes in lens form; therefore, consideration should be given to using a lens measure to check previous base curves before ordering subsequent lenses. The frame bow and tilt should be assessed and matched where possible as these factors influence the effects of distortion.
Sports eyewear and most sunglasses are designed to provide maximum eye protection by fitting at a minimum vertex distance and wraparound in style. Ideally, the lens curve should match the curve of the frame; however, this often results in visual and mechanical problems. The wearer may notice visual disturbance to their curvature of field, complaining of the ‘goldfish bowl effect’ although this effect often reduces with wear. Medium to high plus powered lenses often match the frame curve due to the convex shaped front surface. However, mechanical problems arise with negative prescriptions as they require surfacing on a much steeper base curve.2 The prescription range is often limited as steeply curved lenses may result in an insecure fitting or be cosmetically poor in appearance when glazed. In this case a compensated prescription is often supplied which is calculated by the lens manufacturer to take into account the parameters of the frame and the prescription.2 When checking the completed spectacles, the practitioner will notice a difference between the supplied and ordered prescriptions and optical centre position due to the required compensation.
Problems associated with anisometropia may have been overlooked at the time of dispensing, for example, in the case of contact lens wearers or monocular pseudophakic patients. Patients may report diplopia when viewing near objects due to the induced amount of differential prismatic effect when looking away from the optical centres. The patient will notice a greater problem if not constantly wearing the spectacle prescription, or if alternating between contact lenses and spectacles due to the difference in spectacle magnification. The powers should be checked along the vertical meridians as a difference in power induces differential prism at near, resulting in diplopia when performing near vision tasks.
Spectacle wearers with a large degree of anisometropia and good visual acuities may experience aniseikonia. The symptoms include visual discomfort, visual distortion of space and sometimes difficulty in achieving good binocular vision resulting from a difference in image size. The solution is to dispense lenses that alter spectacle magnification by modifying the base curve and lens thickness.
Generally, an aspheric lens is dispensed to the most positive powered eye in order to reduce spectacle magnification. The retinal image size is reduced due to the aspheric surface and flatter base curve; care must be taken to order both lenses from the same manufacturer to ensure that any MAR coatings or tints match. Alternatively, an iseikonic (‘sized’) lens may be dispensed in order to increase the retinal image size of the least positive eye, although the result may be cosmetically unacceptable due to the lens thickness. An economical solution would be to order a larger uncut lens for the least positive eye in order to increase thickness and spectacle magnification.
Resolving non-tolerance related to anisometropia at near requires calculation of differential prism and deciding upon an appropriate solution. An issue of cost may need to be considered with prism compensated lenses such as Franklin Split bifocals or slab off prism solutions which can be supplied as flat top bifocals, E-lines, single vision and some PPLs.4 For myopes, a slab off lens is an ideal solution to be dispensed to the most negative eye in order to remove base down prism from the lower lens portion.
A cost-effective alternative is to dispense unequal round segment bifocals although some patients dislike the cosmetic appearance. The largest round segment induces the most base down prism and is, therefore, dispensed to the eye with the most positive prescription consequently eliminating the base up prism induced by the main lens.
When a patient presents with a non-tolerance relating to bifocals, careful questioning as to the nature of the problem must be undertaken. Consideration should be given to the suitability of the bifocal segment size, shape, drop, position in relation to the lower limbus and patient use. Round segments induce increased base down prism and, therefore, are suitable for plus powers whereas flat top bifocals induce base up prism and are more suitable for minus powers. The amount of prism induced by the segment is related to the add power, therefore, older patients tend to notice more jump when collecting their new bifocals with an increased addition. Reducing jump-related issues can be achieved by using E-line and D40 segments.
Off axis blur
Following the trend of larger spectacle frames, sunglasses and upswept styles, wearers often notice and complain of compromised vision at the lens periphery. Consideration should be given to the frame tilt, face form angle (bow) and vertical lens centration. Using a lens measure, the base curves of the previous pair and new pair should be compared.
The key to successful resolution of non-tolerance cases is to listen carefully to the patient to understand the main issues, methodically check the appliance to ensure it conforms to the intended specification and consider the range of potential causes in systematic way.
About the author
Sally Bates is a dispensing optician and lectures at ABDO College where she is responsible for teaching all elements of the professional practical examinations, and theory in communication skills. She is the proprietor of Identity Optical Training, offering courses for all sectors of the optical profession.
- Jones D (2012) Common technical enquiries. ABDO Dispensing Optics 4-10
- Jalie M (2008) Ophthalmic Lenses and Dispensing. Butterworth-Heinemann/Optician
- Wilson D (2006) Practical Optical Dispensing. Centre for Learning Innovation, New South Wales Department of Education and Training, Australia
- Norville Prescription Companion (2012)