Refracting the visually impaired patient

The principles of refraction remain unchanged whether the patient is visually impaired or normally sighted. The refractive routine will logically follow the recording of a comprehensive case history covering the following main areas:

Reason for the consultation
Visual details
Ophthalmic details
Medical details
Social, educational and employment details
Task analysis, whereby patients are asked to outline the nature and extent of problems experienced.

Attention must be paid to both the objective and subjective elements of refraction, as evidence suggests that between 8 per cent and 10 per cent of new patients referred for low vision consultations simply require an updated spectacle correction.1 In addition, general population studies have shown that although 30 per cent of the general population aged 65 and older have a visual acuity <0.3 logMAR (Snellen <6/12), between 27 per cent and 72 per cent of these people can have their vision improved through the provision of an up-to-date prescription or basic eye surgery.2,3

Case history
Fundamental to the process of collecting a comprehensive case history is the need to draw out responses that will inform the process by which the patient will be provided with low vision aids and visual rehabilitation advice. Patients must be helped to feel comfortable and at ease in the presence of the practitioner and must be certain that the practitioner has a genuine interest in assisting them to deal with problems resulting from their visual impairment and the clinical expertise to match.
As with a general refraction, it is important to ascertain the reason behind the patient's consultation with you. This will outline the patient's motivation and expectations, which will shape and dictate the 'success' of the consultation. For example, a patient noticing gradually decreasing vision may just desire 'stronger glasses' and may struggle with the concept that their vision cannot be improved due to a condition such as atrophic age-related macular degeneration, making the acceptance of magnification aids difficult.
On the other hand, the patient may have a good understanding of their eye condition and be seeking optimisation of their visual aids or advice on coping strategies.
The psychological status of the patient is important to consider, as unless realistic acceptance of the permanent nature of their visual loss has been reached, patients will struggle with using visual aids and coping strategies. Loss of vision has a profound effect on most patients' state of mind, and has been likened to that experienced when a member of one's family or a close personal friend dies.4
Questioning on visual function will relate to those tasks that a patient is now finding difficult due to their visual impairment. It must be remembered that other disabilities such as failing memory or an unsteady hand with increasing age or arthritis may also impact on task performance, so visual assistance may need to be combined with other coping strategies.
A number of 'standard' questionnaires have been designed to assess quality-of-life loss in someone with visual impairment, but have not become commonly used in clinical practice.5 If the patient can bring to the consultation the tasks they are struggling with, the visual demand of the task can be assessed (obviously for a patient struggling with playing piano music it may not be practical to bring the piano, but the music will help). However, the task lighting and working distances (which can be affected by their chair) involved will need to be estimated by the patient.
Interest in ophthalmic history should focus on those aspects of ocular health that are associated with visual impairment and the availability of medical and rehabilitative treatment. Low vision aids cause compromises to ideal vision, such as aberrations and a limited field of view, so all possible medical treatments (for example, cataract removal) should be investigated and undertaken as is appropriate before low vision rehabilitation is undertaken.
Questions on medical history should likewise concentrate on relevant issues and be directed at those disorders which are likely to produce confounding disability (such as rheumatoid arthritis, Parkinson's disease and multiple sclerosis) and those that are likely to preoccupy the patients' thinking and thus impact on motivation (such as uncontrolled diabetes mellitus, terminal illness, psychotic illness).
In similar fashion, discussion on educational, employment and social issues should concentrate on areas affected by visual impairment and on the likelihood that help can be provided either directly through the clinic or via tertiary referral.

Current optical
corrections
Patients should be encouraged to bring all existing spectacles to the appointment, which should be assessed and the respective prescriptions recorded. Patients should also be asked to comment on the degree of usefulness, or lack of it, which they attribute to each pair of glasses. Apparent idiosyncratic responses should be recorded and subsequently addressed.
The patient who claims glasses, which are worn constantly, are 'absolutely useless' may be encouraging the practitioner to prescribe a minimal change in the hope that new glasses will be synonymous with improved vision. Alternatively, they may simply feel undressed without glasses, which have been part of their attire for as long as they care to remember. Confirmation should be sought as to the origin of the spectacles, as a significant proportion of elderly visually impaired persons resort to using other people's glasses in the hope that they may be of more use than their own.
Elderly patients also often become confused as to which glasses they should be using for different visual tasks. All existing low vision aids should be categorised and once again patients should be asked to outline their uses. As many as 50 per cent of patients referred for low vision consultations already have one or more magnifiers.6 Patients should be encouraged to bring existing aids to subsequent clinical appointments as often they will be reticent to demonstrate homemade or personally-acquired devices, to the one they perceive as 'an expert' in the field.

Measuring visual acuity
Monocular distance acuities should be recorded using the distance spectacles and magnifiers brought, using a logMAR chart (Figure 1). Only in cases where the prescription is minimal or in those cases where patients claim existing glasses are useless, are there any real benefits in assessing unaided acuities.
The more traditionally used Snellen chart has many disadvantages, most importantly:

There are fewer letters at larger letter sizes, so a response (whether accidentally guessed rightly or wrongly) has far more consequence in the visual acuity recorded
The visual acuity steps between successive lines of letters on the chart are much greater for larger letter sizes than smaller letter sizes, resulting in a decreased sensitivity in measuring the visual acuity of the visually impaired
The spacing between letters is not proportional to the letter size, resulting in a change in letter crowding effect between different lines of letters.

The reluctance to convert to logMAR charts despite their design almost 30 years ago7 is probably due to their large size and limited availability. However, the increasing use of electronic vision charts such as the Thomson Test Chart 2000 (Figure 2) leaves little excuse to the practitioner not to convert to using logMAR charts, even if Snellen notation is still used for ease of understanding. In addition, such electronic charts allow randomisation of the letters at the touch of a button, alleviating the problems of memorisation.
Attention should be paid to the optimal working distance. The emphasis should be on eliciting a positive and encouraging response from the patient, as it is disconcerting for them to be presented with a letter chart and realise that they can barely see the largest letter. Ideally, the acuity chart can be moved to the appropriate distance in front of the patient, but if not, the patient may need to be moved to the appropriate distance from the chart.
Those with visual acuities of <1.0 logMAR (6/60 Snellen) should be presented with charts at distances of 1-3 metres, whereas those with acuities of between 0.6 and 1.0 logMAR (6/24 and 6/60 Snellen) should use working distances of 3-4 metres. Six metre charts should only be used with those whose acuities are better than 0.6 logMAR (6/24 Snellen). Where only one chart is available, it may be moved closer to the patient. In this instance, a useful rule is that for every halving of the working distance for a chart, 0.3 needs to be added to the logMAR.
Electronic charts usually allow the text size to be increased beyond the traditional maximum of 1.0 logMAR (6/60 Snellen) dependent on the screen size and distance, so the working distance may not have to be altered. However, the number of letters read by the patient at any acuity size should remain as five, so if only one letter can be displayed at that print size, the randomisation function should be used five times to allow an 'equivalent of task' for the patient, compared to smaller print sizes.
Contact lens wearers should have their lenses assessed and should be encouraged to attend their contact lens practitioner for regular aftercare. Regular wearers will undoubtedly wish to use low vision aids in conjunction with existing contact lenses and as such over refraction should confirm that they are appropriately powered. A base-line refraction should nonetheless be undertaken during the course of a subsequent visit.
Near acuities should be recorded both through the reading glasses and through existing aids, as used in the manner demonstrated by the patient. The same logMAR chart principles that are used in distance visual acuity charts are appropriate for measuring near acuity and several charts that follow these principals are available.8,9 However, near acuity is normally measured with lowercase words rather than uppercase letters and there has been some debate over whether unrelated words and punctuation should be used or not.10

Retinoscopy
Obtaining an accurate retinoscopy result is essential if one is to minimise the fatigue and distress caused to the patient by a protracted and difficult subjective routine. Accuracy will, of course, depend on the clarity of the media, the nature of any uncontrolled eye movements and patient co-operation. In difficult cases the procedure will be considerably improved by ensuring that the retinoscope is serviceable and the batteries are fully charged. A rechargeable instrument with a halogen bulb will prove invaluable.
In those cases where medial opacities are significant, reducing or eliminating background illumination can be beneficial and reveal a reflex previously indistinguishable. The technique of 'Radical retinoscopy', which involves the use of reduced working distances, to improve image brightness, may also prove beneficial.11 Similar effects can be achieved in those with central medial opacities, by moving off axis. In both circumstances care must be taken to compensate for the apparent prescription changes induced. Any off-axis cylindrical components detected must be confirmed subjectively. Understandably, full aperture trial lenses prove advantageous in these circumstances. Cycloplegic refraction may assist in optimising a retinoscopy reflex in an eye with medial opacities. This should, of course, be undertaken as a matter of course when refracting phakic children attending the clinic as new patients. Care should be taken to occlude the dominant eye when refracting a strabismic fellow eye. Throughout the course of the retinoscopy examination it is imperative that the patient be provided with an appropriate fixation target and that those with nystagmus should be allowed to utilise head tilt or turn to minimise movement. 'Near retinoscopy' using illuminated targets attached to the retinoscope can prove an extremely useful technique when estimating the accommodative potential in unresponsive or mentally handicapped children and those with learning disabilities.12
Corneal topography
Although rarely used during the course of the routine low vision assessment, keratometry or videokeratoscopy can help confirm the presence of high degrees of astigmatism alluded to on retinoscopy. Corneal topography can also confirm the presence of irregular astigmatism, a finding that can be particularly beneficial when refracting visually impaired patients with severe learning disability. Twenty per cent of Down's syndrome patients are, for example, likely to exhibit keratoconus.13

Subjective routine
Prior to commencing the subjective refraction, ensure that the trial frame is both comfortable and appropriately fitted in such a way as to resemble the final spectacles. Vertex distance, pantoscopic tilt and both horizontal and vertical centration should all be optimal. As a general rule the spherical lenses should be positioned in the back cell of the trial frame with cylindrical ones in front.
As previously stated full aperture trial lenses are desirable, in this case because they allow the examiner to assess eye movements and fixation as the patient undertakes both distant and near tasks. A refractor head (phoropter) is entirely inappropriate in this respect. In certain circumstances it may be appropriate to refract over existing glasses in order to demonstrate more clearly whether any refractive change recorded, is significant. In these cases a Halberg trial clip should be used.
Throughout the course of the subjective examination, the practitioner must use consistent and clear terminology. Patients will find many of the tests difficult enough without having to respond to ambiguous questions. When working from a reliable retinoscopy result, initial modification to the spherical component should be made in relatively gross steps (2.00D/5.00D). Attempts should be made to encourage patients to make forced choice decisions. In those cases where the ocular media are not clear and a reliable retinoscopy result is not forthcoming, larger steps may be utilised in the early stages (10.00D/20.00D). High degrees of uncorrected refractive error may simply never have been detected by previous examiners. As patient confidence in the decision-making process grows and responses improve, smaller steps can be used (1.00D/0.50D). There is rarely any point in using 0.25D steps when assessing a visually impaired patient.
Assessing the cylindrical component of the refraction is best done using a 1.00DC Jackson cross cylinder with an appropriately chosen target. As large enough circular targets are not always present (typical cross cylinder targets typically have a visual acuity demand of ~0.3logMAR or 6/12 Snellen), a 'U' or 'D' letter may have to suffice. Alternatively, a hand-held Landolt 'C' Acuity Test Chart Panel can prove invaluable. Modification to the cylinder axis should initially be made in 20o steps, whereas power modifications should be made in 1.00 steps. In certain circumstances it may be appropriate to allow the patient to assume control of the rotating cylinder, although elderly patients and those with restricted upper limb movements or poor manual dexterity may find this difficult. In the case of a poor retinoscopy result, manually rotating the cylinder in the trial frame can speed up the process of determining the axis of any astigmatism. The stenopaeic slit may also assist the practitioner in the search for the cylinder axis in conditions such as keratoconus.
In theory, subjective testing should be less time consuming than when performed on normally sighted patients, as fine-tuning may not be possible. In practice, the low vision practitioner must invest more time and energy in the patient, as throughout the course of the examination it is important to show empathy with the patient. Only when the patient believes that the examiner really cares about both them and the outcome of the examination, will optimal results be achieved.
As a general rule, an updated prescription should not be issued until it can be clearly demonstrated that an improvement of two lines or more on a logMAR acuity chart can be achieved through the new correction.14 Those with acuities of less than 1.0 logMAR (6/60 Snellen) are unlikely to appreciate a spherical change of less than one dioptre or a cylindrical change of less than two dioptres. Those with significant medial lenticular opacities may, in addition, find it more difficult to discriminate defocus than patients with macular pathology. This phenomenon has been attributable to the respective gradients of the frequency of seeing curves.15
Final distance acuities should be recorded as described earlier. One may wish to conclude the distance refraction with a pinhole acuity check, although it must be borne in mind that those with central scotomas often find this test difficult. Performance may be improved by using multiple pinholes.

Calculating required enlargement
Near acuities, as determined using word reading charts, should be recorded through both pre-existing reading glasses and with a standard +4.00D reading addition over the optimised distance prescription. The +4.00D reading addition is arbitrarily chosen as it provides unit magnification (Enlargement = F(addition)/4) when the object is placed at the least distance of distinct vision (25cm). This working distance may also represent a psychological barrier to the optometrist unfamiliar with low vision. Practically, most visually impaired patients including the elderly can be encouraged to use a working distance of 25cm for reading if benefit can be demonstrated to them (Figure 3).
Younger patients may understandably choose to utilise accommodation in preference to a near addition, although near acuities should still be recorded at a 25cm working distance as well as their accustomed working distance. As was the case with distance acuities, near acuities should be recorded both monocularly and binocularly.
Adequate auxiliary lighting should be available in the form of an angle poise lamp. Patients must be encouraged to adjust both the lamp to work surface working distances and the angle of incidence of the light. As luminance is proportional to the square of the distance between the light and the task, decreasing the working distance is far more effective (as well as safer and more cost-effective) than increasing the power of the light.
The CIBS code recommendations are that casual readers should utilise surface illuminance of 150lux whereas more dedicated readers, and those involved in sewing, should use up to 300lux. Recommendations for elderly patients are that illuminance levels should be increased by between 50 and 100 per cent.16 During the course of the near acuity assessment, note should be taken of both reading speed and accuracy, both with and without occlusion and using optimal and sub-optimal illumination. Formal methods of assessing reading speed and of comparing oral and textual comprehension recall skills are available, but are generally only used as research tools.17
A number of methods are available by which the near add required to achieve a given acuity can be calculated.18 These are based on dividing the measured distance or near acuity by the acuity demand of the task (enlargement ratio). However, this is only an estimated enlargement and may need to be increased if the patient cannot perform the task with relative ease (acuity reserve), or a lower enlargement magnifier tried if the task can be performed with ease (to increase the field-of-view). No method of calculating the required enlargement seems to outperform another, although increasing the near addition in 4.00D steps to determine the near addition required to perform the desired task nicely demonstrates the use of closer working distances (with the advantages of a wider field of view) to the patient.18

Binocularity
Although the vast majority of visually impaired patients do not have normal binocularity, many are convinced that it is to their benefit to function under binocular conditions. This is usually not a problem when considering distance requirements. In those cases where binocularity can be demonstrated and where the acuities recorded in the fellow eyes are similar, base-in prism can sometimes be applied to facilitate comfortable fusion at near. There are various rules of thumb that can be used to determine the necessary base-in prism to incorporate into the lenses, usually split equally between both the right and left lenses. Typically, the strength of the incorporated prism is equivalent to the strength of the near addition in each eye (Figure 4). The individual using spectacle magnifiers with a +10.00D near addition would therefore require 10 prism-dioptres base-in right and left.
Manufacturing limitations ensure that the maximum near addition through which binocularity can be achieved is +12.00D. Individuals incapable of binocularity must be made aware of the benefits of monocular occlusion and reassured that occlusion will not lead to the deterioration of vision in either the occluded eye through under use, or the preferred eye through over use. Binocular vision tests including the use of fogging lenses and duochrome are generally unhelpful in low vision practice.

Prescribing options
Spectacles
In deciding whether to convert the refractive findings into a prescription, consideration must be given not only to the actual improvement likely to be achieved through a change in prescription, but also to the lens form and design of the appliance. Bifocals incorporating a +4.00D near addition may prove difficult for the elderly patient, whereas the aphakic child may adapt easily to these for general purpose work. Working distances and the characteristics of the desired task must also be considered as, for example, a +4.00D near addition may be entirely inappropriate for bench work even though it improves the acuity substantially over the original +2.00D near addition. The problem of illuminating a work surface at 25cm must also be considered, as elderly patients may find it difficult to evade the shadows cast in the home environment. The instability of the underlying refraction in, for example, corneal graft cases and those with diabetic retinopathy, may cause the practitioner to defer prescribing until such times as confirmation of the result can be achieved.

Contact lenses and
refractive surgery
Many patients will enquire about contact lenses or refractive surgery as if these were magical appliances that, if prescribed, will alleviate the problems imposed by visual impairment. Contact lenses are beneficial in low vision when RGPs are fitted to visually impaired keratoconics or those with irregular corneae.
High myopes may also benefit from the increased retinal image size achieved when compared to spectacles. Those with a highly myopic refractive error lose their in-built 'uncorrected' near proximal magnification when fitted with distance contact lenses or corrected with refractive surgery. Visually impaired children who have high degrees of refractive error may, however, gain great personal confidence from being able to dispense with thick, unsightly glasses. In aphakes, contact lenses may assist with field expansion and ultraviolet light reduction, although this is at a cost as they lose the magnification (20 per cent) induced by a highly positive lens mounted in the spectacle frame. Contact lenses have, in addition, been used as the ocular in contact lens telescopes, although reported success has been limited.

Summary
Obtaining accurate refraction results is fundamental to the success of any low vision consultation. Existing techniques must, however, be modified to achieve an optimal result. Most importantly, patients must feel that the practitioner has a full understanding of their needs and in addition has a genuine desire to help. Attention should be paid to recording distance and near acuities using appropriate charts, working distances and illumination. Optical and non-optical low vision aids and aspects of low vision rehabilitation such as multidisciplinary referral will be covered in more detail in a new book, Low Vision Rehabilitation to be published by Elsevier Science Publishing later this year.

References
A full list of references is available from optician clinical editor: william.harvey@rbi.co.uk

Professor Jonathan Jackson is principal optometrist of the Royal Victoria Hospital, Belfast, and a consultant for the Central Service Agency of Northern Ireland. Dr James Wolffsohn is senior lecturer and director of undergraduate studies in optometry at Aston University