Refraction Principles for Visually Impaired Patients PDF

The principles of refraction remain unchanged whether the patient is visually impaired or normally sighted. The refractive routine logically follows the recording of a comprehensive case history (medical, ophthalmic, visual, social, educational and employment) and task analysis, whereby patients are...

The principles of refraction remain unchanged whether the patient is visually impaired or normally sighted. The refractive routine logically follows the recording of a comprehensive case history (medical, ophthalmic, visual, social, educational and employment) and 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% and 10% of new patients referred for low vision consultations simply require an updated spectacle correction.1 6.1 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 is provided with helpful and appropriate low vision aids and advice on visual rehabilitation. Patients must be helped to feel comfortable and at ease in the presence of the practitioner, and that the practitioner has a genuine interest in helping them deal with the problems resulting from visual impairment, and the clinical expertise to match. The psychological aspects associated with sight loss and with the interaction between patient and practitioner have been explored by Dodds et al2 and are also covered in Chapter Questions on medical history must concentrate on relevant issues and be directed at disorders that are likely to produce confounding disability (rheumatoid arthritis, Parkinson's disease and multiple sclerosis) or preoccupy the patient's thinking and thus impact on motivation (uncontrolled diabetes, terminal illness, psychotic illness). Interest in ophthalmic history should likewise focus on aspects of ocular health that are associated with visual impairment and the availability, or lack of availability, of medical and rehabilitative treatment (cataract, age-related macular degeneration). In similar fashion, discussion of 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. Questioning on visual function will relate closely to the assessment of vision and is thus dealt with in Chapter 7, whereas discussion on task analysis and rehabilitation issues is to be found in Section 4. Practical advice Ensure that questions asked when recording the case history are those likely to elicit responses that will infl uence assessment procedures, prescribing and rehabilitation advice. 6.2 Current optical corrections All existing spectacles should be assessed and the respective prescriptions recorded. Patients should be asked to comment on the degree of usefulness, or lack of it, that they attribute to each pair of glasses. Apparently idiosyncratic responses should be recorded and subsequently addressed. The patient who claims that glasses that 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, the patient may simply feel undressed without glasses that have been part of their attire for as long as they care to remember. Confi rmation should be sought as to the origin of the spectacles, as a signifi cant 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 often becomconfused as to which glasses they should be using for different visual tasks. All existing low vision aids should be categorised and, again, patients should be asked to outline their uses. As many as 50% of patients referred for a low vision consultation already have one or more magnifi ers.3 Patients should be encouraged to bring existing aids to subsequent clinical appointments as often they are reticent to demonstrate home-made or personally acquired devices to a person whom they perceive as an 'expert' in the fi eld. Monocular distance acuities should be recorded, at appropriate working distances, through the most appropriate glasses, using charts from the selection outlined in Chapter 7. Only in cases where the prescription is minimal, or where the patient claims that existing glasses are useless, is there any real benefi t in assessing unaided acuities. Near acuities should be recorded both through the reading glasses and through existing aids, as used in the manner demonstrated by the patient. Contact lens wearers should have their lenses assessed and 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. An accurate over-refraction should confi rm that they are appropriately powered. A baseline refraction should nonetheless be undertaken during the course of a subsequent visit. 6.3 Retinoscopy Obtaining an accurate retinoscopy result is essential if the fatigue and distress caused to the patient by a protracted and diffi cult subjective routine is to be minimised. Accuracy will, of course, depend on the clarity of the media, the nature of any uncontrolled eye movements and patient cooperation. In diffi cult cases the procedure will be improved considerably 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 signifi cant, reducing or eliminating background illumination can be benefi cial and reveal a refl ex that was previously indistinguishable. The technique of 'radical retinoscopy', which involves the use of reduced working distances, to improve image brightness, may also prove benefi cial.4 Similar effects can be achieved in patients 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 confi rmed subjectively. Understandably, full-aperture trial lenses prove advantageous in these circumstances. Cycloplegic refraction may assist in optimising a retinoscopy refl ex 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 fi xation target and that those with nystagmus 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 children and those with learning disabilities (Fig. 6.1).5,6 Figure 6.1 Dynamic retinoscopy using a custom-made, internally illuminated, target. (Reproduced from Jackson & Saunders 19996 with kind permission of Blackwell Publishing.) Practical advice An elusive retinoscopy refl ex may be tamed in an eye with medial opacities by performing 'radical retinoscopy' at a reduced working distance, in virtual total darkness.6.4 Keratometry/corneal topography Although rarely used during the course of the routine low vision assessment, keratometry can help confi rm the presence of high degrees of astigmatism alluded to on retinoscopy. Keratometry can also confi rm the presence of irregular astigmatism, a fi nding that may be particularly benefi cial when refracting visually impaired patients with a severe learning disability. Some 20% of patients with Down's syndrome, for example, are likely to exhibit keratoconus (Fig. 6.2 \[Plate 9\]5 Subjective routine Before commencing the subjective refraction, ensure that the trial frame is both comfortable and appropriately fi tted in such a way as to resemble the fi nal spectacles. Vertex distance, pantascopic 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 stated previously, full-aperture trial lenses are desirable, in this case because they allow the examiner to assess eye movements and fi xation 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 signifi cant. In these cases a Halberg trial clip should be used. 6.5.1 Distance vision Once an appropriate chart has been selected from the wide range of those available to the practitioner (see Ch. 7), 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 the patient to be presented with a letter chart and realise that they can barely see the largest letter. Those with visual acuities of less than 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 be used only for those whose acuities are better than 0.6 LogMAR (6/24 Snellen). Throughout the subjective examination the practitioner must use consistent and clear terminology. Patients fi nd many of the tests diffi cult enough without having to respond to ambiguous questions. When working from a reliable retinoscopy result, initial modifi cation to the spherical component should be made in relatively gross steps (±2.00/±5.00). 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 (±10.00/±20.00). High degrees of uncorrected refractive Low vision assessment 122 error may simply never have been detected by previous examiners. As patient confi dence in the decision-making process grows and responses improve, smaller steps can be used (±1.00/±0.50). There is rarely any point in using ±0.25 steps when assessing a visually impaired patient. Assessment of the cylindrical component of the refraction is best done using a ±1.00 DC Jackson Cross Cylinder with appropriately chosen targets. For this purpose a hand-held Landolt C Acuity Test Chart Panel can prove invaluable. Circular targets are not always present, especially on the more recently produced logarithmic charts, Us or Ds may, however, suffi ce. Initially, modifi cations to the cylinder axis should be made in 20° steps, whereas power modifi cations 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 fi nd this diffi cult. In theory, subjective testing should be less time consuming than when performed on normally sighted patients, as fi ne 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 patients believe 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.4 Those with acuities of less than 1.0 LogMAR (6/60 Snellen) are unlikely to appreciate a spherical change of less than 1 dioptre or a cylindrical change of less than 2 dioptres. Those with signifi cant medial opacities may in addition fi nd it more diffi cult to discriminate defocus than patients with macular pathology. This phenomenon has been attributed to the respective gradients of the frequency of seeing curves. Final distance acuities should be recorded in a manner consistent with the instructions given with individual charts (see Ch. 7). The practitioner may wish to conclude the distance refraction with a pinhole acuity check, but it must be borne in mind that those with central scotomas often fi nd this test diffi cult. Performance may be improved by using multiple pinholes. The stenopicslit may, on rare occasions, assist the practitioner in the search of the cylindrical axis in keratoconus. Practical advice Changes in prescription of less than ±1 DS or less than ±2 DC are unlikely to signifi cantly benefi t the patient with low vision. Changing serviceable spectacles on the basis of a prescription change alone should generally be contemplated only if an improvement of at least two lines on a LogMAR chart can be demonstrated. 6.5.2 Near acuity Near acuities, as determined by word reading charts (see Ch. 7), should be recorded through both existing reading glasses and a standard +4.00 reading addition over the optimised distance prescription. The +4.00 addition is chosen arbitrarily, as it provides unit magnifi cation when the object is placed at the least distance of distinct vision (25 cm). 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 25 cm for reading if benefi t can be demonstrated to them. Younger patients may understandably choose to utilise accommodation in preference to a near addition, although near acuities should still be recorded at a 25-cm working distance, as well as the patient's accustomed working distance. Adequate auxiliary lighting should be available in the form of an anglepoise lamp. Patients must be encouraged to adjust both the lamp to work surface working distances and the angle of incidence of the light. The Chartered Institution of Building Service Engineers' code recommendations are that casual readers should utilise a surface illuminance of 150 lux, whereas more dedicated readers, and those involved in sewing, should use up to 300 lux. Recommendations for elderly patients are that illuminance levels should be increased by 50--100%.8 During the course of the nearacuity assessment, note should be taken of both reading speed and accuracy, both with and without occlusion and using optimal and suboptimal illumination. Formal methods for assessing reading Low vision assessment 124 speed and for comparing oral and textual comprehension recall skills are available, but are generally used only as research tools.9 A number of methods are available by which the near add required to achieve a given acuity can be calculated.10 The actual acuities achieved are, however, often slightly poorer than predicted. Most methods are based on the magnifi cation ratio (MR = Near acuity recorded through a given add/Near acuity required). The predicted addition required to achieve the desired near acuity thus becomes the product of the magnifi cation ratio and the reference add used to determine the present near acuity. The magnifi cation ratio is, in reality, the ratio of the size of the letters read through the standard add to the size of the letters that the patient wishes to read. Practical example A 74-year-old patient using optimal reading glasses incorporating a +4.00 addition achieves N10 print in appropriate lighting at a working distance of 25 cm. His desire is to read N5 newsprint. Magnifi cation ratio = 10/5 = (×2) Predicted add = 2 × 4 = 8 dioptres The process of working through increased near additions in +4.00 steps and thus gradually building up the strength of the 'spectacle magnifi er' in the trial frame can be extremely helpful as it gradually acclimatises the patient with low vision to the concept of the reduced working distance and associated diffi culties. Similar calculations can be performed using the Sloan M Series, Keeler A Series and Bailey Lovie Acuity Charts. As was the case for distance acuities, near acuities should be recorded both monocularly and binocularly. It must, however, be noted that the predicted add required to achieve text of any given size is 'predicted', and thus practical confi rmation is required. 6.5.3 Binocularity Although the vast majority of visually impaired patients do not have normal binocularity, many are convinced that it is to theirbenefi t 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 eye are similar, base-in prism can sometimes be applied to facilitate comfortable fusion at near. The rule of thumb derived from Lebensohn's rule11 is to incorporate base in-prism to both the right and left lenses, equivalent to the strength of the near addition. The individual using spectacle magnifi ers with a +10.00 addition would therefore require 10 prism dioptres base-in right and left. Manufacturing limitations ensure that the maximum add through which binocularity can be achieved is +12.00. Individuals incapable of binocularity must be made aware of the benefi ts of monocular occlusion and reassured that occlusion will not lead to the deterioration of vision in either the occluded eye as a result of underuse, or the preferred eye through overuse. Binocular vision tests, including the use of fogging lenses and duochrome, are generally unhelpful in low vision practice. 6.6 Prescribing options 6.6.1 Spectacles In deciding whether to convert the refractive fi ndings 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 +4.00 addition may prove diffi cult 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 +4.00 addition may be entirely inappropriate for bench work even though it improves the acuity substantially over the original +2.00 addition. The problem of illuminating a work surface at 25 cm must also be considered, as elderly patients may fi nd it diffi cult to evade the shadows cast in the home environment. The instability of the underlying refraction in, for example, patients with corneal grafts and those with diabetic retinopathy may cause the practitioner to defer prescribing until such time as confi rmation of the result can be achieved. Low vision assessment 126 6.6.2 Contact lenses Many patients enquire about contact lenses as if these were magical appliances that, if prescribed, will alleviate the problems imposed by visual impairment. In reality, contact lenses are often benefi cial in low vision only when fi tted to visually impaired keratoconics or those with irregular corneae. High myopes may also benefi t from the increased retinal image size achieved in comparison with spectacles. In aphakics, contact lenses may assist with fi eld expansion, although this is at a cost as they lose the magnifi cation (20%) 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. Patients with a highly myopic refractive error lose their inbuilt 'uncorrected' near proximal magnifi cation when fi tted with distance contact lenses. Visually impaired children who have a high degree of refractive error may, however, gain great personal confi dence from being able to dispense with thick unsightly glasses. 6.6.3 Refractive surgery Practitioners must be prepared to advise on the indications and contraindications of corneal refractive surgery, as visually impaired patients with a signifi cant refractive error may pursue these new procedures in the vain hope that they will not only eliminate the need for spectacles but also cure visual impairment. Practitioners should also be prepared to enter into discussion with surgical colleagues on the optimal choice of intraocular lens power for those listed for cataract surgery. The high myope, for example, may well be most appropriately left undercorrected in order to ensure that unaided near acuities remain usable. Knowledge of the availability of intraocular implantable telescopes and the postsurgical rehabilitative process associated with their use should also be sourced (Fig. 6.3).12 Practical advice Visually impaired patients have the same right to fashionable and serviceable eye wear as the normally sighted. Provided the patient understands that a new correction may not improve acuity, do not deny the patient the opportunity to obtain a new correction if they so desire.