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Low vision aids: keeping it simple: This article takes a back-to-basics approach to using simple low vision aids that can be easily dispensed from the consulting room.

Introduction

There is currently no definition of low vision in UK legislation, but to aid understanding it is often useful to look towards the definition of low vision adopted by the UK Low Vision Consensus Group: 'A person with low vision is one who has an impairment of visual function for whom full remediation is not possible by conventional spectacles, contact lenses or medical intervention and which causes restriction in that person's everyday life.' (1) This definition includes those who are registered as sight impaired and severely sight impaired, but also covers the vast grey area of patients whose vision is impaired enough to significantly affect day-to-day activities but not enough to be eligible for registration.

Most causes of visual loss occur later in life and worsen over time making adaptation much harder. Coping with deteriorating vision involves coming to terms with the emotional side of vision loss as well as developing strategies to utilise residual vision. This process, which has been compared to coming to terms with bereavement, (2) is often referred to as visual rehabilitation, of which a key component for patients is the access to low vision services.

There are between 1.6 and 2.2m people aged 65 years and over in the UK with visual acuity ranging from mild (6/12-6/18) to serious (VA<6/18) levels of visual impairment. Approximately half fall into each category. (3) About 85% of those known to be visually impaired have useful residual vision and could benefit from visual rehabilitation. (4) Currently, 41% of potential low vision service providers, for example, hospitals, optometry practices, universities, offer no low vision service at all with 26% only selling magnifying devices and just 33% actually providing low vision services. Calculations based on this estimate indicate that there are just under 155,000 appointments offered for low vision annually, (2) highlighting a vast shortfall in capacity and poor utilisation of potential service providers.

Low vision services have traditionally been provided in hospitals by optometrists or dispensing opticians, accounting for 65% of total annual appointments. (2) However, increasingly these services are also being provided in multidisciplinary centres and community practice. (5) Most service providers are clustered in urban areas with the highest population densities. Interestingly, in coastal and rural areas that have the highest percentage of elderly, and therefore, correspondingly high levels of visual impairment, there is a distinctive lack of service availability. (2)

In the UK, NHS low vision services provide a range of low vision aids on a long-term loan basis without any charge to the patient; this is normally the best option for patients as the majority have chronic, progressive conditions meaning their vision is likely to deteriorate over time resulting in changes to their low vision requirements. However, in areas where there is no local NHS low vision service or the waiting times are long, practitioners may wish to offer a basic low vision service to patients by providing a consultation and selling a limited range of magnifiers. Although only a minority of practitioners actively participate in low vision clinics, most optometrists will come across patients with some form of visual impairment during routine eye examinations. For this reason, being able to assess, manage and refer people with reduced vision are all core competencies. (5) However, the College of Optometrists' guidelines do recommend: 'If the optometrist is not comfortable with his or her expertise in examining the patient with low vision, or if they cannot offer a low vision service, he/she should refer the patient to someone who can.' (6) This article concentrates on the types of basic magnification aids that can be stocked and easily dispensed in community practice. It details information on their magnification, how to use them and their advantages and disadvantages. For more detail on the theory behind magnification practitioners should read more comprehensive texts.

Finding a starting point

Although it is often time consuming to conduct a comprehensive history for someone with low vision, it can frequently be the key factor to ensuring success in visual rehabilitation. Combining open questions with a few specific closed questions will give an impression of the overall situation and areas of most concern. Topics for discussion may include: reading, cooking, mobility and independence outdoors, social situation, communicating by phone, reading correspondence, managing medication, other sensory loss and glare. The aim is to see which problems you can help with and identify those that may need an onward referral. It is vital that practitioners should have a working knowledge of local services and referral pathways so patients can be helped to access the appropriate services for their needs.

On a more immediate level, carrying out an effective history can provide a useful starting point for dispensing low vision aids. Most people with sight impairment find having a low vision aid invaluable for completing short everyday tasks such as reading correspondence or deciphering instructions. These low vision aids work on a principle of using magnification to increase the retinal image size. For people with a scotoma, this may make an object easier to see because, although the retinal image size increases, the area of visual loss remains the same size. (5)

In practice, one of the challenges can be establishing what the power of the magnification device actually is. Due to the ambiguity of nominal (magnifier power/4) and trade ((magnifier power/4) +1) magnification values and the fact that they can only be truly met under very specific and unrealistic conditions, often the best way is to determine the dioptric power of the magnifier as a starting point only and work from there.

Finding a starting point for dispensing magnifiers can be done in a number of ways but one of the simplest is by calculating the predicted magnification that is required. This is a comparison of what the person can read with their reading add with what they want to read (magnification = actual near acuity/target near acuity). For example, if a patient can read N18 with a +4.00D reading addition but would like to be able to see N6, the magnification needed is 18/6 = 3x. It is important to remember that in practice the predicted and actual acuities do not always correlate due to other variables including lighting, contrast and size of the scotoma. It may also be advisable to aim for a smaller level of print if someone would like the magnifier for fluent reading; this is called the acuity reserve ratio. For spot reading, such as reading instructions on a packet, the near acuity needed is equal to the acuity threshold so the acuity reserve ratio is 1:1. However, for fluent reading, for example a magazine, to be able to read N12 print they will need to be able to read N4 with the magnifier, which gives an acuity reserve ratio of 3:1. (5) Unfortunately accounting for this increase in magnification comes as a trade-off to the field of view.

After settling on a starting magnification, the abilities of the person and the type of task should be taken into account. Factors include: portability, lighting, task duration, ability to change batteries and work the controls of the aid, and capacity to hold the device for the required duration. Starting with simpler, lower powered low vision aids helps the person understand the limitations of magnification and what devices are available. (5) It is advisable to try the low vision aid on a near test chart first and then test it on a task similar to that described by the patient. With this in mind, it is worthwhile keeping a range of everyday reading activities in the consulting room such as a letter of correspondence, instructions on a packet, and a newspaper.

A basic range of low vision aids

High add readers

Mounting plus lens magnifiers in spectacles provides hands-free magnification and the greatest field of view as the lens is close to the eye. (5) Another advantage is the reduced stigma a patient may feel in using something as acceptable as a pair of glasses. It is worthwhile showing all patients with low vision high add readers, although one of the biggest limitations is the uncomfortably close working distance, which is often the reason for rejection. Spectacle-mounted plus lenses can be fitted monocularly or binocularly, using base-in prisms to aid convergence, up to a limit of +12.00D/3x magnification. Anything above this requires use of a specialist lens such as a hyperocular. (7) However, generally above +10.00D, binocularity is unlikely, (5) and a balance, frosted or occluded lens would be advisable in the non-viewing eye. (7) High add reading glasses can be stocked as pre-mounted fixed range powers equal in each eye, available from various manufacturers or can be dispensed as necessary specific to the patient.

Hand magnifiers

Hand magnifiers are another form of low vision aids that are increasingly more socially acceptable and very portable, especially in their folding or pocket magnifier form (see Figure 1). They are often invaluable for short 'spotting' tasks, like price tags, and the range of designs available mean they are generally cost effective and can be used flexibly for a variety of day-to-day activities. They can be internally illuminated and come in a wide range of powers (6D/ 1.5x to 56D/ 14x), (8) making them suitable for the vast majority of patients. However, the biggest drawback is the difficulty in keeping the magnifier stable for long periods of time especially for people with hand tremors or problems holding things.

The principle of all plus-lens magnifiers is the same: the magnification remains constant regardless of the eye-to-magnifier distance providing that the object is placed at the anterior focal point. (7) However, this is rarely done in practice and leads us to another question which is what glasses should patients use with their magnifiers? In theory as hand magnifiers are designed to be used with the object placed at the anterior focal point of the magnifier, parallel rays of light emerge and distance vision spectacles are advised. However, this is rarely the case, as in practice, patients will assume it is a near vision task and automatically put readers on, or subconsciously exert accommodation. In this case, the magnifier will need to be positioned closer to the object to obtain a clear image, (5) thereby reducing the magnification. If the patient also takes advantage of the flexible eye-to-magnifier distance by holding the magnifier further away it means they will also reduce their maximum potential field of view, (7) thereby not getting the best out of their magnifier. The maximum field of view is obtained when the magnifying lens is held closer to the eye.

Stand magnifiers

In order to overcome the issue of placing the object accurately at the anterior focal point of the magnifying lens, the lens can be mounted in a stand to fix the position. However, in general, manufacturers tend to mount the lens at a distance less than the anterior focal length, (5,7) so that patients can intuitively use their reading glasses or pre-presbyopes can exert accommodation. With a similar range of powers to hand magnifiers (8D / 2.8x to 56D / 14x) the biggest advantage of stand magnifiers is the stability they provide due to the fixed lens casing; this means they can be used for more extensive reading such as a book or magazine and it also makes them more comfortable for patients with hand tremors, although it also makes them bulkier, heavier and less portable. In some cases, the magnifying lens can also be mounted with an adjustable screw thread enabling a degree of variable magnification. As with hand magnifiers, the maximum field of view is obtained when the magnifying lens is held closer to the eye.

The majority of stand magnifiers have an internal light source as the casing obstructs ambient light falling onto the page and low-powered magnifiers can sometimes have a space underneath enabling the patient to write or for handicrafts such as sewing (see Figure I)P Around the neck (chest) magnifiers are also a useful way to provide hands-free magnification, although they are only available in limited magnification and can sometimes be heavy.

Bar readers/flat field magnifiers

Although only available in lower magnifications (up to 3.6x) these distortion-free, paperweight-style I magnifiers are invaluable for children, patients in a working environment or those with early vision loss. The single lens hemi-cylindrical (bar-reader) or hemi-spherical (flat field) form of magnifier is designed to be placed directly on an object,5 usually written text (see Figure 3). The dome shape of these magnifiers means that light gathering properties provide a brighter view avoiding the need for an internal illumination source. These magnifiers are lightweight, portable and socially acceptable and also available with a red line to allow for easier tracking of text. Varying the magnifier-eye distance does not change the field of view of these magnifiers, as that is purely dependent on the size of the magnifier. Although great for reading multiple lines of text it is important to remember that bar readers only elongate the text in the vertical meridian, causing a certain amount of distortion. (7)

Distance aids

Only a small proportion of people with low vision actually use distance low vision aids. One of the most likely reasons for this is due to the dynamic nature of distance tasks. Apart from watching the TV or going to the cinema / theatre and occasional spotting tasks like road signs and train times, distance vision is generally done 'on-the-go,' something which distance low vision aids are not suited for due to their restricted field of view and the fact that they tend to distort space and limit movement perception making them unsuitable for walking around. (5) The starting point for distance vision magnification is calculated in a similar way to near vision by dividing what they can see with their distance correction by what they would like to see.

Due to the relatively small uptake, the most useful distance aid to stock in practice is a simple pair of binoculars or a small range of hand-held monocular telescopes, full details of which are beyond the scope of this article. Binoculars are widely available in magnifications of 8x and lOx with varying fields of view. They are simple to use, socially acceptable and cost-effective.

Conclusion

Potential low vision providers are vastly underutilised in the UK producing a large discrepancy in the volume of low vision appointments available compared with the number of patients requiring help. Although referring patients to NHS low vision services is normally the preferred choice, sometimes this option may not be suitable for patients and they may prefer to be seen in a community setting. Being able to provide a basic consultation and supply a core selection of low vision aids is a step forward in helping patients with visual rehabilitation.

Exam questions

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 8 December 2017. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article.

CET points will be uploaded to the GOC within 10 working days. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

Preeti Singla MCOptom, Prof Cert Glau, DipTp(IP)

About the author

* Preeti Singla is a specialist optometrist working for Buckinghamshire Healthcare NHS Trust. She holds an MSc in clinical optometry and is a qualified independent prescriber.

References

Visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.

Learning objectives

* Be able to advise on the use of and dispense simple low vision aids (Group 4.2.1)

* Be able to advise on the use of and dispense simple low vision aids (Group 4.4.1)

* Understand the range of simple low vision aids available for near vision tasks (Group 6.3.1)

Caption: Optometrist

Caption: Dispensing opticians

Caption: Figure 1 Hand magnifiers are readily available in a wide range of sizes and magnifications

Caption: Figure 2 Some stand magnifiers are designed to enable the patient to write or perform other tasks underneath the lens

Caption: Figure 3 Flat field magnifiers provide good illumination without the need for an internal light source
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Title Annotation:Low vision
Author:Singla, Preeti
Publication:Optometry Today
Date:Nov 1, 2017
Words:2757
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