Addendum

IAPB ESSENTIAL LIST: LOW VISION SERVICES—key terms and meanings

Version: FirstEdition (March 2017)

Why do we need Low Vision services?

Despite major advances in eye care, an estimated 124 million people worldwide cannot have their sight fully restored with standard corrective measures.These people are regarded as having low vision.

The majority of them, about 80 million people, however, have some residual vision that can be enhanced with special intervention: rehabilitation strategies, optical and non-optical devices, all of which can be utilized to assist them to carry out tasksthat require vision.

Groups with special low vision needs

Especially children, people of a working age and people who are older may have visual needs that require different interventions.

Most of those with low vision are older and the numbers are expected to double over the next 20-30 years. This is due to an increase in the prevalence of age-related and disease specific chronic conditions and accompanying complications.

Whilst the number of children with low vision is small in comparison with adults, the life years spent with the problem is significantly greater than adults. Low vision not only affects the child’s development, independence and ability to pursue education, but has significant psychosocial and economic impact on the child and the family.

Uncorrected vision impairment has a major effect on a person’s ability to perform tasks fundamental to not only activities of daily living, but also education and gainful employment. It carries with it a higher risk of death, depression, an increased risk of falls, and increased dependence.

Universal health coverage

Coverage of low vision services in most low or medium income countries is, however, below 5%, and under 30% in most high-income countries. Among the reasons for this low coverage is lack of trained professionals, the intensive assessment procedure and little to no availability of appropriate and affordable assistive devices.

Initiatives for low vision services

A number of initiatives provide the foundation for low vision services to reduce risk, promote best practice and so increase the quality of clinical and non-clinical care. These initiatives also emphasise the need for eye health providers to align work within the wider disability and inclusion arenas. Also to engage with a broader base of stakeholders working in education, rehabilitation and the development sector; thus acting as the bridge between these sectors and ensuring a continuum of care. Management of low vision through the provision of quality services and appropriate and affordable devices which are scalable, adaptable, cost effective and responsive to the population remains a priority of 'Universal Eye Health' - the WHO Global Action Plan, Education for All (EFA), Global Campaign for Education (GCE) and United Nations Convention on the Rights of Persons with Disabilities (UNCRDP). The only cost effective source of assessment tests and low vision devices, for those in the developing world, is through the Low Vision Resource Centre based at the Hong Kong Society for the Blind. In addition, WHO established the Global Cooperation on Assistive Technologies (GATE). A global survey resulted in the WHO Priority Assistive Products List (APL), which WHO believes will increase access to assistive products.

Technical notes on the prescription of Low Vision Optical Devices

Low vision optical devices have a variety of designs, with a range of ergonomic characteristics across the magnification range, with and without built-in light source. Restrict range to non-illuminated magnifiers where the availability of suitable batteries or power sources are limited.

Magnifiers

Hand-held Magnifiers

The magnification effect obtained with a hand held magnifier mainly depends on the optical power (Dioptric Power) of the magnifying lens. The manufacturer's ratings of magnification are unreliable because different manufacturers use different formulae and often do not measure powers precisely. Best magnification is obtained when the object of regard is positioned at or close to the focal plane of the lens. (for a +5.00D lens the focal plane is at 20 cm, for +10.00D it is 10 cm, for +20.00D it is at 5 cm, and so forth).

For the following sequence of lens powers, each step represents a predicted one line of improvement on a Logarithmic designed (LogMAR) visual acuity chart (+4D, +5D, +6.25D, +8D, +10D, +12.50D, +16D, +20D, +25D, +32D, +40D).

Stand Magnifiers

Many stand magnifiers incorporate inbuilt lighting systems. These require batteries or an electrical power source. There are many different designs for mountings that hold the lens above the page. Some use transparent cylinder structures, others use four legs, others have a cantilevered arrangement, which allows the person to write or do other manipulations beneath the magnifier lens. Most, but not all, stand magnifiers have a fixed focus arrangement: a screw, or slide focus that allows the lens to be moved closer to or further from the page. For some stand magnifiers, the lens system is mounted on an arm or a gooseneck connected to a base that allows the lens to be suspended above the object of regard.

In choosing a range of stand magnifiers, the following sequence of enlargement ratios can provide a guide. On this sequence of enlargement ratios, each step represents one line of improvement on the visual acuity chart (1.6x, 2.0x, 2.5x, 3.2x, 4x, 5x, 6.3x, 8.0x, 10x, 12.5x, 16x).

A minimum set of magnifiers should have enlargement ratios in the order of 2x, 4x and 8x and also a dome magnifier (1.5x) the enlargement ratios of the magnifiers should be relatively evenly distributed across the range shown above. The range of stand magnifiers should include examples of the various styles of stand magnifiers and about half of them should include illumination systems.

Hemispherical Dome Magnifiers and Cylindrical Bar Magnifiers

Dome magnifiers are sometimes called visolets or bright field magnifiers. The hemisphere domes are solid glass or plastic and are designed to rest on the page. They form an image that is enlarged by the value of the refractive index and the image is in the same plane as the object. This means the enlargement ratio is 1.5x as the refractive index of the plastic or glass used for these devices is very close to 1.5. When patients use hemi-spherical dome magnifiers they gain 1.5 times magnification advantage, and the patient does not have to move closer to or further from the page.

These magnifiers have a special light gathering effect, so the image is quite bright. The curvature of the surface is related to the diameter of the dome –small domes have steep curves and large domes have flatter curves - but the enlargement ratio is always very close to 1.5x. Hemi-cylindrical bar magnifiers operate in a similar way except that they only give magnification in one direction. For reading, the bar is oriented across the page, and as a result the letters are magnified in the vertical direction and the height of the letters is enlarged by 1.5 x but there is no magnification of the letter width.

Spectacle magnifiers

Spectacle magnifiers can be considered to function similarly to hand magnifiers, except that the lenses are mounted in the spectacle plane. The working distances required to obtain optimal focus are inversely proportional to the dioptric power of the lens. The reduced working distance required with high-powered spectacle magnifiers can make them difficult to use by persons with restricted dexterity.

Spectacle magnifiers can facilitate binocular vision in lower powers. As the power of the lens increases, additional base in prism is required to provide binocularity. Spectacle magnifiers incorporating addition powers of > 10 dioptres are really only appropriate for monocular vision.

Modern designs use differing lens designs and diffractive principles can make the spectacles more cosmetically acceptable and lens weight can be reduced. In higher powers, half-eye designs or bifocal formats can be very useful.

Telescopes

Telescopic Magnification

In the following sequence of telescope magnifications, each successive step represents one extra step on a LogMAR visual acuity chart. (2.0x, 2.5x, 3.2x, 4.0x, 5.0x, 6.3x, 8.0x, 10.0x, 12.5x, 16.0). When choosing a range of telescope magnifications to be used in a clinic, it is not necessary to choose precise magnifications that come from this list, but one should be aware of how much visual acuity improvement should be expected when changing from one magnification level to another.

The most commonly prescribed magnification in monocular telescopes is 4x. A minimum series of telescopes magnifications would be 2.5x, 4x, and 6x. There are approximately 2 rows of visual acuity improvement between each of these magnification levels. Within a busy low vision clinic it would be desirable to expand this magnification range by adding extra magnifications from a larger sequence up to x8 or x10.

Telescopes are mainly, but not exclusively, used for distance vision. They enable users to see finer details when the object of regard is distant (say, beyond arms length).

Telescope systems come in many different formats. Some are monocular and others are binocular. While most have some adjustability of focus, some offer a wide range of focus (up to close distances in the range 40 cm to 15 cm), while others do not focus to close distances. Some telescope systems are head-mounted with the telescopes mounted in a spectacle or dedicated frame to hold the telescope system. Some head-mounted telescope systems are available as "over the counter" ready-made systems, but others are custom made by speciality manufacturers or laboratories. Sometimes the telescope system is mounted for bioptic use (to facilitate the interchange from viewing the object directly to viewing the same object through the telescope system) and at other times the head-mounted telescopes are arranged directly before the eyes for continuous viewing of the object or activity of interest. For some telescopes, there are mounting arrangements that allow the telescope unit to be easily attached and removed from a spectacle frame.

For some monocular distance vision telescopes there are "lens caps" for near vision available. These are positive lenses in mountings that attach to the front of the telescope. The power of the lens cap determines the viewing distance. Some telescope systems are designed to be used binocularly for near vision, but then the two telescope units must be angled appropriately so that they converge to the specific viewing distance. Many binocular telescope systems must be specifically designed and manufactured for the individual patient.

Filters

A wide variety of tinted lenses (or filters) are available to help patients with visual impairments.

Prescribing filters

For an individual patient it is difficult to predict the color characteristics and the density (or darkness) of the tint that will give the best visual comfort and performance. There is no widely accepted methodology for prescribing tint colors and densities. Nor are there rigid rules that specify that one kind of filter should be used for patients with a particular ocular disease. Almost invariably, the prescribing of tint characteristics will involve some trial and error and patients identifying their preferences.

For many patients for whom tints are important, it is not uncommon for two or more tints to be prescribed; for example, a light tint for indoors, a dark tint for outdoors and a very dark tint for use at the beach or on snow fields. For a patient needing these filters of different densities, the tints would usually have similar color characteristics.

To test which tinted lenses are most appropriate for individual patients, the clinician should have a diverse range of tinted lenses available. This should include a range of neutral grey filters and a range of “minus blue” filters. These should be filters with four density levels of light, medium, dark and very dark(these would have transmission values of approximately 60%, 35%, 15 % and 5%)

  • For a substantial proportion of visually-impaired patients, the preferred lens tints preferentially reduce the amount of blue light and these lenses are generally orange, brown amber or yellow in their colour appearance. These are often referred to as “minus-blue” tints.
  • Another substantial proportion of visually impaired patients prefer neutral grey tints.
  • And for some individual patients, other colours may be best.
  • There should be dark red lenses as these are best for most individuals with rod achromatopsia, and for blue cone achromatopsia, magenta is often the tint of preference.
  • Many patients with active retinal disease have a strong preference for lenses in the “minus-blue” range. Thus, many patients with retinitis pigmentosa or with macula degeneration will be prescribed yellow, orange or brown filters. Some patients with the same diseases will prefer neutral or other coloured tints.

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