Low Vision Aids

Function

Low vision aids utilize the angular magnification effect by increasing the relative size and relative distance. Angular magnification is apparent object size compared to the actual object size when seen without the device, e.g., a telescope. The relative size makes the object larger and bigger (no accommodation is needed), e.g., CCTV. Relative distance is achieved by bringing the object closer (accommodation is required), e.g., magnifiers.[8]

Classification

LVAPs can be broadly classified as:

• Optical
• Non-optical

Optical Devices for Distance

• Handheld telescope
• Mounted telescope [9]

Optical Devices for Near

Spectacles

• Prismatic half eyes
• Bifocals

Magnifiers –Handheld

• Stand
• Illuminated
• Non-illuminated

Electronic Devices

• Video magnification systems like closed circuit television, portable video magnification
• Computer hardware and software that provides screen magnification, synthesized speech, and tactile display[10]

Non-Optical Devices

• Reading lamp
• Reading stand
• Writing guide
• Reading guide
• Signature guide
• Bold line notebooks
• Black ink bold tip pens
• Soft lead pencil – 2B,4B,6B, etc
• Needle threader
• Notex

Others

• Talking scales
• Talking glucometers
• Color identifiers
• Talking compasses
• Walking sticks

Principle

Low vision aids help in utilizing the vision to the maximum. These are useful in clinical conditions like retinitis pigmentosa, glaucoma, macular degeneration, albinism, aniridia, retinal detachment, diabetic retinopathy, optic atrophy, and chorioretinitis.[11] LVAP's function is based on the principle of angular magnification. This is achieved by increasing the relative size and decreasing the relative distance. Angular magnification is defined as an apparent increase in the object's size compared to its original size, as, for example, with a telescope.[12] Relative size increase makes the object visibility better even with a relatively lower vision. This does not require any accommodation, such as with closed circuit television (CCTV). Relative distance decrease assists by bringing the image of the object relatively closer. This requires good accommodation, as, for example, with magnifiers.[13]

For non-optical devices, the concept of illumination is extremely important. The light source is kept near to the eye, and moving the light source closer will provide higher illumination. A higher level of illumination is required in patients with retinal pathologies like loss of cone function (age-related macular degeneration), glaucoma, diabetic retinopathy, retinitis pigmentosa, and chorioretinitis. Reduced illumination is needed in albinism and aniridia.[14]

Low Vision Optical Devices for Near

Magnifying Spectacles

Optical Principle 

A convex lens produces a magnified image once the object is within the focal length. This results in an erect, virtual, and magnified image. The magnification produced is 1/4 the power of the lens. This is suitable for near and intermediate distances.[2]

Advantages

• Allows a larger field of vision
• Handsfree
• Can be used monocular or binocular
• Improves near and intermediate distance
• Cosmetically better as compared to other LVAPs

Disadvantages

• Suffers from spherical aberration
• The more the power, the shorter the focal length, thus closer the reading distance.
• Close reading leads to fatigue and unacceptable posture
• Not suitable for those with eccentric fixation[15]

Hand Magnifiers

Optical Principle

A convex lens from +4.0D to +40.0D produces an erect, virtual, and magnified image. This is useful for short-time tasks in patients with the field of vision reduced to 10 degrees or more. Hand magnifiers are available in three designs – aspheric, aplanatic, and biaspheric.

Advantages

• No accommodation required
• Working distance is more
• Useful for eccentric viewing
• The additional light source may enhance vision further.

Disadvantages

• Occupies both hands
• Limited field of vision
• Manual dexterity is a prerequisite[16]

Stand Magnifiers

Optical Principle 

A convex lens forms a virtual, magnified image at a short distance from the lens. The magnifier needs to be held over the reading material and moved across it.

Advantages

• Technically simple to use
• Very useful for patients with tremors, arthritis, or constricted visual fields

Disadvantages 

• A small field of vision
• A flat surface is always needed for comfortable reading
• Uncomfortable posture because of close reading posture[17]

Closed Circuit Television Systems (CCTVS)

CCTV comprises a television, a camera, and a platform to put the readable text. We can control the brightness, contrast as well as polarity, and it can magnify the text from 3x-60x.[18]

Low Vision Optical Devices for Distance

Telescope

These devices employ the principle of angular magnification. The magnification can range from 2 to 10x. They can be prescribed for distant, intermediate, and near viewing, reducing the field of view with magnification.

Types

• Hand held monocular
• Clip-on
• Bioptics- Mounted on spectacles[19]

Principal

A telescope has two lenses (two optical systems) mounted in such a manner that the focal point of the objective coincides with that of the focal point of the ocular. The objective lens act as a converging lens. The magnification of the telescope is denoted by the formula M=f/f. The telescopes focus on the near object by altering the distance between the objective and the eyepiece lens or by increasing the power of the lens.

Galilean Telescope

In the Galilean telescope, the eyepiece is a negative lens, and the objective lens is a positive lens. The image formed is virtual and erect. The loss of light causes a reduction in brightness, and the field quality is poor.[15]

Keplerian Telescope

In the Keplerian telescope, the eyepiece and objective are both positive lenses. The image formed is real and inverted. Prisms are mounted to erect the image. There is more loss of light, and field quality is good. The major advantage of the telescope is that it is the only device that increases distance vision. The disadvantages are a restricted field of view, higher cost, reduced depth perception, and apprehension.[20]

Non-Optical Devices

Writing Guide

This is made of black cards having horizontally oriented rectangular cutouts along the cards. A patient with low vision can feel the empty cutout space and write.[8]

Signature Guide

This is made up of a black card with a white rectangular sheet. The white area act as a signature guide.[21]

Typoscope or Reading Guide

This is a masking device with a line cut out from an opaque area, a non-reflecting black plastic paper, or a paper of thicker consistency.[14]

Notex

This is a rectangular piece of cardboard having steps in the right upper corner, which help in identifying the currency of the note. The first cut indicates Rs 500; the second indicates Rs 100. The third indicates Rs 50, and so on.[22]

Relative Size Devices

These work on the principle that a larger object will subtend a larger visual angle and is easier to visualize. Examples include

• Large printing material
• Larger playing cards
• Computer keyboard
• A large clock, telephone, calendar, etc.[8]

Computer Softwares

Various interactive software allows the read-out of documents. Examples of these include

• SuperNova screen reader
• MAGic LVS
• Zoom Text magnifier/reader

Glare Alleviating Devices

These devices reduce unwanted light. These are useful in ocular pathologies like cataracts, corneal opacity, retinitis pigmentosa, and albinism. Various glare-reducing devices include 

• Sunglasses
• Polaroid glasses
• Corning photochromatic filters (CPF glasses)
• Cap
• Umbrella
• NoIR filters[11]

Corning Photochromatic Filters (CPF Glasses)

They block approximately 99% UV-A wavelength and reduce 100% UV-B wavelength. They attenuate 98% of the blue wave light except CPF 450 and 96% of high-energy ambient blue light. The number in the CPF glass indicates the wavelength in nanometres above which the light is transmitted. Examples include

• CPF 550 (red) – useful in retinitis pigmentosa and albinism
• CPF 527 (orange) – useful in retinitis pigmentosa and diabetic retinopathy
• CPF 450 (yellow) - useful in optic atrophy, pseudophakia, and albinism
• CPF 511 (yellow-orange) - useful in optic atrophy, pseudophakia, aphakia, glaucoma, developmental cataract, and macular degeneration.[23]

NoIR Filters

These filters absorb short wavelengths in the visible spectrum, which scatter within the ocular media. These filters absorb infrared rays and ultraviolent light up to 4000nm. These filters help in filtering out excessive light and allow the optimal light to reach the eyes.

Examples of NoIR filters lenses include

• Dark amber (2%) - Useful during bright days and protect 100% from ultraviolet and infrared rays
• Standard grey (13%) - Helpful in patients with glaucoma, diabetic retinopathy, cataract, and corneal transplant patients
• Medium plum (20%) - Good in dim light and also helpful indoors
• Light grey (58%) - Reduces indoor glare light under fluorescent light
• Yellow (65%) - Helpful in patients with retinitis pigmentosa and macular degeneration[24]

Color and Contrast Sensitivity Improvement

These devices improve contrast by utilizing a light color against black or dark color. The colors with high contrast should be chosen in the room.[25]

Pinhole Spectacles

These spectacles have multiple pinholes of approximately 1 mm in size, and the holes should be at least 3-3.5 mm distance, which is the pupil's approximate size. The pinhole spectacles are helpful in patients with corneal opacities or patients having irregular astigmatism. These are less helpful in patients with reduced visual acuity.[26]

Mobility Training Devices

The mobility assisting devices for low vision patients help navigate the distance. Examples include long canes and strong portable lights.[27]

Field Expansion Devices

With the increase in magnification, the field of view decreases. The various methods by which field of view increases is

• Compression of an existing image to include more area available
• Addition of an image that relocates the image from a non-seeing area to a visible area
• Reflection of the image with the help of a mirror from a non-seeing area

Reverse telescopes and Fresnel prisms are other examples of field expansion devices, but reverse telescopes are less accepted due to the minification effect, and prisms are used in the direction of field loss with a power range of 10 to 15 D.[28]

Future of Low Vision Aids

Bionic Eye

This special LVA device is specially designed for patients who are blind due to retinitis pigmentosa and AMD. A bionic eye can also serve the purpose in patients with severe vision loss. The prime functioning depends on a healthy Optic nerve and well-developed visual cortex and association areas. This device is not helpful in patients who have congenital blindness. The device consists of a digital camera inbuilt into a pair of glasses, a video processing microchip unit, a radio transmitter, a receiver implanted in the ear, and a retina implant with electrodes on a chip behind the retina.[29]

Mechanism of Action

The camera in the eyeglasses captures the image and then sends the signals to the microchip, which converts the image into electric impulses of light and dark pixels. The image is also sent to a radio transmitter which transmits pulses wirelessly to the receiver. This, in turn, sends impulses to the retinal implant by a hair-thin walled implanted wire. The electrode generates electrical signals, which are, in turn, transmitted to the visual cortex. Bionic eye application needs training by patients. The patients should learn to interpret the objects' dark and white dots array. This device is still under clinical trial and evaluation.[30]

Issues of Concern

There are various issues of concern with LVA. These include the following:

Non-availability of Low Vision Aids Services

The presence of LVA services in a region does not necessarily mean good coverage of services. A survey in 195 countries revealed that only 115 had coverage of LVA services; among 115, 39% had <10% coverage, 22 had coverage between 11 to 50%, and 8 had coverage of more than 50%. Only 23 countries had coverage in the Asia Pacific region, and five countries had >10% coverage.

The countries or areas with >10% coverage had a higher proportion of older people and a more urbanized population. The survey also revealed that LVA services were primarily monodisciplinary and were available at secondary and tertiary eye care levels.[31]

Poor Socioeconomic Strata

Patients with poor income, daily wage earners, rural areas, small children, women, patients with disabilities, ethnic origin, refugees, and the elderly are also the vulnerable age group who miss LVA services.

Other Barriers

• Long traveling distance to access the services
• Access to cost and affordability
• Lack of awareness
• Lack of referral services
• The communication gap between patients and doctors
• Lack of motivation for visual rehabilitation
• Patient misconceptions and wrong perceptions[32]

Clinical Significance

The most common indications for LVAPs among children include albinism, retinopathy of prematurity, congenital malformation, and optic neuropathy. Common indications among young adults include keratoconus and ocular injuries. Common indications of old age are glaucoma, ARMD, diabetic maculopathy, macular degeneration, retinal degeneration, chorioretinitis, optic atrophy, and myopic degeneration.[33]

Other Issues

• Social stigma
• Difficulty in obtaining the devices
• Unable to understand and learn the functioning of the device
• Long waiting time at the hospital
• High magnitude of denial
• Fear of loss of job
• Lower need for an LVA product[34]

Enhancing Healthcare Team Outcomes

Low vision assistive products are critical in patients' visual rehabilitation and upliftment. Using assistive technology to the best potential is a viable and achievable option that will reduce the dependency of visually handicapped and visually impaired patients.[35]

It is imperative to understand the patient's ocular pathology, the need for LVA products, the type of LVA needed, and the barriers to utilizing these products. The acceptance rate of these products is low in general. Improving the utilization of these products will reduce the burden of blindness. A multidisciplinary approach will help target the low vision patient group and provide quality treatment necessary for visual rehabilitation.[36]

Nursing, Allied Health, and Interprofessional Team Interventions

The nursing team, the allied health staff, and the interprofessional team play a key role in patient evaluation, counseling, and guidance. The nursing and allied health staff trained in handling LVA play a vital role in the visual rehabilitation of these patients. The patients should be taught how to use the device for the best effective outcome. In addition, counseling of the family members plays a crucial role in assisting these patients. The nursing staff also ensures patient follow-up and satisfaction post visual rehabilitation.[36]

Nursing, Allied Health, and Interprofessional Team Monitoring

The nursing team, the allied health staff, and the interprofessional team help monitor the application of these devices to the patients. They also help ascertain the working vision and decide the type of device needed per the indication. The patients should have a healthy optic nerve and visual cortex to utilize the LVA effectively. The allied health staff also helps determine these patients' functional optic nerve and higher mental function.[37]