LOW VISUAL AIDS

PRESENTER : Dr Chethan Murthy

Guide : Dr Santosh Patil

Low Vision

• Low vision essential refers to :

• “Level of vision that prevents a person from performing customary vision activities with standard or conventional optical correction”

• ICD : Visual Impairment – [Maximum Visual Loss less than 6/18 snellen in better eye/ field of vision of 10 degree or less] divided into 5 categories

Causes of Low vision

In Children • Albinism • Optic neuropathy• Trauma• And congenital malformations

In young adults • Late onset congenital disorders• High refractive errors• Ocular injuries

In Old Age• ARMD• Diabetic Maculopathy• Macular and retinal degenerations• Glaucoma• Myopic degenerations

Irreversible damage to ocular media or visual pathway due to any disease results in low vision

Low Visual Aids

Low visual aid refers to optical device that improves , enhances residual vison by magnifying the image at retinal image

Mainly Optical devices, but consists Non optical devices also

Basic Optics :

Four different ways of enlarging an image are used in low vision practice

• Relative distance enlargement

• Relative size enlargement

• Electronic (transverse) enlargement

• Angular enlargement

Relative distance enlargement

Enlargement that results from a reduction in the object viewing distance - usually 25 -20 cm

result of reducing the object viewing distance is to increase the angular subtend of the object at the pupil entrance

Thus, an object observed at an initial reference working distance of 40 cm produces a retinal image twice as large as the original when viewed at a new working distance of 20 cm

This is the principle behind hand and stand magnifiers and all spectacle-mounted magnifiers

Unmagnified object

Decreasing the viewing distance

Magnified object

• Schematic representation of the effect of initial reference working distance on the calculated enlargement

12

Relative size enlargement• Magnification achieved by increasing the size of the object while the working distance

remains the same

• The enlargement ratio is the comparison between the standard text size

• E.g: 12-point print type compared with the enlarged version e.g:- 24 point Measurement is by direct scale comparison which, in the above case, is 24/12 = 2×

• If the use of large print text is combined with a closer working distance, the overall enlargement ratio is the product of the two

• for example, 24-point print read at 20 cm compared with 12-point print at 33 cm provides an enlargement ratio of

• Relative size magnification (24/12 = 2) multiplied by Relative distance magnification (33/20 = 1.6); i.e. 2× 1.6 = 3.2×

• The retinal image size produced by the combination of large print held at a reduced working distance is therefore increased by a factor of 3.2

Electronic (transverse) enlargement• Enlargement is achieved by projecting or electronically changing an image so that the new image

has an increased angular subtend at the eye

• Enlargement of the image is measured directly by dividing the image height measured from the screen by the original object height

• Eg: If the image of a 1 cm object produced on the screen measures 9 cm, the enlargement is 9×.

• This form of enlargement usually allows longer working distances to be utilized

• Eg: the person habitually uses a working distance of 10 cm, but use of the EVES increases the working distance to 30 cm, the overall enlargement is reduced by a factor of 3

• Overall enlargement is thus 9/3 = 3×

• Electronic vision enhancement system (EVES), often referred to as a CCTV (closed-circuit television), provides this form of enlargement

Electronic (transverse) enlargement

Angular enlargement• Ratio of the angle subtended by the image through the optical system to the

angle subtended by the object when viewed directly

• Angular magnification is produced by telescopic LVAs

• when used for near vision tasks, the angular effect is combined with relative distance enlargement (by using a plus lens powered addition)

LVAs

Hand Magnifiers• The hand magnifier is the most familiar of all low vision aids

• These magnifiers are generally suitable only when small amounts of image enlargement are required

• Usually used for near vision problems

• Can be used along with distance and reading spectacles or within bifocal reading addition

• Indications• Primarily indicated for spot and short-time tasks in patients with filed of vision

reduced to 10 degree or more• Also used by patients with low vision as an auxiliary lens for finer jobs

• Types and design • Available from +4 to +40 D or higher• Most patients accepts 8, 12, 16, 20 D depending on task and degree of impairment• They should be preferably have wide field , light weighted, and self contained

illumination

Visolett Magnifiers• Special types of hand magnifiers, that almost double the spectacle magnification

achieved by reading without correction • Diffusely increase illumination, binocularity • Has constant magnification of 1.8X • Used as LVAs in high Myopes

Stand magnifiers

• Most people with visual impairment are elderly and often have hand tremors or suffer fatigue after even relatively short periods of having to support a hand magnifier steadily

• Being able to support the magnifying lens without using the hand to sustain a fixed lens-to-working plane distance

• Although low-powered stand magnifiers are available, they are often bulky as a tall stand is required to ensure that the lens-to working plane distance approaches Fm

• As it is frequently difficult to perform tasks beneath high-powered stand magnifiers, they often have solid sides and built-in illumination.

Practical optics• Most stand magnifiers have a fixed focus, although they can consist of a lens

suspended around the neck on a cord (often termed a ‘chest’ magnifier) or held on an adjustable or flexible ‘arm’

• Variable focus stand magnifiersThe distance between the object plane and the lens can be freely selected by the

patient, so these magnifiers can be used essentially as a ‘steadied’ hand magnifier. This type of low vision aid is particularly valuable to the model maker or crafts

makers, although supplementary lighting is almost essential.

• Fixed focus stand magnifiers• Fixed focus stand magnifiers are typically designed so that the magnifier lens-to-

object plane distance is less than the anterior focal length of the lens, Hence the light leaving the lens is divergent

• The light must therefore be converged before reaching the retina• This can be achieved by ocular accommodation and/or through the use of a near

reading addition (Fa), so that parallel light enters the eye.

Bar and flat-field magnifiers

• Bar and flat-field magnifiers are designed to be placed on an object, where lens is not, however, raised above its base

• Also known as ‘paperweights’, ‘bright field’

• The enlargement produced is real, similar to that of an electronic vision enhancement system

• Flat-field magnifiers • Are a single solid hemispherical lens, • Material removed from the top and bottom to leave a rectangular viewing window,

allowing a larger hemispherical lens to be used (increasing the field-of-view width) without adding excessive weight.

• In practice, enlargement is generally up to 3×

• The image is formed close to the original object, regardless of the bar or flat field magnifier thickness.

• Hence enlargement is not created by a reduction in viewing distance (as with other plus lens magnifiers) and the field of view is not increased by decreasing the eye-to magnifier distance.

• image is formed close to the patient’s normal working distance, so the bar or flat-field magnifier can be used with no change in posture and with normal reading glasses

• Binocular viewing is also possible

• The larger the proportion of a sphere used to create a flat-field magnifier, the higher the magnification, but the smaller the field of view

• Bar magnifiers• single solid lenses of hemi cylindrical shape that magnify only in the vertical direction

when the length of the magnifier is positioned horizontally

Spectacle magnifiers• A spectacle magnifier is any lens mounted in the spectacle plane with a

front vertex power greater than that used as a conventional reading addition

• This magnifying lens may be monocular or binocular, spherical or aspheric, single vision, bifocal or even multifocal

• It differs from a telescope in having no vergence amplification

• Practical optics• Spectacle magnifier provides for an increase in retinal image size by increasing

the front vertex power, enabling a closer working distance to be used effectively• It thus provides a form of relative distance enlargement• An erect, virtual, and magnified image is produced

Types and designs• Half eye glasses : Preferable because they reduce the weight , thickness and

size

• High add bifocals or high add trifocals can be used to read large print at greater distance

• Binocular spectacles: • Prescribed usually vary in power from 4D with 6Δ base in to 12 D with 14Δ base in

• Monocular Spectacles:• Consists of standard aspheric lenses from 4D to 20D in 2D increments and specially

designed microscopic and double lenses from 24D (6X) to 60D (15X)

Instructions : • Patients are instructed to hold material very close and then move it out and scan the

line one by one• Light must be properly adjusted , and patient should be advised to have patience and

learn reading skills, since speed is reduced to word by word pace.

A range of binocular high-powered near addition readers

• Eschenbach spectacle-mounted reading aids• Eschenbach have developed a series of spectacle mounted reading aids including the

lightweight Labo Clip-ons• Available in binocular (×1.7–3) and monocular (×4 and ×7) forms• preassembled Prismatic Bino Comfort, which are reduced aperture, high add, reading

spectacles (4D, 6D, 8D and 10D)• Aplanatic Comfort, which are full aperture spectacle magnifiers (12D, 16D, 20D,

24D, 28D and 32D)

High-powered addition bifocal

• Many high add bifocals are produced, and various ranges of finished lenses are available

• additions of from +16D to +48DS are available• Eg: Norville and Zeiss - SOLA LVA 25• Keeler LVA 12

Telescopes

• Telescopes increase angular subtense without requiring a change in working distance

• Ideal for tasks that have to be undertaken at far and intermediate distances, such as spotting bus numbers and reading music

• useful when undertaking near tasks, as a longer working distance can be achieved than by proximal enlargement

• Telescopes are essentially afocal systems, in which the rays of light entering the telescope from a distant object are parallel, as are the rays leaving the telescope and entering the user’s eyes

• Astronomical telescopes(also known as Keplerian telescopes)• consist of two convex (positive) lenses; the first (Fo) converges the light so

that the rays reverse when they pass the lenses focal length ( fo)• second eyepiece lens (Fe) converges the now divergent rays so that they are

parallel when exiting the system• The image is inverted, a situation that is rectified, both laterally and vertically,

by prisms• Can be achieved using two right-angled (Porro) prisms, as in conventional

binoculars; these also allow light paths to be folded, reducing the length of the telescope

Astronomical telescopes [Keplerian] telescopes

• Consist of two convex (positive) lenses; the first (Fo) converges the light so that the rays reverse when they pass the lenses focal length ( fo)

• Second eyepiece lens (Fe) converges the now divergent rays so that they are parallel when exiting the system

• The image is inverted, a situation that is rectified, both laterally and vertically, by prisms

• For an astronomical telescope, both lenses are convex and therefore both fo and fe are positive

Galilean telescopes

• have a convex objective lens (Fo) to converge the light from the distant target, but a concave (negative) eyepiece lens (Fe)

• This is positioned closer than the focal length ( fo) of the objective lens, thereby diverging the converging light rays to exit the system in parallel

• For a Galilean telescope, the eyepiece lens is concave, an therefore fe is subtracted from fo.

Telescopes designed for mobility

Bioptic telescopes• typically present a magnified retinal image,

but only to part of the field of view

• User views through their refractive correction, utilizing the image enhancement through the compact low-powered telescope, by a change in head/eye position as required

• Disadvantage : cause ring scotoma around the magnified zone

Autofocus telescopes• lightweight autofocus is available on some

designs of telescope to allow users to track objects more easily at changing distances

• If the system is binocular, convergence of the telescope tubes will also have to be motorized, to provide autofocus

Contact lens and spectacle lens combination telescopes

• field of view of a telescope can be increased by decreasing the distance between the objective lens and the eye

• Hence fitting the eye with a negative powered contact lens (typically about −30.00D) in combination with a positive powered spectacle lens creates a Galilean telescope with a length equal to the back vertex distance of the spectacles.

• The field of view is dependent on the power, diameter and vertex distance of the spectacle lens

• A ring scotoma around the lens edge can be minimised by using a blended aspheric lens

Aids for peripheral field loss• Field loss, particularly if less than 20º of the visual field remains, can cause severe

visual impairment

• Retinitis pigmentosa and end-stage glaucoma can severely constrict the visual field, and stroke or other brain injuries can cause hemianopic field loss

• Reversed telescopes• these increase the field of view (although in commercial designs this is limited by the now smaller

objective lens) at the expense of visual acuity

• Negatively powered lens• held 20–30 cm from the eye, this lens acts as the objective of a reversed Galilean telescope with the

patient’s accommodation as the eyepiece.

• Spectacle reflecting systems: alternative to telescopic systems, typically used in hemianopic field loss, can reduce the eye or head movement needed to scan the visual environment

• a mirror in front of the seeing field to reflect objects in the non-seeing area into the seeing field, • a right-angled prism, base towards the field defect in front of the non-seeing field

• Eg: Fresnel Prisms

Electronic visionenhancement systems• Electronic devices that provide electronic (transverse) enlargement

Mouse’-style devices

• Mouse’-style, more portable, EVES were first described in the 1970

• consist of a camera usually mounted on rollers in a small hand-held case that can be moved over the object of interest

• Mouse-style devices are more portable than conventional stand-based EVES

• Newer models have a larger depth of focus, enabling curved surface to be viewed, and they can, in addition, be mounted on a stand or tilted on edge to enable the user to write beneath the camera

Closed circuit televisionCCTV the camera picks up the reading material , magnifies it and displays it on tv screen

CCTV provides excellent contrast and magnification, color and reverse contrast polarity

Can provide magnification electronically upto 60X

Advantages:Provides Distortion free, brighter, magnified image with enhanced contrast on a larger screenReverse Polarity also helps in improvement of image clarity

Disadvantages:Expensive, heavy and difficult to move aroundDifficulty in operation for some patients

Large Print Computers• Can provide Enlarged print on the screen through either standardized and

dedicated software• In some programs the image may be enlarged and moved infront of the

observer's eye

Head-mounted devices Large Print Computers

Intraocular Low vision aids• IO-LVA is the recent advance in low vision rehabilitation

• Involves Surgical implantation of intra ocular aids

• It provides 3X magnification in the fundus, so the resultant retinal image is perceived through peripheral retina

.

implantable miniaturized telescope mounted in the lens capsule using a carrier lens similar to common intraocular lenses an supported anteriorly by the iris.

Carrier system

Section through telescope

multiple-unit artificial retina chipset (MARC) system

retinal prosthesis is based on the fundamentalconcept of replacing photoreceptor function with an electronic device

Finger clipped Reader

Other Optical devicesAbsorptive lenses

Tinted Lenses : Low absorptive high transmission

Photochromatic : help to people who are less light sensitive

Filters : Contrast can be enhanced by using spectacles with yellow and amber filters

Corning CPF filters

Younger Protective lens series

Medical Technology Filters

• Discomfort glare can be removed by reducing light with a tint

• A grey neutral density filter will simply reduce all the light and hence not change the contrast of the retinal image

• Blue light is scattered more than red light by the lens, and is also thought to b more harmful to the macula. Therefore yellow, amber and brown tints, which absorb shorter wavelengths of light, have been suggested as being of benefit to patients with low vision who suffer from disability glare

Non Optical Devices

• Approach Magnification

• Lightning

• Contrast Enhancement

• Increasing the size of the object

• Personal Items : Needle threader

• Auditory aids: talking clocks, watches

• Writing and communication guides : hand writing guides, signature guides, check guides

• Mobility assistive devices : Sighted guide. Dog guides, Night scopes etc

• Sensory substitute assistive and adaptive devices• Auditory substitutions : talking books, reader services and audio descriptive devices• Tactile Substitutions : Braille

Evaluation of the patient with low vision• History taking

• Onset, progression of visual loss• Educational status, Professional requirements• Exact requirements of the patient in terms of near, intermediate or distance vision

• Ocular Examination• Slit lamp /fundus examination• Refraction

• Snellen’s chart• Pinhole test – Pinhole mask – particularly used when Vn is less than 6/18• Special Low vision charts :

• Distance Vision Charts- Bausch ad Lomb, Sloan, Bailey and love log mar and modified ETDRS . • Near vision Charts that are recommended are Sloan’s M chart, Keeler’s chart, modified ETDRS charts with

spectacle correction for near work – Nv Is taken at 40 cm

• Assessment of the magnification required for reading• Slogan’s M chart – M1 to M20• Directly gives the magnification required : If the patient reads comfortably M5 chart , then magnification

required is 5 times…

• Functions tests : Amsler grid , Contrast sensitivity , Glare test, Visual fields, Colour vision, Binocular vision

Prescription of LVA’s

• Guidelines• Aim is to provide maximum vision without compromising mobility of patients• When Magnification increases both working distance and visual filed

decreases• Aid chosen should be simple, light weight, portable, flexible• All the devices with similar magnification should be tried before prescribing a

particular LVA• Patients Visual status , mental status, needs, and motivation should be given

due consideration while prescribing LVA

Dispensing LVA• Magnifiation trial

• Magnification required should be calculated taking into influence of scotometry, contrast and glare data

• Power of reading correction should be adjusted until the patient can read the target size words and text

• Presciption of Appropiate devices• Various LVAs of the calculated power should be testes systematically in sequence : Convex lens first

as a spectacle [either monocular/ binocular with converging prisms]• Second as a hand magnifier and third as a stand magnifier • Complex telescopes, microscopic doublet, electronic systems should be reserved as last choice• Final Choice depends on Patients compability

Training to use appropriate LVA• Patient should be taught how to handle the aid, how to hold the printed material,

Working distance• Should be educated regarding optimal Lightning , take care of the device• Counselling and motivation of the patient for the continue use of LVA• Periodic Follow up – User performance with the aid, amount of progression made

THANK YOU

Bibilography

• Optics and refraction – Khurana

• Clinical Optics : AAO

• Low visual Manual- Jhonathan jackson