You are not logged in.

  


A Review of Low Vision Rehabilitation

Mark E. Wilkinson, OD

Contents

Vision Rehabilitation Devices

Devices should be considered and presented to the patient in a sequence that roughly follows increasing cost and complexity:

Regular spectacles Always start by determining whether a change in the spectacle correction will enhance distance and/or near acuity as determined by a trial frame refraction. Regular spectacles can provide:

Stronger bifocal corrections will be required to use relative distance magnification early in the vision loss process. As higher amounts of reading addition are needed (greater than 6D), a +4.00D add may prove beneficial as an intermediate distance add. Individuals benefiting from this type of intermediate correction are those who need lower amounts of magnification for less detailed tasks such as signing their name, cutting their finger nails, seeing the food on their plate as well as cooking and reading larger print, such as the headlines, etc.

Spectacle Magnifiers can take several forms. These can include:

Figure 13. Reading spectacle designs.

When prescribing reading spectacles, is important to consider whether the patient functions better monocularly or binocularly. If patient is monocular, he or she will not need prism incorporated into the reading spectacles but it may be necessary to occlude/fog the fellow eye if it interferes with the better eye.

The need for occlusion or fogging the poorer seeing eye is often found in situations where the sighting/dominant eye has the greatest vision loss. In this situation, the now poorer seeing/dominant eye confuses the better seeing non-dominant eye resulting in poorer visual performance. Occlusion or fogging of the poorer seeing eye will ameliorate this problem and allow the individual to function at the highest potential.

For those individuals who have similar near acuities between their two eyes and whose binocular acuity is better or the same as their monocular acuity, base-in prism will provide more comfortable, sustained reading ability. Base-in prism is used for adds of +4.00 to +12.00D. The prism power equals the add strength plus 2 prism diopters base-in for each eye. As an example, a +6.00D add would have 8 prism diopters base-in added to each eye's lens.

Advantages of spectacle magnifiers:

Disadvantages of spectacle magnifiers:

Working distance for these lenses is determined by taking the reciprocal of the equivalent add power (+20D lens will have a working distance of 100/20 = 5cm).

Figure 14. Close working distance resulting from use of a high dioptric power lens.

Training considerations for spectacle magnifiers:

Absorptive Lenses These lenses can absorb uniformly across the spectrum (e.g., gray sunglasses) or selectively in certain wavelength bands (e.g., blues, so the lenses appear yellow). Absorption of blue may be advantageous because chromatic aberration and glare can be reduced.

Advantages of absorptive lenses include:

Figure 15. Absorptive lenses with broad and selective absorbance spectra.

Disadvantages of absorptive lenses include:

Hand/Stand Magnifiers

Hand magnifiers are typically positioned so that the material being viewed is at the focal point of the lens. Patients need to be made aware that the larger the lens diameter, the weaker the lens power will be.

Hand magnifiers are used with the individual's distance spectacle correction in place. They come in both illuminated (standard or LED bulbs) and non-illuminated versions.

Hand magnifier considerations include the optical design, which may be:

Figure 16. Hand magnifiers.

For aspheric hand magnifiers, the front surface gradually flattens toward the edge of the lens. This design reduces or eliminates distortions induced when looking away from the optical center of the lens. Aspheric lenses have directionality. This means that the more curved surface should face toward the individual using the magnifier.

Aplanatic magnifier systems are created by using two plano-convex lenses with convex surfaces facing each other. This results in a distortion-free image right up to the edge of the lens. Patients vary in their appreciation of aspheric and aplanatic systems when compared to conventional spherical lenses.

Advantages of hand magnifiers include:

Disadvantages of hand magnifiers include:

Training considerations for use of hand magnifiers include:

Stand Magnifiers are available in illuminated and non-illuminated designs. The larger the lens diameter, the weaker the lens power. Stand magnifiers need to be used with a reading correction.

Figure 17. Stand magnifiers.

Advantages of stand magnifiers include the following:

Disadvantages of stand magnifiers include the following:

Training considerations for stand magnifiers include:

Telescopes can be hand-held or spectacle-mounted, and they can be monocular or binocular. Fixed focus, manual focus and auto focus systems are available with Galilean or Keplerian designs. They can be used for distance, intermediate, or near vision enhancement.

Figure 18. Telescopes.

Figure 19. Spectacle mounted bioptics.

Telescopes are afocal optical systems consisting of two lenses, separated in space by the sum of their focal lengths. Galilean telescopes have a plus power objective lens and a minus power ocular lens. They form an erect/upright image. Keplerian telescopes have a plus power objective lens and a plus power ocular lens. Keplerian (astronomical) telescopes form an inverted image and require an erecting lens or prisms to make them into terrestrial telescopes.

Galilean telescopes have several practical advantages for low vision work. The image is upright without the need for erecting prisms, and the device is shorter than a Keplerian telescope. Galilean telescopes typically are 2, 3, or 4x in strength, inexpensive, lightweight, and have a large exit pupil, which makes centering less difficult.

Four power (4X) telescopes and stronger are usually Keplerian in design, which gives an optically superior image, but they are more expensive with a smaller exit pupil requiring better centering and aiming. Keplerian binoculars, contain prisms to erect the otherwise inverted image.

Advantages of telescopes for vision remediation:

Disadvantages of telescopes:

Training considerations for telescopes:

Telemicroscopes (a.k.a. reading telescopes or surgical loupes) can be hand-held or spectacle mounted. Spectacle-mounted reading telescopes can be in full diameter (center-mounting) or bioptic configurations. They are available in Galilean or Keplerian designs.

Galilean telescopes used as surgical loupes require an add to be combined with the objective lens. The field size is far smaller than that obtained with bifocal spectacles.

Telescopic loupes can produce asthenopia when the patient has any type of refractive error. If binocular loupes are not aligned properly, vertical or horizontal phorias can be induced.

Adopting a working distance too far inside the focal distance of the add can require excessive accommodation, even for a myope.

When viewing a near object through an afocal telescope, the telescope acts as a vergence multiplier. The approximate accommodation required is given by Aoc = M2U, where Aoc equals vergence at the eyepiece which also equals accommodation, U equals object vergence at the objective which equals 1/u (u is the distance between the objective lens and the object being observed), and M equals the magnification of the telescope.

Advantages of telescopes include the following:

Disadvantages of telescopes include the following:

Training considerations for telescopes include:

Video Magnification Devices Closed circuit video magnification systems are available in a variety of different styles ranging from full sized systems with their own or separate monitors, to hand-held camera systems that plug into the user’s own television, to portable battery powered systems.

Figure 20. Video system used for magnification.

Figure 21. Maximum magnification obtained with a video system.

Figure 22. Portable video display system used for reading.

Advantages of video magnification systems include:

Disadvantages of video magnification systems include:

Training considerations for video magnification systems include:

Head-Borne Video Magnification Devices As the name implies, these devices are worn on the patient's head and consequently move as the head turns.

Advantages of head-borne devices include:

Disadvantages of head-borne devices include:

Training considerations for head-borne devices:

Non-Optical Devices Objects used in daily living can be modified to facilitate use by low vision patients. Some modifications and special aids include:

Figure 23. Typoscope reading and signature guides and other special aids for low vision patients.

Reading stands and clipboards can be helpful for maintaining proper placement of reading material. Use of these devices can reduce postural fatigue and facilitate placement of adequate light on reading materials.

Figure 24. Use of a lapboard and magnifier for reading.

Figure 25. Use of a reading stand.

Use of typoscope signature and reading guides can reduce glare from glossy paper and minimize figure-ground confusion.

Some aids make use of relative size magnification, which can be used in conjunction with other forms of magnification (i.e. use of low powered reading lenses with large print). There are a variety of options available.

Illumination is probably the single most important factor in enhancing visual functioning. The median illumination found to give optimum performance in a low vision clinic was 1188 lux, whereas normal home conditions have a median value of only 177 lux. More than 90% of low vision patients showed some improvement in near or distance visual acuity when illumination was improved. (Silver JH, Gould ES, Irvine D, Cullinan TR, Visual Acuity at Home and in Eye Clinics, Trans. Ophthalmol. Soc. UK (1978) 98: 262-266)

Types of illumination that can be used include:

Light fixtures are as important as the bulbs used in them. They must be flexible to allow proximity to the paper and placement at a non-glaring angle. The position of the source must be adjustable to allow maximum comfort/contrast enhancement. Normally, light angled in from the side of the better seeing eye is best.

Figure 26. Light fixture providing proper illumination.

Adaptive Technology A comprehensive review of adaptive technology is beyond the scope of this course, however a brief review of computer systems that can be beneficial for low vision patients is included as an Appendix to this course.

 

 

 

 
< < previous