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A Review of Low Vision Rehabilitation

Mark E. Wilkinson, OD

Contents

Health Assessment

It is obviously important to review the patient's ocular and systemic health with respect to preexisting and new diseases, and to conduct a complete ocular health examination.

When cataracts are adversely affecting visual functioning (they would normally be removed if no other ocular problems were present), and cataract surgery is not contraindicated by some other ocular disease (e.g. active diabetic retinopathy or choroidal neovascular membrane, etc.), surgery should be considered to enhance visual functioning and maximize visual potential.

When considering postoperative refractive error correction for individuals with additional ocular diseases, Lighthouse International suggests the following: individuals undergoing cataract surgery who will require magnification because of coexisting conditions such as macular degeneration or diabetic retinopathy, should almost never be made emmetropic postoperatively.

Lighthouse suggests that a postoperative refractive error of -2.00 D to -3.00 D will leave the individual happier because he or she can remove his or her spectacles to read more comfortably (with or without an optical device) than would be possible with bifocals or separate reading glasses. Additionally, if the patient needs reading glasses, they will be of a lower power. If the patient needs higher amounts of magnification, the reading glasses will be lower powered with less weight and distortion. Additionally, patients will be able to use hand-held magnifiers without their glasses on. Obviously, these individuals will still need a lens correction to have their best distance vision, but the reading advantages more than make up for this.

Applicability of Selected Low Vision Devices

Many individuals that are visually impaired will benefit from the prescription of optical and electronic devices, and from changes in their visual environments.

Magnification

Assuming it has been concluded that an optical aid is appropriate for a patient, the magnification to be provided by the aid must be determined.

There are 4 types of magnification that individuals with visual impairments can employ to enhance their visual abilities. They are: relative size, relative distance, angular, and electronic.

Relative Size Magnification (RSM) enlarges an object while maintaining the same working distance. Numerically, RSM is equal to size after magnification divided by size before magnification.

Consider an example for print:

Because it is difficult to enlarge reading materials much beyond the 2M (18 point) level, this option is of relatively little value for individuals who have experienced a significant loss of vision. Additionally, there are many things individuals who are visually impaired want to read (e.g., newspapers, mail, bank statements, etc.) that are not readily available in large print formats.

Relative Distance Magnification (RDM) The easiest way to magnify an object is to bring it closer to the eye. By moving the object, the image size on the retina is enlarged. Children with visual impairments do this naturally. Adults with less accommodative ability will require reading glasses to keep the object in focus as it is moved closer.

RDM is defined as r/d where r equals the reference or original working distance and d equals the new working distance. For example:

With reading glasses, as the lens power increases, the working distance decreases. For example, a +5D lens focuses at 100/5 = 20cm (40/5 = 8 inches) and a +10D lens focuses at 40/10 = 4 inches (100/10 = 10cm). Reading glasses do not magnify by their power alone when worn in the spectacle plane. Magnification occurs simply because the lens strength requires the individual using the glasses to hold things closer to have them in focus.

Angular Magnification Angular magnification occurs when the object is not changed in position or size but has an optical system interposed between it and the eye to make it appear larger. Examples of devices that produce angular magnification are telescopes and hand magnifiers.

Optical Magnification Ratings

Some companies use lens focal length/4 (defined as Rated Magnification) while others use the quantity (lens focal length/4) + 1 (defined as Conventional Magnification) to specify magnification strength for their devices. This is why dioptric power, which is an absolute value and is the same under all conditions, is a better way to specify the magnification needs of an individual.

Rated Magnification (Mr)

Rated magnification assumes that the individual can accommodate up to 4.00 diopters when doing close work, which gives a working distance of 25cm (25cm is the standard reference distance always used when talking about magnification).

Conventional Magnification (Mc)

The underlying assumption in the equation ((lens focal length/4) + 1) is that the patient is “supplying” one unit (1X) of magnification


Effective Magnification (Me)

Effective magnification is based on the reference distance in meters to the object (image is formed at infinity). (d is reference distance and F is dioptric power of the lens)

Determining Needed Magnification Magnification needs are based on an initial reference value and the desired final value. Clinically, needed magnification is defined as the entrance distance (or near) acuity divided by the goal acuity (VA entrance/VA goal).

Electronic Magnification is magnification that can be provided by a closed circuit television system or computer software. These systems can make an image appear larger and with greater.

Magnification Estimation Techniques Accurate near visual acuity testing is essential for determining magnification needs required for reading and other near point activities. There are several ways to determine a starting point for near magnification. We will discuss the two most common ones that are used today.

Kestenbaum’s Rule To determine the power necessary to read 1M size print (newsprint), take the reciprocal of the patient's distance acuity to establish a starting add power.

As examples:

However, because distance acuity is a poor predictor of near visual functioning, this is not a very accurate method for determining a reading add power. The more accurate and more frequently used approach is the Lighthouse method.

Lighthouse Add Determination To determine the power needed to read 1M print, measure the patient's near acuity at a 16 inch/40 cm working distance (WD). Multiply the M acuity by 2.50D to arrive at the theoretical add power needed to read 1M print

As examples:

The Lighthouse method establishes a good starting power. However, patients may need additional power if their contrast sensitivity is reduced, if they have multiple scotomas, if they need to read smaller than 1M print, or if they have less than ideal illumination when reading. The clinician should adjust the add power using normal reading materials under task lighting to determine how much add power is actually needed.

 

 

 

 

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