Raising the Bar in Low Vision

With the arrival of spring, the eye care professional’s mind turns to new ways to improve the bottom line of the practice. Although the usual thoughts revolve around frames and accessories, one should look at an ever improving product line: low vision aids. This is not only a non-traditional source of revenue, but it is also an area where you can provide much needed and much appreciated specialized care to your patients.

What is Low Vision?

Low vision occurs when there is a visual impairment that does not allow a patient to achieve a visual acuity of 20/60 or better with correction, when the degree of peripheral vision is less than 20, or when there is a loss or contrast – or so the textbooks state. In reality, low vision occurs whenever your patient cannot see what they need or want to see due to decreased acuity. What is low vision to one patient is only a minor inconvenience for another. The classifications of low vision are as follows:

• Moderate low vision- 20/70 to 20/160 VA

• Severe low vision- 20/200 to 20/400 VA. In the United States, this is considered legally blind

• Profound low vision- 20/500 to 20/1000 VA

• Near total blindness

• No Light Perception (NLP) - This is usually less than 25% of the legally blind population

To better understand exactly what 20/200 means, the largest letter or optotype on the standard Snellen chart (E), is 20/200. Since there is no optotype between 20/100 and 20/200 on a standard chart, or an optotype over 20/200, the refractionist usually has to utilize a special hand held visual acuity chart which reduces the testing distance and makes the letters easier for the patient to see. Alternately, the practitioner can have the patient walk slowly toward the Snellen chart until they can see the 20/200 letter. The distance would then be recorded. When the refractionist reduces the distance, he or she must convert the results to a 20 foot notation.

To convert to a 20 foot notation, the refractionist must first document the testing distance and optotype. The reduced distance becomes the top number and the bottom number is the size of the optotype recognized; for example, 4/100. Next, the top number is multiplied by the integer needed to produce the answer of 20. In this case, the number is 5. The bottom is then multiplied by the same number. Therefore, the 20 foot notation for a 4/100 would be 20/500 VA.

Low Vision is on the Rise

The most common causes of severe vision loss are injury, macular degeneration, glaucoma and diabetes.

Macular degeneration is the leading cause of blindness in individuals over the age of 60. It is a disease that affects the central portion of the retina, called the macula. The macula is the area where we have excellent central vision. Age related macular degeneration, or AMD, causes a loss in the patient’s central vision and it occurs in dry and wet forms.

• Dry AMD- Characterized by yellow deposits on the macula called drusen. Although a majority of these patients will never lead to central vision loss, it does need to be monitored carefully. If the drusen increase in size or amount it can cause a distortion in the patient’s vision or a loss of contrast. In advanced cases, the drusen can cause an atrophy of the macula tissues which will lead to loss of vision. Dry AMD can also lead to Wet AMD in about 10% of Dry AMD sufferers.
  

• Wet AMD- Deriving its name from the blood and fluid that accumulates in the retina, Wet AMD is the result of choroidal neovascularization. This is when there is an abnormal growth of blood vessels in the choroid, just behind the macula. When the blood vessels leak fluid, the fluid causes distorted vision, blurry lines and a loss of central vision. As the choroidal neovascularization increases, scar tissue can result and a permanent loss of vision occurs.

Glaucoma is an eye disease that usually involves an increase of intraocular pressure along with other risk factors, in which optic nerve damage occurs. Normally symptom free in the early stages, glaucoma can result in noticeable blind spots, tunnel vision and then total blindness when left untreated. Glaucoma is a major cause of blindness in America, and the leading cause of blindness in African Americans. There are two main categories of glaucoma in adults: open angle glaucoma and narrow angle glaucoma.

Open angle glaucoma is the most common type of glaucoma. It affects 70 to 80 percent of all glaucoma sufferers. It is more prevalent in individuals over the age of 35, African Americans, and those with a sibling history of the disease. Open angle occurs when an individual gradually loses the ability to drain aqueous from the eye or when the eye over-produces the aqueous causing the pressure inside the eye to reach abnormally high levels. Since it is gradual, the patient usually doesn’t realize they have the disease until permanent damage is achieved and vision loss has occurred. Glaucoma vision loss usually occurs at the periphery of their visual field and progresses inward.

Narrow angle glaucoma is less common but is a more serious form of the disease. A medical emergency, narrow angle glaucoma occurs when there is a mechanical closing of the angle and there is an immediate increase in intraocular pressure. The result is ocular pain, nausea, vomiting and the classic halo’s around lights.

Diabetes is a major cause of blindness in adults ages 20 to 74. Diabetes can be a contributing factor in patients developing macular degeneration and is the cause of diabetic retinopathy. Diabetic retinopathy is a progressive disease that affects the blood vessels on and around the retina. Diabetic retinopathy is usually diagnosed in four stages:

• Mild Retinopathy- Blood vessels in the retina develop tiny aneurysms.

• Moderate Retinopathy- Blood vessels that feed the retina are blocked.

• Severe Retinopathy- Areas that are deprived of blood due to blocked blood vessels begin to atrophy and signal the development of new blood vessels. When the new blood vessels actually develop, the last stage begins.

• Proliferative Retinopathy- Weak, abnormal blood vessels are formed along the retina and vitreous surface. When these blood vessels leak blood, vision loss results.

Treatment Options

When surgery, medications and other treatment options are not viable for the patient, then he or she must turn to low vision aids. Low vision aids can range from the simple hand held magnifier, to a high tech computer, and are usually designed around the patient’s needs. Low vision aids in general fall into the categories of optical and non-optical aids.

Low Vision Aids for Near Use

One of the first options is specialized near vision glasses or high add bifocals. These are an excellent way for a practice to offer low vision aids to a patient. Patients are usually already comfortable with wearing glasses, they offer hands free use, they provide a wide field of vision when compared to a magnifier, and they are easier to use for long term work. When fitting a patient with these devices, keep in mind:

• If the patient has excellent binocular vision, keep the add power to a maximum of +5.00 diopters to help avoid convergence problems.
  

• The ECP should adjust the near PD to allow for the greater convergence needed to utilize higher powered plus lenses and bifocals. The easiest technique for this is known as the Gerstman inset. Multiply the add power by .75 to arrive at the inset per eye. Some low vision experts recommend 1.0 mm inset per diopter of add power.
  

• Higher binocular add powers than +5.00 can be used in patients with monocular vision. However, do not inset the segment as much because a monocular patient will turn their head to avoid their nose.

Hand held or stand magnifiers are an excellent low vision device for an ECP to offer to their patients. They are the conventional and most recognizable low vision aid, and are relatively easy to use, when the proper power is used. A good, diversified supply of magnifiers will include dioptric powers of 2, 4, 6, 8, 10, 12, 16, 20 and 24(or .5X, 1X, 1.5X, 2X, 2.5X, 4X, 5X, and 6X). Magnifiers are easy to carry, may allow for a greater working distance than specialized glasses, and the stand type of magnifiers are excellent for older patients with tremors and/or arthritis. Magnifiers that are lighted and can run on either batteries or electricity are also excellent for near use because they increase the amount of usable light to the eye making images easier to see. LCD lighting is becoming standard on these magnifiers.

Telemicroscopes are mounted on an eyeglass frame and can be designed to allow for any near or intermediate working distance. This can be of great benefit to those who use a computer frequently, play an instrument, crochet and other tasks that are more comfortable at a longer working distance.

Low Vision Aids for Distance Use

Telescopes are the primary low vision aid for distance use. When a patient cannot see sporting events, signs, or the black board a telescope can be of great use to them. A telescope can be hand held, placed on a lanyard or mounted on a pair of glasses. The prices can range from inexpensive premade designs as well as custom ordered designs. A standard set of telescope powers to have on hand are 2x, 4x, 6x, and 10x. The strongest telescope, 10x, should only be used for patients with a visual acuity up to 20/600. If a patient has a visual acuity worse than 20/600, special mobility training may be required. Remember, as plus power increases, the patients’ field of view decreases.

Non-Optical Aids

Non-optical aids can include everything from large print media to lighting to high contrast overlays. Most low vision patients can benefit from the large print versions of routine consumer magazines that are readily available. Lighting can also be beneficial. With increased illumination, the patients’ pupil constricts which allows for an improvement on their depth of focus. Using a yellow bulb, or a yellow plastic overlay can also add a high contrast improvement for reading tasks.