Bioptic Low Vision Aids for the Visually Impaired
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Helping you see with macular degeneration (and other central
vision disorders)
(Some content is courtesy the National Eye Institute)
What is macular degeneration?
Macular degeneration (MD) is a disease that gradually damages the macula which is the central part of the retina. The macula is responsible for providing sharp, central vision for reading, identifying objects, recognizing faces and even driving. It works best in bright light and is also responsible for providing most of our color vision.
In some cases, MD advances so slowly that people notice little change in their vision. In others, the disease progresses faster and may lead to a loss of vision in both eyes.
MD is a leading cause of vision loss in Americans 60 years of age and older and hence is also called Age-Related Macular Degeneration (AMD). There are many other types of disorders that can be called “macular degeneration” including Juvenile Macular Degeneration (Stargardt’s Disease), Macular holes, Best’s Disease, Epiretinal membranes (Macular pucker) and there are many other disorders that can also affect central vision including Albinism, Achromatopsia, Diabetes, MS, Optic Atrophy, Nystagmus, Micro-ophthalmia, Coloboma, Rod-Cone Dystrophy, and Myopic degeneration to name just a few.
AMD occurs in two forms: dry and wet. In Dry AMD the retina tissues atrophy slowly over time, while in Wet AMD vessels beneath the retina begin to grow and leak causing swelling and other complications that can cause a sudden and potentially dramatic reduction in vision.
Fortunately, macular degeneration is limited to the central retina, so that the peripheral retina, responsible for side vision and motion detection remains intact. Individuals with AMD will never go totally blind. In fact, individuals with AMD can usually walk and engage in most domestic activities with little difficulty due to their vision.
Where and what is the macula?
The macula is located in the center of the retina, the light-sensitive tissue at the back of the eye. The macula is “ground zero.” When we look at things we look with our macula. The macula represents about 10 degrees of one’s central vision and is the part of the retina that provides our 20/20 vision. As we move further away from the macula, the remaining retina, even when healthy, does not provide the clear, detail vision that we are accustomed to using. In fact, 10 degrees from the center of the macula (fovea), even healthy eyes are only capable of seeing about 20/100.
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Image courtesy National Eye Institute |
This is a photograph of a normal retina. Image courtesy Academy Eye Associates |
10 degrees from the center of the macula (fovea),
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What is dry AMD?
Dry AMD occurs when the light-sensitive cells in the macula slowly break down, gradually blurring central vision in the affected eye. As dry AMD gets worse, you may see a blurred spot in the center of your vision. Over time, as less of the macula functions, central vision is gradually lost in the affected eye.
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This retina has dry AMD. Image courtesy Academy Eye Associates |
The most common symptom of dry AMD is slightly blurred vision. You may have difficulty recognizing faces. You may need more light for reading and other tasks. Dry AMD generally affects both eyes, but vision can be lost in one eye while the other eye seems unaffected.
One of the most common early signs of dry AMD is drusen.
What are drusen?
Drusen are yellow deposits under the retina. They often are found in people over age 60. Drusen alone do not usually cause vision loss. In fact, scientists are unclear about the connection between drusen and AMD. They do know that an increase in the size or number of drusen raises a person's risk of developing either advanced dry AMD or wet AMD.
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This retina has macular drusen. Image courtesy Academy Eye Associates |
What is wet AMD?
Wet AMD occurs when abnormal blood vessels behind the retina start to grow under the macula. These new blood vessels tend to be very fragile and often leak blood and fluid. The blood and fluid raise the macula from its normal place at the back of the eye damaging the macula, often rapidly.
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This is a retina with wet macular degeneration. Image courtesy Academy Eye Associates |
Why don’t eyeglasses help macular degeneration?
When macular degeneration damages central vision, the focus on the back of the eye (retina) usually does not change. It’s the back of the eye that’s not working properly. Imagine visiting a movie theater and seeing the picture perfectly well. Now imagine that black paint is spread across all of one of the actor’s faces. Will changing the focus get rid of the black paint? Of course not. Now imagine that the camera zooms in for a close-up and now the actor’s face is 4 times larger. Now the black paint will cover just the actor’s nose, or cheek or eye, and as a result you can now see much more of the face.
So, what helps individuals with macular degeneration see better is making things larger. Magnification can be useful to read, engage in crafts, see faces, TV and even to drive. Low Vision Care is the specialty that helps individuals with vision loss maximize their remaining vision so they can continue to lead productive and rewarding lives.
What is Low Vision?
Low vision is defined as reduced visual function caused by any disorder of the eye or visual system. This results in a reduced level of vision that cannot be improved to normal with glasses, contacts, medication or surgery. Individuals usually start to notice difficulty engaging in normal day-to-day activities when their vision becomes less than 20/40 (6/12).
Who provides Low Vision care?
While individuals with macular degeneration are likely to be visiting retina specialists to treat their disease, different vision care providers called low vision specialists are specially trained to prescribe the appropriate Low Vision Aids and train the individuals to use them effectively.
What are Low Vision Aids?
The major optical goal in low vision care for central vision loss is to magnify the material sufficiently to make it legible by the visually impaired. Individuals know that they can read the newspaper headlines but not the small print, so in effect we try to make the small print as large as the headlines. Strong reading glasses, magnifiers and electronic magnification devices are used to magnify print to make it legible for the low vision individual.
The activity distance determines the type of management
Optical magnification requires that the material to be viewed be held at the focal length of the optical system. For strong reading glasses and magnifiers this is usually much closer than the habitual working distance of the user, but with training and practice it can usually be accommodated.
Conversely, some activities must be performed at distances defined by the activity such as computer screens, cooking, reading music, seeing faces, theater, TV, signs, blackboards, and traffic signals for example. In activities such as these the optical device must provide a working distance supportive of the activity— and magnify the image enough to be able to resolve it by the use of telescopic aids. Of course, there are instances where simply moving close-enough is an option, but when that is impossible, impractical or inappropriate, telescopic aids provide the only option.
Reading
Distance vision is a social activity
Distance vision, however, is often a public activity, and impacts social interaction and hence quality of life. The loss of the ability to see body language and make eye contact can be isolating and can contribute to depression amongst the visually impaired. Loss of distance vision usually impacts independence and self-worth much more so than loss of reading and it cannot readily be replaced through other modalities.
When distance vision is the best that can be achieved with glasses and one’s vision remains inadequate for the patient’s goals, than there’s only two options left to further enhance distance vision- walk up close enough to see it, or, make it closer optically using miniature telescopic devices.
Low Vision Telescope Optics “101”
Optical telescopes are available in two designs—Galilean and Keplerian. Each has its distinct characteristics and attributes.
Galilean telescopes are small and lightweight due to their rather simple optical design. They produce a bright image but offer rather narrow fields of view (about 5 degrees at 3x) and tend not to be sharp edge-to-edge. They are available as both fixed-focus and focusable versions and are usually prescribed in powers between 1.5x and 3x powers, though other powers are available. Because of their small size and low weight they are convenient for binocular prescriptions.
The Galilean telescope contains a convex objective lens and a concave eyepiece lens. The telescope is short but provides a narrower field of view than Keplerian designs especially in higher powers.
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Galilean
Telescope:
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Keplerian telescopes are longer and heavier as they incorporate prisms to reorient what would otherwise be an inverted image. They offer fields of view at least twice as large (about 12 degrees at 4x) as Galilean telescopes. They usually incorporate larger objective lenses to produce brighter images. All commercially available Keplerian telescopes are focusable and are most frequently prescribed in 3x, 4x and 6x powers, though other powers are also available.
The Keplerian telescope contains a convex objective lens and convex eyepiece lenses. The system is longer and requires prisms to properly orient the image. It provides a wider field of view than Galilean optical designs.
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Keplerian
Telescope:
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What are we trying to achieve when we prescribe a telescopic device?
As discussed above, patients complain that they cannot see far enough away to perform whatever activity they have in mind. They must move closer in order to see it adequately. Patients with 20/40 vision, however, rarely complain of difficulty seeing. Individuals are licensed to drive without restrictions with 20/40 acuity, and children are unencumbered in the classroom with that level of vision.
It is reasonable to assume then, that if we can provide 20/40 acuity through the telescope, most patients should be reasonably satisfied with the functional benefit it provides. So, if the goal is 20/40, a 2x device should be adequate for an individual with 20/80 acuity, 4x for 20/160, and 6x for 20/320. If we prescribe higher magnification to achieve better acuity it will be at the expense of a narrower field of view, which is the major complaint of telescope users. Clinically we find that user’s have increasing difficulty when fields become narrower than 5 degrees. So we always need to balance adequate acuity and field of view with the distance the device will be used. Since the field of view is a cone that widens as we go further out, narrow fields of view will be much more problematic closer-in than they would be at further distances.
Telescopic options for low vision
Low vision telescopes are available as handheld, spectacle clip-on, head born self-contained and bioptic designs. Virtually all styles are available in both Galilean and Keplerian optical designs. When individuals need magnification greater than 6 or 7x, than handheld monoculars (or even binoculars) become a compelling option. If the activity requires that the hands are available, or if the user does not have the dexterity to manipulate the device, or if the telescope would be used for extended viewing purposes, than a head born device is appropriate. Head born designs are available with the telescope centered in the patient’s field of view (Full Diameter) or positioned above the line of sight so that the user can alternate their view between the carrier lens and the telescope (Bioptic), or in a bottom position similar to surgical operating loupes (Telemicroscopes).
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Ocutech VES-II mounted in “Full Diameter” position. |
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Ocutech VES-K mounted in
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Ocutech VES-Mini mounted in
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