Henry A. Greene, OD, FAAO, Invited Author, with Richard J. Shuldiner, OD, FAAO, Chief Clinical Editor
When you think nothing more can be done, tell the patient to Google “low vision help”.
The major goal in low vision care is finding a way for the person to do what they want to do. The major optical goal in low vision care for central vision loss is to magnify the image sufficiently to make it discernible by the patient.
Optical magnification requires that the material to be viewed be held at the focal length of the optical system. For hi- add readers 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 magnification at a working distance supportive of the activity. This can be accomplished with the use of telescopic aids.
Telescopic low vision glasses can be focusable or fixed-focus, the latter using reading caps selected for specific working distances. In instances where simply moving close enough to see the object of regard is NOT an option, telescopic aids currently offer the best alternative.
Distance vision supports social activity and its loss can undermine quality of life. The inability to see body language or 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—no one can see your friends’ and family for you.
The “In’s and Out’s” of Telescopes for Low Vision
Optical telescopes are available in two designs—Galilean and Keplerian. Each has its distinct characteristics and attributes. Galilean telescopes are comprised of a plus objective lens and a minus ocular lens. Keplerian telescopes have plus lenses for both the objective and the ocular.
Galilean telescopes are small and lightweight due to their simple optical design. They produce a bright image but offer rather narrow fields of view (about 5 degrees at 4x) and tend not to be sharp edge-to-edge. They are available as both fixed-focus and focusable versions and are usually prescribed in 1.7x, 2.2x and 3x powers, but are available as high as 6x. Because of their small size and low weight they are convenient for binocular prescriptions.
Keplerian telescopes are longer and heavier as they incorporate prisms to reorient what would otherwise be an upside down and inverted image. They provide fields of view at least twice as large (about 12 degrees at 4x) as Galilean telescopes. All commercially available Keplerian telescopes are focusable (with one auto focusing version) and are most frequently prescribed in 3x, 4x, 5x and 6x powers.
Autofocus Keplerian Bioptic telescope glasses
Manual Focus Keplerian Bioptic Telescopes
Hand Held Focusable Galilean Telescope
Galilean Fixed Focus Bioptic Telescope Glasses
Keplerian Telescope Glasses for near only
Head Borne Non-Rx Keplerian Telescope Glasses
The Telescope Prescribing Goal
As discussed previously, patients complain that they cannot see far enough away to perform whatever activity they have in mind. Patients with 20/40 vision, although they may notice some reduction in visual clarity, can usually see well enough for most activities. In fact, in most states, individuals are licensed to drive without restrictions with 20/40 acuity, and children are unencumbered in the classroom with that level of acuity. 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.
Field of View
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 users have increasing difficulty when fields become narrower than 10 degrees. So, we always need to balance adequate acuity while maximizing field of view with the telescopic device that will be prescribed.
The patient’s perspective on low vision telescopes
When we place a telescope in front of the patient their immediate reaction will be that everything appears closer. Of course, that’s true because the image is enlarged by a factor of the power of the device. The value to the patient, however, is not that the image is larger, but that they can see it further away. A 4x device will allow a target normally only visible as far as 10 feet away now be seen as far as 40 feet away.
However, not all patients receive a geometric acuity gain from telescopes. This is especially true in individuals with poorer acuities such as in neovascular AMD, diabetic retinopathy, and media issues. Those with a retinal “ring” scotoma (not to be confused with the ring scotoma produced by a telescope) may experience a reduction in acuity with magnification!
Telescopic options for low vision
Low vision telescopes are available as handheld, spectacle clip-on, head born self-contained, and as eyeglasses in full diameter and bioptic designs. Virtually all styles are available in both Galilean and Keplerian optical designs. When individuals need magnification greater than 6 or 7x, handheld monoculars (or even binoculars) become a compelling option.
If the telescope would be used only infrequently and in episodic activities often a handheld device will be acceptable. If the activity requires that the hands are available, or if the user does not have the dexterity to aim the device, or if the telescope would be used for extended viewing purposes, then a head born device is appropriate.
Head born designs are available with the telescope centered in the patient’s field of view (full diameter position), positioned inferiorly and converged in the reading position, or located above the line of sight so that the user can alternate their view between the carrier lens and the telescope (Bioptic position) to allow for mobility. The bioptic design is prescribed for drivers needing to see signs and traffic lights where legal and appropriate. Actually, the telescopes can be custom mounted in any position and for any distance, as the task requires.
Determining a prognosis for a successful telescope prescription
As a result of NIH-funded clinical trials, a Telescope Prescribing Protocol has been developed to help establish a prognosis for successful telescope prescribing. It is divided into two parts—Hard Signs, those that can be measured or otherwise determined, and Soft Signs, those that are judgments and are developed through patient interaction.
- Visual Acuity: Best corrected visual acuity through the telescope is in the 20/40 to 20/50 range.
- Contrast sensitivity: Able to see facial features while looking through the telescope at a distance of 12-15 feet under normal room illumination.
- Ocular Dominance: That the better seeing eye is the dominant eye, OR, that while looking through the telescope the dominant eye sees better than the fellow eye.
- Appropriate activity goals for use of the device: Telescopes are primarily used for distance and midrange visual activities.
- Dexterity with the device: Patients who show a natural skill and an intuitive familiarity with using the device will be more successful in their adaptation. Evaluation of such skills should be done out of the examining room and in an environment more akin to a normal setting.
- Motivation and enthusiasm: No matter how natural it is for the patient to use the device, and no matter how well it enhances their vision, patients must be personally motivated to seek to improve their vision. They must be prepared to confront the challenges integrating such a device into their daily activities.
The five questions you should ask your patient to identify a promising telescopic candidate.
If your patient can answer YES to the following questions than they may be a good candidate for telescopic systems.
- Is your vision loss due to macular degeneration or a similar disorder that causes a loss of central, detail vision?
- Is vision in the better eye with your best eyeglass prescription (if used) 20/300 (6/90) or better?
- Can you read headlines on a newspaper?
- Can you watch TV or recognize people’s faces from farther than 2 feet away?
- Do you understand that this device is designed to help you see better at distance and midrange for faces, signs, TV, computer screens, shopping and traveling?
Telescopic low vision devices are often misunderstood or maligned because of their reputation for the small field of view they produce. However, surgeons and dentists use them daily. Drivers needing to see a road sign, AMD patients needing to see a facial expression, or a musician required to read music do not need a large field of view for those activities. Low vision telescopes, prescribed in the right form for the right task, with the right patient, can turn anxiety and depression into joy and independence.