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Spectacle-mounted Telescopic Lenses: A Practitioner’s Guide

Maximizing success in prescribing and fitting customized spectacle-mounted telescopic systems can be enjoyable, challenging… and profitable.

By Bruce P. Rosenthal, OD, FAAO

The clinician should consider distance telescopic systems when the low vision patient mentions that she or he enjoyed going to the theater, museums, shows, playing bridge, or sporting events, but has given these activities up because of vision problems.
Variables in Selecting a
Spectacle-mounted Telescope
• Type – Galilean or Keplerian.

• Amount of magnification needed.

• Size of patient pupil.

• Dimension of exit pupil of telescope.

• Ability to include a prescription.

• Available as a bioptic/full diameter/reading
telescope.

• Fixed or variable focus.

• Weight of system.

• Light-gathering quality.

• Binocularity.

• Cosmetics.

• Manual or autofocusing.

• Cost of system.
Factors to Consider in the Successful Fitting of Spectacle Telescopic Systems

• Identifying important and achievable patient objectives.

• Selecting suitable patients.

• Obtaining the relevant findings of the low vision evaluation.

• Understanding the variables and parameters of telescopic systems.

• Knowing the range of available telescopic systems.

• Making the proper selection and accurate fitting of the telescopic system.

 
Successful fitting usually means bringing the eye as close to the ocular as possible to provide the largest field of view.

Most clinicians specializing in low vision feel comfortable in prescribing near low vision devices, as do many new practitioners to the field. But when it comes to distance or intermediate low vision needs, they often rely on simple handheld telescopes or stock off-the-shelf telescopes.

Developing one’s proficiency with a variety of low vision aids will certainly enhance the practice’s profile and service levels to low vision patients.

The following are some ways to maximize success in prescribing and fitting customized spectacle mounted telescopic systems. Some of these special devices will not only help patients achieve special objectives, but will make your practice more enjoyable and challenging … and increase income too.

Getting Started

The careful recording of a patient’s history is a vital part of obtaining distance objectives and patient selection, and is the first step in successfully prescribing spectacle-mounted telescopic systems. The clinician should consider distance telescopic systems when the low vision patient mentions that she or he enjoyed going to the theater, museums, shows, playing bridge, or sporting events, but has given these activities up because of vision problems. Contraindications include a history of severely decreased vision, poor contrast, rapidly progressive vision loss, hemorrhaging, or multiple pathology.

Evaluating Low Vision

Visual acuity and contrast sensitivity are two important tests of visual function that will guide the telescopic system prescription and the power of the system. The ETDRS visual acuity charts, available from Lighthouse International and Precision Vision, are the preferred choices because they are standardized and offer high contrast. The Vision Contrast Test System (VCTS) from Vistech and Stereo Optical or the Pelli-Robson (PR) charts from Clement-Clark are good ways to test the contrast sensitivity function (CSF). A poor candidate would have a response of less than 3 on the first three lines of the VCTS and 5 triplets or less on the PR chart.

How to Pick a Telescope

There are many variables to consider when choosing a spectacle-mounted telescopic system. Telescopic design is a key one. The Galilean telescope is made up of a high minus ocular (eyepiece) and a high plus objective. It is lightweight, compact, provides an erect image, has a large exit pupil, has good light gathering qualities, but is generally limited to 3.0x magnification.

The Keplerian telescopic system is heavier than the Galilean telescope because the eyepiece and objective are composed of two plus lenses. As a result, it is necessary to use erecting prisms or mirrors in order to re-erect the image, which consequently adds to the weight.

The size of the exit pupil of the telescope is also a factor. The Galilean telescope has a relatively large exit pupil (the image of the objective as seen through the ocular). Successful fitting usually means bringing the eye as close to the ocular as possible to provide the largest field of view. Keplerian telescopic systems are usually more difficult to fit because the exit pupil is generally smaller than a Galilean and must coincide with the entrance pupil of the eye.

Selecting a System

Once you’ve chosen the basic design, you’ll want to choose a device. The Beecher, Ocutech, Designs for Vision, and Eschenbach Optik spectacle-mounted telescopes systems should fit almost any patient who is a candidate.

Beecher telescopic devices are lightweight, Keplerian mirror-design systems. They are available as distance monocular or binocular systems in powers of 4, 5.5, 7, 8x, and 10x magnification, and a near power of 5.5x. The 4, 7, and 10 have objectives that are 20, 30, and 35mm respectively, which enhances the light gathering ability of the system.

Here are some tips to successfully fit the Beecher mirage telescopic spectacle:

• Use extended eyepieces to get the exit pupil closer to the eye.

• Utilize Rx stock eyepieces, which are available in powers of +/- 4, 6, 8, 10, and 12 diopters.

• Employ the Lighthouse silicone adjustable comfort bridge that comes in 8mm, 11mm, and 14mm tabs.

• Hold a transilluminator light in front of the objective to make sure that the exit pupil of the telescope is aligned with the entrance pupil of the eye.

Ocutech, Inc. offers a variety of telescopic systems, including manual, autofocusing, and binocular arrangements. Vision Enhancing System® (VESII®) is a monocular, across-the-bridge, Keplerian telescopic system that is available in a 4x and 6x manual system. Ocutech also offers the VES®-Autofocus, a 4x autofocus that has a continuous and immediate focus from 12" to infinity.

The VES-K® is a new “K mounting style” system mounted on conventional metal frames. The VES® Mini is another Keplerian telescopic system that can be mounted in a conventional frame as a binocular system.
Since the VES® - Autofocus, VES®-K and VES®II are Keplerian telescopic systems, it is important to make sure that the exit pupil of the telescope coincides with the entrance pupil of the eye. Use a transilluminator in front of the telescope to make sure that the beam of light falls in the center of the pupil.
The newly redesigned Designs for Vision “Robin fitting system” is designed to ensure maximum success in fitting bioptic telescopic systems, both monocularly and binocularly. Fitting frames that hold 2.2, 3, and 4x Galilean telescopes, as well as 2x to 6x Keplerian telescopes, eliminate much of the guesswork in successful fitting.

Here are some ways to successfully fit telescopic lenses with the Robin system:

• Select an appropriate size Yeoman special fitting frame, and move the fitting slots so they center over the patient’s pupil. Check the pupillary reflex with a trans-illuminator.

• Thread the Galilean telescope in the fitting slot and recheck the pupillary alignment by holding the transilluminator in front of the telescope and looking at the pupillary reflex. Make sure it is centered.

• Order the appropriate-size Yeoman frame and specify monocular PDs as well as jumbo pads.

• Make sure that the patient understands that the final telescopic system will look virtually the same as the fitting system.

Eschenbach Optik and Designs for Vision have recently collaborated on an exciting new method of fitting the Galilean telescopic system. This new fitting system is available in Galilean design of 2, 3, and 4x powers. Eschenbach’s frame No. 1631 holds the telescopic lenses to determine the appropriate PD. Fitting frames, in which the telescopic lenses can be snapped into, provide a close exit pupil for maximum field of view. The system, which can be ordered in the bioptic and field diameter position, comes both monocularly and binocularly in a stylish metal frame.

Prescribing and fitting low vision telescopes for distance and intermediate can be professionally challenging and fun. The next time you have a suitable candidate, leave the handheld and off-the-shelf telescopes alone, and try one of the professional devices above. You and your patient will be glad you did.

Bruce Rosenthal, OD, is the Chief of the Low Vision Programs at Lighthouse International in New York City and Adjunct Professor at the Mt. Sinai Hospital. He is a Low Vision Diplomate of the American Academy Optometry and on the Low Vision Executive Council of the AOA.

 

 

 

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