focus on low vision
Placement Tips… Spectacle-Mounted Devices
by Alex Yoho, ABOM
There are many spectacle-mounted devices that are used for low vision needs. Here are tips for placement of several of these devices.
• Press-on materials. Some devices are relatively simple, such as cutting and adhering press-on prism material and placing it over a scotoma to expand the visual field.
Put the apex of the prism just at the edge of the usable field with the base toward the scotoma. This is sometimes referred to as a Jack-in-the-box effect since the area that cannot be seen jumps into view without turning the eye as much.
• Mirrors. Hemianopic mirrors can be custom made to address the scotoma or hemianopsia. There are a number of ways to approach placement. The mirror can be completely or partially silvered. If partially silvered, the patient looks through the mirror, but is able to get a partial reflection of the blind area.
A fully silvered mirror will be mounted like a prism in the scotoma, or it can be mounted on the opposite side. When the patient glances at the mirror, he sees the reflection of the blind area in the opposite direction. The more adjustable the mirror, the better, since most patients like to fine-tune them.
• Microscopes. Microscopes are generally placed higher in the frame than a normal bifocal would be. Sometimes the low vision specialist will recommend an offset position since looking straight at an object may not yield the best results. When testing, have patients touch their nose with the reading card and slowly move it away until it clears.
Also, be sure they understand focal length limitations. Full-field microscopes and prismatic spectacles have similar characteristics.
• Telescopic devices. One of the most challenging tasks in low vision dispensing is measuring for and adjusting a spectacle-mounted telescopic device. Position the scope so the axis of the scope coincides with the visual axis of the eye, but offset viewing positions must also be accounted for.
A person who has lived with low vision for a while will probably look at you using their optimum eccentric fixation, which you can observe to your advantage. Some pathologies, such as nystagmus, may require that the scope be positioned at a null point, a gaze in which the patient has the most control of eye movement.
After determining the primary function of the scope, you can decide which position will best accomplish the patient's needs.
For spotting, orientation, and driving, a superior bioptic position is generally in order. The scope is positioned high in the frame and angled upward two degrees for every millimeter above pupil center at primary gaze. With this configuration, the patient can negotiate his surroundings with gross vision and has only to tip his head down slightly to align the scope with his visual axis for more detail.
| Quick Tips |
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| An important way to learn about low vision is by experiencing it first hand. For a first-hand experience, use an old pair of eyewear and have someone spot the pupil center at primary gaze. Have your telescope mounting service drill a superior and inferior hole, and then you can experiment with various configurations. Also, use a strong magnifier to read the smallest print you can find. That will give you an idea of the inconveniences that people with low vision endure. |
| SCOPE PROBLEM SOLVING |
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| If a patient complains of a shadow that is crescent shaped on an edge of the field while looking through a scope, two things could correct it. First, the angle of the scope can cause it to occur. To correct it, angle the scope in the opposite direction of the dark edge. If the angle seems correct, it could be the scope position in relation to the visual axis. This means you would move the scope toward the dark edge by raising or lowering the eyewear, or, perhaps moving one nosepad in and one out, to create a lateral displacement. |
If your state allows driving with a bioptic telescope, the patient might want to try the telescope out before receiving proper training behind the wheel. This could put you in a sticky situation if an accident should occur. Consider having an attorney write up a waiver for the patient to sign at dispensing.
Regardless, it is imperative the patient understands the telescope is not to be used for constant viewing while driving since it severely alters depth perception.
The primary gaze or full diameter position is intended for constant use during tasks such as watching television, movies, or sporting events at distance; or, computer screens, sheet music, or easel at intermediate distance For close distances, an inferior bioptic position is used. Like a bifocal, the patient can often move about a bit and still zero in on a near task.
• Adjustments. To adjust eyewear with spectacle-mounted scopes, keep in mind the general rule that every millimeter of adjustment up or down requires two degrees of pantoscopic or retroscopic tilt.
For example, if you want to raise a superior bioptic up two millimeters to get it up and out of the way, you must add four degrees of upward inclination (either by retroscopic tilt or re-angling the scope) to maintain alignment with the visual axis in relation to the point of rotation of the eye. Though frequently challenging, working with these devices is both interesting and rewarding. EB
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