focus on low vision
Fitting Low Vision Telescopes
by Alex Yoho, ABOM
Done right, fitting a low vision patient with a telescope can be one of the most fulfilling experiences for both fitter and wearer.
There are three different mounting positions that are commonly used.
■ SUPERIOR BIOPTIC, where the scope is mounted high in the frame and angled upward. This position is used for briefly spotting objects at a distance.
■ FULL DIAMETER position is for continuous use and is mounted in a straightforward position aligned with the visual axis.
■ INFERIOR BIOPTIC position is used primarily for near tasks.
| QuickTips |
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| When you first begin to use telescopes, use a felt tip pen to draw the diameter of the scope on the lens. This will help you visualize how much room you will have under the scope for the eye to see straight ahead. |
MEASURING
When measuring for the position of the new telescope, the frame should be fitted as it will be on the patient's face. Put a dot at pupil center while the patient is in a straight-ahead gaze. This dot, along with vertex distance, will provide the scope mounter the necessary information to angle the scope so that it aligns with the visual axis when the wearer's head is tilted downward.
In full diameter position, the scope will be centered at the dot. This position can be used for distance, intermediate, or near. If used for near or intermediate, you will need to specify a dedicated working distance in binocular systems. Fusion in binocular scopes will only occur at the working distance.
In superior bioptic position, the scope will be placed as high in the frame as possible. In selecting a frame, allow ample room for the scope. When mounting in this position, you should measure the diameter of the scope that will fit through the lens. One half of that diameter will be the distance from the edge of the scope to the center.
You will need an additional 3mm from the scope edge to the top edge of the lens. If the ocular barrel of the scope is 14mm in diameter, there will be 7mm from the center of the scope to the edge of the scope, plus 3mm to equal a total distance of 10mm from the edge of the lens to the center of the scope.
Basically, the same parameters apply for the inferior bioptic position except the eye is above the scope. You will still need about 3mm of space below the scope for a sturdy mounting.
| WORKING DISTANCE |
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| You will need to provide your mounting service with a working distance. If the scopes are to be used at infinity, they will be placed in a straight-ahead position. If they are used for reading or other near tasks, they will be converged for the working distance. The scopes will not function at any distance other than the working distance provided. If focusable scopes are used for a near system, they will only fuse at the working distance provided. If the individual would like to use a near system for distance as well, they can use one scope focused at distance, but will need to refocus for binocular near viewing. |
FRAME SELECTION AND FITTING
Frame selection is an important factor. A frame with a larger vertical measurement will allow more space for the eye under the scope. Reasonably heavy metal frames that allow good end piece adjustment and have adjustable nosepads are preferable. The patient should see a sample scope held roughly in place on the frame they want. When they see this approximation of the finished product, style will probably become secondary to function.
When you are ready to fit, slight adjustment may be necessary. For a monocular telescope in a bioptic position, two adjustments may be necessary. Nosepad adjustment should be done first for comfort and approximate position of the scope. The frame may also need to be adjusted for pantoscopic or retroscopic tilt.
First, have the patient look straight ahead, then tilt the head down to engage the scope. The eye should fall naturally into the center. If it does not, you will need to angle the end pieces of the frame up or down until the eye centers vertically in the scope. If you raise the frame with nosepads, you will have to add retroscopic tilt to align the scope with the visual axis and vice versa.
If the image jumps to one side, this can be corrected by adjusting the face form in the bridge area. Have the patient move his head up and down while fixated on a distant object. When the scope is adjusted laterally, the patient should be able to engage the scope without the image moving from side to side.
A binocular system may also require some minor face form adjustment to merge the images to one. If there is a vertical disparity, "X" the bridge slightly. If this has to be done more than a few degrees, you should return the scopes to the mounter for correction.
These adjustments will be slight, but will make a profound difference in comfort and vision. Be sure to schedule enough time for scope fitting. The first fitting could take up to half an hour. But, with experience, this could be reduced to 10 minutes. EB
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