Photograph by Mark Babushkin; Frame: Jalapenos Gordo from A&A Optical
Miss Amelia, a retired elementary teacher, was capable, spritely and 82 years of age. She was in my exam chair complaining of an intermittent blur when looking up at traffic lights. “I’m so discouraged,” she said, “I just got these new glasses from another doctor and that’s when the trouble started. When I look up at traffic signals they are blurred. It isn’t always there, but it keeps coming back and it has me scared.”
“Did you go back and tell him you were having trouble,” I asked, knowing full well that the doctor she had seen was a highly competent professional and a missed pathological finding was a remote possibility.
“I called him and he said to bring the glasses in so he could check them again. I did that, but he said the prescription was exactly right, so I came to you to see if you could figure out what was wrong.”
I took the old eyewear, her written Rx and her new eyewear to our in-office lab and soon concluded that the change in prescription seemed reasonable and the written Rx had been correctly filled. The new prescription was “better in everyway,” as she put it, except for looking up at traffic signals. I had her view a fixation light in the six cardinal directions and although there was no blur at first, she
suddenly commented that when the light was up and centered her vision got blurry. The riddle was solved! She had a small area of mascara smudges on the upper central area of each lens. We were able to explain the problem and adjust the frame sufficiently for her lashes to clear the flatter curve of her new lenses.
Lash Relief
Optical instruments are designed so that there is a comfortable distance from the eye of the viewer when the instrument is in use. This distance is termed the “lash relief” by optical instrument designers. It can be a factor in choosing your next binoculars, telescope or lensometer (see sidebar, “Getting the Best View”). Lash relief is also needed when spectacle lenses are fitted. It’s an easy-to-overlook part of good eyewear design.
Patients who have problems with lash relief tend to have certain things in common. It should be no surprise that a short vertex distance is often a factor, but other predisposing causes include lashes that are unusually long, either naturally or otherwise, frame wrap, facial anatomy, lens design (asphericity or base curve), position of the lens bevel (on minus lenses), lens material, lens prescription and cold weather.
Cold Weather
In cold, dry weather organic (plastic) lenses tend to build and maintain a static electricity charge that can be sensed when the lashes brush close to the back of the lenses without actually contacting the lens’ surfaces. If you have ever experienced this, you know how annoying it can be. This is most likely to happen with new lenses for several reasons: 1. patients are more diligent about cleaning new lenses so there is a greater static charge, 2. new lenses have relatively pristine, oil-free surfaces contributing to building a greater static charge, and 3. patients are much more aware of each and every attribute of new eyewear until the newness wears off. In most cases, the annoying sensation caused by static electricity slowly decreases as the lenses are worn. This is probably due to a reduction in the static build-up as the lens surfaces become coated with oils from the skin. If the lashes are just shy of touching the lens surfaces, the frames can usually be realigned to give a bit more lash relief and the problem will be solved.
A more difficult situation is the one in which the lashes actually make contact with the lenses. Younger patients can usually tell you their lashes are touching. Elderly patients may not feel the lashes lightly contacting the lenses, but they may complain of difficulty in keeping their lenses clean. A simple diagnostic test consists of cleaning the lenses, having the patient blink a few times with the eyewear in place and then looking for smudges that are usually located centrally and slightly below center. The best way to avoid lash relief problems is to predict them in advance and head them off at the pass, as the saying goes.
Questions For The Ladies
“Did you curl your lashes today?” “Do you always curl your lashes?” “Do you ever use anything to thicken and lengthen your
lashes?” “Have you ever had problems with your lashes touching or smearing your lenses?” If you anticipate a lash relief problem, it’s best to tell the patient in advance that this may present a design issue. You may even want to tell them you have several alternatives to avoid the problem, each of which have some benefits and some draw backs.
The Alternatives
• Increase the eye size. This increases the sag depth and gives a slightly greater vertex distance. The increase must not unduly increase the weight or be undesirable from a cosmetic standpoint. Increasing the eyesize works best for lenses with steeper curvatures. For a lens with a plano posterior curve there is no advantage to increasing the eye size, yet for a lens with a -6.00D posterior curve the increase in lash relief can be significant.
• Decrease the bridge size. This can work well with the “increased eye size” solution because it offsets the overall increase
in the width of the eyewire. For fixed-bridge frames it will also tend to increase the vertex distance. Be sure that the bearing area and appearance will be acceptable with a narrower bridge.
• Adjust the frame’s vertical positioning so that the MRPs fall at about the point at which the lashes are most extended. It’s important to remember that the greatest cornea-to-back-of-the-the-lens distance will almost always fall at the MRP level unless the patient has a prism prescription.
• Select a frame with minimal wrap. Frames with a strong wrap place the lenses closer to the eyes.
• Request your lab to give you special bevel positioning for patients with minus lenses. Labs sometimes call this a 1/3-2/3 bevel because the apex of the bevel is positioned about 1/3 of the way back from the front surface of the lenses. This moves the back surface of the lens away from the eye.
• Ask the lab to give you lenses with base curves that are steeper than recommended by the lens manufacturer. Be aware that this will increase peripheral aberrations and will decrease the performance of the lenses. Your patient should be cautioned about this also. This alternative is especially undesirable if the patient’s Rx is not close to the next steeper base curve on the manufacturer’s recommended curve chart. You should also recognize that steepening the base curve has very limited value for small diameter lenses. See Fig. 1.
• Lash relief problems can be created by using lenses with flatter curvatures, so you should use caution when going to higher-index lenses. For minus lenses there is not only a flattening of the curvature, but there is also thinner edge that limits the amount you can move the bevel.
Occam’s Razor
The principal of Occam’s Razor, that the simplest solution (e.g. picking one alternative) is the best solution, does not strictly apply when solving lash relief problems. Using a combination of the above alternatives, each in moderation according to your patient’s needs, will usually give the most acceptable result.
If All Else Fails
Because using a longer vertex distance can dramatically narrow the reading area and the corridor width of PALs and also because of other limitations related to the patient’s anatomy or prescription, it is sometime impossible to give adequate lash relief. In such cases patients can be offered the alternative of curling or cutting their lashes...or they might wish to consider contact lenses.
Palmer R. Cook, OD, is director of professional education for Diversified Ophthalmics in Cincinnati
