Why Do Glasses Fail?
Most of the time, you can go to the eye doctor and get a pair of glasses without any trouble getting used to the them. There are occasions when a patient can have trouble adapting to the changes in the prescription written by the doctor. A prescription for glasses is similar to a prescription for medication. Sometimes side effects can occur after a doctor writes a new prescription. Yes, even lenses can have side effects.
Whenever a pair of glasses does not seem to work, we like to perform a failure analysis to determine the cause. This process helps us to prevent from making the same mistake in the future as well as provide you with a clear explanation. As you can see from the list of causes below, glasses design can be complex with many reasons for problems.
Below are reasons why new prescriptions don’t work for some people.
Getting new glasses during or after vision loss Patients with vision loss from conditions such as a cataract, diabetes, glaucoma, macular degeneration or other conditions may not have had the real impact displayed to them until they get new glasses.
Despite counseling on part of a doctor or staff on the limitations to expect from new glasses, people may not process the information we try to tell. It’s as if a psychological block, a type of denial exists, with a subconscious desire that new glasses fix all cases of blurred vision. Their past experience with visual blur had always been improved with new glasses. They remember the positive impact new glasses made. This expectation is probably strong and seems to persist in some patients despite even careful counseling.
When they see for the first time that new glasses didn’t help, it can hit one like a brick wall. This is the first real impact of their vision loss. Despite the education, the emotional wall now breaks down. Denial can turn into anger (see the 12 stages of healing) for the eye care professional who are trying to do their best. Sometimes we as eye doctors may not have counseled our patients well enough for them to be prepared.
Stress in one’s life can influence our adaptability to new lenses. Going through a major illness, family changes, grieving, compound our ability to get used to new eyewear. See another post titled Can Emotions Affect Vision?
Large prescription change A large change in prescription can challenge our ability to adapt to the new prescription. Doctor’s will often have to cut the strength of the lens to aid in adaptation. The prescription can be increased again later to further improve your vision. We try to anticipate how much you can adapt and prescribe the amount which gives the greatest benefit. Sometimes we find that this amount can be too much.
Astigmatism or prism not tolerated in the prescription While a refraction may reveal the presense of astigmatism, it may not mean that a person should be prescribed the full amount or even any of it. This and other conditions may produce a “pulling sensation” of one or both eyes while you attempt to wear the glasses. Eye alignment problems such as convergence insufficiency can create similar trouble as well and may mean that vision therapy is required to correct the condition.
Awareness of optical material difference A few people can notice an improvement in the clarity of glass compared to plastic or between plastic and some other material. The advantages of a lighter weight lens may not be worth the change you notice.
Awareness of frame or edge of lens Some individuals can become aware of the edges of the frame. This effect is more noticeable if a dark color or a bold, plastic frame is chosen. Some have described a sensation of being “closed in”. This is often unpredictable and occurs more frequently in people who are getting glasses for the first time. Selecting a rimless or a metal frame with a thin eyewire can improve this problem. Contact lenses or refractive surgery are options also.
Monocular cataract (cataract in one eye) A cataract in one eye can affect how we see with both eyes working together as a team. Some people still have problems with their eyesight because the cataract still obscures their vision and interferes with their ability to see comfortably with both eyes. Monocular cataracts can also result in prescription shifts which can cause other problems with adaptation which may be checked off elsewhere.
Binocular cataracts (cataracts in both eyes) Cataracts in both eyes can deteriorate vision in mild to severe ways. Eyeglass prescriptions change more frequently when cataracts are present. An examination may reveal that glasses may help still even with the cataract. When the glasses do not seem to help however and your problem still continues, cataract surgery becomes the best solution to eliminate the problem if all other eye health factors are normal.
Binocular interference Glasses are usually designed to give each eye ideal vision by itself and under binocular (two-eyed) conditions. When new glasses produce problems with eyestrain, headaches, double vision or dizzyness, the problem may be be due to the effect that the glasses are stimulating the brain to attempt to use information from both eyes whereas before only information from one eye was being processed at any given moment. Unequal refractive error, eye turns, weak or unstable eye muscles and monocular cataracts all can create this type of problem. Either the predisposing problem needs to be resolved to allow normal binocularity or the glasses will have to be remade to continue the “one-eye only” input style the brain is used to.
Eye alignment problem A new pair of glasses can result in double-vision in some individuals. This can happen while looking up or down the lens away from it’s optical center. Symptoms can also occur while looking at near for a period of time. Each individual has their own tolerance to how much deviation it can tolerate before giving headaches, eyestrain or double-vision.
Non-Optical problem Some conditions can not be improved with a regular pair of glasses. We do our best to try to confirm that glasses will help a person. Conditions such as cataracts, macular degeneration, glaucoma, diabetic retinopathy can create decreased vision to the point which eyesight can not be improved with regular glasses. “I just want a pair of glasses that will work,” is the most common comment mentioned to a problem which can not be fixed by glasses alone. Special visual aids may be needed in these cases. See the first section Getting new glasses during or after vision loss andCan Emotions Affect Vision?
Changes in vision due to a base curve change in the lens design Most eyeglass prescriptions allow the manufacturing laboratory to decide on the base curve of the lens to give the best optical performance. The base curve of a lens refers to the inherent curvature of the front surface. Some prescription lenses while having the same power may have different curvatures because of the base curve selected by the lab. Most people adapt to this new curve readily but some can not tolerate the change. Remaking the same new prescription with the base curve of the old glasses solves this problem.
Image size difference between each eye If prescription lenses of significant unequal powers are put into a pair of glasses, the image on the retina may not be the same size. This creates a form of binocular interference as mentioned before. This can occur by itself, with the development of cataracts or from eye surgery including cataract and retinal surgery.
Bifocal Power – Working Distance Problems Bifocal strength is determined by several tests during your vision exam. Each different power of a bifocal has a specific focal range – a zone of clear vision for that power. The stronger a bifocal is, the closer in the material must be held. A person’s natural reading distance may not equal the optimal bifocal strength determined in the exam. Print held at your natural reading distance appears blurred until you hold it closer in. Remaking the glasses with a weaker bifocal moves the zone of clear vision out to a person’s natural reading distance. Computer screens often sit further out than a person’s natural reading distance and may require a trifocal, a progressive or a special pair of glasses designed for the computer’s distance. Some people may notice that a blurred vision zone develops at distances beyond an arm’s distance which can be made clear with neither the top or the bottom of a bifocal lens. When this happens, a trifocal with a third power segment or a progressive multifocal is needed to keep vision clear at this intermediate distance. There are some cases where vision may have deteriorated to the point where normal print needs to be larger to see it. An easy way to magnify the print when this happens is to prescribe a strong bifocal. This stronger bifocal requires a person to hold the material closer than they naturally would in order to help them see well enough to read.
Bifocal height problem A bifocal can be set too high or too low. The best height can vary from person to person. Whenever possible we try to match the current relative height of the bifocal of your last pair of glasses if you are having no problems.
Bifocal line awareness Some people can become perceptually aware of a bifocal line to the point of distraction. This awareness normally goes away after the regular two week adaptation period. If problems continue past this period, a prescription can often be remade in a pair of lineless bifocals or two pairs of glasses with one set for distance and one set for near.
Refractive shift since exam Cataracts, diabetes, pregnancy and an unstable eye-focusing system are the most common causes for changes in prescriptions which occur in a short time. Usually the underlying cause needs to be cured before a stable prescription can be determined.
For diabetes, one’s prescription can change as the blood sugar level varies. Even after control, it takes 5 weeks or longer sometimes to get a reliable and repeatable refraction.
Cataracts can also cause prescription shifts in relatively short amount of time (weeks or months).
Medications can sometimes dramatically affect prescriptions. We once observed a 7.00 D shift during the use of Topamax, a migraine treatment medication.
Specific Progressive Lens/”Lineless” Bifocal problems
Progresssive lenses offer changes that first time wearers must take time to adapt – many taking 3 to 4 weeks. This involves a neurological adapation. After age 40, this adaptation is not as swift as it is for age 20.
The first change involves peripheral vision appearing to swim below the line of sight due to the variable lens optics. During this time, objects like stairsteps may seem deeper or more elevated and reaching for items like the salt shaker may need adjustment to learning a new distance.
The second involves learning to use the corridor efficiently – point your nose at the item you wish to view and raise or lower the chin to adjust the focus.
Don’t go with your first impressions when you get new progressive lenses. That initial impression dissapates with time again up to 3 or 4 weeks although many show rapid improvement in the first week.
Time is needed to adapt for new progressive lens wearers
Patience rewards you with more efficient vision with the best multifocal lens design. Ninety five percent of people truly adapt. Failures can be traced towards nervous systems that show very sensitive motion sickness symptoms – another lens either a flat top or two pair of prescriptions will be needed.. Failures have also been traced to impatient patients wanting a quick fix. For them, no eye doctor will be successful with a progressive. Each decade of life slows down this neurological adaptation – it will take longer at age 70 than age 40 to adjust to prescription changes.
The following can be optical reasons for lens problems unique to the optics of a progressive lens that would require a doctor adjustment..
Near area too narrow – The usable width of most progressive lenses is not as wide as most conventional “lined” designs. This can be a problem for people who tend to move their eyes only as they read and do not move their head. If an acceptable wide near area progressive can not be found then a traditional bifocal or reading glasses may be needed.
Trouble adapting to lens periphery – The periphery of a progressive lens is blurry and can produce a swimming motion which may not be well tolerated by some. This can produce motion-sickness like symptoms especially in people prone to this problem. Traditional bifocals or single vision based glasses (one pair for distance and/or one pair for near) may be needed.
Misalignment of the lens / frame fit to the face – We have to fit these lenses to the millimeter. Abnormal tilting, X-ing of the lens, wrong interpupillary or optical center height measurements and other misalignments would require an office visit for the doctor to fix.
Spherical aberration Some newer lens designs can get rid of distortions which make walls appearred bowed in or out. Aspheric lenses can correct for this distortion but can in some create symptoms similar to base curve changes. Traditional spherical designs can help correct this problem.
Chromatic aberration Colored fringes on lights may be noticed with high prescriptions in lens materials described as “lighter and thinner” Chromatic aberration is caused by high index lenses that refract light differently for each wavelength which results in images splitting up and developing colored fringes. This effect is more pronounced the further away from the optical center you view an object.
Waves Waves are defects in the surface of the lens. The lens is not perfectly spherical in shape and distorts light in specific places creating a distortion in the image which can appear like waves.
How much change can humans adapt? A study looked into the ability to adjust to new viewing conditions. This experiment tested the absolute limits the human visual system could adapt.
The study paid college students to wear special glasses every waking moment of the day. The students were told to put the glasses on from the first moment they awakened to when they went to bed. They were to do everything with them on even shower and bathe with them on.
One half of the glasses were a simple lens with no prescription in it for a control. The experimental half had glasses that turned the world upside down. Up was Down and Left was Right.
The experimental group had a terrible time. Imagine trying to navigate the world that’s upside down. Walking was awkward and unsteady with poor balance. Subjects developed headaches, eyestrain even nausea and vertigo. Their handwriting was awful. They had trouble reading, eating, typing and moving around. These symptoms lasted about 3 weeks. Then remarkabily they all adapted. In the second half of the experiment the glasses were taken away and the symptoms came back while adjusting back to the normal world. Their recovery was much shorter – 3 days instead of 3 weeks.
This experiment showed that with extreme prescriptions, adaptation is possible. It also showed that a motivated individual will work at keeping their glasses on to allow adaptation. In the experiment, college age students were being paid to participate. They had a motivation to succeed to earn the money.
In the real world
In the real world, we can’t expect people to change to this dramatic amount. Fortunately no eyewear prescription is ever this disruptive. We can ask that they try to give it 3 weeks of real committed effort.
Another factor involves age. Subjects age 20 will adapt more easily than subjects age 30 and age 30 subjects will adapt more easily than age 40 and so on. Neurological adaptation slows over the decades.
A 70 year old with real vision problems caused by a retina disease will be a real challenge. There will never be an instant fix in acclimatizing with glasses. Their expectations are based on previous glasses instantly sharpening vision and feel disappointed when this is not achieved. Or the glasses may swim and swirl around because somehow they are “too strong” or “too weak” or “not right.”
Even folks age 40 can have trouble adjusting to their first progressive lens. The same observations have been reported in people getting used to a no line bifocal. This effect is more common if a history of motion sickness or a condition like Meniere’s disease exists. Some correlations with fibromyalgia have been correlated as well with success with Rx changes but this relationship needs further research.
Hang in there. Eye care professionals do the very best we can to help.