Many people believe that the longer you wear spectacles the worse your eyesight becomes. But are they right? Claudia Hammond takes a clear look at the evidence.
There are lots of reasons why people don�t always wear their glasses. They might dislike the way they look, get teased or simply feel more comfortable without them.
Beyond comfort and aesthetics, though, some fear that wearing glasses too often will weaken their eyesight, and that they will increasingly rely on them more often than when first worn.
A study from Nigeria published last year found 64% of students believed that wearing glasses can damage eyes. Research in the Indian state of Karnataka put the figure at 30%, and in Pakistan 69% of people feel the same way.
In Brazil, even medical staff believed that your eyes would gradually get weaker as a consequence of wearing glasses. Is there any evidence to suggest they are right?
There are, of course, two very different reasons why people wear glasses � short-sightedness, or myopia, where things in the distance are blurry; and long-sightedness, or hyperopia, where you can�t focus on things close up.
Long-sightedness is often age-related: many people begin noticing in their 40-50s that it�s difficult to read in low lighting. As we age the lenses in our eyes gradually stiffen, making it harder to adjust to different distances.
When people get to the stage where their arms aren�t long enough to hold a book or menu far enough away to focus on the text, they opt for reading glasses.
What�s surprising is how few trials have been conducted on the prolonged effect of wearing glasses. And from what we know there�s no persuasive evidence that wearing reading glasses affects your eyesight.
Why then do so many people become convinced, anecdotally, that glasses have made their eyesight worse? People may gradually find themselves more and more dependent on their specs, but it�s because their lenses have continued to deteriorate with age.
So people find themselves needing their glasses more often, leading them to conclude that the glasses must have made their sight worse, where in fact, there�s no causal relationship.
Whether or not you choose to wear your reading glasses will make no difference to your eyesight in the long run (although if you have to strain your eyes to read, you might get headaches or find that your eyes feel sore).
Corrected vision
However, the situation is not the same with children. Not wearing the right glasses, or any glasses at all if they are needed, can have a long-term impact. For decades it was thought that deliberately under-correcting for short-sightedness � by giving children weaker glasses than they really needed � might slow down the elongation of the eyeball over time and thus slow down the progression of myopia.
The idea was that if you wear glasses to allow you to see clearly in the distance, your eyeball tries to elongate itself when you focus on a close object in order to see it properly.
But a trial conducted in Malaysia in 2002 proved this hypothesis was so wrong it had to be halted a year early. A group of 94 children with myopia were randomised at the toss of coin either to wear the correct glasses for their prescription, or to wear glasses that left them slightly short-sighted.
When the study began the children were between the ages of nine and 14, and for the next two years the length of their eyeballs were measured at regular intervals.
Contrary to an earlier, smaller study from the 1960s, the children who wore the weaker glasses showed a greater elongation of the eyeball over time. In other words their eyesight was gradually getting worse.
Some argue that there�s still not enough evidence to come to any firm conclusions. But a Cochrane review from 2011 of studies of interventions in children with myopia concluded that the limited evidence available suggests it is better to give children the correct glasses, rather than deliberately trying to under-prescribe.
There�s no suggestion that wearing the correct glasses will make their eyesight worse than not wearing them at all. In fact the longest-ever study of the progression of myopia, which has just published its 23-year findings suggests the contrary.
Back in 1983 a group of children in Finland with myopia were randomised to various conditions, including reading without spectacles. Their myopia progressed a little faster than those who wore their glasses continuously. After the initial three years of the study, they were all advised to wear glasses all the time.
Twenty years on, there was no difference between the groups.
The benefits of wearing glasses if you�re a child who needs them, are clear. Children�s eyes need to learn to see, so if they don�t have the right glasses they can develop so-called �lazy-eye� or amblyopia because they�ve never had a sharp image on their retina.
The correct prescription has also been shown to improve your reading speed and reduce the risk of developing a squint.
But, returning to adults, what I find curious is the lack of studies that have been carried out in this area. We might expect science to have all the answers, but sometimes the studies that seem the most obvious to conduct haven�t been done.
Studies requiring children with myopia not to wear glasses would be unethical because of the effects it�s known to have on educational attainment and on the developing eye.
But, in principle, this kind of study could be carried out on long-sighted or short-sighted adults. So we�re left with the question of why no one wants to do it.
Professor Ananth Viswanathan, Consultant Surgeon at Moorfields Eye Hospital in London, believes the lack of research is probably down to the absence of any physiological reason why glasses might damage eyesight. Research needs not only to look for associations, but for plausible mechanisms.
So it sounds as though this type of study won�t be done any time soon.
In the meantime we�ll have to go on the anatomical evidence. And while there are plenty of reasons to choose not to wear glasses, the fear that you might be damaging your eyesight isn�t one of them.
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