What Causes Myopia
For others with severe high myopia, the consequences can be much more than the nuisance of thick heavy glasses, but actual blindness due to retinal degeneration.
Myopia has become a plague of western society over the last 100 years, with significant increases even over the last 1-2 decades amongst children. In Singapore for instance the rate of myopia amongst school kids is about 75%, up from about 25% just 30 years ago. In Israel between 1990 and 2002 the rate of myopia among school children increased from 20% to 28%. Amongst the Jewish Ultra-orthodox male population in Israel the prevalence has reached up to 80%.
Myopia is much more common where one parent is myopic, and more common still with two parents. In other words genetic make up is a very important factor.
However environmental factors are also crucial. For instance 70 per cent of 18-year-old men of Indian origin living in Singapore have myopia, while in India itself the rate is roughly 10 per cent. The same can be seen in Inuit and Pacific Islanders where myopia rates have shot up in recent decades. A study in Nepal found that Sherpa children who led a rural lifestyle had a prevalence of myopia of only 2.9% compared with 21.7% for Tibetan children (with a similar genetic ancestry to the Sherpas) who led an urban lifestyle.
Some are convinced that the cause of the increasing myopia is the increased hours spent focusing close up (known as accommodating), reading and sitting in front of a computer screen. One study in Singapore showed that spending more than 20.5 hours reading and writing a week was found to be positively associated with myopia.
Ultra-orthodox Jewish males spend many hours from a young age studying texts and swaying backwards and forwards, accommodating and re-accomodating. Some 80% of this community are myopic whilst their female counterparts, and secular Jewish males have rates no higher than in other Caucasian populations.
What we do know is that myopes have larger, longer eyeballs. It may be that close work requiring intensive focusing in the young developing eyeball causes it to elongate. How this happens is also a bit of a mystery although there is some evidence to show that intensive accommodation causes a pressure rise inside the eyeball, which may cause the soft tissues of the young eye to stretch- all a nice theory but unproven.
However, in contrast, we have the island of Vanuatu where children study for 8 hours a day and their rates of myopia are extremely low, around 2%. Here scientists have put forward a dietary theory saying that Vanuatans eat food low in refined starches, in contrast to a typical Western diet, and this is somehow protective. The authors of the Nepalese study mentioned above were unable to explain their findings, and could not relate it to hours of schooling. Other studies in Singapore, where the rates of myopia are having a serious socioeconomic impact, have also failed to show a statistical association with hours of close work.
Whatever, it is known that myopic children tend to be more studious than non myopic classmates, and children who play more sport are less likely to be near sighted and require glasses. But what is cause and what is effect is debatable.
Confused? So is the medical profession-which is why I can’t tell you how to prevent your children’s myopia. But our knowledge of what does not work and what just might work is increasing. Watch this space!
Andrew Fink MD FRCOphth