The Economist - USA (2022-06-11)

78 Science&technology TheEconomistJune11th 2022

For  decades,  researchers  had  thought

myopia was mostly genetic. It runs in fam­

ilies, and genomic studies have turned up

several  gene  variants  which  increase  the

risk  of  developing  the  condition.  There

were  early  hints,  though,  that  this  could

not be the whole story. A study of Inuit in

Alaska, published in 1969, found that myo­

pia  was  virtually  unknown  in  those  mid­

dle­aged or older, but that rates were above

50%  in  older  children  and  young  adults.

Such a change is much too fast to be purely

genetic,  and  it  had  happened  just  as  the

study  participants  had  begun  to  adopt  a

more  settled,  Westernised  way  of  life.  But

the  results  went  against  the  dogma  of  the

day, says Dr Morgan, and were ignored.

The spike in East Asia, which occurred

as  places  there  industrialised,  was  harder

to dismiss. Short­sightedness is stereotyp­

ically  an  affliction  of  the  bookish,  and  a

procession  of  studies  has  confirmed  a

strong,  reliable  link  with  education.  “The

more  educated  you  are,  and  the  higher

your grades, and the more you participate

in  after­school  classes  and  tutorials—the

more likely you are [to be myopic]”, says Dr

Morgan. And an intriguing study on ortho­

dox Jewish children in Israel, in the 1990s,

confirmed the link with long school hours.

It  showed  that  boys—who  receive  inten­

sive religious education in addition to the

normal  curriculum—were  more  myopic

than their sisters, who do not. 

Since there is no obvious way in which

learning sums, spelling or even the Talmud

could cause short­sightedness directly, the

assumption  was  that  education  was  a

proxy  for  something  else.  One  possibility

is  the  popular  notion  (raised  by  Johannes

Kepler, a German astronomer who himself

needed  glasses,  more  than  400  years  ago)

that myopia is linked to too much close­in

work, such as reading and writing.

That  theory  remains  popular,  says  Dr

Rose,  but  evidence  for  it  is  mixed  at  best.

Instead,  the  dominant  hypothesis  now  is

that exposure to daylight is the main vari­

able. A study of Californian children, pub­

lished in 2007, found that time spent out­

doorswasstronglyassociatedwitha lower

riskofmyopia.Anotherpaper,published

thefollowingyearbyDrsRoseandMorgan

andtheircolleagues,followedmorethan

4,000childreninSydneyforthreeyears

andcametoa similarconclusion.Thetype

ofactivity—sports,walking,picnics—did

notseemtomatter.Simplybeingoutdoors

was the crucial point. The researchers

cross­checked theclose­workhypothesis

andfoundthatbeingoutsidedrasticallyre­

ducedtheriskofshort­sightedness,even

forchildrenthatdida lotofit.

Thistheoryfitsthedataneatly.Itex­

plains whymyopia seems,likediabetes

andheartdisease,tobewhatdoctorscalla

“diseaseofaffluence”—morecommonin

richcountriesthanpoorones—sinceeco­

nomic  growth  brings  with  it  more  educa­

tion,  and  therefore,  for  children,  more

time inside. It explains why rates are high

in East Asia in particular, says Dr Morgan,

since  the  ubiquity  of  private  tutoring  and

after­school  classes  mean  schoolchildren

there routinely work longer days than their

Western counterparts. Most South Korean

students,  for  instance,  attend  private  tu­

toring schools called hagwonsin which les­

sons often stretch well into the evening. 

Seeing the light

The  daylight­exposure  theory  is  also  bol­

stered by animal studies, in which that ex­

posure can be carefully controlled, and in

which  dimness  reliably  produces  short

sight. Researchers have a putative mecha­

nism, too. Exposure to bright light appears

to stimulate the production of dopamine, a

neurotransmitter, in the retina. Dopamine,

in  turn,  seems  to  help  regulate  the  rate  at

which the eye grows. Too little, and the eye

grows too long to focus properly.

Human  trials,  too,  confirm  the  theory.

One of the biggest, led by Pei­Chang Wu of

the  Chang  Gung  University  College  of

Medicine,  in  Taiwan,  was  published  in

2020.  It  reported  results  from  millions  of
      Taiwanese primary­school pupils who had
      gone  through  the  school  system  between
      2001 and 2015. In 2010 the government in­
      stituted  a  programme  called  “Tian­Tian
      Outdoor 120”, which encouraged schools to
      take  pupils  outside  for  two  hours  a  day.
      After it was implemented, rates of myopia
      fell, slowly but steadily, from 49.4% in 2012
      to 46.1% in 2015—reversing a decades­long
      trend of rising rates. 
      Exactly how much light is needed is un­
      clear,  though  Dr  Morgan  reckons  10,000
      lux is in the right ballpark. That is about as
      much  as  you  might  get  in  the  shade  on  a
      reasonably  sunny  day.  (Direct  sunlight  in
      the tropics can exceed 100,000 lux.) Levels
      indoors,  by  contrast,  rarely  exceed  1,000
      lux. It is technically possible to light class­
      rooms  to  10,000  lux,  notes  Dr  Rose.  But
      even  with  led lighting,  she  says,  the
      amount  of  heat  produced  would  require

specialistairconditioning,andtheglare

mightbeenoughtomakereadingtricky.

Researchershavealsobeenworkingon

waystoslowmyopia’sprogressiononceit

hasstarted.Oneistouselowdosesofatro­

pine,a poisonouschemicalfoundindead­

lynightshade—thejuiceofwhichwasonce

usedbywomentodilatetheirpupils,inor­

dertomakethemselveslookmoreattrac­

tive. Another is specially designed “or­

tho­k”contactlenses,whichareintended

to reshapethecornea whileworn. (The

corneaisthefront,transparentpartofthe

eye,whichdoesmostoftheworkoffocus­

inglightontheretina;theeye’sso­called

lensismainlyforfinetuning.)Thesealso

seem effective, though Dr Rose worries

abouttheside­effectsfromgivingcontact

lensestochildren,sinceifmisusedthey

cancauseirreversiblecornealscarring.

Sophisticated pairsofspectaclesmay

help,too.In 2020 thebmjpublisheda Chi­

nesetrialof“defocusincorporatedmulti­

plesegments”(dims)glasses.Thelensesof

thesehavea centralzonedesignedtocor­

rectthewearer’ssight,whichissurround­

edbyhundredsofothersmallzonesofdif­

ferentopticalpower.Theideaistoprovide

bothclearvisionthroughthecentralpart

ofthelensanddeliberatelydistortedvi­

sion through the smaller zones, since

poorlyfocusedvisionisthoughttosignal

totheeyetoslowitsrateofgrowth.Wear­

ingdimsglassesappearedtocuttherateof

myopicprogressionroughlyinhalf.

Eyedrops, more sunlight and clever

glassesmightbeabletopreventorslow

myopiainfuture generationsofschool­

children.Butbythetimesufferersreach

adulthood, the condition is permanent.

Thismeansthatinsomecountries,a pub­

lic­healthproblemisalreadybakedin.For

thosewithreallyseriousmyopia,saysDr

Rose,andwhoareatthehighestriskofbad

complications,unhealthychangesinthe

eyecanstarttooccurwhenpatientsarein

their40s.“Andsomeofthosearenotinany

waytreatable.”n

Clear as daylight

Myopia, estimated prevalence at age 20, %

Source:“Theepidemicsofmyopia:Aetiology and prevention”,

byIanMorganetal.,ProgressinRetinaland Eye Research, 201

1

100

80

60

40

20

0

11200090807060501938

South Korea

Hong Kong

Singapore

Taiwan

Look out!

Prevalence of myopia and severe myopia

in school-leavers, 201 or latest available, %

Source:“Theepidemicsofmyopia: Aetiology and prevention”,

byIanMorganetal.,Progress in Retinal and Eye Research, 201

2

Ejina(China)

Singapore

Jeju(SouthKorea)

Shandong(China)

Taiwan

Guangzhou(China)

Seoul(SouthKorea)

100806040200

Myopia Severe myopia