Abstract
From population-based surveys around the world it is now
clear that untreated cataract is by far the major cause of
global blindness, accounting for over 20 million bilaterally
blind people. Cataract is associated with ageing, but there
appear to be several factors which increase the risk of an
earlier onset. Until these risk factors are reduced by changes
in life style or a specific preventive measure can be introduced,
the treatment is by cataract surgery. A successful cataract
service requires operating on sufficient numbers (output),
good visual results (outcome) and adequate financial arrangements
(outlay). Cataract surgery is the top priority of the new
World Health Organisation Vision 2020 programme.
Keywords
World blindness, cataract risk factors, cataract surgery, Cataract Surgical Rate (CSR), Vision 2020
Historical perspective
When attention turned after World War II, in the 1950 and
60s, to the problem of untreated blindness in the developing
countries, it was assumed that trachoma, the commonest infectious
disease at that time, was also the leading cause of blindness.
Much research was developed on the biology of the causative
organism, Chlamydia trachomatis, the epidemiology of
the disease, and how it could be controlled. It was not until
1981 that the first population-based survey of the magnitude
and causes of blindness in a whole country, Nepal, was completed.
This survey revealed that 72% of the 0.84% blindness was caused
by cataract or the complications of cataract surgery. Trachoma
accounted for only 2.4% of the total. In the next few years
several other population-based epidemiological surveys were
published, and in all of these cataract was found to be the
largest cause of blindness. Table 1 gives examples of the
prevalence of blindness and causes in some representative
surveys.
The magnitude of the cataract problem
For international comparisons the World Health Organization
(WHO) devised a definition of blindness: less than 3/60 in
the better eye, with best correction (an alternative definition
- visual field constriction of less than 10° around
fixation - was included to allow for the disability produced
by onchocerciasis and glaucoma). Other degrees of vision loss
were also described: severe visual impairment is defined as
corrected vision less than 6/60 but more than 3/60; visual
impairment is defined as corrected vision less than 6/18 but
more than 6/60. (It will be appreciated that these definitions
automatically excluded refractive error as a cause of blindness
or low vision. Therefore 'presenting' or 'functional' vision
is more commonly recorded in recent surveys.)
|
Country
|
Date
|
Prevalence
Blindness by
WHO
definition (%)
|
Causes
of Blindness
|
|
Cataract
and complications
|
Trachoma
|
Other corneal
opacities
|
Glaucoma
|
Nepal
Chad
The Gambia
The Gambia
India
SE Turkey
Morocco |
1981
1985
1986
1996
1986-89
1989
1992
|
0.84
2.31
0.7
0.42
0.7
0.4
0.76
|
72.1
48.0
55.0
58.0
80.1
50.0
54.6
|
2.4
23.0
17.0
5.0
0.39
0.3
3.9
|
5.0
14.0
11.0
16.0
1.52
12.0
6.5
|
3.2
15.0
2.0
9.0
1.7
12.0
14.3
|
| Table 1. Representative population
surveys of blindness |
|
Any of these levels can be used to indicate the magnitude
of visual reduction due to cataract.
There is now no doubt that unoperated cataract remains far
and away the major cause of both blindness and low vision
throughout the world, making up at least 50% of blindness
in most developing countries. The number of blind in the world
in 1998 was estimated at 45 million, with 20 million due to
cataract. Blindness is thought to be 50 million in 2001 and
is projected, if present trends continue, to reach 75 million
by 2020. Of these, unoperated cataract may be expected to
account for at least 35 million. This figure is equivalent
to the combined present total populations of Australia, New
Zealand, Sweden and Denmark. Three times as many will have
visual impairment or severe visual impairment, another 100
million. The burden of cataract is increasing remorselessly,
as illustrated in (Figure
1)1. There is very little
information on the incidence of new cases of cataract, but
Minassian and Mehra measured the number of new cases over
4 years in a central area of India: when projected to the
whole population of India, 3.8 million people were becoming
blind from cataract each year.
Causation and risk factors
Cataracts consist of an aggregation and denaturation of the
lens proteins which are normally regularly dispersed and which
have molecules small enough relative to the wavelength of
light not to interfere with its transmission. It is probable
that if we lived long enough, we would all eventually develop
lens opacities. A recent population survey in Melbourne found
that by the age of 90, 100% of males had a significant degree
of nuclear cataract and 60% had cortical lens opacities as
well, with females having prevalences almost as high2.
But while age is the major risk factor, some people develop
lens opacities at an earlier age than others. It has recently
been shown by studies of monozygotic and dizygotic twins in
the UK that in the case of nuclear cataract 50% of this variation
can be accounted for by genetic factors3.
The other factors are presumed to be environmental.
Risk factors are characteristics, attributes or exposures
that increase an individual's risk of developing a cataract.
There are many studies, in different countries, and these
have been reviewed4; a list
of probable and possible factors is given in Table 2. Almost
none of the factors are found consistently across all the
studies in different continents. Some factors are associated
more with cortical, some with nuclear and some with posterior
subcapsular (PSC) lens opacities.
|
Definite
|
Age
|
| Probable |
Gender (female)
Smoking
Diabetes
Corticosteroid use
Sunlight exposure
Dehydrational crisis/heatstroke
Low social class |
Possible
|
Low height
Low weight
Low body-mass
Limited education
Alcohol
Oestrogens
Hypertension
Renal failure
Myopia
Rural residence |
| Possible protective factors |
Aspirin
Antioxidant vitamins |
| Table 2. Risk factors for
cataract. |
|
Cigarette smoking has been associated with nuclear cataract
both in the UK and USA. Nutritional factors are likely to
play a part, and cataract is more common in diabetics and
people with renal failure. The role of ultraviolet light is
controversial. Individual exposure to ambient UVB in sunlight
has been associated with cortical cataracts in otherwise fit
fishermen in Chesapeake Bay and with cortical, nuclear and
PSC in Australia. But other studies are not consistent, and
it is not clear how much this factor contributes to the overall
global burden of blindness due to cataract.
A cardinal question is, 'Why is cataract so much more frequent
in developing countries such as India, and why does it come
on at an earlier age than in industrialised countries?' A
consistent finding in studies of aetiology in India has been
the association with low socio-economic status and low educational
level. This factor may point to malnutrition in infancy, possibly
malnutrition in the mother and repeated severe infections
in infancy. An important finding in the hot central part of
India (where temperatures go up to 45°C and sometimes 50°C)
is an association with severe dehydrational crises (a cholera-like
illness) and heatstroke. Thus the early presentation of cataract
in these poor people may represent repeated insults to the
body proteins earlier in life. As with ageing in general,
lens opacity comes about through the gradual build-up of unrepaired
faults in the cells and tissues of our lenses as we live our
lives.
Prevention of blindness from cataract
Prevention of a bad functional outcome of any disease is
typically described at 3 levels:
1. Primary prevention: preventing the disease occurring
in the first place;
2. Secondary prevention: once the disease has occurred,
preventing it giving rise to a disability; in the case of
cataract, we have various surgical procedures to remove the
damaged lens;
3. Tertiary prevention: where a loss of function cannot
be prevented, providing remedial measures to prevent it becoming
a handicap.
Primary prevention
If we can discover more precisely the nature of these faults
in the lens, we can hope to slow their accumulation. It may
be expected that general public health measures, and a better
standard of living, will delay both the age of onset and the
rate of progression of cataracts, especially for poorer people
in developing countries. Improved living standards include
good nutrition in pregnancy and childhood, and provision of
clean water. Gastro-intestinal infections leading to severe
diarrhoea should be treated promptly before dehydration sets
in. If there were not already enough reasons to stop cigarette
smoking, cataract is another one.
There is not yet evidence from clinical trials that any
of the drugs that are claimed to delay the progression of
cataract are effective.
Secondary prevention
Until improved personal lifestyles become widespread, or
until a specific method of prevention becomes available, the
only way to reduce unnecessary visual impairment due to cataract
is by surgery. Even in the most optimistic scenario, cataract
extraction will continue to be required for the oldest segment
of the population. The questions which must be addressed are:
'How many cataract operations need to be done in different
settings?', 'How good are the results?', and 'How can these
operations be paid for and the service be sustained?'
Output
We have already noted above that in 1998, WHO estimated that
there was a backlog of 20 million people blinded by cataract,
and it is thought that globally at least 25 million eyes reach
a vision of less than 6/60 each year due to cataract.
The number of cataract operations performed each year per
million inhabitants is referred to as the Cataract Surgical
Rate (CSR). At present, this varies from 100 in Nigeria and
450 in Kenya to 3,100 in India, 3,800 in the UK and 5,500
in USA. Australia at present appears to have the highest rate
at 6,300 operations per million people each year. Table 3
gives estimated CSR for different WHO regions1.
|
WHO Region
|
Population
(millions)
|
No of Cataract Operations
per year (millions)
|
Cataract Surgical Rate (operations/million/year)
|
Africa
Eastern Mediterranean
Western Pacific
Europe
Western
Eastern & Central
South East Asia
Americas
North
Central & South |
650
500
1650
900
400
500
1500
800
300
500
|
0.2
0.5
1.65
2.1
1.6
0.5
3.6
2.15
1.65
0.5
|
300
1000
1000
2300
4000
1000
2400
2700
5500
1000
|
Table 3. Estimates of cataract
surgery (1999)1*
* updated by A. Foster |
|
The relatively low level for Europe is explained by the
CSR of 1000 for the 500 million people of East and Central
Europe (including Russia and the independent Asian republics)
compared with a CSR or 4000 for the 400 million people of
Western Europe. Similarly the rate for Central and South America
is 1000, compared with 5,500 for USA and Canada. The level
of visual acuity at which a cataract is considered operable
will vary according to the circumstances in a particular country.
Thus in some African countries there are so few cataract surgeons
that it is appropriate to operate only on those blind (<3/60).
Taylor has calculated that the less than 6/60 CSR will have
to double if the cut off moves from less than 6/60 to less
than 6/24, and 5 times if it moves to less than 6/125.
The traditional type of surgery in eye camps in India and
neighbouring countries has been intracapsular cataract extraction
with aphakic spectacle correction. Increasingly, even in eye
camps, surgeons are changing to extracapsular cataract extraction
with an intraocular lens. This change may result in people
coming forward for surgery at earlier levels of visual impairment,
and surgical services tend to go to men who can afford the
IOL at the expense of women and people with more severe visual
impairment. There has been a tremendous effort in India to
increase the number of cataract operations to more than 3
million per year (CSR over 3,000) but this rate is not yet
keeping pace with the backlog of cataract blindness plus the
incidence of new cases.
Even where surgeons are available and there is the capacity
to do more surgery, many people are not coming forward. The
barriers, which include cost of surgery and cost of transport,
as well as fear of surgery, are being intensively studied
in different countries so that new approaches can be put in
place. The cost of surgery has been much reduced since high
quality intraocular lenses have become available for $7 or
less by local manufacture in India (Aravind Eye Hospital)
or Nepal, Eritrea and Vietnam (Fred Hollows Foundation).
Outcome
Poor visual results will also lead to low take-up of surgery.
Recent measurement of the results of large volume cataract
surgery have shown that the visual outcome is often not as
good as had been thought6. The
emphasis of programmes has therefore shifted to some extent
from numbers to quality. Factors which lead to poor quality
of final vision can be considered under four headings:
Selection: pre-existing disease such as advanced
glaucoma or macular degeneration.
Surgery: complications at the time of surgery or
in the early post-operative period.
Spectacles: refers to the uncorrected refractive
error, whether with a standard power of aphakic glasses, or
inaccurate IOL power which has not been corrected further.
Sequelae: long-term surgical complications such as
uveitis or posterior capsular opacification (20% of eyes after
2 years, 50% after 5 years). These complications emphasizes
the need for good follow-up.
Careful attention must be paid to each one of these elements
if consistently good results are going to be obtained. Work
is presently going on to develop simple tally sheets and more
elaborate pro-formas and computer software so that everywhere
cataract surgeons can monitor their own results.
Outlay
In order to have sustainable services year after year, the
costs must be kept under control and revenue maintained. As
well as lower costs for IOLs, inexpensive locally produced
sutures and other consumables are becoming available. Many
different models of cost-recovery are being tried, such as
asking patients to pay for the IOL and charging for the use
of private rooms.
Tertiary Prevention
Residual disability can be reduced to a minimum by providing
best possible refraction or low vision aids where there is
co-incident disease or complications. Thus the disability
may often be prevented from becoming a severe handicap.
The Vision 2020 Programme
A new programme for the global elimination of avoidable blindness
was launched in 1999 by the WHO and the International Agency
for the Prevention of Blindness. It is called 'Vision 2020
- the Right to Sight'. The 3 elements of this programme are
the control of major causes of blindness; human resource development;
and development of infrastructure and appropriate technology.
Amongst the 5 high-priority causes targeted in the first five
years the most important is cataract (the others are trachoma,
onchocerciasis, childhood blindness, and refractive errors,
including provision for low vision services). For the next
several years the major initiative will be to focus on the
biggest burden of blindness - that due to cataract.
|