AGEING: PQPULATIONS, ORGANISMS AND CELLS.
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Ageing is a reproducible and highly recognizable process. You only have to look at friends or TV and cinema stars and you can guess their age usually correctly to Within a few years. But the process is very variable. Some people look much older than their years and others much younger. What is clear is that the same changes occur but at different rates.
An
ageing population is defined epidemiologically as one in which there is an increase
in age –specific death rates. Death rates start to increase remarkably early in
life from the early 20s. Medical and nursing students are already well into the
ageing process by the time they have finished their courses! Male rates are
higher than female rates but this is usually attributed to men’s tendency to
smoke, drink too much and take part in risky activities, like riding
motorcycles.
AGEING OR DISEASE?
What
is ageing?
Ageing
is a developmental process, part of the cycle beginning at conception and
ending with death. Ageing is not the same as disease, but diseases do become
more common among older people. This association between ageing and disease is the cause of much confusion about what is in
store for us as we age. And it is a confusion that tends to affect doctors more
than their patients. Patients complain to their doctors of pain and immobility only
to be told that its due to their age – which is seldom true.
For
example, a patient develops difficulty getting in and out of the bath. This may
be due to ageing: a decline in the number of functioning muscle cells leading
to Weakness. It may be due to a disease affecting the peripheral nerves, a
metabolic disease (eg. diabetes or osteomalacia), or disease of large joints
(e.g. osteoarthritis).
MEDICAL CARE VERSUS
SOCIAL CARE
In
the example given, a social model of care would lead to assessment of the
problem and provision of a bath board and rails around the bath. A medical
model would sort out disease from ageing, and attempt to treat any underlying
disease process. A collaborative approach would lead to both treatment (in its
widest sense) of any disease underlying the disability and provision of any
aids or lifestyle modifications to overcome the disability. Collaboration and
teamwork are the hallmarks of health care for elderly people.
The
medical model is often criticized for its preoccupation with diagnosis and drug
treatment, for its focus on organ systems, and for its super-specialization. But
the medical model has much to offer elderly people. Disease does become so much
more common with increasing age and deserves to be diagnosed and treated.
Despite much disease being Chronic, degenerative and irreversible, a great deal
can be done for its disabling and handicapping consequences. However, the doctor
is seldom able to combat disease consequences single-handedly. Teamwork is
necessary, and the concept of diagnosis has to be extended to assessment, treatment
becomes management, and prognosis becomes monitoring and evaluation of
achievement of goals.
An
exclusively social model, bypassing the question is this problem due to disease
or not?’, is just as inadequate for solving problems as a solely medical model.
Collaboration is essential, and is best achieved through comprehensive services
for elderly people.
IMPAIRMENTS, DISABILITIES
AND HANDICAPS
Although these terms
are often used interchangeably, they do have precise meanings that can help in
understanding what comprehensive care
means.
Impairment:
Damage caused to a
cell, organ or system by a disease
Process.
Disability:
Difficulty in
carrying out tasks - such as activities of daily
living, or driving a
car -that are a consequence of the
disease.
Handicap:
Effect the disease
has on the lifestyle of the patient -- the
disadvantage caused
by the disease
Some
treatment may reduce impairment. For example, exercise against progressively
increased resistance may increase muscle strength and joint flexibility in an arthritic
joint. The measurement of muscle strength and joint range of movement would be
the best indicators of success and failure.
Other
treatments may have no effect whatsoever on the impairment caused by disease.
For example, no amount of physiotherapy will reduce the amount of brain damaged
by a stroke. However, therapy may well reduce the time taken to get
independently mobile again. In this case the correct measure of success is the measure
of an aspect of disability - mobility.
For
many diseases, neither the impairment nor the disability is affected by
rehabilitation teamwork. This is not to say that useful management is
impossible. It is this type of patient who is often viewed as a ‘no-hoper’ on
an acute medical ward, but is transformed into a ‘major rehabilitation
challenge’ on a health care of the elderly ward! There is no special magic in
this, but simply a more realistic appraisal of the possibilities by the team,
and a tendency to focus on handicap or lifestyle.
It
is vital that services for elderly people understand their contribution to the
overall picture, and do not attempt to demonstrate the supremacy of their
particular skills over those of other equally needed and important skills. The
impairments, disabilities and handicaps framework is a useful way of ensuring a
comprehensive approach.
POPULATION AGEING
Most
people know that we are living in an ageing society, indicated by such phenomena
as the greying of America, the rise of the Grey Panthers, the growth of retirement
and nursing homes. One of the major justifications for health care for elderly
people is the increased number of them. This is highlighted by the tremendous
increase in the number of centenarians in the UK who receive a birthday
telegram from the Queen each year.
The
spectacular population changes over the last century are due to two major
factors: declines in fertility and decreases in death rates at all ages. Fertility
declines because of a smaller number of babies born and increasing maternal age
at first birth. These trends began with the industrial revolution and the
associated socio-economic development which made large families less
advantageous. Family planning has accelerated the process in many developing
countries. A willingness to use family planning methods depends on culture,
education and religion, but with increased economic wealth a move to smaller
families is inevitable. Prosperity is no longer measured by the size of the
family.
Deathrates have also declined dramatically over the last century and the decline has
affected the old and the young. The explanation for the decline is found in
improved nutritional status, better social and economic conditions and the
major public health reforms of clean water and sewage disposal. Medical care
was not responsible for the changes. Over the last century, the world has -
despite appearances - become a healthier place to live, and more of us are
doing it for longer.
It
is paradoxical that an ageing population is a consequence of living in a
successful society that ensures that babies do not die, that promotes the
autonomy of women to control their fertility, and that has reduced the chances
of death over the entire lifespan. The large numbers of elderly people are our
heritage, and the rewards of socio-economic development. Turning back the clock
to the turn of the century when old age was not a ‘problem’ is impossible, and
countries where high fertility and infant mortality rates occur do not wish to stay
at that point in development. A better understanding of ageing, removing the
myths and stereotypes, and at improving the health, social and economic lot of
elderly people are keys to the solution.
RECTANGULARIZATION OF SURVIVAL
Survival
curves have changed in shape over the last few hundred years. This process has been
called the ‘rectangularization of survival’.
One Of the Consequences of the increased chances of survival is that average
life expectancy begins to approach maximum lifespan. The maximum lifespan is
around 115 years, but the average expectation of life at birth is around 80 years
for women in the most successful countries (Japan, USA). Deaths, instead of being
spread across a fairly long part of the human lifespan, are now compressed into
a shorter time. This should mean that people are sicker for a shorter time as
well, and in essence will tend to be healthier for longer.
This
optimistic scenario for the next century may come true, but for the foreseeable
future it is more likely that increased survival will be associated with
increased risk of non-life-threatening, but disabling chronic diseases. At
present, the rectangularization of survival is associated with a far greater number
of very old people, which is producing an explosion of morbidity, and an increased
pressure on health and social care resources.
INDIVIDUAL AGEING
We
have become used to hearing how things change, usually for the worse as we get
older. Blood pressure goes up with age, hearing and vision get worse, bone density
declines, intelligence declines, and so on. As humans grow and develop they
reach a peak in physiological and anatomical capacity in their late teens, but subsequently
capacity declines. It seems to matter little which type of capacity is
considered: muscle strength, stamina, lung volume, kidney function, brain
volume. All show age-related declines.
Cross-sectional and longitudinal studies
There
is a large trap for the unwary here. Most of the studies of age-related
declines are not really studies of ageing at all. In fact they are studies of a
number of young people, a number of middle-aged people, and a number of old and
very old people, all examined at the same point in time. None of the subjects
has aged at all during the study, they are simply of different ages at the time
of the study. Such studies are cross-sectional - snapshots -- and we have to
make some assumptions before we can accept that they are a good indication of
true ageing changes.
The
first assumption is that what is true for 60-year- olds now will be true for
50-year-olds in I() years’ time. We have to believe that the effects of
belonging to a group _ or cohort - of people born around the same time, and
living through a unique set of life experiences, is not relevant to the factor
(e.g. blood pressure, brain volume) under study. The effects of belonging to a
cohort have been extremely important over the last century. For example, women
born at the turn of the century had far fewer chances for marriage than those born
later because of World War I. Men who went to light in that war, if they
survived, were subsequently much more likely to die of lung cancer because of smoking
habits acquired during the war. Cohort effects can be powerful, and are
impossible to detect in a simple cross-sectional study.
The
second assumption to be made is that conducting the study at a particular point
in time is irrelevant. It is unlikely that a cross-sectional study of changes
in, say, muscle strength and age conducted in 1989 would come up with different
results to one conducted in 1990. But over a period as short as five years,
differences in the relationships between blood pressure and age have been
found.
A
third assumption is that older subjects are disease free. For example
reductions in air flow with age may be due to changes in the elasticity of the
lung, but inclusion of older people with chronic bronchitis will lead to much
greater age-related reductions in air flow due to the combined effects of age
and disease.
The
reason that scientists continue to examine the effects of age by
cross-sectional studies is because such studies are generally much cheaper than
cohort _ follow-up or longitudinal – studies. Provided such studies give more
or less the same answer, no harm is done. However, in the few cases where
comparisons can be made between cross-sectional and cohort studies, the answer
can be strikingly different. The best example of a discrepancy between cross-sectional
and cohort effects are the changes in performances on intelligence tests at
different ages.
The
general conclusion of cross-sectional studies is that intelligence declines
with age, supporting stereotyped images of confused old people. The cohort
changes are strikingly different: far from getting more stupid over the years,
people tend to improve on some aspects of intelligence and remain more or less
the same on others, and decline in only a few areas (Fig. 1.3).
So
why the difference? It probably arises out of a cohort effect. People born in
1920 had limited schooling and access to educational mass media like television
and newspapers. People born in 1930 enjoyed better education. When studied
cross-sectionally in 1990, those born in 1920 are 70 years old and will do
worse on the intelligence tests than the 60-year-olds born in 1930.
Describing true ageing effects
Describing
true age effects requires complicated studies of successive cohorts of people
over long periods of time – so – called ‘cohort-sequential’
studies. This is because a single cohort invariably experiences a unique set of
circumstances thrown up by the age they live through (e,g. wars, economic
depression) – period effects that have an impact at all ages. The task is to separate
the effects of ageing from the effects of the era in which people age. This can
only be done by studying a succession of cohorts to determine whether age-related
changes are consistent across each cohort. Very few studies like this have ever
been set up, so we have to make do with the information from cross- sectional
studies. However, never take such
information at face value: always ask – could this be due to cohort or time
period effects?
Statistically,
ageing can be defined as facing an increased risk of death over time. What does
this mean? If you take 1000 glasses and count how many ‘survive’ over a period
of years you would not be surprised if many of them ‘died’. The death rate of
the glasses can be calculated as the number broken divided by the number at the
outset. However, a small adjustment has to be made for the fact that once a
glass is broken it cannot be broken again, and must be removed from the denominator
of the calculation of the ‘death’ rate. If this is done, a glass survival curve
can be drawn, giving the percentage of glasses surviving at the end of each year
(Fig. 1.4).
Now
glasses do not age: their molecules contain no ability to replicate, they are
inert. And yet a population of glasses appears to ‘age’, and has a survival
curve. The key is that the rate of breakage of glasses is constant – there is a
constant accident hazard that systematically removes glasses until they are all
gone. Death rates of populations do not show a constant rate, but an ever increasing
rate with increasing chronological age. This gives population survival curves
their characteristic rectangular shape. The point at which death rates start to
rise can be viewed as the start of human ageing. It comes as a surprise to see
how early in life this starts - late teens, early 20s!
Many
factors change as people get older. Some people get richer, most get poorer.
Their homes age with them, often becoming hazardous. Social contacts and status
within the family alter. These factors are called ‘extrinsic’.
THE MECHANISMS OF
HUMAN AGEING
Ageing has fascinated
scientists, philosophers and poets for centuries. What is ageing? Why do we
age? What is ageing for? How can we slow down or stop the process? These are weighty
questions and need unravelling into smaller questions that are more easily
tackled. An inherent difficulty in the study of ageing is that the findings
from easy-to-study animal species (bacteria, paramecium, mice) are not directly
transferable to humans.
Strehler’s framework
of ageing states that, for a process to be truly due to ageing (rather than due
to pathology), it must be universal, progressive, intrinsic and deleterious.
Cellular and molecular ageing
Cells get old and die
just like the whole organism, but they do it at a more rapid rate. The cells of
the epidermal layer of the skin shed themselves within a few days. The red
blood cell lives for about four months. Theories of ageing have to accommodate
the following observations about ageing.
Hayflick limit to cell division
The Hayflick limit is
the number of cell divisions that human fibroblasts - connective tissue cells –
grown in a cell culture can undergo before stopping. They stop after about 50
cell divisions. By contrast, many human cancer cells continue to replicate unchecked,
and are called immortal.
Mercyvill.
