Stockholm
Gerontology Research Center,
NEUROTEC, Karolinska Institute,
Huddinge University Hospital,
Sweden.
The
relationship between smoking and
neurological diseases has always
been controversial. Even the
expected association between
smoking and increased risk for
cerebrovascular disease has been
debated for years. It was at the
end of the 1980s that smoking
became definitively accepted as
a risk factor for ischemic
stroke. More recently, two other
neurological diseases have been
studied in relation to smoking:
Parkinson's disease (PD) and
Alzheimer's disease (AD). Many
epidemiological studies have
found a highly significant
negative association between
cigarette smoking and these two
neurodegenerative disorders. The
risk of AD or PD in nonsmokers
has generally been about twice
that of smokers. That is,
patients with AD or PD are
approximately 50% less likely to
have smoked cigarettes during
their lifetime than are age- and
gender-matched controls.
Alternatively, cigarette smokers
are 50% less likely to have PD
or AD than are age- and
gender-matched nonsmokers. This
statistically significant
negative association has been
interpreted as suggesting that
cigarette smoking exerts an
undefined, biologic,
neuroprotective influence
against the development of PD
and AD. A review of all studies
that either support or refute
this hypothesis is presented
separately for PD and AD.
A few major studies have established a correlation between smoking and
developing Alzheimer's Disease, but none are widespread or detailed
enough to fully understand the link. Some early studies were
retrospective, meaning they examined the smoking habits of those who had
already developed Alzheimer's. Prospective studies followed smokers and
non-smokers, administered tests, and measured mental acuity. As it
stands, the decline in mental skills of the elderly is worse among
smokers. However, in people who carry a gene that makes them susceptible
to developing Alzheimer's, smoking seems to neither prevent nor speed
the onset of the disease.
In 1998, a prospective study from Erasmus Medical School in the
Netherlands, showed that smokers were twice as likely as those who never
smoked to develop dementia associated with Alzheimer's Disease. All of
the participants started out without symptoms of dementia, and were
tested two years later to determine their mental acuity. This study also
considered the "gene-environment" factors. It looked separately at
people who carried the apolipoprotein E-4 gene that indicates they are
more likely to develop Alzheimer's in their lifetime. Surprisingly,
these carriers who smoked were not more likely than non-carriers to get
dementia.
The same Medical Center published another study in 2004, showing that
year to year, the rate of mental decline was significantly worse among
those who smoked. In fact, they could even measure a difference between
individuals who had smoked in the past, but since quit, and those who
had not smoked their entire adult lives. The study was much larger than
previous studies, involving almost 10,000 people over 65 years old.
There is also increasing evidence that Alzheimer's Disease, as a
neurological disorder, might also be considered a vascular disease.
There are numerous, indisputable studies that prove smoking is
detrimental to vascular health.
One complication in these studies is the tendency of smokers to die
earlier than non-smokers from stroke, cancer, or heart disease. Thus,
the studies are skewed toward those relatively healthy smokers that have
not suffered serious health problems. Also, these studies rely on
people's own reporting about their smoking habits, rather than
collecting independent verification. Lastly, it has been shown that
nicotine, when injected and not inhaled, can improve mental faculties,
such as memory recall, of Alzheimer's patients. Certainly, further
studies are needed to fully understand the causal relationship between
Alzheimer's Disease and smoking.
