Researchers
are keen to establish whether or not a smell test is a reliable indicator in
identifying individuals with pre-motor Parkinson’s, that is who currently show
no signs of Parkinson’s, but will go on to develop the illness later. In the same way that detecting heart disease
can help to prevent heart attacks, it is hoped that knowing there is a high
risk of Parkinson’s developing will allow time for preventative,
neuroprotective action to be taken. At
present diagnosis tends to come only when motor symptoms are observed and it is
to late to introduce preventative or neuroprotective strategies.
In a
study in Hawaii, the Honolulu-Asia Aging Study1, an odour
identification test was carried out in 2,267 men and over the following eight
years, 35 of them were diagnosed with Parkinson's. After taking into account any influence age
might have, the incidence of Parkinson's was 8.4 per 10,000 people per year for
those with the highest smell identification scores (good smell function),
whereas for those with the lowest smell identification scores (impaired smell
function) the incidence of Parkinson’s was 54.5 per 10,000 people per year.
Researchers
concluded that those with the lowest smell recognition scores were more than
five times more likely to develop Parkinson's compared to those with the
highest smell test results. In other
words, smell impairment may be useful in identifying individuals with a high
risk of developing Parkinson’s. They also concluded that reduced sense of smell
may occur between two and seven years before the first symptoms of the
condition emerge.
A study
from Germany2 evaluated 30 people with anosmia (complete loss of
smell function) for which the cause was uncertain and who showed no motor
symptoms. They underwent transcranial ultrasonography and
specialised brain imaging. Eleven out of the 30 individuals had an abnormal
transcranial ultrasonography, characteristic of Parkinson’s. Ten out of the 11
went on to have brain imaging: five were abnormal, suggesting that they might
be in a pre-motor phase of Parkinson’s.
Two out of the five subjects (7%) with anosmia and abnormal scans have
now developed clinically confirmed Parkinson’s.
A study
in Amsterdam3 looked at 361 asymptomatic (who showed no symptoms of
the condition), first-degree relatives of people with Parkinson’s in a two-step
approach combining smell testing and specialised brain imaging. Unexplained
hyposmia or impaired sense of smell alone was associated with a 12.5% risk of developing Parkinson’s
within a five-year period.
A study
in a larger population4 which began in 2008, led by the Institute
for Neurodegenerative Disorders at the University
of Pennsylvania, USA, hopes to
determine if smell is an accurate predictor of who will develop
Parkinson’s. The University of Pennsylvania’s
Smell Identification Test (UPSIT) checks for the first signs of
neurodegenerative disorders using 40 common odours, ranging from turpentine to
pizza and roses. People with a normal
sense of smell can usually correctly identify around 35 smells whereas those
with Parkinson’s can typically identify 20 or less.
Around
15,000 participants will undergo the UPSIT, as well as neuroimaging to check
for changes in brain activity that are associated with Parkinson’s. Follow up, by a movement disorder specialist,
will continue for between three to five years.
Researchers anticipate that many of those who had low scores in the
smell test will go on to develop Parkinson’s in the next two to three years.
Interestingly,
the study will also look into whether participants have sleep difficulties, in
particular Rapid Eye Movement (REM) Behaviour Disorder which often involves
acting out violent dreams, as this is also thought to be another risk indicator
for developing Parkinson’s. This
information, combined with the smell test results, may help to establish how
much greater is the risk of developing Parkinson’s when compared to normal
smell results and no sleep disorders. It
is hoped that this will enable preventative steps to be taken based on a
measured level of risk.
