A new study has found that smell impairment could be an early indicator of Parkinson's disease (PD).
In the study, led by G. Webster Ross of the VA Pacific Islands Health Care System and the Pacific Health Research Institute in Honolulu, Hawaii, researchers found that an impaired sense of smell could precede the development of PD in men by at least four years.
The researchers included 2,267 men from the Honolulu-Asia Aging Study who received olfactory testing at least once, either between 1991 and 1993 or between 1994 and 1996, and were followed for up to eight years to find out if they developed PD.
During the course of follow-up, the researchers found that 35 men developed the disease.
The findings showed that an odour identification deficit could predate the development of PD by at least four years, although it was not a strong predictor beyond this time period.
Odour identification deficit was linked to older age, smoking, more coffee consumption, less frequent bowel movements, lower cognitive function and excessive daytime sleepiness.
However, even after taking into account these factors, those with the lowest olfactory scores, meaning they had the poorest odour identification, had a five times greater risk of developing PD than those with the highest scores.
"One interpretation of this finding is that the relationship of olfactory deficits to higher risk of future PD begins to weaken beyond a threshold of approximately four years between testing and diagnosis," the researchers said.
The fact that the time from olfactory testing to diagnosis was shortest among those with the lowest olfactory scores supports this.
Also, previous studies suggest that olfactory impairment starts between two and seven years before the diagnosis, and according to neuroimaging and pathological studies, there is a period of about five to seven years between the onset of nerve loss in an area of the brain affected by PD and diagnosis of the disease.
Although, the pathology of impaired sense of smell in PD is not completely understood, nerve loss and the formation of Lewy bodies, abnormal clumps of proteins inside nerves cells that are thought to be a marker of PD, are known to take place in the olfactory structures of patients with the disease.
The researchers note that one study involving brain dissection of deceased patients with neurological disease found that olfactory structures are the earliest brain regions affected by Lewy degeneration, which contributes to the idea that an impaired sense of smell could be one of the earliest signs of the disease.
Impaired nerve cell formation might be responsible for olfactory deficits in PD. The olfactory bulb is one of two regions in the brain that receives new neurons throughout life, and dopamine depletion, which occurs in PD, has been shown to impair nerve cell growth in this structure in rodents.
There is a also a possibility that olfactory deficits are not directly related to the structures themselves, but originate in the amygdala, an area of the brain affected by PD that is known to be involved in smell function.
Impaired sniffing might be another motor symptom of PD, for it could also cause an impaired sense of smell.
The study is published in the Annals of Neurology.