Impaired Olfaction May Point to Impending Parkinson's

By Charles Bankhead
HONOLULU, March 21 -- One of the earliest signs of impending Parkinson's disease could be right under a patient's nose, according to a study here.Impaired olfactory function predated clinical Parkinson's disease by at least four years and conferred as much as a five-fold increased risk of the neurologic disorder, G. Webster Ross, M.D., of the University of Hawaii, and colleagues reported in the February issue of Annals of Neurology."Olfactory testing together with screening for other potential early indicators of Parkinson's disease . . . could provide a simple and relatively economic means of identifying individuals at high risk for development of Parkinson's disease who could participate in trials of medications designed to prevent or slow disease progression," the authors concluded.
Olfactory dysfunction has a well-recognized association with Parkinson's disease. Moreover, an accumulation of evidence suggests that impaired olfaction may precede classic motor manifestations of the condition by several years, the authors said. However, proof of the association in a general population has been lacking.
Seeking the missing confirmation of the olfactory link, Dr. Ross and colleagues reviewed data from the Honolulu-Asia Aging Study, which began in 1965 as the Honolulu Heart Program. Evaluation of dementia Parkinson's was incorporated during 1991-1993, with three follow-up assessments from 1994 to 2000.
The current study comprised data on 2,267 men who underwent olfaction assessment during 1991 to 1996 by means of the Brief Odor Identification Test. The men were ages 71 to 95 at the time of olfaction testing and were free of clinical Parkinson's disease and dementia at the time.
During eight years of follow-up, 35 study participants had Parkinson's diagnoses, a rate of 24.6 per 10,000 person-years. The average age at diagnosis was 82.9, and the time from olfaction assessment to diagnosis was four years.
After stratification of the men by olfactory test results, patients in the lowest quartile of odor identification had an age-adjusted incidence of Parkinson's disease of 54.5 per 10,000 person-years. That compared with rates of 26.6, 8.2, and 8.4 per 10,000 person-years among men in the second, third, and fourth quartiles (P<0.001). To rule out the possibility that undiagnosed dementia might have affected olfaction tests, investigators excluded men with low scores on a standardized test of cognitive function.
Adjustment for potential confounders resulted in a Parkinson's odds ratio of 5.2 (95% CI 1.5 to 25.6) for men in the lowest quartile of odor identification versus men in the top two quartiles. The association between olfactory function and Parkinson's disease was not evident beyond four years of follow-up.
"This is the first population-based prospective study to demonstrate that odor identification deficits can predate the development of clinical Parkinson's disease in men by at least four years," the authors said. "These results remained significant when restricting the at-risk population to those without cognitive impairment."
The authors noted several potential limitations to the study including that the population studied was all male and that the smell test may not be completely applicable to Japanese-American men.
The authors reported no disclosures.
Primary source: Annals of NeurologySource reference:Ross GW, et al "Association of olfactory dysfunction with risk for future Parkinson's disease" Ann Neurol 2008; 63: 167-173.