An at-home olfactory test helps to identify people with an increased risk of having dopamine transporter (DAT) imaging indicative of early Parkinson’s disease (PD), show data from the Parkinson Associated Risk Syndrome (PARS) study.
DAT deficit, defined as putamen [123I] 2β-carbomethoxy-3β-(4-iodophenyl) tropane binding of 65% or less of that expected for the individual’s age, was present in 11% of participants with hyposmia, compared with just 1% of those without, report Danna Jennings (Institute for Neurodegenerative Disorders, New Haven, Connecticut, USA) and co-workers.
The participants were drawn from 4999 people who were contacted by mail and returned a completed 40-item University of Pennsylvania Smell Identification Test. Of the nearly 700 people who were hyposmic, 203 agreed to further assessments and were matched by age and gender to 100 normosmic participants.
Participants with and without hyposmia were similar in terms of age, gender, smoking status and family history of PD. Their Unified Parkinson’s Disease Rating Scale scores were not significantly different; however, more hyposmic than normosmic participants received a neurological diagnosis (22 vs 12%), with 14% versus 7% receiving a clinical diagnosis of prodromal PD.
Although the 24 participants with DAT deficit comprised just 0.24% of the more than 10,000 people who were originally sent the olfactory test, the findings “successfully demonstrated the proof of concept that the study design and prescreening assessments are highly feasible, inexpensive, and scalable”, say Jennings et al.
Besides having a higher likelihood of a DAT deficit, the group with hyposmia also had an increased proportion of indeterminate scans (DAT binding between 65 and 80% of that expected for age), at 17% versus 7% in the normosmic group.
“Longitudinal assessment of the subjects with indeterminate scans will be particularly informative in refining the DAT imaging cutoff used to define risk of motor PD”, observes the team in Neurology.
The findings also suggest that accounting for other prodromal symptoms could increase the likelihood of identifying people with DAT deficit. For example, the highest rate of DAT deficit in the hyposmic group occurred among men with constipation, at 43%. And on logistic regression analysis, these three factors (male gender, hyposmia, constipation) were significant predictors of DAT deficit.
However, the researchers caution that combining biomarkers will reduce generalisability and increase the number of people that would need to undergo olfactory screening to identify the same number of people with DAT deficit.
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