People with Parkinson’s disease lose not just their ability to detect smells but their capacity to enjoy pleasant ones — a distinction that could allow doctors to diagnose the condition more cheaply and quickly than current methods permit.
Researchers led by Noam Sobel at the Weizmann Institute of Science in Rehovot, Israel, found that measuring how people perceive and respond to odours — rather than simply whether they can detect them — identified Parkinson’s patients with 88 per cent accuracy, rising to 94 per cent when participants were matched by age and sex.
The study recruited 94 participants, most in their late 50s to late 60s. 33 had been diagnosed with Parkinson’s, 33 had no known medical conditions, and 28 had smell dysfunction unrelated to the disease. Standard smell tests could identify decline across all groups but could not separate those with Parkinson’s from those with other causes of smell loss. Only what the researchers call an “olfactory perceptual fingerprint” made that distinction.
What the Smell Test Actually Measures
Participants rated three jars — one containing a high concentration of citral, which smells of lemon; one with a heavily concentrated mix of asafoetida and skatole, which smelled faecal; and one empty. The team recorded both pleasantness ratings and sniff duration.
People with Parkinson’s perceived the citrus smell as equally intense as the healthy group did, and more intense than the group with unrelated smell problems. But their pleasantness ratings for the lemon scent dropped, placing them closer to those with other smell disorders. The sniffing behaviour was also telling: Parkinson’s patients sniffed nearly 2 per cent longer in response to the unpleasant smell compared with the pleasant one, while both other groups cut their sniff duration by 11 to 12 per cent when moving from pleasant to unpleasant.
The team speculates that olfactory processing in the nose itself may remain largely intact in Parkinson’s patients, but that the brain handles the signals differently — blunting the enjoyment of pleasant odours and decoupling the automatic sniffing reflex from scent quality. According to the announcement, this is likely tied to changes in the anterior olfactory nucleus, an area suspected to be among the earliest sites of Parkinson’s brain pathology.
A Gap the Test Could Fill
Loss of smell affects 75 to 90 per cent of Parkinson’s cases, often preceding tremors by years or even decades. Previous attempts to use smell loss as a diagnostic marker stalled because the sense also declines with normal ageing, making it hard to distinguish one cause from another.
Michał Pieniak of the Smell & Taste Clinic at Dresden University of Technology notes that roughly one in every ten patients who arrive at his clinic with unexplained smell loss will eventually develop Parkinson’s. “Anything that would bring us closer to helping identify their personal risk would be great,” he said.
Charles Greer at Yale School of Medicine called the approach promising but said larger trials are needed. Validation could take considerable time, given that smell loss can precede other Parkinson’s symptoms by many years.
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