People with Parkinson's disease experience reduced enjoyment from pleasant smells like lemon, according to a new study. This difference in smell perception could help diagnose the condition earlier and more simply. Researchers found that brain processing of scents varies in those affected, distinguishing them from others with smell loss.
A study led by Noam Sobel at the Weizmann Institute of Science in Rehovot, Israel, suggests that individuals with Parkinson's disease perceive pleasant smells differently from healthy people or those with unrelated smell issues. The research involved 94 participants, mostly aged in their late 50s to late 60s: 33 diagnosed with Parkinson's, 33 without medical conditions, and 28 with smell dysfunction not linked to the disease.
Standard tests assessed participants' ability to detect and identify smells, while custom olfactory perceptual fingerprint tests required rating the intensity and pleasantness of odors from three jars. One jar held a high concentration of citral, smelling like lemon; another contained a mix of asafoetida and skatole, creating a fecal odor; the third was empty.
All tests identified general smell decline, but only the perceptual fingerprints separated Parkinson's patients from the other smell-loss group with 88% accuracy. This improved to 94% when accounting for age and sex. Those with Parkinson's rated the citrus smell as intense as the healthy group did—more so than the unrelated group—but scored lower on pleasantness, similar to the unrelated group.
Additionally, Parkinson's participants sniffed the unpleasant odor nearly 2% longer than the lemon one, unlike the other groups, who reduced sniffing by 11 to 12% for the unpleasant scent.
The researchers propose that nasal smell detection functions normally in Parkinson's, but brain processing alters enjoyment and sniffing responses. This may relate to changes in the anterior olfactory nucleus, an early site of Parkinson's pathology that shrinks without smell signals.
Michał Pieniak at the Smell & Taste Clinic at Dresden University of Technology notes the potential value: of 10 clinic patients with unexplained smell loss, about one develops Parkinson's. Charles Greer at Yale School of Medicine sees promise but calls for larger studies, noting validation could take years since smell loss precedes other symptoms by decades.
Loss of smell affects 75 to 90% of Parkinson's cases, often years before tremors, but distinguishing it from age-related decline has proven challenging.
