Loss of smell, or anosmia, can precede neurodegenerative diseases such as Alzheimer's and Parkinson's by years, offering potential for early warning and diagnosis.

A notable case is Joy Milne, who could smell Parkinson's in her husband years before diagnosis, illustrating the potential for scent-based clues in early detection.

Evidence points to different mechanisms: in Parkinson's, the olfactory bulb may be involved early, while in Alzheimer's the locus coeruleus is implicated, helping explain why smell loss can occur before motor or cognitive symptoms.

In Parkinson's disease, up to about 90% of patients experience early smell loss, which could serve as a biomarker for earlier diagnosis and guiding treatment, though it is not exclusive to Parkinson's and can result from aging, stress, or other conditions.

Smell is a powerful, often overlooked sense that can trigger memories and emotions, and its loss can signal neurological issues.

Parkinson's-related smell loss can be selective, with trouble detecting neutral or unpleasant odors while pleasant smells may remain detectable, and some patients may experience olfactory hallucinations.