Loss of smell, often dismissed as a minor inconvenience, could be an early warning sign of neurodegenerative diseases such as Parkinson's and Alzheimer's, appearing years before other symptoms. Research indicates that up to 90% of Parkinson's patients experience olfactory decline, which may precede motor symptoms like tremors and stiffness by several years.
In Parkinson's disease, damage may begin in the olfactory bulb, a brain region near the nose that processes scents, before spreading to areas controlling movement. This suggests that inhaled viruses, pesticides, or toxins could trigger the disease. For Alzheimer's, a recent study points to initial damage in the locus coeruleus, a brainstem region linked to alertness and smell, disrupting the connection between scents and emotions.
The challenge is that smell loss is not exclusive to these diseases; it can result from ageing, stress, or infections like COVID-19. However, given that neurodegenerative damage is often advanced by the time classic symptoms appear, identifying early biomarkers such as olfactory dysfunction could enable earlier diagnosis and more effective treatment.
While a temporary loss of smell due to a cold is common, persistent or distorted smell—such as perceiving oranges as burnt plastic—may warrant medical attention. Experts emphasise the need for greater awareness of this subtle but significant symptom.
