Smell is often the quietest of the senses, but its loss could be an early warning sign of neurodegenerative diseases such as Parkinson's and Alzheimer's, potentially appearing years before other symptoms. Research indicates that up to 90% of Parkinson's patients experience a reduced sense of smell, which may serve as a biomarker for early diagnosis.
In Parkinson's disease, by the time classic symptoms like tremors and stiffness emerge, more than half of the dopamine-producing neurons have already been lost. Identifying smell loss early could allow for earlier intervention and more effective treatments. However, the symptom is not exclusive to Parkinson's and can also result from ageing, stress, or other conditions.
The connection between smell and neurodegeneration may lie in the olfactory bulb, a brain region near the nose that processes scents and links to memory and emotion areas. In some Parkinson's patients, the disease may start in the olfactory bulb before spreading to movement control areas, possibly triggered by inhaled viruses, pesticides, or toxins.
For Alzheimer's, damage may begin in the locus coeruleus, a brain stem region linked to alertness and the olfactory bulb. When this connection is disrupted, smell problems can precede dementia symptoms by years. Despite this, smell loss is often underestimated, partly because it is subjective and gradual.
While a temporary loss of smell from a cold is common, persistent or unexplained changes should not be ignored. Scientists hope that recognising smell loss as an early sign could lead to screening tools for neurodegenerative diseases, offering a window for earlier diagnosis and treatment.
