People suffering from Alzheimer’s and Parkinson’s disease may experience a loss of smell long before other symptoms manifest.
A damaged sense of smell is often one of the first indicators of brain damage associated with these diseases. Notably, the symptom presents differently depending on the specific brain disease.
Dr Ronald Postuma, a professor of neurology and neurosurgery at McGill University, said that those with Parkinson’s or dementia with Lewy bodies often doubt they are being given a scent to identify at all.
In contrast, Alzheimer’s patients tend to have a different reaction, as he told the New York Times: “They go, ‘What a lovely smell. It smells so sweet. It must be gasoline.'”
The link between smell Alzheimer’s and Parkinson’s
While it’s common for our sense of smell to decline as we age, this can also be an early warning sign of neurodegenerative conditions.
According to research conducted in 2023 by the National Institute of Aging in the US, maintaining a good sense of smell is linked to slower brain volume loss and cognitive decline in older adults.
The study found that for those who developed dementia or cognitive impairment, the connection between smell and changes in the brain and cognition might be “especially pronounced”.
This was based on a comparison of odour identification scores and brain MRI scans from 567 participants in the NIA’s Baltimore Longitudinal Study of Aging.
“Participants who developed cognitive impairment or dementia had worse odour identification scores than those who did not. Better odour identification scores were associated with slower loss of brain volume, particularly in the frontal and temporal regions – areas important for thinking and memory.”
They further explained: “Better scores were also associated with slower decline in memory, attention, processing speed, and sensorimotor integration skills over time. However, when data points after a diagnosis of cognitive impairment or dementia were excluded from analysis, the associations between sense of smell with brain volume and cognitive functioning were not as strong.
Smell and Parkinson’s disease
Anosmia, defined as the loss of smell or reduction of smell, is a common symptom experienced by people suffering from the disease. Although it’s somewhat of a “hidden” symptom, Parkinson’s UK reveals that up to 95% of people with Parkinson’s experience anosmia to some degree.
The charity highlights recent research which discovered that the ‘olfactory bulb’ – the part of the brain responsible for processing smell – is smaller in a group of people diagnosed with Parkinson’s disease.
Hyposmia, otherwise known as a reduced sense of smell, is “often an early sign of Parkinson’s”, according to the Parkinson’s Foundation.
Experts highlight a subtle but crucial link between smell loss and Parkinson’s disease, clarifying: “Not all people with reduced sense of smell will go on to develop Parkinson’s disease (PD), but most people with PD have some loss of their sense of smell. Looking back, you may realize you were losing your sense of smell several years, or even decades, before you received a Parkinson’s diagnosis.”