Using the skills of a “super smeller,” researchers have identified how Parkinson’s disease alters the way a person smells. They hope that the discovery will help aid early diagnosis.
Older adult holding hands
A recent study investigated the links between odor and Parkinson’s.

Parkinson’s is a progressive neurodegenerative condition.

The National Institutes of Health (NIH) estimate that around half a million people in the United States are living with the condition.

Parkinson’s disease tends to affect older adults.

The population of the United States is growing older, so the number of Parkinson’s cases is likely to increase in line.

Despite decades of intense research, there is still no cure for the condition, and there is no reliable diagnostic test.

The need for new diagnostics

Currently, treatment cannot begin until the telltale motor signs, such as tremor and rigidity, appear. However, the breakdown of nervous tissue starts around 6 years before people notice any clinical signs.

Finding a reliable way of diagnosing Parkinson’s disease earlier would mean that treatment could begin sooner and, perhaps, that we could keep the condition at bay for longer.

Doctors have used odor to aid their diagnoses for centuries. For instance, some people claim that scrofula smells similar to stale beer, while typhoid fever produces an aroma reminiscent of baked bread.

However, until recently, no odor had been associated with a neurodegenerative condition of any kind.

Introducing Joy Milne

Joy Milne is a “super smeller.” These people are hypersensitive to aromas and particularly adept at discriminating between them. In 1986, doctors diagnosed Parkinson’s disease in Milne’s husband, Les. Since then, Milne has been able to distinguish a specific odor associated with Parkinson’s.

Recently, a group of researchers teamed up with Milne to try and distinguish what chemicals might be causing this particular odor. They recently published their findings in the journal ACS Central Science.

Firstly, the scientists needed to pin down where the smell originated. They noticed that it was most intense on the upper back and forehead but not the armpits. This means that the odor is probably not from sweat but from sebum, which is a waxy liquid that sebaceous glands in the skin create.

Scientists already know that the production of sebum increases in Parkinson’s diesease; they call this seborrhea. Led by Perdita Barran, the researchers set about trying to understand what chemicals in the sebum might be causing the changes in odor.

Next, the team collected sebum samples from the upper backs of 60 individuals. Some people had Parkinson’s disease and some did not.

A ‘musky’ aroma

Using mass spectrometry, the scientists analyzed the sebum samples to identify any chemicals that were elevated in people with Parkinson’s disease. They demonstrated that there was a significant difference between the volatile chemicals in the sebum of people with Parkinson’s disease and those without.

Three compounds seemed to play a key role in the distinct aroma: hippuric acid, eicosane, and octadecanal.

Crucially, there were no significant differences between people with Parkinson’s who were taking medication and people with Parkinson’s who had never taken medication for the condition. This means that the change in odor is probably not due to medication.

When the team presented these chemicals to Milne, she was able to identify the “musky” aroma of Parkinson’s disease.

The scientists carried out this study using a limited number of participants, so they will need to continue their work. However, they are hopeful that this might be a unique way of detecting Parkinson’s much earlier than is currently possible. They write:

Identification and quantification of the compounds that are associated with this distinctive [Parkinson’s disease] odor could enable rapid, early screening of [Parkinson’s disease] as well as provide insights into molecular changes that occur as the disease progresses.”

Why the change in smell?

The researchers did not design their study to find out why hippuric acid, eicosane, and octadecanal levels are elevated in the sebum of people with Parkinson’s disease. However, the authors discuss some possible causes.

For instance, earlier studies had confirmed that there are links between various skin conditions and Parkinson’s disease. The authors explain how some research suggests that certain microbes are more common on the skin of people with Parkinson’s.

Malassezia spp. — a yeast present on human skin — often appears in increased amounts in people with Parkinson’s.

According to the study authors, these changes in yeast and bacterial populations may alter skin microflora and physiology in ways that are “highly specific” to Parkinson’s disease.

These findings open the door to an entirely new way of approaching the diagnosis of Parkinson’s disease; they might also offer fresh insight into how the condition progresses.