A subtle sign could predict Alzheimer’s or Parkinson’s years in advance

In a magical scene towards the end of 2007 film Ratatouille, notoriously harsh food critic Anton Ego is instantly transported back in time, his childhood memories revived by a spoonful of the film’s eponymous dish. No pictures were needed – just the taste and scent of the food.

Smell is the quietest of our senses, but also one of the most powerful. Indeed, it can even open the floodgates to memories we thought long forgotten.

It is also the most personal and subjective sense, which is what makes it so hard for people to agree on what smells good. However, there does seem to be consensus on the worst smell in the world.

In 1889, in a German laboratory, a chemical reaction involving a compound called thioacetone produced a stink so terrible that it caused people to vomit and faint half a kilometre away, forcing the partial evacuation of the city of Freiburg. We still don’t know what exact reaction caused this stench, but it seems nobody is in a hurry to find out.

But what happens when our smell starts to fail? And what if losing smell were more than just a passing symptom?

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A shortcut to our emotions

Smell is an evolutionary advantage: it warns us of invisible dangers and puts us on alert. Scent can also influence our decisions, and big brands are well aware of this, perfuming their shops with fragrances that tap into our emotions and invite us to stay.

Smells have the ability to awaken memories and intense emotions, and the olfactory bulb is responsible for this. This small area of the forebrain, located very close to the nose, receives olfactory signals and sends them directly to the areas of the brain that manage our memory and emotions.

Despite its importance, smell remains the least well-understood of the senses, and is often underestimated. When it fades, it usually goes unnoticed, but we may not realise how important it is until we lose it.

This is what happened to Michele Crippa, a renowned Italian “super taster” who lost his sense of smell during the pandemic. Although he regained it weeks later, his personal nightmare had only just begun, as when his sense of smell came back, it was distorted. Oranges smelled like burnt plastic, peaches smelled like basil, and vanilla made him feel sick. This was likely because the neurons in his olfactory bulb had been damaged.

While any loss of smell is unpleasant, it could have a greater significance: a warning signal from deep with our brains.

A common cold, or Parkinson’s?

Most of us have lost our sense of smell at some point, usually thanks to a simple bout of cold or flu. However, this symptom can also be an early symptom of neurodegenerative diseases like Alzheimer’s or Parkinson’s. This has been known for some time, but the surprising thing is that the loss of smell occurs years before the symptoms of these diseases appear.

So, could loss of smell be used to predict Parkinson’s disease? The answer, unhelpful though it may be, is: it depends.

An early warning

One of the major problems with neurodegenerative diseases is that the damage is already very advanced by the time they can be diagnosed. In the case of Parkinson’s disease, when the first symptoms (stiffness, tremors, and so on) appear, more than half of the neurons that produce dopamine, the neurotransmitter that controls movement, have already been lost.

Identifying early symptoms like loss of smell – which affects up to 90% of patients – could serve as a biomarker, alerting us to the presence of the disease. This would allow us to diagnose it much earlier, and provide access to more effective treatments.

The problem is that this symptom is not exclusive to Parkinson’s: it can also appear with ageing, stress, or other conditions. This means we tend to downplay its importance.

We still do not know for certain why neurodegenerative diseases cause loss of smell, although we have some clues. In some Parkinson’s patients, the disease may begin in the olfactory bulb long before spreading to the areas that control movement. This is because certain viruses, pesticides or toxins that we inhale could damage it and cause alterations to the area.

In the case of Alzheimer’s disease, damage could begin in a tiny bluish region of the brain stem called the locus coeruleus, as a recent study has revealed. This “alert button” keeps us awake and focused, and its connection to the olfactory bulb is what links smells to emotions. When that connection is broken, problems occur with smell long before the first signs of dementia emerge.

In short, the loss of the ability to smell would not be a symptom of the disease itself, but rather a warning sign that the degenerative process has begun.

Diagnosing through smell

When a patient comes into the clinic, it is not always easy to distinguish between Parkinson’s disease and other similar movement disorders. A loss of smell, combined with other tests and indicators, could help confirm the diagnosis. It could also help us predict the disease’s progression, as it is related to more severe forms of the disease.

Furthermore, the loss of smell in Parkinson’s disease is selective. Patients perceive pleasant smells such as chocolate without issue, but have difficulty detecting neutral or unpleasant odours like soap, smoke or rubber.

Other patients, especially women, experience something even stranger: olfactory hallucinations. This means they perceive “phantom” odours, such as tobacco or burning wood, which are not really there.

Incredible though it may seem, Parkinson’s disease even has its own scent, which has been described as woody and musky. We know this thanks to Joy Milne, a Scottish woman with a heightened sense of smell – she was able to recognise this particular odour on her husband 12 years before he was diagnosed with the disease.

The loss of smell may seem like like something confined to the nose, but it is actually a window into the brain. It allows researchers to peek inside the brain to decipher its secrets, and to gather valuable information that will help us care for and improve the quality of life of those who suffer from neurodegenerative diseases.

This article is republished from The Conversation under a Creative Commons license. Read the original article.