Dec 22, 2025
3 min read
Losing a sense of smell can be an early warning bell for Parkinson and Alzheimer's disease, as this sign appears years before diagnosis.Research reveals that a decline in odor is associated with damage to sensitive brain areas, making this sense a window to understand neurodegenerative diseases.
Loss of smell can be an early warning sign for Parkinson's and Alzheimer's, as this symptom appears many years before the disease is diagnosed.Research shows that loss of smell is linked to damage to sensory brain regions, making this a window into understanding neurodegenerative diseases.
In a magical scene at the end of the 2007 film Ratatouille, infamous food critic Anton Ego is instantly transported back in time as one spoonful of the film's eponymous dish brings back childhood memories.No pictures needed, just the taste and smell of the food.
The sense of smell is the quietest of our senses, but it is also one of the most powerful. It can even open the door to memories that we thought were long forgotten.
It is also the most unique and subjective idea, which is difficult to accept in good fragrances.However, this seems to be the idea of the worst smell in the world.
In 1889, in a German laboratory, a chemical reaction with a compound called thioacetone caused a stench that caused people to vomit and faint half a kilometer away, forcing the evacuation of part of the city of Freiburg.
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We still don't know the exact reaction that caused this stench, and no one seemed to want to know why.
But what happens when our sense of smell starts to deteriorate?What if the loss of smell is more than just a passing symptom?
A shortcut that touches our emotions
The sense of smell is an evolutionary advantage that warns and alarms us of unseen dangers.Smell can also affect our decisions, and the big brands are aware of this, so they invite us to perfume their stores with scents that appeal to our emotions.
Smells have the power to evoke strong memories and emotions, and the key to this is the "olfactory bulb". This small area of the forebrain, very close to the nose, receives smell signals and sends them directly to the parts of the brain that control our memories and emotions.
Despite the importance of this meaning, it remains the least understood and often underestimated. When they disappear, it usually goes unnoticed because we can only realize their importance after they are gone.
This is what happened to the famous Italian supertaster Michele Creppa, who lost his sense of smell during the pandemic.Although he regained it weeks later, his personal nightmare had only just begun, as his sense of smell returned distorted.Oranges smell like burnt plastic, peaches like basil and vanilla made him sick.This was likely the result of damage to the neurons in the olfactory bulb.
Although no loss of smell is troubling, it may have more meaning than a warning signal comes from deep inside our brains.
A common cold or Parkinson's disease?
Most people have lost their sense of smell at some point, usually from the common cold or flu.But this sign can also be the first sign of neurodegenerative diseases such as Alzheimer's or Parkinson's disease.Although this has been known for some time, it is surprising that the loss of smell occurs years before the symptoms of these diseases.
So, can loss of smell reliably predict Parkinson's disease?The answer, although not conclusive, is: it depends on the situation.
A major conundrum of neurodegenerative diseases is that by the time they are diagnosed, the damage is so advanced.In the case of Parkinson's disease, when the first symptoms appear (stiffness, tremors, etc.), more than half of the neurons that produce dopamine, the neurotransmitter responsible for controlling movement, have already been lost.
Recognizing early symptoms, such as loss of smell, which affects up to 90 percent of patients, can be an important indicator of the presence of the disease.This allows us to detect it early and provide effective treatment.
The problem is that these symptoms are not limited to Parkinson's disease, but can also occur with age, stress or other conditions.This means that we often underestimate its importance.
We still don't know for sure why neurodegenerative diseases damage the sense of smell, although we do have some clues.In some cases of Parkinson's disease, the disease may begin in the auditory bulb before it spreads to areas that control movement.This is because some viruses, pesticides, or toxins that we swallow can damage this area and cause changes in it.
In the case of Alzheimer's disease, the damage may begin in a small blue area inside the brainstem called the locus coeruleus, according to a recent study.This area acts as an alarm button to keep us awake and focused, and its connection to the olfactory bulb is what links smells to emotions.When that connection is broken, problems with smell occur long before the first signs of dementia appear.
In other words, losing the ability to smell is not a symptom of the disease itself, but rather a warning sign that the deterioration process has begun.
Diagnosis by nesting
When a patient comes to the clinic, it is not always easy to distinguish between Parkinson's disease and other similar movement disorders. Therefore, in combination with tests and other indicators, the loss of the sense of smell can confirm the diagnosis.It can also help predict the course of the disease, as it is associated with more severe forms.
In addition, the loss of the sense of smell in Parkinson's disease is selective: the patients perceive pleasant odors such as chocolate without any problem, but have difficulty identifying neutral or unpleasant odors such as soap, smoke or rubber.
Other patients, mainly women, suffer from a more peculiar disease: olfactory hallucinations, which cause them to smell imaginary odors such as tobacco or burning wood, even though they are not there.
ومهما بدا أن الأمر لا يصدق، فإن لمرض باركنسون رائحته الخاصة، التي وصفت بأنها تشبه رائحة الخشب والمسك. ونعرف ذلك بفضل جوي ميلن، السيدة الأسكتلندية التي تتمتع بحاسة شم فائقة، إذ استطاعت تمييز هذه الرائحة بالتحديد لدى زوجها قبل 12 عاماً من تشخيص إصابته بالمرض.
Perhaps the loss of the sense of smell is limited to the nose, but it is a window to the brain. It allows researchers to look inside the brain to unlock its secrets and collect important information that will help us to care and improve the quality of life of those suffering from neurodegenerative diseases.
Janet Rodríguez Pallares is Professor of Human and Embryonic Anatomy at the University of Santiago de Compostela.This article is reprinted from The Conversation under a Creative Commons license.
