# Can Dementia Cause Loss of Smell Sensitivity
The relationship between dementia and the sense of smell is one of the most fascinating discoveries in modern neuroscience. For years, scientists have known that memory loss and confusion are the hallmark signs of dementia, but recent research has revealed something surprising: the loss of smell might actually come before these more obvious symptoms appear. This discovery is changing how doctors think about detecting dementia early and could potentially save lives by catching the disease in its earliest stages.
## Understanding the Connection Between Smell and the Brain
To understand why dementia affects smell, we first need to know how smell works in the brain. When you smell something, odor molecules enter your nose and trigger special cells called olfactory receptors. These receptors send signals directly to a part of your brain called the olfactory bulb, which is located deep inside your brain near areas that control memory and emotion. This is why smells can bring back vivid memories so powerfully – the smell system is directly connected to the memory centers of your brain.[1]
The olfactory bulb is not just a simple relay station for smell information. It is deeply connected to other brain regions that are crucial for thinking, remembering, and processing emotions. This direct connection between smell and memory is why researchers have become so interested in studying what happens to smell when dementia develops.
## How Dementia Damages the Smell System
Recent research has shown that dementia, particularly Alzheimer’s disease, damages the brain in ways that directly affect the sense of smell. One of the most important discoveries came from a study published in Nature Communications, which revealed that Alzheimer’s disease often starts with non-memory symptoms, and one of the earliest signs is a reduced sense of smell.[2]
The damage begins in a small but critical brain region called the locus coeruleus, which produces a chemical messenger called noradrenaline. This chemical is extremely important for processing smells and sending smell signals throughout the brain. In people with early-stage Alzheimer’s, the nerve fibers that carry signals from the locus coeruleus to the olfactory bulb – the brain’s smell center – are lost before any of the typical signs of Alzheimer’s appear.[2]
What makes this discovery even more important is that this nerve damage happens before amyloid plaques, which are the hallmark protein clumps associated with Alzheimer’s disease, even begin to form in the brain. This means that smell loss could be an even earlier warning sign than scientists previously thought possible.
## The Timeline of Smell Loss in Dementia
One of the most significant findings from recent research is that smell loss does not happen at the same time as memory loss in dementia. Instead, it comes much earlier. A relevant number of studies suggest that olfactory identification impairment – which means difficulty recognizing and identifying smells – may predict the progression of cognitive decline from normal aging to mild cognitive impairment and eventually to dementia.[1]
This timeline is crucial because it means that if doctors could test a person’s sense of smell early enough, they might be able to identify Alzheimer’s disease before the person even realizes they have a problem. The severity of smell loss has been associated with the rate of cognitive decline, meaning that people who lose their sense of smell more severely may experience faster mental decline.[1]
Researchers have found that the connection between smell loss and cognitive decline is particularly relevant in people with inflammatory upper respiratory diseases, including conditions related to COVID-19 and chronic rhinosinusitis. These conditions can damage the olfactory system and may accelerate cognitive decline in vulnerable people.[1]
## Why This Matters for Early Detection
The discovery that smell loss precedes memory loss in dementia is revolutionary for medical diagnosis. Currently, doctors typically diagnose Alzheimer’s disease only after a person has already experienced significant memory problems and cognitive decline. By that time, substantial damage has already occurred in the brain, and treatment options are limited.
However, if doctors could identify smell loss as an early warning sign, they could potentially catch Alzheimer’s disease much earlier, when interventions might be more effective. The study authors suggest that testing a person’s sense of smell, along with brain imaging, could help diagnose the disease much earlier than current methods allow.[2]
A study by the National Institute on Aging in the United States found that a decline in sense of smell was connected to a faster build-up of Alzheimer’s proteins in the brain.[5] This finding suggests that smell loss is not just a symptom of dementia but may actually be linked to the underlying biological processes that cause the disease.
## The Mechanism Behind Smell Loss in Dementia
Understanding exactly how dementia causes smell loss requires looking at the specific brain chemistry involved. The locus coeruleus, the brain region where damage begins in Alzheimer’s disease, is responsible for producing noradrenaline throughout the brain. This chemical is essential for many brain functions, including attention, memory, and the processing of sensory information like smell.
When Alzheimer’s disease damages the locus coeruleus, the production of noradrenaline decreases. This means that the signals carrying smell information from the nose to the brain become weaker and less organized. The nerve fibers that normally carry these signals degenerate and die, leaving the brain unable to properly process smell information.
In research conducted on mice with early-stage Alzheimer’s, scientists found that when they blocked this whole process of nerve fiber loss, both the nerve fibers and the animals’ sense of smell were preserved.[2] This finding suggests that the damage to the noradrenaline system is the key mechanism behind smell loss in Alzheimer’s disease, and it also suggests that future treatments might be able to prevent or reverse this damage.
## The Broader Impact of Smell Loss in Dementia
While the connection between smell loss and Alzheimer’s disease has received the most research attention, smell loss is also associated with other forms of dementia and cognitive decline. The relationship between olfactory dysfunction and cognitive decline is relevant across multiple types of dementia, not just Alzheimer’s disease.[1]
Smell loss can have profound effects on a person’s quality of life, even beyond the cognitive symptoms of dementia. The sense of smell is crucial for enjoying food, detecting danger like smoke or gas leaks, and maintaining social connections through the enjoyment of pleasant scents. When someone loses their sense of smell, they often lose their appetite, which can lead to weight loss and malnutrition. They may also become isolated because eating and socializing become less enjoyable.
For people with dementia, the combination of cognitive decline and smell loss can be particularly devastating. They may forget to eat because they cannot smell food, leading to further health problems. They may also become more anxious or depressed because they lose the comfort that familiar smells once provided.
## Current Research and Future Possibilities
The scientific world is still working to understand the full
