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Olfactory Health

Smelling is our ability to detect airborne volatile chemicals. If the ability to smell declines, it can affect everyday safety, diet and nutrition, physical well-being, and relationships. Olfactory tests assess odor processing pathways within the brain and provide insight into the health of those brain pathways and abilities in primary odor detection. This can be thought of as our OLFACTORY HEALTH.

Quick Facts

  • In Alzheimer’s disease, smell declines before memory problems occur.

  • Poor sense of smell predicts future development of mild cognitive impairment as well as conversion from mild impairment to dementia.

  • Smell testing offers a quick, easy, and inexpensive way to give people some reassurance, by telling them whether they are in a low- or high-risk category.

  • In Parkinson’s, smell declines before motor symptoms in most affected.

Why is Smell Important?

Smelling is our ability to detect airborne volatile chemicals. If the ability to smell declines, it can affect everyday safety (e.g., food spoilage, gas), diet and nutrition (e.g., less nutritious choices of higher salt and sugar which lead to other medical conditions, such as high blood pressure, kidney disease, or diabetes), and physical well-being, and relationships. Odors can influence many aspects of our lives, such as memory, mood, and emotions. You smell hormones and can even detect genetic complexity and these molecules we smell support sustained intimacy and relationships. Smell is needed in order to form close personal bonds.

This means that smell is a very important and often overlooked part of everyone’s lives.

Most importantly, smell is a measure of brain health and can be
used to screen for risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s long before other symptoms show up.
 

Impacts

Research tells us that our ability to smell impacts our daily lives as well as our overall well-being. It is also one of the best early indicators of dementia, Alzheimer’s disease, Parkinson’s disease and other neurodegenerative disorders. Improving public awareness of the connection between sense of smell and cognitive health is imperative for wellbeing and the early detection of disease so we can take steps towards solutions.

Why is Smell Important in Dementia?

Dementia is an umbrella term used to describe a decline in cognitive function that affects memory, language, problem-solving, and other mental abilities. There are several different types of dementia, the most common of which is Alzheimer's disease. There is currently no cure for dementia, and the exact causes are still not fully understood. However, early detection and diagnosis is crucial, as it can lead to earlier treatment and better outcomes (more information on dementia can be found here: Alzheimer's Society of BC).

 

The loss of the sense of smell is one of the first indications of neurodegeneration in the brain. The reason for this is that Alzheimer's pathology build up in a predictable way through the brain and the olfactory areas are one of the first to be impacted. One of the main pathological changes in Alzheimer's disease is the buildup of beta-amyloid plaques in the brain. These plaques can be found in the olfactory bulb and olfactory cortex, as well as other areas of the brain, and they are thought to disrupt the normal functioning of nerve cells. The accumulation of beta-amyloid plaques in the olfactory system is thought to lead to damage and loss of nerve cells, which can cause a decline in the sense of smell.

Another pathological change in Alzheimer's disease is the accumulation of tau protein in the brain. These tangles are found in the olfactory bulb and olfactory cortex, which are thought to disrupt the normal functioning of nerve cells, which leads to damage and loss of nerve cells, and a decline in the sense of smell and other functions.

 

Research has shown that the olfactory system is closely connected to other brain regions that are involved in memory and emotion, such as the hippocampus and the amygdala. As Alzheimer's disease progresses, it affects these regions as well, and this can lead to a decline in memory, as well as other cognitive and emotional symptoms.

Linking Smell and Memory

The olfactory system, which is responsible for the sense of smell, and the memory system are closely connected in the brain. The olfactory pathways, which include the olfactory bulb and the olfactory cortex, are responsible for processing and interpreting smells. These pathways are connected to several other brain regions that are involved in memory and emotion such as the hippocampus and the amygdala.

The hippocampus is a critical brain region for memory formation and consolidation. It plays a key role in the formation of new memories, particularly for events and experiences. Research has shown that the hippocampus is activated when people encounter new odors, and that the strength of this activation is related to the ability to remember the odor. The hippocampus is also thought to play a role in the formation of emotional memories, and the olfactory system is closely linked to emotional processing.

The amygdala, another brain region, is also closely connected to both the olfactory pathways and memory. The amygdala is involved in the processing of emotional information, including positive and negative emotions. It also plays a critical role in the formation of emotional memories, and it is activated when people encounter new odors.

The olfactory system and memory pathways are so closely linked that the olfactory system can act as a powerful trigger for memories. Further, the sense of smell is closely linked to emotion, and it is thought that the emotional content of a memory is related to the olfactory information associated with the memory.

These connections between memory and olfactory pathways in the brain have led researchers to investigate whether building and strengthening these neural connections in the brain have an impact on cognition. 

What is Olfactory Testing?

Olfactory testing, also known as olfactometry or smell testing, is the process of measuring a person's sense of smell. The goal of olfactory testing is to evaluate the function of the olfactory system, which is responsible for the sense of smell, and to identify any problems or impairments. Olfactory testing is typically used to diagnose and evaluate conditions such as anosmia (loss of the sense of smell), hyposmia (decreased sense of smell) and other olfactory disorders. It is also useful for evaluating the health of the brain pathways responsible for smell and memory and can help to detect early signs of neurodegeneration in the brain. Repeated testing is then used to monitor the effectiveness of treatment for these conditions, and to evaluate the potential impact of these conditions on a person's quality of life.

There are several different methods for olfactory testing, each with their own advantages and disadvantages, some of which include:

 

  • Sniffin' Sticks: This test uses small plastic sticks that contain different odors. The person being tested is asked to identify the odor, or to match the odor to a list of options.

  • Odor Threshold Test: This test uses a series of increasingly dilute solutions of a particular odor. The person being tested is asked to identify the odor at each concentration, and the lowest concentration at which the odor is still detected is used to determine the person's threshold.

  •  Odor Discrimination Test: This test uses two or more different odors and the person being tested is asked to identify the odors and discriminate between them.

  • Odor Identification Test: This test uses a series of different odors, and the person being tested is asked to identify each odor, either by naming it or selecting it from a list of options.

  • Scratch and Sniff test: this test uses small cards with different odors, and the person being tested is asked to scratch and sniff the card and identify the odors.


The results of olfactory testing are usually reported as a score, which can be used to compare the person's sense of smell to normal values. Olfactory testing is used to diagnose and evaluate conditions such as anosmia (loss of the sense of smell), hyposmia (decreased sense of smell), and other olfactory disorders. It is also used to monitor the effectiveness of treatment for these conditions, and to evaluate the potential impact of these conditions on a person's quality of life.

What is Olfactory Training?

Olfactory training is a form of therapy that aims to improve a person's sense of smell. The training  involves smelling different odors and identifying them, in order to help the person better recognize and distinguish different scents. This type of therapy is often used to help people who have lost their sense of smell due to injury or illness, such as those with anosmia (a loss of the sense of smell) or hyposmia (a decreased sense of smell).

There have been several studies that have investigated the cognitive effects of olfactory training. One study found that olfactory training improved the ability of older adults to identify and distinguish odors, as well as their overall cognitive function. Another study found that olfactory training led to improvements in memory and attention in people with mild cognitive impairment.

Some researchers also think that olfactory training may help to delay the onset of age-related cognitive decline and other neurodegenerative diseases. For example, some studies have found that people with Alzheimer's disease have a decline in their sense of smell earlier than the decline of other cognitive abilities. It is hypothesized that olfactory training may help to slow the progression of these diseases by preserving the sense of smell, which is thought to be linked to other cognitive functions.

It's worth noting that the research on olfactory training and cognitive effects is still emerging, fur
ther research is needed to understand the underlying mechanisms and the best ways to implement olfactory training as a therapy. However, olfactory training is considered a promising approach for helping individuals with olfactory dysfunction. The treatment has potential to improve quality of life, it's also non-invasive and relatively low-cost. In addition, it's thought to have potential as a preventative measure to protect against age-related decline and neurodegenerative diseases.

Research Articles

1 / Olfaction and impacts on the Brain

It is well known in the research community that the olfactory system can be successfully rehabilitated with regular, intermittent stimulation during multiple daily exposures to selected sets of odors, i.e., olfactory training. Olfactory training has been repeatedly shown to be an effective tool of olfactory performance enhancement but many have thought that its beneficial effects exceed olfaction and extend to specific cognitive tasks. So far, studies on olfactory training provided compelling evidence for its effectiveness, but there was still a need to search for an optimal training protocol. The study linked to below examined whether increased frequency of olfacotyr leads to better outcomes in both olfactory and cognitive domains.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8051546/

2 / Effects of COVID-19 on Olfaction

Reduced olfactory function is the symptom with the highest prevalence COVID-19 with nearly 70% of infected individuals experiencing partial or total loss of their sense of smell at some point during the disease. The exact cause is not known, but beyond peripheral damage, studies have demonstrated insults to both the olfactory bulb and central olfactory brain areas.

The following article suggests that COVID-19 might cause long-term reduction in olfactory bulb volume and altered functional
connectivity.

Link to article

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