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The Unexpected Symptoms That May Warn of Alzheimer’s Looming After Diminishing Sense of Smell

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Diminishing sense of smell influences many aspects of our health – and this may be the one that puts our life at greater danger of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease, as well as early death.

Hyposmia refers to a reduction in the sense of smell or a diminished ability to detect odors through the nose. On the other hand, anosmia denotes a complete inability to perceive any smells.

While hyposmia is relatively uncommon, anosmia has been particularly prevalent during the COVID-19 pandemic.

The sense of smell holds immense significance in our lives as it allows us to savor new experiences, recall past memories, and even detect potentially hazardous scents like smoke or spoiled food.

Loss of smell should be considered a medical condition that requires evaluation and treatment by an ENT (ear, nose, and throat) specialist, also known as an otolaryngologist.

However, hyposmia is a more severe condition with longstanding associations with Alzheimer’s disease and other forms of dementia in older individuals. Recent research further reinforces its connection to various ailments experienced in later stages of life.

In a study spanning eight years and involving over 2,000 older adults residing in the community, researchers at Johns Hopkins Medicine have uncovered compelling evidence linking a decreased sense of smell to an increased risk of developing late-life depression.

Depression is highly prevalent among individuals with Alzheimer’s, particularly during the initial and middle stages. It is estimated that up to 40 percent of people with Alzheimer’s disease experience significant depressive symptoms.

While their findings, published on June 26 in the Journal of Gerontology: Medical Sciences, do not establish a causal relationship between the loss of smell and depression, they do suggest that hyposmia may serve as a powerful indicator of overall health and well-being.

“We’ve seen repeatedly that a poor sense of smell can be an early warning sign of neurodegenerative diseases such as Alzheimer’s disease and Parkinson’s disease, as well as a mortality risk,” comments Dr. Vidya Kamath from the Johns Hopkins University School of Medicine.

“This study underscores its association with depressive symptoms. Additionally, this study explores factors that might influence the relationship between olfaction and depression, including poor cognition and inflammation.”

The study utilized data obtained from the Health, Aging and Body Composition Study (Health ABC), a federal government study comprising 2,125 participants. These individuals were initially aged between 70 and 73 when the eight-year study commenced in 1997-98. At the beginning of the research, the participants did not experience any difficulties in walking a quarter of a mile, climbing ten steps, or performing regular activities. They underwent annual in-person assessments and six-month phone evaluations. The assessments included tests to measure their ability to detect specific odors, evaluate depression levels, and assess mobility.

During the first measurement of smell in 1999, it was observed that 48% of the participants had a normal sense of smell, 28% exhibited a reduced sense of smell (hyposmia), and 24% suffered from a complete loss of smell (anosmia). Participants with a stronger sense of smell tended to be younger compared to those reporting significant loss or hyposmia. Throughout the follow-up period, 25% of the participants developed notable depressive symptoms. Further analysis revealed that individuals with decreased or complete loss of smell had a higher risk of developing significant depressive symptoms during the follow-up period, compared to those in the normal olfaction group. It was also observed that participants with a better sense of smell tended to be younger than those experiencing significant loss or hyposmia.

The researchers identified three distinct patterns of depressive symptoms within the study group: stable low, stable moderate, and stable high depressive symptoms. A weaker sense of smell was associated with a greater likelihood of falling into the moderate or high depressive symptoms groups. In other words, individuals with a more impaired sense of smell exhibited higher levels of depressive symptoms. These findings remained consistent even after adjusting for factors such as age, income, lifestyle, health conditions, and the use of antidepressant medication.

“Losing your sense of smell influences many aspects of our health and behavior, such as sensing spoiled food or noxious gas, and eating enjoyment,” remarks the researcher.

“Now we can see that it may also be an important vulnerability indicator of something in your health gone awry. Smell is an important way to engage with the world around us, and this study shows it may be a warning sign for late-life depression.”

The human sense of smell is one of the two chemical senses we possess. It operates through specialized olfactory neurons located in the nasal cavity. These neurons contain odor receptors that detect molecules released by substances in our surroundings. The information gathered by these receptors is then transmitted to the brain for interpretation. The strength of a smell depends on the concentration of these odor molecules, and different combinations of molecules create distinct sensory experiences.

The processing of smell occurs in the brain’s olfactory bulb, which is closely connected to other brain structures like the amygdala, hippocampus, and others responsible for memory, decision-making, and emotional responses.

According to the researchers at Johns Hopkins, their study indicates a potential link between olfaction and depression, involving both biological factors (such as altered serotonin levels and changes in brain volume) and behavioral mechanisms (such as reduced social functioning and appetite).

To further investigate this connection, the researchers plan to replicate their findings in additional groups of older adults. They aim to examine any alterations in the olfactory bulbs of individuals diagnosed with depression, determining whether this system is indeed affected. Additionally, they intend to explore the potential use of smell in intervention strategies aimed at reducing the risk of late-life depression.

Image Credit: Shutterstock

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