A recent study concludes that the loss of smell associated with COVID-19 may have links to an increased risk of depressed mood and anxiety. The authors hypothesize that this might be due to the virus interacting with the central nervous system (CNS).

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Does COVID-19 impact mental health by interacting with the CNS?

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The current pandemic is heavily impacting the mental health of healthcare workers and the public at large. However, scientists have conducted little research into the mental health impact on individuals with COVID-19.

Recently, a group of researchers set out to investigate this matter. They explain that “[w]hile the full impact of COVID-19 on patients’ emotional well-being has not yet been determined, it is reasonably hypothesized that mental health disorders will be a major challenge in survivors of COVID-19.”

The scientists set out to quantify the association of depressed mood and anxiety with COVID-19. They also wanted to identify any factors that have links to these changes in mood.

They explain how “it is reasonably hypothesized that mental health disorders will be a major challenge in survivors of COVID-19,” and, therefore, it is important to identify “associated factors that may modulate that burden.” Their findings appear in the journal The Laryngoscope.

To investigate, the researchers took data from an earlier study that focused on olfactory disturbances in people with COVID-19. In all, they had data for 114 people who had received diagnoses of COVID-19.

The “classic” symptoms of COVID-19 are fever, cough, and shortness of breath. However, researchers also note that a loss of smell and taste is a common feature of the disease.

Up to 80% of people with COVID-19 report olfactory disturbance, and around 1 in 4 people with COVID-19 report that loss of smell is the first symptom they experience.

Each participant completed questionnaires designed to quantify feelings of anxiety and depressed mood on the day of enrollment and how they had felt immediately before they developed the symptoms of COVID-19.

Participants also rated the severity of their symptoms, including loss of smell, shortness of breath, and fever. They rated their symptoms at disease onset, at their worst, and before they contracted COVID-19.

Researchers expected the results to indicate that contracting COVID-19 adversely impacts mood. The surprising finding was that loss of smell had the most significant associations with depressed mood and anxiety.

“We found that depressed mood and anxiety were positively associated with COVID-19 symptoms of decreased sense of smell and taste,” explain the authors, “In contrast, and surprisingly, depressed mood and anxiety were not associated with more symptoms of COVID-19, such as fever, cough, or shortness of breath, which may be harbingers of more dire COVID-19 outcomes.”

So, although one might expect mood to worsen for individuals who are finding it hard to breathe, only those who found it hard to smell experienced a significant drop in mood.

This, the authors hypothesize, might infer that SARS-CoV-2 affects mood by impacting the CNS. Previously, scientists have observed how the new coronavirus affects the CNS in terms of agitation, confusion, and seizures. Researchers have also detected SARS-CoV-2 in cerebrospinal fluid.

Although there is a mounting body of evidence to suggest that COVID-19 affects the CNS, there are still a lot of unanswered questions. With the current study, as the authors clarify, it is not possible to infer that COVID-19-induced loss of smell causes emotional disturbances.

Another important limitation of the study is its reliance on patient recall of symptoms and mood before the onset of COVID-19. Human memory is rarely perfect and liable to error, especially in times of trauma.

Finally, the researchers used subjective measures of olfactory dysfunction rather than an objective method. It is probable that participants over- or underestimated the level of dysfunction.

Perhaps the most significant problem with interpreting this study is that mood and olfaction are deeply linked. It is possible that the loss of smell, regardless of viral infection, caused the changes in mood.

In one mouse study, for instance, researchers knocked out the animals’ sense of smell. This increased depressive behavior and decreased anxiety-like behavior.

Another study involving older adults in Korea found that individuals “with severely impaired olfactory function showed a significantly higher degree of depression.”

To further increase the complexity, the relationship between depressed mood and loss of smell appears to be a two-way street. In one paper, the authors explain that depression might impair olfaction “as a consequence of reduced olfactory attention and diminished olfactory receptor turnover rates.”

The fact that a loss of smell appears to influence mood means that, from the current study, it is impossible to determine that any change in mood is due to SARS-CoV-2 influencing the CNS.

The study, if nothing else, brings home the complexity of COVID-19 and shows how much we still have to learn about this new disease. Because the mental health fallout from a pandemic is likely to involve a long battle, understanding the details of how this disease impacts every aspect of health is vital work.

As the authors explain in their paper, “detection of infected individuals is critical to stopping [the] spread of the disease, and thus it remains of great importance to continue to learn and understand the many disease manifestations of COVID-19.” No door should be left unopened.