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Mental health professionals typically look for well-documented signs like sleep disturbances, appetite changes, and social withdrawal when diagnosing depression. However, recent research has highlighted a lesser-known indicator: impaired sense of smell.

Termed dysosmia when smell is distorted and anosmia when it's completely lost, these olfactory impairments have been linked to depression, alongside more recognized symptoms like irritability and suicidal thoughts, as per Psychology Today.

The relationship between olfactory dysfunction and depression isn't straightforward. Some studies suggest that declines in smell and taste may directly contribute to mood disorders, potentially explaining higher depression rates among older adults as senses degrade. Others propose a shared neurological basis, where structural changes in brain regions like the hippocampus affect both mood and olfaction.

Research also points to external factors like respiratory infections and allergies, which can inflame nasal passages and impact both smell and mood. Notably, COVID-19 patients with impaired smell have shown elevated rates of depression, indicating a possible link between inflammation and mood disorders.

Despite these correlations, the exact causal relationship remains elusive. Functional imaging studies have shown limited direct correlation between depression severity and olfactory bulb volume, suggesting a complex interplay rather than a direct cause-effect scenario.

"We are therefore in favor of a top-down mechanism originating in higher cortical areas explaining parts of the relation between depression and olfaction," the authors of the study concluded.

Implications for diagnosis and treatment are significant. Integrating olfactory function assessments into depression screenings could aid in early detection, especially among older adults where declining smell may signal forthcoming depression.

Olfactory interventions such as olfactory training (OT) and enhancement (OE), involving exposure to varied odors, show promise in alleviating depression symptoms. While these therapies suggest potential benefits, the exact causal relationship between dysosmia and depression requires further exploration.