When mental health professionals look for signs of depression, they assess well-known signs such as altered sleep patterns, loss of appetite, disinterest in everyday activities, sadness, lack of energy, drug and alcohol abuse, difficulty concentrating, irritability, social withdrawal, suicidal thoughts, and other indicators that correlate with clinical depression.
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But a much less well-known phenomenon associated with depression is impairment of the sense of smell [1,2,6].
These impairments include elevated thresholds for detecting odors and difficulties identifying and differentiating different odors [2,6]. Impaired olfaction is called dysosmia, while complete loss of sense of smell is termed, anosmia.
What causes what?
As with many symptoms of mental illness, the determination of causes and effects is not straightforward when it comes to the strong correlation between dysosmia and depression.
There is mounting evidence that odors can directly affect mood [11], as when the smell of your favorite childhood cookies evokes fond memories. And a healthy sense of smell, and associated sense of taste, is important for enjoying life [3].
Thus, one school of thought is that a degraded sense of smell and taste can directly lead to depression. Some researchers [1] have even speculated that one reason the incidence of depression tends to increase with age is that sense of smell also degrades with age [1].
However, the association between olfaction and depression may be more correlative than causative, according to other researchers {2.4,8]. Olfaction and mood have common neural underpinnings in structures such as entorhinal cortex, hippocampus, amygdala, and orbito-prefrontal cortex [4]. Therefore, anatomical changes in these structures, such as atrophy associated with stress and excess corticosteroids could simultaneously affect mood and olfaction, without there being a direct causal link between the two.
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Respiratory Infections and allergies, which are a leading cause of degradation of the sense of smell, also bring inflammation, which has been strongly linked to mood disorders [12]. A recent study of COVID-19 patients who suffered an impaired sense of smell also demonstrated a higher incidence of depression in these patients [7].
Thus, both depression and olfactory dysfunction may arise from a third factor such as stress, inflammation, or anatomical changes in shared neural structures, and not be causally related to each other, one way or another.
And yet, some researchers speculate that depression might trigger olfactory deficits due to declines in cognitive and information processing abilities in depressed patients, instead of from direct decreases in the functioning of olfactory structures. Functional imaging studies of depressed patients with and without olfactory symptoms reveal very little correlation between depression and olfactory bulb volume [5], leading the authors of the study to conclude: “We are therefore in favor of a top-down mechanism originating in higher cortical areas explaining parts of the relation between depression and olfaction [5].”
Implications for diagnosis and treatment
It remains unclear whether an impaired sense of smell helps trigger depression, depression by itself produces olfactory deficits, factors such as inflammation and stress both contribute to dysosmia and depression, smell and mood disorders are mutually reciprocal—or some complex interactions among all of the above factors is responsible for the depression-olfaction connection.
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But even without knowing the causal linkage between olfaction and depression, the nexus of smell and mood could be important for both diagnosis and treatment of depression.
Adding olfactory function to the signs and symptoms employed to diagnose depression might facilitate earlier diagnosis in some patients or help determine the severity of the disorder. For instance, one longitudinal study of adults with depression, in which some members of the cohort did not have depression when the study began, but developed it during the study, provides evidence that in adults over 60, a decrease in olfactory function might be an early warning sign of depression [1]. Also, in older adults, the severity of depressive symptoms has been found to increase with the severity of dysosmia [13].
Regarding treatment for depression, therapies such as olfactory training (OT) and olfactory enhancement (OE), in which subjects are repeatedly exposed to a wide variety of odors and concentrations of those odors, have shown promise for relieving some symptoms of depression [9,10]. By themselves, these results don’t prove that dysosmia triggers depression, but the new findings do hint at a novel treatment modality for depression, especially when other treatment methods have not proven as effective as desired.
Depression Essential Reads
As a former sufferer of depression myself, I am encouraged by new insights emerging from the link between olfaction and depression, and I wish researchers in this emerging field more than just a whiff of success.
Source link : https://www.psychologytoday.com/za/blog/long-fuse-big-bang/202406/a-surprising-early-warning-sign-of-depression?amp
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Publish date : 2024-06-30 16:39:26
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