A literature review published in the Journal of Affective Disorders failed to draw a solid conclusion about the role of olfactory deficits as a marker in bipolar disorder (BD) because of the incompleteness of evidence on BD and olfaction. Olfactory dysfunction is a marker for several neuropsychiatric disorders, including Parkinson disease, Alzheimer disease, and schizophrenia. Studies have found associations between olfactory impairment and depressive symptoms, and animal studies have noted that ablation of olfactory bulbs in animals induces a depressive phenotype.
Chantal Henry, MD, of the Unité Perception et Mémoire, Institut Pasteur, Université de Paris, and the Department of Psychiatry, Service Hospitalo-Universitaire, Paris, France, and colleagues conducted a systematic literature review using PRISMA guidelines. They included all studies assessing olfaction with objective measures in patients with BD. All 9 studies identified assessed odor identification, with 3 reporting deficits primarily in patients with psychotic features or elements of illness severity, such as cognitive impairment or high score on the Brief Psychiatric Rating Scale, in comparison with healthy subjects.
The researchers found no difference in threshold of perception between BD patients and controls. Among 7 studies comparing olfactory function in patients with BD and healthy controls, 4 studies found no deficits in odor identification. While 4 studies assessed olfaction during euthymic phases, another 4 did not report mood status and only 1 study assessed olfaction in depressed patients.
There were also no differences related to discrimination of odors. However, 1 study reported that BD patients with psychotic features rated pleasant odors as less pleasant than controls did, whereas another study of euthymic BD patients found the opposite association. A study that assessed olfaction during depressive episodes found no difference between patients with BD and controls.
Overall, the researchers determined that there is not sufficient evidence to claim olfactory features as markers of BD. The review was limited by the small number of patients included in the studies.
The researchers suggested that “it should be clarified whether the identification deficits are related to a history of psychotic symptoms or to an overall higher severity of the disease or to a cognitive decline” in BD. They noted that impaired identification in Parkinson disease is correlated with mild cognitive impairment, adding that “it would be interesting to know if deficits in [odor] identification could be a marker of neuro-progression of the disease and if changes in identification could represent early signs of cognitive impairment” for patients with BD.
Henry C, Meyrel M, Bigot M, Alonso M, Lledo P-M, Dargel AA. Can olfactory dysfunction be a marker of trait or states of bipolar disorders? A comprehensive review. J Affect Disord. 2020. doi. org/10.1016/j.jad.2020.01.081.