Parosmia

Parosmia refers to an olfactory dysfunction characterized by a distorted sense of smell in which smells that were previously enjoyable may now be deemed foul. It is often experienced following a viral infection. Olfactory dysfunctions are categorized into quantitative (e.g., anosmia and hyposmia) disorders, which measure an objective variation in the ability to smell, and qualitative (e.g., parosmia and phantosmia) disorders, which describe a subjective change to smell quality.  Anosmia describes the complete loss of the sense of smell, and hyposmia occurs when there is a decreased sense of smell. Comparatively, phantosmia occurs when an individual may be able to detect an odor that is not present. 

 In general, smell is sensed through the olfactory bulb, which resides in the nasal cavity on the anterior surface of the frontal lobe (i.e., lobe of the brain that controls behavior and emotion). Neurons of the olfactory bulb send signals to the primary olfactory cortex within the cerebral hemispheres (i.e., the outermost portion of the brain). Damage to the olfactory bulb or disruption along this neuronal pathway may cause parosmia.

Parosmia may occur after infection with the novel coronavirus (i.e., COVID-19). Although the mechanism is not completely clear, parosmia after COVID-19 is thought to be due to damage to the neuroepithelium (i.e., the stem cells that become neurons) and olfactory bulb (i.e., the nerves responsible for transmitting smell) through both viral injury and continuous inflammation. Studies hypothesize that this neuronal death causes an alteration of receptor-mediated targets, thereby altering the ability to properly classify odors.