Smell Loss After COVID19 Infection is a Metabolic Pathology
Smell loss after COVID19 infection is usually transient in most patients. However, in about one third of patients it becomes permanent with loss of ability to smell most odors and loss of ability to obtain flavor from most foods.
Most physicians attribute this change to an abnormality related to neural function and call these changes anosmia (total smell loss) and ageusia (total taste loss). This formulation is incorrect.
First, even if the smell loss after COVID19 is permanent, it is a relative loss of smell defined as hyposmia, not a total loss of smell, defined as anosmia. Similarly, the loss of taste is also relative and is called hypogeusia, not ageusia, which is a total loss of taste.
This relative loss of smell and taste is more consistent with a metabolic rather than a neural change. Indeed, we know that the growth factors which stimulate stem cells in both the smell and taste systems are inhibited by COVID19 infection and this effect on stem cells is metabolic in nature and does not directly affect smell function.
These metabolic effects can be corrected by activation of these growth factors by pharmacological means mainly through the action of phosphodiesterase inhibitors which can increase secretion of these growth factors and restore smell and taste function to or toward normal.