The nose does not always know: COVID-19 induced smell loss

Abstract

Anosmia and parosmia–both olfactory disorders–are commonly associated with COVID-19 infection. Anosmia refers to a complete, or quantitative, loss of smell, and can be caused by an obstruction to the olfactory cleft, damage to the olfactory epithelium, or a combination of the two. Meanwhile, parosmia is a qualitative smell distortion whose origin is not well understood. There are, however, two main hypotheses: the peripheral hypothesis and the central hypothesis. The peripheral hypothesis suggests that damaged olfactory sensory neurons and nasal stem cells cause olfactory dysfunction, and the central hypothesis posits that there is a neurotrophic effect on the olfactory epithelium which travels through the olfactory nerve into the central nervous system. Patients with parosmia seem to share similar trigger scents, and after much research, scientists have discovered that parosmia triggers are generally potent, have low odor thresholds, and belong to one of four different categories: thiols, trisubstituted pyrazines, methoxypyrazines, and disulfides. Unfortunately, there are very few effective treatment options for either one of the above smell disorders. Olfactory training, which takes advantage of the brain’s neuroplasticity, is effective in improving scent in both anosmia and parosmia patients. However, it often takes at least three months to see results. Platelet-rich plasma also seems promising in repairing damaged tissue and ultimately restoring normal olfaction. However, the claim that stellate ganglion block (SGB) helps treat parosmia is contrary to parosmia’s mechanism and has no proven clinical significance in treating the condition. Stellate ganglion block targets the sympathetic nervous system, and since parosmia has no sympathetic nervous system involvement, any improvement in parosmia symptoms post SGB is likely placebo. Further research should be done both in order to definitively determine whether or not SGB is an effective treatment for parosmia and to explore new treatment options for both parosmia and anosmia.