Anosmia (the inability to smell) and hyposmia (a decreased ability to smell) describe the range of olfactory dysfunction or smell disorders. The ability to smell is a complex process involving the nose and brain. When air passes into the nose, odor molecules bind to the receptors of olfactory nerves. These nerves are found in a specialized lining at the top of the nasal cavity called the olfactory epithelium. The stimulation of olfactory nerves causes them to transmit a signal to the brain, where it is processed into a scent that a person can recognize and identify.
Temporary loss of smell, or anosmia, is the main neurological symptom and one of the earliest and most commonly reported indicators of COVID-19. Studies suggest it better predicts the disease than other well-known symptoms such as fever and cough, but the underlying mechanisms for loss of smell in patients with COVID-19 have been unclear. Now, an international team of researchers led by neuroscientists at Harvard Medical School has identified the olfactory cell types in the upper nasal cavity most vulnerable to infection by SARS-CoV-2, the virus that causes COVID-19.
How does the virus attack the sense of smell? Most scientists agree that SARS-CoV-2, like the earlier known SARS-CoV, uses the angiotensin-converting enzyme 2 (ACE2) receptor to gain entry to cells via binding with spike protein. SARS-CoV-2 additionally appears to need TMPRSS2, a protease, to help prime the spike protein in the process of gaining entry to cells and a few other proteins. This means that cells must express all of these proteins for the virus to be able to infiltrate them and hijack their machinery to replicate.
Inflammation in the sensory epithelium could restrict airflow to the relatively small olfactory cleft, high up in the nose, without causing the feeling of a stuffy nose or interruption of breathing, as demonstrated in a published case report. Damage to the sustentacular cells in the SE could also affect the functioning of the sensory neurons in many ways (metabolic, structural, inflammatory), so that even if odors can get to the neurons, they may not be able to transmit signals. So, this is the discussion of articles from KMC Clinic this week which we extract from several sources. In this pandemic situation, keep thinking positive, and stay safe, stay healthy.