Covid-19 causes loss of smell: upper respiratory tract viruses, and conditions like traumatic brain injury, rhinosinusitis, Alzheimer’s, and Parkinson’s disease, can also cause it.
Apart from fever, headache, malaise, cough, and shortness of breath, several patients with Covid-19 report loss of smell or taste, either a continuum or an isolated symptom in an otherwise asymptomatic person. Indeed, loss of smell or taste has become synonymous with a diagnosis of Covid-19: but, does it only occur due to the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection?
In today’s article, we highlight the mechanisms underlying the loss of smell. We also delineate other common causes besides SARS-CoV-2.
The olfactory nerve, the first cranial nerve, is responsible for conveying information about the smell from the nose to the brain at the olfactory neuro-epithelium through the olfactory cleft located in the upper portion of the nasal cavity. When airflow odorants (odour generating substances) reach the neuro-epithelium, they attach onto the olfactory receptor proteins, which are activated and interpret the signal to give the person that particular sense of smell. There are about 5 to 30 million receptor nerve cells within the neuro-epithelium. They express up to 350 various olfactory receptor proteins . It is this variety that enables us to pick every type of smell around us.
Covid-19 isn’t the only illness that can cause loss of smell. Loss of smell is existent in more than 60% of persons with common colds and acute rhinosinusitis . Adenovirus, rhinovirus, coronavirus, and influenza are associated with loss of smell in upper respiratory infections. Traumatic brain injury, neurodegenerative diseases are other major aetiologies. Other causes include tumours within the skull (cranium) or nose and sinuses, drugs, irradiation, et cetera .
Inability to smell occurs when impulses along the olfactory nerve are blocked. It can occur due to mucosal inflammation or neurodegenerative changes along the nerve or at the neuro-epithelium. Cells surrounding the olfactory neuroepithelium highly express angiotensin-converting enzyme receptor-2 (ACE2) and transmembrane protease serine 2 (TMPRSS2) . The two proteins enable SARS-CoV-2 to enter the cells, replicate, and cause carnage. The olfactory receptors express them to a lesser extent. It may explain the loss of smell that occurs during Covid-19.
It is imperative to test for Covid-19 if you experience a sudden reduction in the smell pattern with or without constitutional symptoms during this period.