Loss of taste and smell are common with COVID-19 infection, and many people are finding that alterations in taste and smell can last well after the resolution of other COVID-19 symptoms. Otolaryngologists are less surprised by this finding than most people, as olfactory dysfunction has long been a known sequela of viral upper respiratory infections. SARS-CoV, the virus that caused the SARS pandemic of 2004-2005, has been linked with prolonged anosmia, so the current wave of patients reporting olfactory alterations after infection with SARS-CoV-2, the virus that causes COVID-19, seems predictable in hindsight (Int Forum Allergy Rhinol. 2020;10:814-820).
Explore This IssueJanuary 2021
But COVID-19-related smell and taste alterations are different from previously known post-viral olfactory issues in a number of significant ways. First, “the incidence of olfactory loss in patients who become infected with COVID-19 is dramatically higher,” said Abtin Tabaee, MD, associate professor in the department of otolaryngology at Weill Cornell Medical College. “The percentage of patients diagnosed with COVID-19 who develop self-reported symptoms of olfactory dysfunction has been reported between 60% to 80% in multiple studies. That’s significantly higher than prior viral upper respiratory infections.”
The key question to ask patients is, ‘Does your shortness of breath or cough wake you up at night?’ If the answer is no, it’s almost always vagal. —Jonathan Aviv, MD
Secondly, “patients with olfactory dysfunction in the setting of COVID-19 typically do not present with other viral upper respiratory infection symptoms, even if other COVID-19 symptoms are present,” Dr. Tabaee said. “In fact, olfactory dysfunction may represent an isolated symptom without any other COVID-19 manifestations.”
Parosmia and phantosmia also seem to appear more commonly in patients affected by COVID-19 than other viral upper respiratory illnesses, said Zara Patel, MD, director of endoscopic skull base surgery and an associate professor of otolaryngology–head and neck surgery at Stanford University Medical Center in Stanford, Calif. Rather than perceiving the smell of coffee when sniffing a morning cup of joe, patients might smell paint thinner or the scent of burning chemicals. Some patients report smelling offensive scents, such as sulfur, although they aren’t near anything that emits that particular scent.
Physicians and researchers are still working to understand the pathophysiology underlying COVID-19-associated smell and taste alterations. Previous human and animal studies have revealed that there are multiple virus-specific potential sites of injury along the olfactory tract, and the site of injury may vary from virus to virus, Dr. Tabaee said. “In the case of COVID-19, we know that the SARS-CoV-2 virus attaches to the ACE-2 receptor and that these receptors are found primarily on the sustentacular cells of the olfactory epithelium, not on the olfactory nerve bodies,” Dr. Patel said. The fact that the virus affects the supporting cells rather than the body of the olfactory nerve likely explains why most people who experience COVID-19-related loss of smell eventually regain olfactory function.
A recently published meta-analysis of 27 studies found that estimated global pooled prevalence of loss of smell among COVID-19 patients was 48.47%; the estimated pooled prevalence for loss of taste was 41.47%. Approximately 35% of patients experienced combined loss of smell and taste (OTO Open. 2020;4(3)1-13). A Korean study of more than 3,000 people reported that most patients with COVID-19-related anosmia or ageusia fully recovered their senses within three weeks; interestingly, patients aged 20-39 were more likely to experience prolonged anosmia than older patients (J Korean Med Sci. 2020;35:e174). Dr. Patel estimates that “about 70 to 75 percent of patients regain their usual olfactory function.”
Although most patients will recover their sense of smell and taste without intervention, a significant number won’t, and persistent sensory alternations negatively impact quality of life and safety. Otolaryngologists can improve patients’ lives by educating their communities about evidence-based treatment options for olfactory dysfunction, including olfactory training.
Essentially, olfactory training is the process of recreating appropriate neural pathways (Int Forum Allergy Rhinol. 2020;10:814-820). Patients are instructed to sniff four different scents—rose, eucalyptus, lemon, and clove—twice a day, while focusing their thoughts on memories of those scents. Olfactory training should continue for at least six months.