The loss of smell and taste that can occur with COVID-19 is not a new phenomenon, having been observed in patients affected by the common cold or influenza. It is one of the early symptoms of these diseases, although in most cases it resolves after a short period of time. However, for many COVID-19 patients smell and taste disturbances persist for weeks and months after the resolution of other acute symptoms. This prolonged symptom negatively impacts quality of life and leads to a variety of adverse effects such as anorexia, food aversions, malnutrition, anxiety and depression.
The underlying cause for smell and taste disorders is not yet fully understood, but it is likely that the virus directly infects the cells that support and assist the olfactory neurons called sustentacular cells. The viruses can also infect the olfactory nerves, which transmit information from the brain to the taste buds. Despite this direct damage, most patients recover from smell and taste dysfunction quickly, because the olfactory nerves can regenerate. However, the taste buds are not so resilient and a permanent loss of taste can be expected in some patients.
In a recent study, Mahmoud and colleagues analyzed data from 18 published observational studies of adult COVID-19 patients who experienced smell or taste disturbances following the infection. This study was the first to provide an in-depth analysis of the prognosis for COVID-19-related long-term smell and taste impairment.
All participants in the study experienced anosmia or diminished sense of smell during the COVID-19 infectious phase and were followed up for a maximum of 24 months to evaluate for long-term residual smell or taste impairment. They found that olfactory impairment is significantly longer-lasting than taste impairment in the same group, and that the duration of the symptoms was related to the severity of anosmia or diminished sense or smell. However, the results should be interpreted with caution due to the small sample size, restricted ethnic background and simplified taste evaluation and self-reporting methodologies.
Interestingly, the authors found that the most commonly impaired taste component was salty, compared to sweet and bitter. This finding may be related to the heightened immune response seen in COVID-19 patients, which can induce metabolic changes that lead to salt-related taste disorders.
As the number of Covid-19 taste disorders cases continues to grow worldwide, it is important to keep in mind that a significant portion of people will continue to experience disabling symptoms for months to come. Health systems should be prepared to identify and care for these patients who require specialized treatment for their persistent smell and taste dysfunction. This may include the use of medications to manage nausea and vomiting, dietary modifications and psychosocial support.