Taste loss as a COVID-19 symptom: a review

A new review reports that alterations in taste affected about a third of individuals with a coronavirus disease 2019 (COVID-19) diagnosis, with females being affected more than males.

Study: Taste loss as a distinct symptom of COVID-19: A systematic review and meta-analysis. Image Credit: Nicole Rerk/ Shutterstock

Background

A few months into the pandemic, specialists in otorhinolaryngology reported high rates of sensory loss in the form of an altered sense of smell and taste. Some patients lose these sensations completely (anosmia or ageusia, respectively), while a partial loss is termed hyposmia or hypogeusia. Others have distortions in their sensations, called parosmia and dysgeusia.

While not dangerous, the affected individuals may find it distressing. The symptoms typically resolve within weeks to months. By six months, the majority of patients report that they have regained normal sensation.

Earlier studies have examined one or a few aspects of this symptom set, or only as part of general neurological symptoms in COVID-19. Many have covered only a limited period, thus excluding many relevant articles. Some researchers claim that self-reported sensory loss is not reliable.

The current study, available as a preprint on medRxiv*, provides a fuller survey of literature in this area to help resolve the question of whether taste loss is as real as the loss of smell, rather than being falsely reported because of confusion with the latter.

In particular, the study focused on direct estimates based on individual taste tests vs. self-reported alterations in taste alone. Due to restrictions imposed by pandemic conditions, the latter is the preferred methodology in many settings.

The researchers tested the postulate that direct tests would confirm the presence of taste loss in this condition but could even produce higher estimates than self-reported symptoms even though in the latter, the loss of smell is often conflated with that of taste.

What did the study show?

The researchers studied 241 papers with a pooled patient set of almost 139,000 patients with a positive COVID-19 diagnosis. Of these, approximately 33,000 patients had taste loss in some form, with a pooled percentage of 37%. The prevalence estimates were similar by either direct assessment or by self-report.

The age-group-based analysis showed that the prevalence estimate among those below 18 years was 11%, vs. the highest prevalence of 44% in those between 36-50 years. Both the youngest and oldest groups had the lowest prevalence, while between 18 and 65 years, the prevalence ranged from 32% to 44%.

Thus, age may have affected the different estimates for the prevalence of taste loss in different studies. With sex, again, males were found to contribute towards a lower prevalence of taste loss.

However, there was no significant difference in the estimates obtained by testing with taste strips, taste solutions, and other methods. The highest mean prevalence was obtained using solutions.

Conclusion

Though taste loss occurs in many medical conditions, it can be confused with smell loss. In COVID-19, however, taste loss is a unique feature of the illness, affecting millions of people and reducing their quality of life. In this study, well over a third of individuals had taste loss, as expected from other studies.

The self-report-based prevalence estimates correspond to those obtained by direct testing, indicating the validity of these reports in this condition. In the face of pandemic-related restrictions, innovative testing methods were used, including home testing. However, the sheer variety of testing meant that it was non-standardized and thus led to difficulty in comparing results.

Nonetheless, there is a trend towards increasing sensitivity with the use of taste solutions compared to taste strips, which side-to-side comparisons must verify.

The higher prevalence in females may be due to their increased sensory capacity, while the reason for the highest prevalence in middle age is currently unclear.

The biological mechanism underlying these chemosensory symptoms remains an open research question, though some studies suggest that increased viral shedding in the saliva is linked to more taste loss. This may indicate direct viral damage to the taste cells.

This is supported by the presence of the angiotensin-converting enzyme 2 (ACE2), which is the viral receptor on the host cell, and of TMPRSS2, the protein required to produce the processed form of the viral spike, on the taste receptors as well as the supporting cells of the taste buds on the tongue.

Another putative mechanism is direct viral effects on the brain that lead to taste alterations.

Future work may focus on the standardization of methods used to examine this sense. Loss of taste, which occurs due to disease, advancing age, or loss of chemosensory receptors, can significantly reduce the quality of life.

Secondly, there are no available preventive or screening measures for these chemosensory disorders. The lack of standardized screening and testing guidelines has made it difficult to track these symptoms during the current pandemic.

Clinicians often neglect to assess taste in patients suspected to have or confirmed to have COVID-19. These tests are important because they would provide a picture of how these symptoms evolve over the course of the pandemic and allow researchers to differentiate between smell and taste loss by using objective measures.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.