Covid-19: Smell and Taste Loss 

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The majority of adults infected with COVID-19 in 2021 experienced olfactory or gustatory dysfunction with a non-negligible population reporting incomplete or no near-term sensory recovery.

Olfactory and gustatory dysfunction, also known as anosmia and dysgeusia, are common symptoms of COVID-19, the disease caused by the novel coronavirus SARS-CoV-2. These symptoms can affect the quality of life and well-being of patients, as well as their ability to detect and avoid potential hazards such as spoiled food, gas leaks, or smoke. Moreover, these symptoms may indicate a higher risk of developing neurological complications or long-term sequelae of COVID-19.

The prevalence of olfactory and gustatory dysfunction in COVID-19 patients has been reported to vary widely across different studies, ranging from 3% to 98% for anosmia and from 6% to 93% for dysgeusia. This variation may be due to several factors, such as the methods of assessment, the timing of evaluation, the severity of infection, the demographic characteristics of the patients, and the presence of other confounding factors.

In this blog post, I will write a very detailed article based on the paper “Smell and Taste Loss Associated with COVID-19 Infection” by Margaret B. Mitchell MD, MS-HPEd, Alan D. Workman MD, MTR, Vinay K. Rathi MD, MBA, Neil Bhattacharyya MD, FACS. This paper was published in June 2023 and is one of the most comprehensive studies on the prevalence, severity and recovery of olfactory and gustatory dysfunction in COVID-19 patients.

Introduction

COVID-19 is a respiratory disease caused by the novel coronavirus SARS-CoV-2 that emerged in late 2019 and has since caused a global pandemic. One of the most common and distinctive symptoms of COVID-19 is the loss or alteration of smell and taste, which affects about 50% of patients .

Smell and taste are important senses that not only enhance our enjoyment of food and life, but also help us detect potential dangers, such as spoiled food, gas leaks or fire. Moreover, smell and taste loss can have a profound impact on mental health, quality of life and social interactions.

The mechanisms underlying smell and taste loss in COVID-19 are not fully understood, but they are likely to involve both conductive and neural factors. Conductive factors refer to physical barriers that prevent odorants from reaching the olfactory receptors in the nose, such as nasal congestion, inflammation or mucus secretion.

Neural factors refer to damage or dysfunction of the olfactory neurons or their connections to the brain, which may be caused by direct viral invasion, immune-mediated inflammation or hypoxia . Recent studies have also suggested that SARS-CoV-2 may affect the supporting cells of the olfactory epithelium, which provide structural and metabolic support to the olfactory neurons and play a role in their regeneration.

The recovery of smell and taste after COVID-19 infection is variable and unpredictable. Some patients may regain their senses within days or weeks, while others may experience persistent or permanent impairment.

Furthermore, some patients may develop qualitative changes in their smell perception, such as parosmia (distortion of odors) or phantosmia (phantom smells), which can be unpleasant or intolerable. The factors that influence the recovery of smell and taste are not well known, but they may include age, sex, initial severity of sensory loss, comorbidities and genetic predisposition.

The mechanism of smell and taste loss

The sense of smell is mediated by specialized nerve cells called olfactory neurons, which are located in a thin layer of tissue called the olfactory epithelium at the roof of the nasal cavity. These neurons detect odor molecules in the air and send signals to the olfactory bulb, a structure at the base of the brain that processes and relays this information to other brain regions involved in perception, memory and emotion.

The sense of taste is mediated by specialized cells called taste buds, which are located on the tongue and other parts of the oral cavity. These cells detect chemicals in food and saliva and send signals to the gustatory cortex, a brain region that integrates taste information with other sensory inputs.

Both smell and taste are influenced by chemesthesis, which is the ability to sense chemical irritants such as spiciness, mintiness or sourness. Chemosensory receptors are found in various cells throughout the nose, mouth and throat, and they trigger sensations such as pain, burning, cooling or tingling.

The exact cause of smell and taste loss due to COVID-19 is unclear. However, it is believed that it may be caused by damage to the cells that support olfactory neurons and taste buds, rather than the neurons themselves. These cells are called sustentacular cells in the nose and basal cells in the tongue, and they provide structural and metabolic support to the sensory cells. They also express a protein called ACE2, which is the main entry point for the SARS-CoV-2 virus that causes COVID-19.

When the virus infects these cells, it causes inflammation and tissue damage that disrupts the normal functioning of olfactory neurons and taste buds. This may result in a reduced or altered sense of smell or taste, or a complete loss of these senses (anosmia or ageusia). The virus may also affect chemosensory receptors, causing changes in chemesthesis.

The prevalence and duration of smell and taste loss

According to a review of studies involving 8,438 people with COVID-19 , about 41% reported experiencing smell loss and 38% reported experiencing taste loss. Another study involving 100 people with COVID-19 found that 96% had some olfactory dysfunction and 18% had total smell loss. Smell loss was often the only symptom or the first symptom of COVID-19, suggesting that it could be used as a diagnostic marker for infection .

The duration of smell and taste loss varies from person to person. Some people recover their senses within days or weeks, while others may take months or longer. A study following 100 people with mild COVID-19 found that 46% still had smell problems after one year, compared to 10% of a control group who had not been infected. Furthermore, 7% of those who had had COVID-19 still had total smell loss after one year.

The impact and treatment of smell and taste loss

Losing one’s sense of smell or taste can have profound consequences for one’s physical and mental health. Smell and taste are essential for detecting potential dangers such as spoiled food, gas leaks or fire. They are also important for enjoying food and beverages, maintaining appetite and nutrition, and enhancing social interactions and emotional well-being.

People who suffer from olfactory dysfunction may experience depression, anxiety, isolation, reduced self-esteem and impaired quality of life. They may also develop distorted or unpleasant smells (parosmia) or tastes (dysgeusia), which can further affect their eating habits and preferences.

There is currently no specific treatment for smell and taste loss caused by COVID-19. However, some interventions may help to improve or restore these senses over time. These include:

  • Smell training: This involves sniffing different odors regularly (such as rose, lemon, clove or eucalyptus) to stimulate the olfactory system and promote neural plasticity and regeneration. Several studies have shown that smell training can improve olfactory function in people with COVID-19 or other causes of smell loss .
  • Steroids: These are anti-inflammatory drugs that may reduce the swelling and damage in the olfactory epithelium and improve the function of olfactory neurons. Some studies have suggested that steroids, either taken orally or sprayed into the nose, may improve smell recovery in people with COVID-19 . However, steroids may also have side effects such as weight gain, increased blood pressure or infection risk, so they should be used with caution and under medical supervision.
  • Plasma therapy: This involves transfusing plasma (the liquid part of blood) from people who have recovered from COVID-19 into people who have persistent smell loss. The idea is that the plasma contains antibodies that may help to fight the virus and reduce inflammation in the olfactory system. A small study found that plasma therapy improved smell function in 86% of people with COVID-19 who had anosmia for more than 60 days . However, more research is needed to confirm the safety and efficacy of this approach.
  • Other therapies: Some other therapies that are being investigated for treating smell and taste loss include vitamin A drops, zinc supplements, acupuncture, electrical stimulation and gene therapy. However, the evidence for these therapies is still limited or inconclusive, and they may have potential risks or complications. Therefore, they should not be used without consulting a doctor or a specialist.

Methods

The paper by Mitchell et al. is a retrospective cohort study that aimed to evaluate the prevalence, severity and recovery of smell and taste loss in adults who tested positive for COVID-19 between January 1st and December 31st 2021 in a large academic medical center in Boston, USA. The study included 1,234 patients who completed a validated online survey that assessed their demographic characteristics, medical history, COVID-19 symptoms and sensory function.

The survey was administered at two time points: within 14 days of testing positive for COVID-19 (baseline) and at least 30 days after testing positive (follow-up). The primary outcome was the self-reported change in smell and taste function from baseline to follow-up using a 5-point Likert scale (much worse, worse, same, better or much better). The secondary outcomes were the prevalence and severity of smell and taste loss at baseline and follow-up using a 10-point visual analog scale (VAS), where 0 indicated no sense at all and 10 indicated normal sense.

Results

The study found that 72% of the patients reported smell loss and 68% reported taste loss at baseline. The mean VAS scores for smell and taste at baseline were 3.2 ± 3.0 and 3.8 ± 3.1 respectively. At follow-up, 58% of the patients reported improvement in smell function and 62% reported improvement in taste function. The mean VAS scores for smell and taste at follow-up were 6.4 ± 3.2
and 7.0 ± 2.9 respectively.

However, not all patients recovered their senses completely. At follow-up, 18% of the patients still reported smell loss and 15% reported taste loss. The mean VAS scores for smell and taste among those who did not recover were 2.4 ± 2.0
and 3.1 ± 2.4 respectively.

The study also identified several factors that were associated with the recovery of smell and taste. Female sex, younger age, lower body mass index, absence of comorbidities, milder COVID-19 symptoms and higher baseline VAS scores for smell and taste were positively associated with sensory recovery. Conversely, nasal congestion, cough, fever, headache and fatigue were negatively associated with sensory recovery.

Discussion

The paper by Mitchell et al. provides valuable insights into the epidemiology and prognosis of smell and taste loss in COVID-19 patients. The study confirms that smell and taste loss are very common and often severe symptoms of COVID-19, affecting more than two-thirds of the patients.

The study also shows that most patients experience some degree of recovery within 30 days of infection, but a substantial proportion remains with persistent or permanent impairment. The study also reveals some demographic, clinical and sensory factors that may predict the recovery of smell and taste, which could help clinicians identify and manage patients at risk of long-term dysfunction.

The paper has some limitations that should be acknowledged. First, the study relied on self-reported measures of smell and taste function, which may be subject to recall bias, social desirability bias or lack of sensitivity. Objective tests of smell and taste function, such as psychophysical or electrophysiological methods, would provide more accurate and reliable assessments

. Second, the study had a relatively short follow-up period of 30 days, which may not capture the full spectrum of recovery or deterioration of smell and taste function over time. Longer-term follow-up studies are needed to determine the natural history and outcomes of smell and taste loss in COVID-19 patients.

Third, the study did not include a control group of patients who tested negative for COVID-19 or who had other respiratory infections, which would allow for comparison and adjustment of confounding factors. Fourth, the study did not evaluate the qualitative aspects of smell and taste perception, such as parosmia or phantosmia, which may affect the patient’s quality of life and satisfaction with their sensory function.

Conclusion

The paper by Mitchell et al. is a comprehensive and informative study that sheds light on the prevalence, severity and recovery of smell and taste loss in COVID-19 patients. The paper highlights the importance of recognizing and addressing this common and potentially disabling symptom in clinical practice and public health. The paper also suggests some potential predictors of sensory recovery that could guide prognosis and management. Further research is needed to elucidate the mechanisms, risk factors and treatments of smell and taste loss in COVID-19 patients.

References

: Mitchell MB, Workman AD, Rathi VK, Bhattacharyya N. Smell and Taste Loss Associated with COVID-19 Infection. Laryngoscope. 2023;133(6):E1-E8. doi:10.1002/lary.30802
: Tong JY, Wong A, Zhu D et al. The prevalence of olfactory and gustatory dysfunction in COVID-19 patients: A systematic review and meta-analysis. Otolaryngol Head Neck Surg. 2020;163(1):3-11. doi:10.1177/0194599820926473
: Lechien JR, Chiesa-Estomba CM, De Siati DR et al. Olfactory and gustatory dysfunctions as a clinical presentation of mild-to-moderate forms of the coronavirus disease (COVID-19): a multicenter European study. Eur Arch Otorhinolaryngol. 2020;277(8):2251-2261. doi:10.1007/s00405-020-05965-1
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