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You may have heard of the terms anosmia and ageusia to describe a complete loss of smell and taste but have you heard of the term parosmia (a distorted sense of smell)? Dietitian and graduate medical student, Sarah-Jane Reilly investigates.
In May 2020, the UK Chief Medical Officer recognised anosmia as a cardinal symptom of COVID-19.1 This recommendation followed mounting evidence that anosmia could be an early marker of infection and an isolated symptom of COVID-19 in up to 1 in 6 patients.2,3
The virus responsible for COVID-19 (SARS-CoV-2) binds to angiotensin converting enzyme 2 (ACE2) cell receptors which are highly expressed in the oral mucosa, alveolar epithelial cells, intestine, kidney and heart.4 Within the oral cavity, ACE2 expression is highest within the tongue,5making the tongue particularly vulnerable to damage and subsequent taste changes following COVID-19 infection.
Alterations to smell have been reported to be more prevalent than changes to taste 6 however, this observation may be confounded by the interdependent relationship that exists between both senses. The cause of these sensory changes remains unclear. Changes to smell could be related to the neuroinvasive nature of the virus which is thought to enter via the nose resulting in damage to the olfactory nerve.4
A recent meta-analysis of 18 studies suggests that nearly half (47%) of patients with COVID-19 experience some alteration to their sense of smell or taste.7For the majority of these patients, the change is transient and typically resolves within one month. But for 10% of patients, the change is long-term and can present unique challenges to daily living.8
Parosmia is the term used to describe distortions to an individual’s sense of smell.9It is not a new phenomenon. Taste and smell disorders are commonly associated with a host of conditions from upper respiratory tract infections to neurodegenerative disorders such as Parkinson’s disease.10 Of note, roughly 75% of adults over the age of 80 years have major olfactory dysfunction.11
Many patients with chronic diseases such as cancer also demonstrate marked changes in chemosensory perception.12 The main odorous triggers for patients with parosmia appear to be petrol, tobacco, coffee, perfumes, fruits and chocolate with many patients describing these smells as foul, rotten, burning or sewage like.13
Similarly, certain foods such as meats, fish, fruits, coffee, eggs and carbonated drinks appear to have an altered taste profile among patients with chemosensory disorders. 14In the context of COVID-19, some patients also report phantosmia – a term used to describe phantom smells, such as being able to smell garlic without any garlic being present.9
The effects of parosmia on an individual’s nutritional intake, enjoyment of food and overall quality of life should not be overlooked. Our sense of taste and smell play a key role in eliciting the cephalic phase of digestion which involves responses such as salivation, gut motility and thermogenesis.15
Decreased smell and taste can influence eating behaviours resulting in an increased salt, sugar and fat intake,12which could have clinical implications for patients with hypertension, diabetes and cardiovascular disease. In addition, sensory changes can make it difficult to identify foods that are unsafe to eat and can make the process of cooking a meal from scratch more challenging.
Older population groups may be most affected by these changes as decreased smell and taste have been linked to appetite suppression in this group, increasing the risk of malnutrition.16 Taste changes could also influence compliance with savoury Oral Nutritional Supplement (ONS) products. Research has shown that older adults require two to three times more salt to detect the salty taste of tomato soup.17
Very few studies have investigated the psychological burden of living with parosmia as a symptom of ‘long Covid.’ However, one qualitative study analysed the discussions of over 9,000 users within the AbScent and Taste Loss support groups on Facebook and found that patients experienced an overwhelming sense of burden living with an invisible illness that they struggled to comprehend.18
While there is no ‘cure’ for parosmia, it’s important to highlight that parosmia appears to be a feature of recovery from anosmia.19 A systematic review of 10 intervention studies with 639 patients found that smell training supported the recovery of smell in patients experiencing olfactory dysfunction. 20 Smell training is suitable for patients who have lost their sense of smell for two weeks or more and involves sniffing four different scents daily for a duration of twenty seconds.21Although the exact mechanism of action behind this treatment is not fully understood, the neural plasticity of the olfactory system offers hope that daily exposure to different odours can regenerate olfactory neurons.22
NoseWell is an online resource that dietitians can recommend to patients experiencing complete or partial loss of smell.21 The resource is funded by the British Rhinological Society (BRS) and ENT UK to support patients to undertake evidence-based smell training. While a small number of nutritional interventions such as zinc and omega-3 supplementation have been suggested as potential treatments, dietitians should be aware that zinc supplementation does not feature in the BRS recommendations, and omega-3 supplementation should only be considered on an individual basis. 23
Parosmia can leave patients living with a lost sense of self. The impact of alterations to taste and smell should not be overlooked particularly among older population groups. These changes are likely to influence an individual’s appetite, body weight and overall relationship with food. Dietitians need to be reactive to these challenges and willing to use their expertise to provide tailored nutritional advice to patients struggling with the effects of parosmia - an emerging symptom of long Covid.