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.
Sensory Problems - The Extent of the Problem
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
A Spotlight on Parosmia
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
Parosmia and Nutritional Intake
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
A Final Note
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.
- UK Department of Health and Social Care. Statement from the UK Chief Medical Officers on an update to coronavirus symptoms [Internet]. London: Department of Health and Social Care; 2020 May 18 [cited 2021 March 16]. Available from: https://www.gov.uk/government/news/statement-from-the-uk-chief-medical-officers-on-an-update-to-coronavirus-symptoms-18-may-2020.
- Hopkins C, Surda P, Kumar N. Presentation of new onset anosmia during the COVID-19 pandemic. Rhinonology. 2020 Jun 1; 58(3): 295-298.
- Boscolo-Rizzo P, Menegaldo, A, Fabbris C et al. High presentation of long-term psychophysical olfactory dysfunction in patients with COVID-19. MedRxiv [journal on the internet]. 2021 Jan 8 [cited 2021 March 16]. https://doi.org/10.1101/2021.01.07.21249406 [Epub ahead of print].
- Mehraeen E, Behnezhad F, Salehi MA. et al. Olfactory and gustatory dysfunctions due to the coronavirus disease (COVID-19): a review of current evidence. Eur Arch Otorhinolaryngol. 2020 Jun 17; 278: 307–312.
- Xu H, Zhong L, Deng J. et al. High expression of ACE2 receptor of 2019-nCoV on the epithelial cells of oral mucosa. Int J Oral Sci. 2020 Feb 24;12(1):1–5.
- Russell B, Moss C, Rigg A. et al. Anosmia and ageusia are emerging as symptoms in patients with COVID-19: what does the current evidence say? Ecancermedicalscience. 2020 Apr 3;14:103332.
- Dorsetto D, Hopkins C, Philips V. Self-reported alteration of sense of smell or taste in patients with COVID-19: a systematic review and meta-analysis on 3563 patients. Rhinology. 2020 Oct 1; 58(5): 430-436.
- Boscolo-Rizzo P, Borsetto D, Fabbris C. et al. Evolution of Altered Sense of Smell or Taste in Patients with Mildly Symptomatic COVID-19. Otolaryngol Head Neck Surg. 2020 July 2;146(8):729-732.
- Fifth Sense. Parosmia And Phantosmia [internet]. Fifth Sense; 2021[cited 2021 Mar 16]. Available from: //www.fifthsense.org.uk/parosmia-and-phantosmia/
- Walker A, Pottinger G, Scott A. et al. Anosmia and loss of smell in the era of COVID-19. BMJ. 2020 Jul 21; 370. doi: https://doi.org/10.1136/bmj.m2808
- Doty RL, Shaman P, Applebaum S. et al. Smell identification ability: changes with age. Science. 1984 December 21; 226(4681): 1441-1443.
- Kershaw JC, Mattes RD. Nutrition and taste and smell dysfunction. Otolaryngol Head Neck Surg. 2018 March 4; 4(1): 3-10.
- Bonfils P, Avan P, Faulcon, P et al. Distorted Odorant Perception – Analysis of a series of 56 patients with parosmia. Arch Otolaryngol Head Neck Surg. 2005 February 1;131(2): 107-112.
- Mattes RD, Cowart BJ. Dietary assessment of patients with chemosensory disorders. J Am Diet Assoc. 1994 Jan 1; 94(1):50-56.
- Smeets P, Erkner A, De Graaf. Cephalic phase responses and appetite. Nutr Rev. 2010 Nov 11; 68(11):643-655.
- Boyce JM, Shone GR. Effects of ageing on smell and taste. Postgrad Med J. 2006 Apr 22; 82(966):239-241.
- Stevens JC, Cain WS, Demarque A.et al. On the discrimination of missing ingredients: aging and salt flavour. Appetite. 1991 Apr 14;16(2):129–140.
- Burges Watson D, Campbell M, Hopkins C. et al. Altered Smell and Taste: anosmia, parosmia and the impact of long Covid-19. MedRxiv [journal on the internet]. 2020 Nov 30 [cited 2021 March 16]. doi: https://doi.org/10.1101/2020.11.26.20239152[Epub ahead of print].
AbScent. Parosmia [internet]. Abscent; 2021[cited 2021 Mar 16]. Available from: https://abscent.org/about-smell-loss/parosmia
- Kelly P, Rakesh K, Chandra M et al. Efficacy of olfactory training in patients with olfactory loss: a systematic review and meta-analysis. J Aller and Rhinology. 2015 Dec 01; 6(3): 299-307.
- AbScent. Welcome to smell training [internet]. Abscent; 2021[cited 2021 Mar 16]. Available from: https://abscent.org/learn-us/smell-training
- Sorokowska A, Drechsler E, Karwowski, M. et al. Effects of olfactory training: a meta-analysis. Rhinology. 2017 Mar 01; 55(1): 17-26.
- Hopkins C, Alanin M, Philpott C. et al. Management of new onset loss of sense of smell during the COVID‐19 pandemic ‐ BRS Consensus Guidelines. Clinical Otolaryngology. 2020 August 27; 46(1):16-22.