Food, taste and appetite

Olfactory dysfunction has been reported to gravely impair the QoL of the affected individual because of numerous day-to-day activity limitations. The most noticeable change is the decrease in flavour perception from impaired retronasal olfaction, which leads to reduced food pleasure, found in approximately 69 per cent of individuals, based on a study by Nordin et al.^{Reference Nordin, Blomqvist, Olsson, Stjärne and Ehnhage13} The ramifications from diminished food enjoyment result in a grievous situation, as food is a significant part of one's life.

Previous studies have revealed that permanent olfactory dysfunction reduces appetite in almost 56 per cent of patients, although nearly 20 per cent of patients consume more food.^{Reference Ferris and Duffy14} In the same vein, these patients have demonstrated difficulties with cooking and preparing food, as well as detecting rotten food. In our review, reduced ability to taste food was present in 86 per cent of cases.^{Reference Coelho, Reiter, Budd, Shin, Kons and Costanzo9,Reference Elkholi, Abdelwahab and Abdelhafeez10} Loss of appetite was reported in two studies, affecting 67.3 per cent of cases.^{Reference AlShakhs, Almomen, AlYaeesh, AlOmairin, AlMutairi and Alammar8,Reference Elkholi, Abdelwahab and Abdelhafeez10}

Safety issues

Odours serve as the first warning signal, alerting one to various harmful conditions such as smoke or fire, spoiled food, or hygiene-related issues. Patients with olfactory dysfunction may not be aware of life-threatening situations associated with the inability to detect harmful odorants such as fire, gas leaks and smoke, which has been demonstrated to afflict 61 per cent of such patients, as described by Nordin et al.^{Reference Nordin, Blomqvist, Olsson, Stjärne and Ehnhage13} Safety-related concern was reported in one study in our review, involving 57 per cent of their cohort. Croy et al. revealed that patients with olfactory dysfunction are involved in more household accidents as compared with normosmic individuals.^{Reference Croy, Negoias, Novakova, Landis and Hummel15} Patients with olfactory dysfunction tend to be more insecure and constantly worry about safety issues.^{Reference Croy, Nordin and Hummel16}

Personal hygiene

Individuals with olfactory dysfunction are affected by hygiene-related concerns, as they are unable to detect their own body odour. This leads to further insecurity, and could result in isolation and damage to self-esteem. None of the patients included in this review reported on hygiene impairment.

Occupation issues

Intact olfactory function is crucial in many olfaction-reliant occupational groups, notably chefs, gas fitters, firefighters, perfumers, sommeliers, coffee- and tea-tasters, grocers, chemical industry workers, and domestic helpers.^{Reference Werner and Nies17} Loss of smell is also regarded as a criterion for the discharge of US military and reservists, as well as coastguard employees.^{Reference Doty18} These groups of affected individuals are among millions who may have to find a new source of income and relocate, as olfaction is a central part of their job. None of the included studies mentioned job-related concerns associated with olfactory dysfunction. This may be attributed to the fact that the outcome of olfactory dysfunction in Covid-19 remains elusive.

Mental health

Olfactory dysfunction has been traditionally linked to depression.^{Reference Kohli, Soler, Nguyen, Muus and Schlosser19} Interestingly, olfactory function has been regarded as a marker for depression.^{Reference Rochet, El-Hage, Richa, Kazour and Atanasova12} AlShakhs et al.^{Reference AlShakhs, Almomen, AlYaeesh, AlOmairin, AlMutairi and Alammar8} found that 42 per cent of Covid-19 patients with olfactory dysfunction felt isolated, and a study by Coelho et al.^{Reference Coelho, Reiter, Budd, Shin, Kons and Costanzo9} revealed that depression affected 43 per cent of such patients. Although the link between olfactory dysfunction and depression remains unexplained, an overlap between the brain areas implicated in both depression and the olfactory process has been postulated, notably the orbitofrontal cortex, anterior and posterior cingulate cortices, insula, amygdala, hippocampus and thalamus.^{Reference Naudin and Atanasova20} In the same vein, odours are known to have a profound effect on the emotions, mood and behaviour, which could explain depression amongst these patients.^{Reference Seo, Roidl, Müller and Negoias21}

What is next for ‘scentless’ survivors?

It is salutary to note that although spontaneous recovery from olfactory dysfunction has been linked to post-infectious olfactory dysfunction, we are yet to unravel the outcome of olfactory dysfunction post-Covid-19. Currently, olfactory dysfunction following Covid-19 is possibly underreported in the available data, as most studies are based on self-reports rather than a more objective assessment. The implications of long-standing olfactory dysfunction in terms of the economic burden have been reported in previous studies.

To date, no validated medication exists specifically to treat post-infectious olfactory dysfunction, although numerous trials involving systemic and topical steroids, vitamin B, caroverine, acupuncture and alpha-lipoic acid have been attempted, with varying results.^{Reference Damm, Pikart, Reimann, Burkert, Goktas and Haxel22} None of the included studies proposed a treatment regimen.

Olfactory retraining therapy has gained popularity recently because of the rising numbers of patients afflicted with olfactory dysfunction post-Covid-19. This non-invasive intervention, directed at the rehabilitation of olfactory dysfunction, has demonstrated promising outcomes in relation to olfactory dysfunction from various causes.^{Reference Oleszkiewicz, Hanf, Whitcroft, Haehner and Hummel23,Reference Choi, Jeong, Noh, Park, Cho and Kim24} As well as being easy to administer, olfactory retraining therapy has been reported to decrease depressive episodes amongst patients.^{Reference Birte-Antina, Ilona, Antje and Thomas25} The concept behind olfactory retraining therapy is based on daily repeated exposure to selected odorants. Four odours are traditionally incorporated, based on the odour prism proposed by Henning, of rose, eucalyptus, lemon and clove,^{Reference Hummel, Rissom, Reden, Hahner, Weidenbecher and Huttenbrink26} which correspond to one from each category: fruity, flowery, resinous and spicy.^{Reference Whitcroft and Hummel27} Olfactory retraining therapy is believed to strengthen the olfactory nerve through repeated exercise. The only drawback of olfactory retraining therapy is the need to repeat the training exercise for months, which may be tiresome to most patients. Nonetheless, favourable results have been demonstrated amongst Covid-19 patients with olfactory dysfunction, whereby early olfactory retraining therapy corresponds to a better olfactory outcome.^{Reference Yan, Gao, Sun, Guo, Yao and Liu28}

Several coping strategies directed to managing olfactory dysfunction from various causes have been carried out successfully over the past decade. These include safety measures such as installing gas or smoke detectors.^{Reference Croy, Nordin and Hummel16} Olfactory dysfunction patients tend to rely on family members to test food or check perfume usage. Family members and close friends need to play an important role in supporting these Covid-19 survivors, as they may be vulnerable and fearful of having permanent smell loss. Dysregulated appetite can be addressed by scheduled eating hours.^{Reference Croy, Nordin and Hummel16} Strict adherence to manufacturing dates on food and drink is required to avoid food poisoning. Additionally, there are numerous social support groups that can play an important role in targeting emotional adaptation.^{Reference Nordin, Blomqvist, Olsson, Stjärne and Ehnhage13} Currently, the ongoing pandemic has witnessed charity-based support groups such as Fifth Sense and AbSent coming forward to help afflicted patients. Online patient support groups enable patients newly affected with olfactory dysfunction to share their struggle with others who have managed olfactory dysfunction from other causes. Finally, the patients themselves need to come to terms with and gain emotional acceptance of the deficit to allow adaptive adjustment to the disease.