The sense of smell plays an important role in everyday life: it warns us of sometimes life-threatening dangers such as spoiled food, smoke or fires as well as toxins. This warning function is lost in patients with a long-term loss of smell [1]. Treatment options are limited, depending on the cause. This makes it all the more important to diagnose and educate patients.
If you have no perception of smell, you are more insecure, miss out on a lot of social information and have difficulties eating and drinking or enjoying food, explained Prof. Dr. Thomas Hummel from the Interdisciplinary Center for Smell and Taste, Department of Otorhinolaryngology, Carl Gustav Carus University Hospital in Dresden (Germany). As a consequence, a loss of smell can also lead to a severe impairment of quality of life and even depression [2,3]. The prevalence of anosmia, i.e. a severe reduction or even complete lack of olfactory perception, is reported in European studies to be around 5%, with around 15% of the population suffering from a mild to moderate reduction (hyposmia) [4,5]. The reasons for olfactory dysfunction can be manifold, ranging from age-related disorders to sinonasal problems, consequences of traumatic experiences and post-infectious effects, explained Dr. Julien Hsieh from the Department of Clinical Neurosciences, University of Geneva. One of the most common causes of olfactory loss of function is a disease of the paranasal sinuses, such as chronic rhinosinusitis with or without nasal polyposis.
Post-traumatic anosmia and post-infectious loss of smell
Up to 56% of patients can suffer from loss of smell after a traumatic brain injury. Olfactory training is initially recommended for this group: Different odors such as rose, eucalyptus, lemon or clove are presented three times a day over a period of 12 weeks for olfactory testing. In a randomized control study, an improvement was observed after 3 months. However, this effect was lost again after a further 12 weeks, so that the two groups had the same values again after a total of 24 weeks [6]. In a position paper, olfactory training is nevertheless recommended for these patients with head injuries, as it can improve neuroregeneration and neuroplasticity and has no side effects [7]. Post-infectious loss of smell has come into focus in connection with the COVID-19 pandemic, but it has also been observed in other diseases such as influenza that all symptoms usually recover after the infection has subsided, but the loss of smell can persist for years or even for life. Olfactory training is also the first measure in such cases: The likelihood of clinically significant improvement is three times higher if olfactory training is carried out in post-infectious olfactory disorders [8–11]. Corticosteroids are not recommended in the position paper on olfactory dysfunction [7].
Diagnosis of the olfactory disorder
The olfactory test is the cornerstone in the treatment of olfactory disorders. The quantitative determination of olfactory performance can be carried out by subjective assessment, psychophysical tests or electrophysiological methods. Although subjective assessment is the quickest and simplest method for assessing olfactory function, it is often inaccurate – probably due to differences in suffering and self-esteem – and usually does not correspond to objective olfactory ability, as Prof. Dr. Basile Landis, Head of the Department of Rhinology and Olfactology at the Hôpitaux Universitaires Geneva, has shown in a paper (Fig. 1) [12,13]. There are many different odor tests worldwide, which can be divided into three categories:Threshold tests enable the determination of the lowest concentration at which an odorant can be detected [14,15]. Thediscrimination test assesses the ability to distinguish odors: Subjects are given, for example, three different odor samples, two of which are identical, and must state which sample does not match the other two. In the identification test, odours are usually characterized using four terms [15]. The participant must select the term that corresponds to the odor. According to Dr. Hsieh, it is always better to test than to just ask. He recommends testing different odor categories, among other things for a better quantification of the odor function and to differentiate between etiologies.
Clinical clarification
An idiopathic loss of smell is rather unusual and can be an early sign of Parkinson’s disease [16] and also of dementia [17]. Idiopathic loss of smell may also indicate increased mortality. Over 90% of men and women with idiopathic Parkinson’s disease (IPD) have an olfactory disorder, which is used as a supporting diagnostic criterion in the clinical diagnosis of IPD. The olfactory disorder can occur more than 10 years before the onset of motor symptoms [16]. Severe olfactory disturbances can also occur in various forms of dementia [18,19]. They are considered an early symptom of Alzheimer’s disease and occur in patients whose cognitive abilities are not yet severely impaired.
Therapeutic measures
Drug treatment options are available for underlying sinonasal diseases. Underlying chronic inflammation such as chronic rhinosinusitis with nasal polyposis (CRSwNP) can be treated with topical steroids, which also has a significant effect on olfactory function [20]. The administration of systemic oral corticosteroids over a period of two weeks can achieve a temporary improvement in odor, but the hyposmia or anosmia slowly returns after about 50 days [21]. If corticosteroids do not work for CRS, the expert recommended functional endoscopic sinus surgery (FESS) as an alternative. Monoclonal antibodies (biologics) offer a third treatment option for CRSwNP. CRSwNP patients who meet at least three of the criteria listed in Figure 2 – one of which should be a significant loss of the sense of smell as measured by an olfactory test – may be eligible for biologic therapy. For CRSwNP or nasal polyps, 3 biologics are currently approved in Switzerland: Dupilumab, mepolizumab and omalizumab [23– 25]. Most of the studies analyzed the identification of odors. There is a need for further real-life studies with more robust tests to investigate the effects on odor sensitivity and discrimination, explained Dr. Hsieh. If an olfactory disorder is not due to a sinonasal disease, there are few treatment options and recommendations [26, 27]. Only olfactory training, i.e. consciously smelling different odors several times a day, has been shown to have therapeutic value [28].
Literature:
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23. technical information Dupixent®,
www.swissmedicinfo.ch
24. Information for healthcare professionals Nucala®, www.swissmedicinfo.ch
25. Information for healthcare professionals Xolair®, www.swissmedicinfo.ch
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Text/editing: Jens Dehn
This report was realized with the kind support of Sanofi (Suisse) S.A.
Source: Symposium “Oh what a smell!” as part of the SGORL/SSORL Spring Meeting 2024, Lugano, 13.06.2024; Organizer: Sanofi. sanofi-aventis (switzerland) ltd, 3, route de Montfleury, 1214 Vernier
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