We do not pay enough attention to a key symptom: smells disorders. Some people develop subtle olfactory alterations; for example, they can smell some foods or substances but not others; this can be associated with changes in tasting too. It might happen after a cold, a flu or an episode of allergy.
If the alteration of the sense of smell persists for more than two weeks, you should see the doctor, since this is a sign of some underlying conditions, a predictive marker of risk, has safety implications, and affects quality of life.
Let’s begin with key definitions.
Definitions
Normosmia: indicates a normal ability to smell.
Dysosmia, an olfactory dysfunction that affects the ability to detect and identify odors: it comprises two types of distortion (1):
Parosmia (or troposmia): The perception that an element, for example, gasoline, tobacco or coffee smells “rotten” or “burn” or “disgusting”; often, patients cannot easily describe the odor.
Phantosmia: A perception of an unpleasant smell when there is no element that provokes it, also described as an olfactory hallucination.
Other related distortions include (2):
Anosmia: A complete loss of the ability to smell, evidenced by a validated test.
Hyposmia (or microsmia): A partial loss of smell.
Dysosmia is an early sign of important risks
Increased risk of neurodegenerative disease: In many cases, dysosmia is one of the first manifestations of Parkinson’s disease, it appears 4-6 years before the typical muscle-skeletal symptoms. In the case of Alzheimer’s disease, dysosmia is present 5 years earlier (3),(4).
Increased risk of mortality: Some studies demonstrated that impairment in odor identification is associated with a higher mortality that might be related to the development of a neurodegenerative disease or to other mechanisms (2).
Implications of dysosmia
A dysfunction of the ability to smell affects individuals to a different extent in various dimensions (2):
Psychological:
Interpersonal relationships: A Body odor that informs about hormonal and emotional estates in a chemical level is distorted by anosmia, resulting in disconnection from familiar interactions with a parent, a child (because of its relation with affective bonding), or a romantic partner (given the role of pheromones in signaling attraction and sexual experiences).
Emotional state: Feelings of sadness and even depression, since a lack of pleasant stimuli from everyday aromas coming from flowers, trees, animals, perfumes, and food, leads to isolating behaviors or anxiety. Dysosmia can also affect personal hygiene.
Safety: An individual with dysosmia may be uncapable of detecting odors from fire, spoiled food and/or dangerous chemicals such as gas.
Prevalence
Despite dysosmia being very common, there are no definitive statistics of it in the United States, underreporting and a lack of test standardization to diagnose it contribute to the variability of information.
A study published in 2016 calculated that in the US about 20.5 million (13.5%) of ≥40 year/old individuals had a smell disorder, based on a test were they could not identify 6 or more odors from a sample of 8 including onion, soap, smoke, natural gas, chocolate, strawberry, leather and grape (5).
A more recent study estimated a prevalence of 13 million adults in the United States (6).
They differ because of the variables and populations used to measure olfactory perception.
Causes of dysosmia
There are many dysosmia causes, including (7):
Rhinosinusitis with or without nasal polyposis.
Neurological or neurodegenerative diseases: In some, the disease is a cause of dysosmia, in others, dysosmia is one of the symptoms associated to the disease. Examples of therese are: Parkinson’s, Alzheimer’s disease, multiple sclerosis, dementia, temporal lobe epilepsy, acute depressive episode, schizophrenia.
Viral infections of the upper respiratory tract: COVID-19 and viruses that cause common cold.
Medications.
Toxic substances.
Brain tumor of the frontal o basal area.
Craniocerebral trauma.
Congenital: Kallmann syndrome, congenital insensitivity to pain.
Radiation therapy.
Surgery near the area of the olfactory system.
Age: Up to 75% of individuals in their eight decade have some degree of dysosmia.
Idiopathic: This is a rule-out diagnosis.
Duration and prognosis
Dysosmia can persist from a few days to several months or years. And recovery times depend on the cause, the amount of time since the loss, age, the intervention to help it, parosmia, history of smoking, and other variables (2), (7).
Younger non-smokers with a recent viral infection as the cause of the dysosmia have a more favorable prognosis.
For individuals with a loss of smell longer than 18 months, about 30% will improve spontaneously withing a year (7).
Treatment
The timing of diagnosis and treatment onset is crucial for the recovery of the olfactory function. Nevertheless, while studies support that earlier interventions increase the probability of some recovery of the olfactory function, the specific timing when it is too late to start a treatment remains unknown (2).
Treatment for dysosmia is related to the cause, it may involve surgery (of nasal polyps), antibiotics (for a bacterial infection), steroids and even monoclonal antibodies (for inflammatory processes) or a specific treatment of an underlying disease. Some cases resolve spontaneously.
Olfactory training, a treatment that “teaches” the brain how to smell again, is effective in dysosmias caused by conditions not related to the sinuses or nasal passages (non-sinonasal olfactory dysfunction) (7).
References
1. Mathis S, Le Masson G, Soulages A, Duval F, Carla L, Vallat JM, et al. Olfaction and anosmia: From ancient times to COVID-19. J Neurol Sci. 2021;425(March).
2. Patel ZM, Holbrook EH, Turner JH, Adappa ND, Albers MW, Altundag A, et al. International consensus statement on allergy and rhinology: Olfaction. Int Forum Allergy Rhinol. 2022;12(4):327–680.
3. Adams DR, Kern DW, Wroblewski KE, McClintock MK, Dale W, Pinto JM. Olfactory Dysfunction Predicts Subsequent Dementia in Older U.S. Adults. J Am Geriatr Soc. 2018;66(1):140–4.
4. Ramos-Casademont L, Martin-Jimenez D, Villarreal-Garza B, Sánchez-Gomez S, Callejon-Leblic MA. The Value of Subjective Olfactometry as a Predictive Biomarker of Neurodegenerative Diseases: A Systematic Review. Life. 2024;14(3):1–24.
5. Liu G, Zong G, Doty RL, Sun Q. Prevalence and risk factors of taste and smell impairment in a nationwide representative sample of the US population: A cross-sectional study. BMJ Open. 2016;6(11):8–11.
6. Schlosser RJ, Desiato VM, Storck KA, Nguyen SA, Hill JB, Washington BJ, et al. A Community-Based Study on the Prevalence of Olfactory Dysfunction. Am J Rhinol Allergy. 2020;34(5):661–70.
7. Hummel T, Liu DT, Müller CA, Stuck BA, Welge-Lüssen A, Hähner A. Olfactory Dysfunction: Etiology, Diagnosis, and Treatment. Dtsch Arztebl Int. 2023;120(9):146–54.
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