Smell training

Smell training or olfactory training is the act of regularly sniffing or exposing oneself to robust aromas[1] with the intention of regaining a sense of smell. The stimulating smells used are often selected from major smell categories, such as aromatic, flowery, fruity, and resinous.[1] Using strong scents, the patient is asked to sniff each different smell for a minimum of 20 seconds, no less than two times per day, for three to six months or more.[2][3] It is used as a rehabilitative therapy to help people who have anosmia or post-viral olfactory dysfunction, a symptom of COVID-19.[4] It was considered a promising experimental treatment in a 2017 meta-analysis.[1]

Efficacy

Along with olfactory implants,[5] smell training is a promising but experimental treatment option.[1]

Several individual studies have indicated that smell training can increase olfactory sensitivity.[6][7][8] In 2021 a meta-analysis was published that examined research studies of olfactory training for treating loss of smell as a consequence of a viral infection. It found clinically significant improvements and supported its use as a treatment option.[9] As of March 2021, there have been no studies of smell training's efficacy for children.[10]

In 2017, the International and European Rhinologic Societies recommended smell training for treating loss of smell due to various conditions.[11] In 2020, the British Rhinological Society published consensus guidelines for the treatment of smell loss due to COVID-19.[12] Although no specific studies were available at that time, the expert panel made recommendations regarding treatment options and concluded that "olfactory training was recommended for all [COVID-19] patients with persistent loss of sense of smell of more than 2 weeks duration."[12]

Critics such as Richard Doty have pointed to the small sample sizes in the studies and the potential for the observed improvements to have been the result of nerve regeneration that would have occurred without intervention as reason to be skeptical.[13][14]

Mechanism

Smell training likely achieves results because the olfactory nerve and olfactory bulb have neural plasticity and are able to regenerate.[1]

History

The idea was first written about by Thomas Hummel, a German psychologist at the Dresden University of Technology, in his 2009 paper "Effects of olfactory training in patients with olfactory loss".[15] In his original study, Hummel instructed patients with olfactory dysfunction to follow a twice-a-day routine for twelve weeks. The routine included inhaling the odor of rose, lemon, clove, and eucalyptus (phenyl ethyl alcohol, citronellal, eugenol, and eucalyptol respectively) essential oils for ten seconds each. These intense odors each correspond to a different odor category in Henning's odor prism.[7]

Hummel's paper built on a 1989 study by the Monell Chemical Senses Center in Philadelphia. The study showed that after repeated exposure to androstenone, a chemical which half of all humans cannot detect, some subjects gained the ability to smell it.[16]

Alternatives

In addition to smell training, other treatments for anosmia that have been researched include systemic steroidal and non-steroidal oral medications, topical medications, and acupuncture.[17]

References
  1. Sorokowska A, Drechsler E, Karwowski M, Hummel T (March 2017). "Effects of olfactory training: a meta-analysis". Rhinology. 55 (1): 17–26. doi:10.4193/Rhin16.195. PMID 28040824.
  2. Olfactory dysfunction in COVID-19: diagnosis and management. Whitcroft KL, Hummel T. JAMA. 2020;323:2512–2514. [Abstract] [Google Scholar]
  3. Chabot A, Huntwork M. Turmeric as a Possible Treatment for COVID-19-Induced Anosmia and Ageusia Cureus. ;13(9). PMCID: PMC8502749.
  4. Zhang Y, Mei T, Chen Y, Wang L, Jiang L, Liu K, et al. (February 2021). "Smell disorders in COVID-19 patients: role of olfactory training: A protocol for systematic review and meta-analysis". Medicine. 100 (8): e24862. doi:10.1097/MD.0000000000024862. PMC 7909207. PMID 33663108.
  5. Holbrook, Eric H.; Coelho, Daniel H. (1 February 2020). "Cranial Nerve Stimulation for Olfaction (Cranial Nerve 1)". Otolaryngologic Clinics of North America. 53 (1): 73–85. doi:10.1016/j.otc.2019.09.014. ISSN 1557-8259. PMID 31685237. S2CID 207904053.
  6. Besser G, Oswald MM, Liu DT, Renner B, Mueller CA (July 2020). "Flavor education and training in olfactory dysfunction: a pilot study". European Archives of Oto-Rhino-Laryngology. 277 (7): 1987–1994. doi:10.1007/s00405-020-05950-8. PMC 7286942. PMID 32248300.
  7. Hummel T, Rissom K, Reden J, Hähner A, Weidenbecher M, Hüttenbrink KB (March 2009). "Effects of olfactory training in patients with olfactory loss". The Laryngoscope. 119 (3): 496–9. doi:10.1002/lary.20101. PMID 19235739. S2CID 5239574.
  8. Liu DT, Sabha M, Damm M, Philpott C, Oleszkiewicz A, Hähner A, Hummel T (March 2021). "Parosmia is Associated with Relevant Olfactory Recovery After Olfactory Training". The Laryngoscope. 131 (3): 618–623. doi:10.1002/lary.29277. PMID 33210732.
  9. Kattar, Nrusheel; Do, Triet M.; Unis, Graham D.; Migneron, Matthew R.; Thomas, Andrew J.; McCoul, Edward D. (February 2021). "Olfactory Training for Postviral Olfactory Dysfunction: Systematic Review and Meta-analysis". Otolaryngology–Head and Neck Surgery. 164 (2): 244–254. doi:10.1177/0194599820943550. PMID 32660334. S2CID 220519030.
  10. Rodriguez CH (25 March 2021). "Nosing In on Kids Who Had Covid and Lost Their Sense of Smell". Kaiser Health News. Retrieved 25 March 2021.
  11. Hummel, T.; Whitcroft, K.L.; Andrews, P.; Altundag, A.; Cinghi, C.; Costanzo, R.M.; Damm, M.; Frasnelli, J.; Gudziol, H.; Gupta, N.; Haehne, A.; Holbrook, E.; Hong, S.C.; Hornung, D.; Huttenbrink, K.B.; Kamel, R.; Kobayashi, M.; Konstantinidis, I.; Landis, B.N.; Leopold, D.A.; Macchi, A.; Miwa, T.; Moesges, R.; Mullol, J.; Mueller, C.A.; Ottaviano, G.; Passali, G.C.; Philpott, C.; Pinto, J.M.; Ramakrishnan, V.J.; Rombaux, P.; Roth, Y.; Schlosser, R.A.; Shu, B.; Soler, G.; Stjarne, P.; Stuck, B.A.; Vodicka, J.; Welge-Luessen, A. (1 March 2017). "Position paper on olfactory dysfunction" (PDF). Rhinology Journal. 54 (26): 1–30. doi:10.4193/Rhino16.248. PMID 29528615. Retrieved 5 May 2021.
  12. Hopkins, Claire; Alanin, Mikkel; Philpott, Carl; Harries, Phil; Whitcroft, Katherine; Qureishi, Ali; Anari, Shahram; Ramakrishnan, Yujay; Sama, Anshul; Davies, Elgan; Stew, Ben; Gane, Simon; Carrie, Sean; Hathorn, Iain; Bhalla, Raj; Kelly, Chrissi; Hill, Nina; Boak, Duncan; Nirmal Kumar, B. (January 2021). "Management of new onset loss of sense of smell during the COVID‐19 pandemic ‐ BRS Consensus Guidelines". Clinical Otolaryngology. 46 (1): 16–22. doi:10.1111/coa.13636. PMC 7461026. PMID 32854169.
  13. Doty, Richard L. (2019). "Treatments for smell and taste disorders: A critical review". Smell and Taste. Handbook of Clinical Neurology. Vol. 164. pp. 455–479. doi:10.1016/B978-0-444-63855-7.00025-3. ISBN 9780444638557. PMID 31604562. S2CID 204330823.
  14. Gamble, Maeve. "People Are Trying a Questionable Therapy to Get Back Their Sense of Smell". Slate. Retrieved 5 May 2021.
  15. Shapouri B (17 November 2020). "The Weird World of Olfactory Training". Allure. Retrieved 30 March 2021.
  16. Wysocki CJ, Dorries KM, Beauchamp GK (October 1989). "Ability to perceive androstenone can be acquired by ostensibly anosmic people". Proceedings of the National Academy of Sciences of the United States of America. 86 (20): 7976–8. Bibcode:1989PNAS...86.7976W. doi:10.1073/pnas.86.20.7976. PMC 298195. PMID 2813372.
  17. Hura, Nanki; Xie, Deborah X.; Choby, Garret W.; Schlosser, Rodney J.; Orlov, Cinthia P.; Seal, Stella M.; Rowan, Nicholas R. (25 June 2020). "Treatment of post‐viral olfactory dysfunction: an evidence‐based review with recommendations". International Forum of Allergy & Rhinology. 10 (9): 1065–1086. doi:10.1002/alr.22624. ISSN 2042-6976. PMC 7361320. PMID 32567798.

Further reading
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