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Sixty seconds on . . . smell training

BMJ 2021; 373 doi: https://doi.org/10.1136/bmj.n1080 (Published 26 April 2021) Cite this as: BMJ 2021;373:n1080

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Re: Sixty seconds on . . . smell training

Dear Editor,

Although it is often stated that 60% of patients with Covid-19 experience varying degrees of smell impairment this information is based on questionnaire analysis. Where standardised olfactory testing is employed the figure is almost 100% (Hawkes 2021)

There have been several attempts at olfactory retraining over the past 12 years. On basic principles one would expect that smell training could work, given the parallels with the other special senses and physiotherapy for motor and cerebellar deficits. Healthy people without smell impairment can lower (improve) their thresholds by training and become wine tasters or perfumers. The first formal trial of smell training was based on a small non-blinded case-control study of 56 people with smell deficits from various causes (Hummel et al 2009). The cases had to repeatedly inhale odours of rose, cloves, eucalyptus and lemon before bedtime and on wakening for 12 weeks. One of 16 control subjects showed lower thresholds after training compared to 11 of 40 patients. Neither group improved on identification scores. Five patients (21%) with smell impairment resulting from upper respiratory infection showed improvement. As reviewed by Doty (2019) there have been several further trials, mostly involving small numbers and without blinding. Some cases were studied for longer periods (e.g. 32 weeks) others used 12 odours instead of 4. In a meta-analysis Sorokowska et al (2017) concluded that there was a positive effect especially for longer training periods and suggested that the benefit derived from enhanced cognitive processing of olfactory information and increased attention to odours. Overall there is no robust scientific evidence for improvement and clearly there is a need for a large double blind trial of olfactory training.

Other approaches designed to accelerate smell recovery that fail to show clear benefit include: acupuncture, alpha-lipoic acid, caroverine, steroids, rasagiline, sodium citrate, theophylline, venlafaxine, vitamins A, B12, D, E and zinc sulphate (Doty 2019).

Where there are olfactory illusions (phantosmias) or distortions (cacosmias) there is anecdotal evidence of benefit with pregabalin, gabapentin or sodium valproate.

Around two-thirds of people with impaired smell sense after Covid-19 recover within two months (Hawkes 2021). So, if your smell sense is persistently impaired by all means try training, it’s harmless and might just help a little. Ideally, relatively pure aromas should be used such as rose, lavender, vanilla, pear, peach, cut grass, orange, chocolate, banana. It is important not to use odours that have significant additional stimulant effect on the non-olfactory nasal epithelium as this region is innervated by the trigeminal nerve. Trigeminal stimulant odours include: onion, alcohol, mint, menthol, chili, ammonia. Any improvement with these ‘impure’ smells could result from training of a healthy trigeminal nerve and indeed this mechanism may be the explanation of benefit observed in smell training trials so far.

References

Doty RL. Treatments for smell and taste disorders: a critical review. Handbook of clinical neurology. 2019 Jan 1;164:455-79.

Hawkes CH. Smell, taste and COVID-19: testing is essential. QJM: An International Journal of Medicine. 2021 Feb;114(2):83-91.

Hummel T, Rissom K, Reden J, Hähner A, Weidenbecher M, Hüttenbrink KB. Effects of olfactory training in patients with olfactory loss. The Laryngoscope. 2009 Mar;119(3):496-9.

Sorokowska A, Drechsler E, Karwowski M, Hummel T. Effects of olfactory training: a meta-analysis. Rhinology. 2017 Mar 1;55(1):17-26.

Competing interests: No competing interests

10 May 2021
Christopher H Hawkes
Honorary Professor of Neurology & Honorary Consultant Neurologist
Queen Mary, University of London & Barts School of Medicine and Dentistry
Neuroimmunology Unit, Blizard Institute, 4 Newark St , London E12AT