Increasing incidence of parosmia and phantosmia in patients recovering from COVID-19 smell loss
Authors: Ohla Kathrin , Veldhuizen Maria Geraldine , Green Tomer , Hannum Mackenzie E. , Bakke Alyssa J. , Moein Shima T. , Tognetti Arnaud , Postma Elbrich M. , Pellegrino Robert , Hwang Liang-Dar , Albayay Javier , Koyama Sachiko , Nolden Alissa , Thomas-Danguin Thierry , Mucignat-Caretta Carla , Menger Nick S. , Croijmans Ilja , Öztürk Lina , Yanık Hüseyin , Pierron Denis , Pereda-Loth Veronica , Nunez-Parra Alexia , Pineda Aldair M. Martinez , Gillespie David , Farruggia Michael C. , Cecchetto Cinzia , Fornazieri Marco A. , Philpott Carl , Voznessenskaya Vera , Cooper Keiland , Dominguez Paloma Rohlfs , Calcinoni Orietta , de Groot Jasper , Boesveldt Sanne , Bhutani Surabhi , Weir Elisabeth M. , Exten Cara , Joseph Paule V. , Parma Valentina , Hayes John E. , Niv Masha Y. 
Published: August 2021

AbstractImportanceSudden smell loss is a specific early symptom of COVID-19, with an estimated prevalence of ~40% to 75%. Smell impairment affects physical and mental health, and dietary behavior. Thus, it is critical to understand the rate and time course of smell recovery.ObjectiveTo characterize smell function and recovery up to 11 months post COVID-19 infection.Settings, ParticipantsThis longitudinal survey of individuals suffering COVID-19-related smell loss assessed disease symptoms and gustatory and olfactory function. Participants (n=12,313) who completed an initial respiratory symptoms, chemosensory function and COVID-19 diagnosis survey (S1) between April and September 2020 and completed a follow-up survey (S2) between September 2020 and February 2021; 27.5% participants responded (n=3,386), with 1,468 being diagnosed with COVID-19 and suffering co-occurring smell and taste loss at the beginning of their illness.Main Outcomes & MeasuresPrimary outcomes are ratings of smell and taste function on a visual analog scale, and self-report of parosmia (smell distortions) and phantosmia (unexplained smells). Secondary outcomes include a checklist of other COVID-19 symptoms.ResultsOn follow-up (median time since COVID-19 onset ~200 days), ~60% of women and ~48% of men reported less than 80% of their pre-illness smell ability. Taste typically recovered faster than smell, and taste loss rarely persisted if smell recovered. Prevalence of parosmia and phantosmia was ~10% of participants in S1 and increased substantially in S2: ~47% for parosmia and ~25% for phantosmia. Persistent smell impairment was associated with more symptoms overall, suggesting it may be a key marker of long-COVID. During COVID-19 illness, the ability to smell was slightly lower among those who did not recover their pre-illness ability to smell at S2.Conclusions and RelevanceWhile smell loss improves for many individuals who lost it due to COVID-19, the prevalence of parosmia and phantosmia increases substantially over time. Olfactory dysfunction is also associated with wider COVID-19 symptoms and may persist for many months after COVID-19 onset. Taste loss in the absence of smell loss is rare. Persistent qualitative smell symptoms are emerging as common long term sequelae; more research into treatment options is strongly warranted given that conservative estimates suggest millions of individuals may experience parosmia following COVID-19. Healthcare providers worldwide need to be prepared to treat post COVID-19 secondary effects on physical and mental health.Trial registrationThis project was pre-registered at OSF: https://osf.io/3e6zc.Graphical abstractKey PointsQuestionWhat are the characteristics of smell and taste recovery of COVID-19 patients?FindingsIn this preregistered observational study of 1,468 participants, smell loss is associated with a higher number of COVID-19 symptoms, and may persist for at least 11 months following disease onset. While a majority of participants report quantitative improvement in their ability to smell, the prevalence of parosmia and phantosmia increases substantially at follow-up. Taste recovers faster than smell, suggesting taste and smell recover separately and can be distinguished by the respondents.MeaningOlfactory dysfunction appears to be a component of long-COVID, with parosmia as a prominent symptom in almost half of those with smell loss. More research into treatment is needed, especially given that olfactory dysfunction is associated with depression and loss of appetite. Health professionals should be aware of these common and long lasting effects.