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disorders like Parkinson ’ s and Alzheimer ’ s disease , even prior to the appearance of significant clinical symptoms .
Acute onset of anosmia under the current environment should strongly suggest COVID-19 infection and trigger further testing . Temporary anosmia appears to be the most common neurological symptom in COVID-19 . Up to 25 % of patients may experience sudden onset of hyposmia or anosmia as the first symptom . 1 The prevalence of anosmia and / or ageusia ( loss of taste ) was 48 % and 41 % respectively in an extensive meta-analysis of COVID-19 patients . 2 Recent studies suggest that the virus does not necessarily infect olfactory neurons and that the loss of smell is secondary to involvement of supporting cells such as the olfactory epithelial sustentacular cells and olfactory bulb pericytes . 3 This may be the reason why the anosmia is transient in most patients with COVID-19 infection .
It may be of interest to look at the opposite situations , where the sense of smell is accentuated ( hyperosmia ) or occurs as transient hallucination ( phantosmia ). In seizures originating in the temporal lobe , phantosmia may occur , often accompanied by confusion and sometimes convulsive seizure . I vividly remember my first such patient , a 45-year-old woman who was concerned about brief episodes of experiencing the smell of gasoline ( she initially thought that her students were playing a prank , hiding gasoline in the classroom ). During some episodes she felt confused , and the last one was accompanied by a convulsive seizure . Such seizures with olfactory aura are known as “ uncinate fits ” indicating their likely origin from the uncus , a part of the olfactory pathway . Needless to say , the story did not end well , as the underlying cause was found to be a malignant glioma in the anterior temporal lobe . She was dead within 18 months , despite surgical resection and radiation treatment . It is surprising and sobering that almost every one of my patients who has experienced an olfactory aura described it as a foul smell , not as a pleasant aroma .
The use of olfaction as a useful tool in the detection of diseases , although known from the time of Hippocrates , has seen sensational debuts in the past few years . Much attention has been focused on a few with highly sophisticated sense of smell , the so-called “ super-smellers .” The most fascinating story is that of Ms . Joy Milne , a retired nurse from Perth , Scotland , who realized that she could “ smell out ” Parkinson ’ s disease ( PD ) while she was attending a support group meeting . Her husband had suffered from PD for many years and she was well aware of a strange smell that emanated from him . She noted that she could perceive the same smell in other patients with PD attending the support meeting . She passed the “ T-Shirt test ” with flying colors , identifying correctly , which shirts were worn by patients with PD . Further research confirmed that the ability to detect “ PD odor ” depended on chemicals in the sebum of PD patients . 4 Efforts are underway to develop a skin swab test , which may detect PD even before clinical signs and symptoms occur .
Let us turn attention to our canine friends who have exquisite olfactory sensitivity and can outperform the “ e-nose .” A patient with epilepsy whom I saw decades ago used to brag about his dog , which apparently could sense when a seizure is imminent . I did not trust him at first , but his parents did corroborate the story . In the next several years , this phenomenon has been well-authenticated , and seizure alerting dogs ( SADs ) are being put to use more and more . There has been no convincing explanation for this phenomenon , but recent studies suggest that olfaction is the underlying mechanism . Disease-specific volatile organic compounds ( VOCs ) are likely to be the clue for the dogs capable of detecting seizures . 5 A similar mechanism may underlie ability of dogs to detect other disorders including certain types of cancer . An aspect of current interest is the discovery that trained dogs may have the potential to “ sniff out ” COVID-19 patients . 6 Hopefully it won ’ t be long before COVID-19 detecting dogs will be doing their “ sniff duty ” at the airports , offering a better experience to air travelers , when compared to the intrusive nasopharyngeal swabbing .
References :
1 . Sedaghat AR , Gengler I , Speth MM . Olfactory Dysfunction : A Highly Prevalent Symptom of COVID-19 with Public Health Significance . Otolaryngol Head Neck Surg . 2020 ; 163 ( 1 ): 12-15 .
2 . Ibekwe TS , Fasunla AJ , Orimadegun AE . Systematic review and meta-analysis of smell and taste disorders in COVID-19 . OTO Open . 2020 ; 4 ( 3 ): 2473974X20957975 .
3 . Brann D , Tsukahara T , Weinreb C et al . Non-neuronal expression of SARS-CoV-2 entry genes in olfactory system suggests mechanisms underlying COVID-19-associated anosmia . Science Advances . 2020 ; 6 ( 31 ): DOI : 10.1126
4 . Poewe W : Smelling Parkinson ’ s Disease : New Metabolomic Biomarker for PD . ACS Cent Sci . 2019:5 ( 4 ): 575 – 576
5 . Catala A , Grandgeorge M , Schaf J et al . Dogs demonstrate the existence of an epileptic seizure odor in humans . Scientific reports . 201:9 ; 4103 https :// doi . org / 10.1038 / s41598-019-40721-4
6 . Jendrny , P ., Schulz , C ., Twele , F . et al . Scent dog identification of samples from COVID-19 patients – a pilot study . BMC Infect Dis . 2020 ; 20 : 536 https :// doi . org / 10.1186 / s12879-020-05281-3
Dr . Iyer practices at the Neurodiagnostic Center of Louisville and is a retired professor of neurology at the University of Louisville School of Medicine .
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