brain sciences Case Report Comparison of Brain Activation Patterns during Olfactory Stimuli between Recovered COVID-19 Patients and Healthy Controls: A Functional Near-Infrared Spectroscopy (fNIRS) Study Roger C. Ho 1,2 , Vijay K. Sharma 3 , Benjamin Y. Q. Tan 4 , Alison Y. Y. Ng 4 , Yit-Shiang Lui 5 , Syeda Fabeha Husain 1,*, Cyrus S. Ho 1, Bach X. Tran 6,7, Quang-Hai Pham 8,9, Roger S. McIntyre 10,11,12 and Amanda C. Y. Chan 4 1 Department of Psychological Medicine, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 119228, Singapore; [email protected] (R.C.H.); [email protected] (C.S.H.) 2 Institute of Health Innovation and Technology (iHealthtech), National University of Singapore, Singapore 117599, Singapore 3 Department of Medicine, Yong Loo Lin School of Medicine, National University of Singapore, Singapore 119228, Singapore; [email protected] 4 Department of Medicine, National University Hospital, Singapore 119228, Singapore; [email protected] (B.Y.Q.T.); [email protected] (A.Y.Y.N.); [email protected] (A.C.Y.C.) 5 Department of Psychological Medicine, National University Health System, Singapore 119228, Singapore; [email protected] Citation: Ho, R.C.; Sharma, V.K.; Tan, 6 Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD 21205, USA; [email protected] B.Y.Q.; Ng, A.Y.Y.; Lui, Y.-S.; Husain, 7 Institute for Preventive Medicine and Public Health, Hanoi Medical University, Hanoi 116001, Vietnam S.F.; Ho, C.S.; Tran, B.X.; Pham, Q.-H.; 8 Institute for Global Health Innovations, Duy Tan University, Da Nang 550000, Vietnam; McIntyre, R.S.; et al. Comparison of [email protected] Brain Activation Patterns during 9 Faculty of Medicine, Duy Tan University, Da Nang 550000, Vietnam 10 Olfactory Stimuli between Recovered Mood Disorders Psychopharmacology Unit, University Health Network, University of Toronto, COVID-19 Patients and Healthy Toronto, ON M5G 2C4, Canada; [email protected] 11 Department of Psychiatry, University of Toronto, Toronto, ON M5T 1R8, Canada Controls: A Functional Near-Infrared 12 Canadian Rapid Treatment Center of Excellence, Mississauga, ON L5C 4E7, Canada Spectroscopy (fNIRS) Study. Brain Sci. * Correspondence: [email protected] 2021, 11, 968. https://doi.org/ 10.3390/brainsci11080968 Abstract: Impaired sense of smell occurs in a fraction of patients with COVID-19 infection, but Academic Editor: Wissam El-Hage its effect on cerebral activity is unknown. Thus, this case report investigated the effect of COVID- 19 infection on frontotemporal cortex activity during olfactory stimuli. In this preliminary study, Received: 1 June 2021 patients who recovered from COVID-19 infection (n = 6) and healthy controls who never contracted Accepted: 20 July 2021 COVID-19 (n = 6) were recruited. Relative changes in frontotemporal cortex oxy-hemoglobin during Published: 23 July 2021 olfactory stimuli was acquired using functional near-infrared spectroscopy (fNIRS). The area under curve (AUC) of oxy-hemoglobin for the time interval 5 s before and 15 s after olfactory stimuli Publisher’s Note: MDPI stays neutral was derived. In addition, olfactory function was assessed using the Sniffin’ Sticks 12-identification with regard to jurisdictional claims in test (SIT-12). Patients had lower SIT-12 scores than healthy controls (p = 0.026), but there were no published maps and institutional affil- differences in oxy-hemoglobin AUC between healthy controls and patients (p > 0.05). This suggests iations. that past COVID-19 infection may not affect frontotemporal cortex function, and these preliminary results need to be verified in larger samples. Keywords: COVID-19; functional near-infrared spectroscopy (fNIRS); olfactory stimuli; sniffin’ sticks Copyright: © 2021 by the authors. 12-identification test Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons 1. Introduction Attribution (CC BY) license (https:// Globally, as of May 6, 2021, there have been 154,640,649 confirmed cases of Coron- creativecommons.org/licenses/by/ avirus disease-2019 (COVID-19), including 3,232,285 deaths, reported to the World Health 4.0/). Brain Sci. 2021, 11, 968. https://doi.org/10.3390/brainsci11080968 https://www.mdpi.com/journal/brainsci Brain Sci. 2021, 11, 968 2 of 10 Organization (WHO) [1]. Since the outbreak of the COVID-19 pandemic, medical profes- sionals have a better understanding of the pathogenesis and clinical features of COVID-19 infection. COVID-19 creates severe respiratory distress and other systemic complications involving multiple organ systems, due to a cascade of immunological responses [2]. Pa- tients who suffered from COVID-19 infection may develop neurological complications, including acute myelitis, cerebrovascular accidents, cerebral venous thrombosis, Guillain- Barré syndrome, meningoencephalitis, posterior reversible encephalopathy syndrome and seizures [3]. Common neuropsychiatric symptoms of COVID-19 infection include change in mental state, delirium, giddiness, gustatory impairment, myalgia, headache [3] and cognitive impairment [4]. In addition to the above symptoms, COVID-19 infection is known to cause olfactory dysfunction [5]. Recent studies have focused on developing questionnaires and a smell identification test that assess olfactory dysfunction [6,7]. It has been reported that the prevalence of olfactory dysfunction is 73.1%, with a male to female ratio of 1:3 [8]. It was also reported that complete recovery of olfactory dysfunction occurs in 31.8% of COVID-19 survivors [8]. Olfactory dysfunction may be due to olfactory cleft opacification and olfactory bulb degeneration, detected by magnetic resonance imaging (MRI) of the olfactory bulb and computer tomography of the paranasal sinus [9]. Furthermore, there has been a report on the association between opacification in the olfactory cleft and the degree of loss of smell [10]. Olfactory dysfunction is thought to manifest when viral replication in the non-neural olfactory cells indirectly cause damage to the olfactory receptor nerves [11]. The COVID-19 virus binds to the human angiotensin converting enzyme ACE2 receptor, and it may thus target other cells that express this protein, such as neurons in the brain [5]. Viruses have been shown to be transported along synapses connecting the peripheral olfactory epithelium and the central nervous system. The first target regions are part of the olfactory system, including the olfactory bulb and amygdala. The virus may then spread to other tissues and induce neurodegenerative symptoms such as epilepsy, motor and cognitive symptoms [5]. In addition, recent studies have linked the incidence of acute respiratory failure with COVID-19 infection of the brainstem. It has been postulated that the COVID-19 virus causes dysfunction of the respiratory center by spreading from the olfactory bulb to the olfactory nerves, the rhinencephalon and, finally, the brainstem [12]. Despite the potential neuropathogenesis, neuroimaging studies on patients with COVID-19 and other respiratory infections are limited. Electroencephalography of patients with COVID-19 infection have shown frontal lobe abnormalities [13,14], while positron emission tomography (PET) of these patients has identified altered glucose metabolism in several brain areas. These include hypometabolism in the bilateral parahippocampal and fusiform gyri, left insula [15], dorsolateral prefrontal cortex, bilateral frontal eye fields and right anterior cingulate cortex, as well as hypermetabolism in the left orbitofrontal cortex, right posterior parietal cortex and right thalamus [16]. Grey matter volume loss detected by MRI occurred in the right orbitofrontal cortex of patients with post-infectious olfactory loss [17], decreased functional connectivity of the chemosensory network occurred in pa- tients with chronic peripheral smell loss [17] and hypometabolism of the medial and lateral temporal cortex occurred in patients with olfactory dysfunction after an upper respiratory tract infection [18]. Taken together, the literature suggests that COVID-19 infection may affect brain function and that viral-associated loss of smell may be linked to long-term changes in the brain. Still, there is a gap in the literature about the neurophysiological response to direct olfactory stimulation in patients with viral-associated smell loss. Hence, neurobiological investigations with technologies that allow neuroimaging data to be ac- quired during direct olfactory stimulation, such as functional near-infrared spectroscopy (fNIRS), are needed. fNIRS enables real-time monitoring of hemodynamic changes in the cerebral cortex, because light in the near-infrared spectrum has the unique property of passing though tissues and being preferentially absorbed by hemoglobin in the cerebral cortex [19]. The absorbance spectra of hemoglobin is dependent on its binding with oxygen, which en- Brain Sci. 2021, 11, 968 3 of 10 ables fNIRS devices to continuously detect relative changes in both oxy-hemoglobin and deoxy-hemoglobin in the cortical regions being studied [20]. According to a phenomenon called neurovascular coupling, fNIRS signals are believed to be a surrogate measure of the underlying neural activity [21]. Regional neuron activity triggers an increase in blood flow and volume that is many times higher than the metabolic demand. Therefore, cere- bral hemodynamic response typically involves a large increase in oxy-hemoglobin and