One of the symptoms considered most characteristic for SARS-CoV-2 infection is smell alteration, ranging from transitory olfactory dysfunction, hyposmia to anosmia. Although the COVID-19 pandemic enters the third year, there are still many questions to be asked and answered regarding the pathology progress of the SARS-CoV-2 infection. One question regards the possible mechanism for producing the olfactory dysfunction. There are still debates concerning the intricate events leading to hyposmia. Moreover, the timespan for suffering from hyposmia is variable, some patients recovering, unfortunately, after long periods of time, and in a very small percentage developing long-COVID syndrome and anosmia. Understanding these processes could lead to improving the recovery of cases with prolonged smell disruption after COVID-19. Encouraging is the fact that during recent waves of SARS-CoV-2 infections there is a decreasing occurrence of hyposmia.
In a study enlisting 288 patients, almost two thirds presented anosmia and in almost one third of these cases the loss of smell was actually the first symptom(1). Regarding the persistence of anosmia, a recent study from France identified 17.6% of the cases suffering from anosmia at six months after the acute phase(2). Using data from a large cohort of almost 13,000 patients, anosmia had a 90% predictivity of RT-PCR positivity, raising questions whether for cases with this symptom there is necessary the financial effort of testing(3).
Olfactory system – structural changes
These two years of pandemics recorded various approaches and responses for understanding the mechanism of hyposmia/anosmia during the SARS-CoV-2 infection(4):
olfactory cleft syndrome;
local inflammation in the nasal epithelium;
early apoptosis of olfactory cells;
changes in olfactory cilia and odor transmission;
damage to microglial cells;
direct effect on olfactory bulbs;
epithelial olfactory injury;
impairment of olfactory neurons and stem cells.
Regarding the olfactory cleft, a study analyzed the differences between patients infected with SARS-CoV-2 who developed anosmia and those without smell disorders. The initial hypothesis was that a smaller anatomy conformation of the olfactory cleft width and volume measured using CT scans will predict the onset of anosmia. However, the study proved, on the contrary, that a lower surface area, meaning a lower possible contact between the virus and the olfactory cleft, will prevent from developing anosmia(5).
MRI studies were the most common imaging tools for visualizing the olfactory bulb (OB). Although 31.2% of the subjects presented normal olfactory bulb images, the rest of the cases recorded polymorphous findings such as: enhancement following gadolinium injection in 8.25% of cases, T2 hyperintensity in the OB/tract in 13.30% of cases, and microhemorrhage/methemoglobin deposition in 7.8% of cases, suggesting that a combination of pathologic changes concur for the prolonged hyposmia(6).
When the OB types were compared between the anosmia and control groups, type D presenting of asymmetric contour lobulation or hyperintense focus of >1 on T2 images and type R, the shrunken or flattened OBs without deformity were more common in the anosmia group, with a statistically significant difference. This implies a structural anatomic predisposition to anosmia(7).
However, fMRI tractography brought into discussion the fact that the structural changes take time and that the imaging studies are performed with priority in COVID-19 cases compared with past cases with anosmia induced by other viruses. Also, there was no significant difference in orbitofrontal and entorhinal activity between COVID-19-related anosmia and post-infectious anosmia, whereas trigeminosensory activity was more robust in COVID-19-related anosmia(8).
Taking the analysis to a microscopic level, the virus is involved in the physical destruction of the structure of the nasopharyngeal epithelium. Scanning electron microscopy enabled the visualization of the viral budding in the apical membrane of the microvilli(9).
The next question is regarding the possibility of a neural invasion by the olfactory route due to the close contact with the olfactory tracts and the presence of receptors for the SARS-CoV-2 at the level of the neurons. For answering this question, there are possible in vivo and in vitro post-mortem study designs. One approach was of analyzing the presence of SARS-CoV-2 in the cerebrospinal fluid (CSF) using PCR testing. Although only 6% of the patients presented a positive PCR test from CSF, this confirmed the rare possibility of the direct invasion to the central nervous system in cases with altered olfactory function(10).
The other approach in order to visualize the neuronal invasion is by analyzing specimens from fresh cadavers suffering from COVID-19. SARS-CoV-2 could be clearly identified in all layers of the olfactory mucosa, up to the level of the leptomeningeal layers, but without clear signs of neuronal invasion, and the possible route could also be hematogenous(11).
The structural changes may be produced by capillary dysfunction, with a domino effect on the homeostasis of the olfactory mucosa. Also, in the olfactory bulb, the capillary pericytes present ACE2 surface receptors, the endothelial cells present the same receptors, and the capillaries may be obstructed by neutrophils. All these mechanisms concur to the development of microthrombosis along the olfactory pathway(12).
The early data postulated that the ACE2 receptors are expressed by the sustentacular supporting and basal cells of the nasal olfactory epithelium. Thus, the disruption of the cellular microenvironment led to apoptosis of these supporting cells and to subsequent neuronal suffering. This was a convenient statement backed by the great number of cases in which the smell recovery was quick and, therefore, not a direct neuronal involvement which otherwise would have implied a very long recovery time. Recent studies take into consideration the neuronal involvement, with the propagation of the virus by retrograde axonal transport in the olfactory bulb(13).
ATP exocytosis hypothesis
This is one of the newest theories trying to explain the mechanism behind the smell disorders reported by COVID-19 patients. This theory focuses on the mechanism used by SARS-CoV-2 to actively exit the infected cell. The oxidative damage produced by the virus affects the mineralocorticoid receptor (MR) and activates the ATP accumulation in lysosomes. Cortisol activates the unprotected MR which stimulates the release of ATP into the basolateral compartment of the cell. This then acts on purinergic receptors that open Ca++ channels which will lead to a vicious circle, amplifying the high levels of extracellular ATP and virus exocytosis(14).
In the natural course of an infection of the olfactory epithelium, there would be an immune reaction with the release of specific cytokines or chemokines in the olfactory mucosa that could variably affect cells structurally or functionally. The selective upregulation of interferon inhibits the neuronal odorant receptor protein expression and induces anosmia along with a surge in tumor necrosis factor alpha (TNF-a), known to promote neuronal cell death in cascade(15).
COVID-19 patients present deregulated kynurenine metabolism through the depletion of TRP along with increased kynurenine levels, suggesting increased indoleamine-pyrrole 2,3-dioxygenase (IDO) expression and elevated neurotoxic metabolites of kynurenine such as 3-hydroxykynurenine (3-HK) and quinolinic acid (QA). This is actually a biochemical induced neuronal inflammation with subsequent hyposmia. Future studies are necessary for understanding this hypothesis(16).
Trigeminal connection hypothesis
Headache is another symptom appearing early in SARS-CoV-2 infection. Clinically, it seems to somehow precede the onset of hyposmia but, due to its scarce specificity, it is often overlooked by the patients. TNF-a and IL-1b increase calcitonin gene-related peptide (CGRP) levels in neurons from the trigeminal ganglion. It has experimentally been shown that CGRP released from activated trigeminal fibers inhibits the response of olfactory receptors to olfactory stimuli. The irritation of trigeminal afferents injured by SARS-CoV-2 may contribute to both headache and concomitant anosmia(17).
Serotonin deficiency hypothesis
There is a dense serotonergic innervation of the olfactory bulb coming from the median raphe nucleus in brainstem. Serotonin has an important role in stimulating tufted cells and in depolarizing mitral cells. This theoretical hypothesis is backed by clinical observation of a decreased severity of symptoms in patients receiving psychiatric treatment targeted on the homeostasis of serotonin(18).
Unfortunately, the olfactometric evaluation correlated with the electrophysiological investigations remains rather in an experimental setting. Thus, the objective quantification of anosmia and the exact anatomic site of disruption are very laborious to be determined and possible only on a very limited number of patients. Any case of smell disorder spanning more than 30 days has a very slim chance of recovery, as this is the period for regenerating the olfactory sensory neurons. Longer than this time frame it seems that the main cause of impaired recovery is the neuromodulation of the sensory pathways, with long-term neurological sequelae(19).
Recent data contradict the popular belief and empirical observational studies in healthcare facilities which stated that the loss of smell early in the infection with SARS-CoV-2 would predict a less severe evolution of the disease. Olfactory dysfunction or taste impairment does not have a prognostic importance compared with the CT scan grading(20).
One of the most recent interesting approaches towards understanding anosmia in COVID-19 patients focused on gut hormone GLP‐1, enhancing insulin secretion and functions as a neurotransmitter, coordinating communication between the olfactory epithelium, the taste buds on the tongue and the brain. Low serum levels of GLP‐1 may be associated with anosmia/hyposmia and dysgeusia amongst COVID‐19 patients(21).
Another aspect is that longer recovery periods could be associated with the exacerbation of allergies, and that a chronic modified immune response could lead to undifferentiated allergic reactions. The efficacy of adding antiallergic medicine in the follow-up treatment still lacks definite clinical studies(22).
The treatment options for olfactory disfunction could be(23):
the inflammation in the nasal cavity can be controlled with corticosteroids, statins and melatonin;
phosphodiesterase inhibitors (pentoxifylline, caffeine, and theophylline) prevent the damage to the olfactory sensory neurons;
neuroprotective agents such as statins, minocycline, intranasal vitamin A, intranasal insulin, omega-3 and melatonin could regenerate the olfactory receptor neurons.
Currently, the approach for alleviating smell disorders associated to COVID-19 is olfactory training in newly developed smell clinics. These facilities will enable the long-term follow-up of the cases, along with quantitative measurements of olfactory dysfunction using the University of Pennsylvania Smell Identification Test (UPSIT). Moreover, the progress of each case can be measured quantifying the improvement of the quality of life using ODQ – Olfactory Disorders Questionnaire(24).
Smell training could be performed in outpatient settings using essential oils with bioactive properties and recreating the classic four types of smell: flowery, fruity, aromatic and resinous. One possible combination of four types of essential oils could be, for example, lavender (flowery), lemongrass (fruity), and hinoki cypress (woody/resinous)(25).
Understanding these clear mechanisms behind the progression of SARS-CoV-2 infection could lead to a better support and therapy for the increasing number of cases developing post-COVID-19 syndrome, defined as a suffering continuing for more than 12 weeks and which cannot be attributed to other clinical situations(26).
Unfortunately, there is an old saying: The more you write and speak about a disease, you do not know the disease. This seems to be the status of the SARS-CoV-2 infection after two years of arduous research amidst a turbulent pandemic. The present review tried to gather many recent ideas of research and possible hypotheses regarding the most debated symptom of anosmia. The focus shifted from structural changes to the intricate molecular mechanisms that are disrupted by the viral activity in a domino effect.
Conflict of interests: The authors declare no conflict of interests.