xRead - Olfactory Disorders (September 2023)

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PATEL et al.

TABLE VII.5 (Continued) Study Year LOE

Study design Case-control

Study groups 22 patients had

Clinical end point

Conclusions

Presence of smell dysfunction is associated with reduced OBVs

SS-TDI MRI using CISS sequence

Mueller et al

2005

4

post-URI olfactory deficits

9 patients had

posttraumatic olfactory deficit

17HCs

Yaoet al

2018

4

Case-control

19 controls 19 cases

Volumetric

Decrease in size of the OB is negatively correlated with duration of olfactory loss A secondary outcome showed decrease of the right olfactory cortex in the case group

measurements of theOB

Chunget al

2018

4

Retrospective cohort

34 patients with subjective OD

SS-TDI MRI ofOB

10 patients were normosmic Those who were

hyposmic/anosmic on SS test had a higher detection rate of OB atrophy

Henkin et al

2013

3

Case-control

59 patients (26 men and 33 women) who had varying degrees of smell loss 9 controls (5 men and 4women)

OF measured by detection thresholds and

Plasma levels of IL-6 were significantly elevated in patients with OD compared with controls

recognition thresholds Plasma, urine, parotid

saliva, and nasal mucus samples

CISS = constructive interference in steady state; ERP = event-related potential; HC = healthy control; IL = interleukin; LOE = level of evidence; MRI = magnetic resonance imaging; NA = not available; OB = olfactory bulb; OC = olfactory cleft; OD = olfactory dysfunction; OBV = olfactory bulb volume; OE = olfactory epithelium; OF = olfactory function; OT = olfactory training; PIV3 = parainfluenza virus 3; PVOD = postviral olfactory dysfunction; SS = Sniffin’ Sticks; SS-TDI = Sniffin’ Sticks threshold, discrimination, identification combination; URI = upper respiratory infection.

2 COVID-19 related Otolaryngologists were the first to draw attention to COVID-19–related smell loss and champion its role as an early, and often only, sign of COVID-19 infection. 2,3,258 Despite the rapidly growing evidence base, the exact mech anisms underpinning the pathophysiologic basis for OD related to this viral process are still under investigation, and our understanding is likely to continue to evolve as evidence accrues. Three mechanisms have been proposed and likely coexist: conductive loss caused by OC obstruc tion, injury to the OE, and injury to the OB. Conductive anosmia Impairment of nasal airflow caused by nasal obstruc tion will restrict delivery of odorants to the OE, a

mucosa of patients with hyposmia. IL-6 is a known proin flammatory cytokine that is present in other chronic diseases. 181 Although there is much work to be done to elucidate the contributions of the immune response, there appears to be a correlation between the immune response andPVOD. Ultimately, more studies need to be performed to iden tify the exact underlying mechanisms of chronic OD fol lowing viral infections, and whether this is consistent or varies depending on the infecting virus. The complexity of olfaction allows for many possible pathways. Nonetheless, current data suggest that the changes to the neuroepithe lium and OB may be the key areas in the pathophysiology of postinfectious OD (PIOD). OD can occur after viral infection. Aggregate grade of evidence : C (Level 2b: two studies; Level 3c: two studies; Level 4c: eight studies).