Table 3.

Summary of the association between OB volume, OS depth, and TDI score across anosmia with various etiologies

No.	Main findings		Notes
Various etiology
(Chung et al. 2018)	- Positive correlation between OB volume and TDI score - Negative correlation between OB volume with a duration of olfactory loss and age - Negative correlation between an olfactory function with aetiology and age
(Hummel et al. 2015)	- Positive correlation between OB volume and TDI score - Positive correlation between OB volume and olfactory function - Positive correlation between R OS depth and olfactory function but NOT L OS depth - Negative correlation between OS depth and age		Assessment of the OB volume and OS depth produces useful clinical indicators of olfactory dysfunction
(Goektas et al. 2009)	- Positive correlation between OB volume and olfactory function - Negative correlation between OB volume and TDI score - Negative correlation between OB volume and duration of olfactory loss - Negative correlation between OB volume with etiologist		*These findings are a sharing finding between anosmia and hyposmia
(Fonteyn et al. 2014)	- Positive correlation between orthonasal and retronasal score		*These findings are a sharing finding between anosmia and hyposmia
(Rombaux et al. 2006b)	- Positive correlation between OB volume with both orthonasal and retronasal score
(Mueller et al. 2005)	- Positive correlation between OB volume with olfactory function		Reduced OB volumes may also be characteristic of parosmia
(Bitter et al. 2010b)	- Positive correlation between GM volume with disease duration - Positive correlation between GM volume with olfactory function		GM areas involved were nucleus accumbens, MPC including the MCC and ACC, DLPFC, piriform cortex, insular cortex, orbitofrontal cortex, hippocampus, parahippocampal gyrus, supramarginal gyrus, and cerebellum Patients with disease duration longer than 2 years showed stronger atrophy in these areas compared with patients who were anosmic less than 2 years
(Peng et al. 2013)	- Positive correlation between GM and WM volume with disease duration - Positive correlation between GM volume with olfactory function		Different GM and WM areas have different sensitivities to olfactory injury GM areas involved were ACG, MTG, STG, fusiform gyrus, SMG, SFG, MFG, MOG, anterior-insular cortex, cerebellum, piriform cortex, ITG, precuneus, and subcallosal gyrus
(Rombaux et al. 2012)	- Positive correlation between OB volume with both orthonasal and retronasal score - Positive correlation between OB volume with olfactory functions		OB volume is suggested to be a predictor of olfactory recovery in patients with postinfectious and post-traumatic olfactory loss With larger volumes → greater improvement of olfactory function
Idiopathic olfactory loss
(Liu et al. 2018)	- No difference in OS depth between IOL and HC in both sides of the brain - No difference in OB volume between in both hemisphere of the two groups
(Rombaux et al. 2010a)	- Positive correlation between OB volume with odour thresholds in both groups - Positive correlation between OB volume with olfactory function
(Yao et al. 2014)	- Negative correlation between the two olfactory tests: higher scores for T&T and lower scores for SS compared to HC - Positive correlation between GM volume with olfactory function		GM areas involved were OFC ACC, insular cortex, parahippocampal cortex, and piriform cortex
Upper respiratory tract infection
(Yao et al. 2018)	- Negative correlation between R OB volume with a duration of olfactory loss - Negative correlation between R OFC volume with a duration of olfactory loss
(Rombaux et al. 2006a)	- Positive correlation between OB volume with olfactory function and duration of olfactory loss
(Rombaux et al. 2009a)	-Positive correlation between OB volume with orthonasal and retronasal score
Congenital anosmia
(Karstensen et al. 2018)	- Positive correlation between OB volume and OS depth with olfactory function - Positive correlation between an olfactory function with a volume of R posterior cingulate and parahippocampal cortex		Lifelong olfactory deprivation trigger changes in GM of prefrontal and limbic cortices - Larger GM volume of the L MFG and in R SFS - Larger GM volume of the R piriform cortex - Smaller GM volume in the L posterior olfactory sulcus within the mOFC Whole-brain analyses - Larger GM volume of the R SFS and L MFG
(Yousem et al. 1996b)	- Positive correlation between an olfactory function with temporal and/or frontal lobe volume loss - Absence of OB and OT (68–84%) in all Pt		Eight individuals had Kallmann’s syndrome (hypogonadotropic hypogonadism with anosmia) Congenital anosmia or hyposmia appears to be an olfactory bulb olfactory tract phenomenon rather than a cerebral process *These findings are a sharing finding between anosmia and hyposmia
(Frasnelli et al. 2013)	- Negative correlation between GM volume with olfactory function (thicker medial orbitofrontal cortex bilaterally, denser, and thicker left piriform cortex, left entorhinal cortex, smaller cortical thickness of the posterolateral) - Negative correlation between WM volume with olfactory function (increased WM density of the left superior longitudinal fasciculus in an area posteromedial to the left insula, as well as of an area posterior to the parietal operculum)
(Huart et al. 2011)	- Positive correlation between OS depth with olfactory function (decrease OS depth)		The depth of the OS is a useful clinical indicator and 8 mm clearly indicates isolated anosmia 10 Pt. with IA had OS deeper than 8 mm, and 26 Pt. had smaller than 8 mm None of the healthy controls exhibited a depth of 8 mm
(Abolmaali et al. 2002)	- Positive correlation between OS depth with olfactory function (decrease OS depth) - Depth of the OS was more significant on the right than on the left, and there was no overlap - Depth of the OS differed between those with and those without visible OT		The present study speculates that olfaction may be processed predominantly in the right hemisphere
(Peter et al. 2020)	- Negative correlation between GM volume with olfactory function (increased GM volume and cortical thickness in the medial orbital gyri; regions associated with olfactory processing, sensory integration, and value-coding) - Positive correlation between GM volume with olfactory function (GM atrophy in bilateral olfactory sulci, explained by decreased cortical area, curvature, and sulcus depth)
Traumatic brain injury
(Han et al. 2018)	- Negative correlation between OB volume and TDI score - Positive correlation between GM volume with disease duration (frontal and temporal gyrus) - Positive correlation between GM volume with olfactory function (in primary and secondary olfactory areas including gyrus rectus, medial OFC, anterior cingulate cortex, insula, and cerebellum)		Pt. with anosmia more frequent lesions in OB, OFC, and temporal lobe pole compared to hyposmia and HC
(Miao et al. 2015)	- Positive correlation between OB volume with olfactory function - Positive correlation between R OS depth with olfactory function - Positive correlation between OB volume and OS depth with age
(Rombaux et al. 2006b)	- Positive correlation between OB volume and retronasal - Negative correlation between OB volume and orthonasal		*These findings are a sharing finding between anosmia and hyposmia Pt. with anosmia had smaller OB volumes than Pt. with hyposmia
(Yousem et al. 1999)	- Positive correlation between left OB volume, OT, and total UPSIT scores		OB, OT, and frontal lobe encephalomalacia coexist in many Pt *These findings are a sharing finding between anosmia and hyposmia
(Yousem et al. 1996a)	- Negative correlation between OB volume and individual olfactory test scores		Pt. without smell function had more significant volume loss in OB and OT than did those trauma Pt. who retained some sense of smell
(Jiang et al. 2009b)	- Positive correlation between OB volume and olfactory loss		Findings indicate that a loss of peripheral sensory input may have resulted in reduced OB volume in Pt A high incidence of sub-frontal lobe damage concurred in Pt
(Doty et al. 1997)	- Positive correlation between OB volume and OT volume with olfactory loss in male patients (but not female)		Pt. complaint olfactory dysfunction typically have anosmia and rarely regain the normal olfactory ability Frontal impacts produced less dysfunction than back or side impacts

Pt. patient, IA isolated anosmia, CR Chronic rhinosinusitis, RVI respiratory viral infection, IOL idiopathic olfactory loss, NP nasal polyposis, HC healthy control, OB olfactory bulb, OS olfactory sulcus, OT olfactory sulcus, GM gray matter, WM white matter, SFS superior frontal sulcus, MFG = middle frontal gyrus, MPC medial prefrontal cortex, DLPFC dorsolateral prefrontal cortex, ACC anterior cingulate cortex, MTG middle temporal gyrus, STG superior temporal gyrus, SMG supramarginal gyrus, SFG superior frontal gyrus, MOG middle occipital gyrus, MCC middle cingulate cortex, ACC anterior cingulate cortex, ITG inferior temporal gyrus, SFS superior frontal sulcus, UPSIT University of Pennsylvania Smell Identification Test, T&T Toyota and Takagi, SS Sniffin’ Sticks

*Retronasal = odorant was presented to the oral cavity so that orthonasal and gustatory stimuli were avoided

*Orthonasal = e.g. Sniffin’ sticks task