A systematic review of olfactory-related brain structural changes in patients with congenital or acquired anosmia.

Brain structural features of healthy individuals are associated with olfactory functions. However, due to the pathophysiological differences, congenital and acquired anosmia may exhibit different structural characteristics. A systematic review was undertaken to compare brain structural features between patients with congenital and acquired anosmia. A systematic search was conducted using PubMed/MEDLINE and Scopus electronic databases to identify eligible reports on anosmia and structural changes and reported according to PRISMA guidelines. Reports were extracted for information on demographics, psychophysical evaluation, and structural changes. Then, the report was systematically reviewed based on various aetiologies of anosmia in relation to (1) olfactory bulb, (2) olfactory sulcus, (3) grey matter (GM), and white matter (WM) changes. Twenty-eight published studies were identified. All studies reported consistent findings with strong associations between olfactory bulb volume and olfactory function across etiologies. However, the association of olfactory function with olfactory sulcus depth was inconsistent. The present study observed morphological variations in GM and WM volume in congenital and acquired anosmia. In acquired anosmia, reduced olfactory function is associated with reduced volumes and thickness involving the gyrus rectus, medial orbitofrontal cortex, anterior cingulate cortex, and cerebellum. These findings contrast to those observed in congenital anosmia, where a reduced olfactory function is associated with a larger volume and higher thickness in parts of the olfactory network, including the piriform cortex, orbitofrontal cortex, and insula. The present review proposes that the structural characteristics in congenital and acquired anosmia are altered differently. The mechanisms behind these changes are likely to be multifactorial and involve the interaction with the environment.

Improved Odor Identification Ability and Increased Regional Gray Matter Volume After Olfactory Training in Patients With Idiopathic Olfactory Loss.

Idiopathic olfactory loss (IOL) is thought as an early marker for neurodegenerative disease. This study investigated the effect of olfactory training (OT) on regional gray matter volume (GMV) among patients with IOL. A total of 24 patients (mean age 64.6 years, 11 male) with IOL and 30 control participants with normal olfaction (mean age 62.6 years, 13 males) were included in the study. Voxel-based morphometry was performed to compare the GMV between patient and control groups. Only the patients received OT (averaged duration 7 months), and a longitudinal approach was used to examine the GMV change from pre- to post-OT. Moreover, the effect of OT on GMV change was explored for patients with different severity of olfactory loss (anosmia vs. hyposmia). Olfactory performance was measured alongside using the "Sniffin' Sticks." Patients had improved odor identification and larger GMV in the bilateral cerebellum, bilateral thalamus, left precentral gyrus, right gyrus rectus, and medial orbitofrontal cortex after OT. However, no correlation was found between changes of odor identification and increased regional GMV. Besides, patients with anosmia, compared with patient with hyposmia, demonstrated increased GMV in the left precuneus, left superior frontal medial cortex, and left midcingulate cortex after OT. The study showed improved odor identification ability among patients with IOL after OT, which is unlikely related to spontaneous recovery. In this specific patient group, the GMV alterations may be associated with factors not directly predicted by the currently performed measurements, but possibly higher order olfactory-related functional changes.

Evaluation of idiopathic olfactory loss with chemosensory event-related potentials and magnetic resonance imaging.

BACKGROUND: Idiopathic olfactory loss (IOL) accounts for a sizable fraction of olfactory dysfunction, but very little is known about its etiology and electrophysiological changes in the olfactory pathway. METHODS: We analyzed the physiology of IOL using chemosensory event-related potentials (ERPs) (olfactory and trigeminal: oERP and tERP) and olfactory pathway magnetic resonance imaging (MRI) measured in adult patients with IOL and healthy controls. Subjective olfactory function was measured by Toyota and Takagi (T&T) olfactometry and Sniffin' Sticks (SS). RESULTS: Olfactory function was worse in patients with IOL compared to controls (T&T, p < 0.001; SS, p < 0.001). oERPs could be evoked in 17 IOL patients. Signals in these patients showed lower amplitude in the N1 and P2 waves than controls (p < 0.05 for both), but there were no difference in latency between the 2 groups (p > 0.05). tERP were detected in all patients and controls; there were no differences in latency and nor amplitude between the 2 groups (p > 0.05). The olfactory bulb (OB) volume was significantly smaller in the IOL group than controls (p < 0.001), but there was no difference in the olfactory sulcus depth between groups (p > 0.05). Better olfactory function was associated with increasing magnitude of N1 amplitude in oERPs (p < 0.05) and increasing OB volume (p < 0.05). CONCLUSION: IOL patients show neurophysiologic deficits and some anatomic differences compared to healthy controls.

[Preliminary study of DTI on cerebral white matter micro-structure of patients with idiopathic olfactory loss].

OBJECTIVES: To investigate the cerebral white matter micro-structure in patients with idiopathic olfactory loss using diffusion tensor imaging (DTI). METHODS: Sixteen patients with idiopathic olfactory loss and sixteen normal subjects matched by age and sex were recruited in this study. Sniffin'Stick olfactory test was performed to evaluate the olfactory function of all subjects. We acquired diffusion tensor images with an echo planar imaging (EPI) sequence from all subject on a 3T scanner. The fractional anisotropy (FA) images were performed using DTI-studio, and bilateral Piriform cortex, Orbitofrontal cortex, Hippocampus and Insula cortex adjacent white matter and Capsula interna were delineated as the region of interesting (ROI), the FA for each ROI was calculated. Independent sample t test analysis was used to compare the FA value of all ROIs between the controls and patients. In addition, correlation analysis between FA value and MMSE score in patients were conducted. RESULTS: Compared with the controls, patients showed significantly decreased FA value in the adjacent white matter of bilateral Piriform cortex, Orbitofrontal cortex, Hippocampus and Insula cortex (P<0.05). There is no significant difference of FA value in bilateral Capsula interna between two groups (P>0.05). CONCLUSIONS: The patients with idiopathic olfactory loss show the damage of white matter micro-structure in the olfactory center, which could be important for the pathogenesis study and early intervention of idiopathic olfactory loss.

Preliminary study of DTI on cerebral white matter micro-structure of patients with idiopathic olfactory loss / 临床耳鼻咽喉头颈外科杂志

OBJECTIVES@#To investigate the cerebral white matter micro-structure in patients with idiopathic olfactory loss using diffusion tensor imaging (DTI).@*METHODS@#Sixteen patients with idiopathic olfactory loss and sixteen normal subjects matched by age and sex were recruited in this study. Sniffin'Stick olfactory test was performed to evaluate the olfactory function of all subjects. We acquired diffusion tensor images with an echo planar imaging (EPI) sequence from all subject on a 3T scanner. The fractional anisotropy (FA) images were performed using DTI-studio, and bilateral Piriform cortex, Orbitofrontal cortex, Hippocampus and Insula cortex adjacent white matter and Capsula interna were delineated as the region of interesting (ROI), the FA for each ROI was calculated. Independent sample t test analysis was used to compare the FA value of all ROIs between the controls and patients. In addition, correlation analysis between FA value and MMSE score in patients were conducted.@*RESULTS@#Compared with the controls, patients showed significantly decreased FA value in the adjacent white matter of bilateral Piriform cortex, Orbitofrontal cortex, Hippocampus and Insula cortex (0.05).@*CONCLUSIONS@#The patients with idiopathic olfactory loss show the damage of white matter micro-structure in the olfactory center, which could be important for the pathogenesis study and early intervention of idiopathic olfactory loss.