xRead - Olfactory Disorders (September 2023)

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PATEL et al.

Intervention : MRI is considered the gold-standard imaging procedure for the evaluation of patients with OD from nonsinonasal inflammatory causes and may be con sidered as an option. The use of MRI is potentially valuable in patients with congenital and posttraumatic anosmia. It can be considered in patients with IOD to exclude intracra nial pathology. Its use in PIOD is debatable considering its low added value to clinical history with regard to manage ment of patients. It should be further investigated whether the use of MRI changes the management and outcome of a select group of these patients, and consequently define which patients with OD would benefit most from MRI. 3 Advanced MRI techniques (requiring research facility/environment) Advanced morphological MRI or fMRI techniques have also been used to investigate olfactory-brain–related mor phology and function (Table VIII.4: 18 prospective cohort studies, one case series). These techniques are usually not feasible or useful in clinical routine practice and require a specific research environment and the use of specific devices and software. We found seven fMRI studies (six prospective cohort studies, one case series) related to OF. These studies found that brain activation is related to OF, with decreased acti vation of primary and secondary olfactory cortices follow ing olfactory stimulation in patients with posttraumatic anosmia. 1075,1076 Moreover, brain activation was found to be negatively correlated to the duration of the disease, 1076 and recruitment of neural network was associated with OF. 1077 In contrast, a study specifically assessing hypos mic patients showed similar central olfactory processing compared with controls. However, hyposmic patients had higher activation in regions associated with odor memory and motivation, possibly as a result of compensation. 1078 In patients with long-term OD, fMRI demonstrated changes in functional connectivity after 12 weeks of olfactory train ing (OF), albeit in a series including only a very small number of patients. 1079 Recently, one study aimed to eval uate the clinical usefulness of fMRI for the evaluation of patients with OD. It has shown that BOLD signal is not able to discriminate between patients with OD and con trols, because of large interindividual variability. More over, there was no correlation between OF and fMRI parameters. 1080 Studies using resting-state fMRI to study functional con nectivity found either no difference in functional connec tivity in the olfactory network in patients with congenital anosmia 1081 or changes in olfactory and global brain net work connectivity in patients with PTOD. 1082 We found 10 prospective cohort studies based on advanced morphological MRI. Among these studies, nine

evaluated patients based on voxel-based morphometry. Assessing patients with congenital anosmia, one study found that congenital anosmia was associated with mor phological alterations at the level of the secondary olfac tory cortex, but not to the POC 920 ; another found that con genital anosmics have larger gray matter volume in both primary and secondary olfactory cortices. 1083 In patients with postinfectious olfactory loss, it has been reported that there is a gray matter volume loss in diverse brain related olfactory areas (notably in the OFC) 1029,1084 and that OF is associated with a regain in the volume of affected regions. 1084 Patients with IOD were also found to exhibit gray matter volume loss in primary and secondary olfactory areas. 1029 Based on OF, patients with anosmia and hyposmia exhibited decreased gray and white mat ter volume 1085–1087 and it has been found that patients with parosmia have gray matter volume loss in regions associated with olfactory discrimination and memory. 1088 Moreover, it has been described that disease duration influenced brain atrophy since atrophy increased with duration 1062,1085 in patients with PIOD and IOD. Finally, using a deep learning model, a prospective cohort study suggested that MRI could be useful for the differen tial diagnosis between Parkinson-related OD and non Parkinson OD. 1089 Diffusion MRI has been investigated in two prospective cohort studies. 1090,1091 One study investigated patients with congenital anosmia and found that these patients have network dysfunction but intact structural integrity. 1090 Another study investigated patients with idiopathic olfac tory loss, considered as at risk for developing PD, in com parison to patients with PD and normosmic controls. 1091 This study found that, on a group level, fractional anisotropy measured at the level of the substantia nigra was decreased in idiopathic patients and patients with PD in comparison to controls. This finding suggests a reduced integrity of the substantia nigra in patients with idiopathic smell loss, supporting their PD at-risk status. However, there is no follow-up of these patients and whether they developed PD. Moreover, the authors mention that their analysis was not satisfactory when performed on an indi vidual level. Use of advanced MRI techniques for evaluation or management of OD Aggregate grade of evidence : C (Level 3: 18 studies; Level 4: two studies). Benefit : Clinical value at an individual level has not been demonstrated. Benefit in research realm only at this time. Harm :Minimal. Cost :High. Benefit-garm assessment : Balance of benefit and harm.