Caroline Huart

Plasticity of the human olfactory system: the olfactory bulb.

In the last years, an increasing interest has been paid to the olfactory system, particularly to its abilities of plasticity and its potential continuous neurogenesis throughout adult life. Although mechanisms underlying adult neurogenesis have been largely investigated in animals, to some degree they remain unclear in humans. Based on human research findings, the present review will focus on the olfactory bulb as an evidence of the astonishing plasticity of the human olfactory system.

Increased olfactory bulb volume and olfactory function in early blind subjects.

It has been shown that the volume of the olfactory bulb (OB) changes with function. The aim of this study was to investigate whether the OB volume and the olfactory function in early blind (EB) subjects increase compared with controls. Psychophysical testing of olfactory performances and OB volumetric measurements assessed by an MRI scan were studied. Quantitative olfactory function expressed in the odor discrimination and odor-free identification scores was higher in EB subjects compared with controls. The mean of right, left and total OB volume was 65.40, 75.48, and 140.89 mm3, respectively for the EB subjects and 54.47, 52.11, and 106.60 mm3, respectively for the controls, with these differences being significant. EB subjects have superior olfactory abilities and presented with significantly higher OB volume than the sighted controls. OB plasticity may explain this compensatory mechanism between visual deprivation and enhanced olfactory perception.

Prognostic Value of Olfactory Bulb Volume Measurement for Recovery in Postinfectious and Posttraumatic Olfactory Loss

Objectives Several prognostic factors influencing the recovery from olfactory dysfunction have been described. The aim of this study was to investigate whether olfactory bulb volume could be used as a new predictor of olfactory recovery in postinfectious and posttraumatic olfactory loss. Study Design Cohort study; Level of evidence, 4. Setting Tertiary university clinic, department of otolaryngology. Subjects and Methods A cohort of 60 patients with postinfectious (n = 28) and posttraumatic olfactory loss (n = 32) was investigated. Assessment of olfactory function was performed using orthonasal (Sniffin’ Sticks test) and retronasal psychophysical olfactory tests, at the time of the diagnosis (t1) and 15 months later (t2). All patients were examined on 3 tesla magnetic resonance imaging, and the olfactory bulbs volume was assessed using planimetric contouring at the time of the diagnosis (t1). Results Recovery rate was 25% in patients with posttraumatic olfactory loss and 36% in patients with postinfectious olfactory loss. There was a correlation between both orthonasal and retronasal olfactory testing and the initial measurement of the total olfactory bulb volume. In addition, we observed a significant correlation between changes in olfactory functions and initial measurement of the total olfactory bulb volume, with larger volumes relating to higher improvement of olfactory function. Finally, we found that none of the patients with a total olfactory bulb volume of 40 mm3 or less exhibited recovery of olfactory function. Conclusion Olfactory bulb volume seems to be a predictor of olfactory recovery in patients with postinfectious and posttraumatic olfactory loss.

Time-Frequency Analysis of Chemosensory Event-Related Potentials to Characterize the Cortical Representation of Odors in Humans

Background The recording of olfactory and trigeminal chemosensory event-related potentials (ERPs) has been proposed as an objective and non-invasive technique to study the cortical processing of odors in humans. Until now, the responses have been characterized mainly using across-trial averaging in the time domain. Unfortunately, chemosensory ERPs, in particular, olfactory ERPs, exhibit a relatively low signal-to-noise ratio. Hence, although the technique is increasingly used in basic research as well as in clinical practice to evaluate people suffering from olfactory disorders, its current clinical relevance remains very limited. Here, we used a time-frequency analysis based on the wavelet transform to reveal EEG responses that are not strictly phase-locked to onset of the chemosensory stimulus. We hypothesized that this approach would significantly enhance the signal-to-noise ratio of the EEG responses to chemosensory stimulation because, as compared to conventional time-domain averaging, (1) it is less sensitive to temporal jitter and (2) it can reveal non phase-locked EEG responses such as event-related synchronization and desynchronization. Methodology/Principal Findings EEG responses to selective trigeminal and olfactory stimulation were recorded in 11 normosmic subjects. A Morlet wavelet was used to characterize the elicited responses in the time-frequency domain. We found that this approach markedly improved the signal-to-noise ratio of the obtained EEG responses, in particular, following olfactory stimulation. Furthermore, the approach allowed characterizing non phase-locked components that could not be identified using conventional time-domain averaging. Conclusion/Significance By providing a more robust and complete view of how odors are represented in the human brain, our approach could constitute the basis for a robust tool to study olfaction, both for basic research and clinicians.

Volume of olfactory bulb and depth of olfactory sulcus in 378 consecutive patients with olfactory loss.

The aim of this study was to investigate the olfactory bulb (OB) and sulcus (OS) in a large group of patients who have been well-characterized in terms of olfactory function, with a specific focus on the comparison between patients with olfactory loss due to chronic rhinosinusitis, head trauma, or acute infections. A retrospective study of 378 patients with olfactory loss was performed. Orthonasal olfactory function was assessed with the“Sniffin’ Sticks” test kit, including tests for odor threshold, odor discrimination, and odor identification. Magnetic resonance imaging analyses were focused on OB volume and OS depth. Major results of the present study included the (1) demonstration of a correlation between olfactory function and OB volume across the various pathologies in a very large group of subjects; (2) the three functional tests exhibited a similar degree of correlation with OB volume. (3) The right, but not the left OS correlated with olfactory function; in addition, (4) OS was negatively correlated with age. In contrast to OS, (5) no side differences were found for the OB. Finally, (6) the three different causes of olfactory loss exhibited different patterns of results for the three olfactory tests. The present data suggest that the morphological assessment of the OB volume and OS depth produces useful clinical indicators of olfactory dysfunction.

Non-sinonasal-related olfactory dysfunction: A cohort of 496 patients.

INTRODUCTION AND AIM There is a high prevalence of olfactory dysfunction in the general population. Several causes of olfactory dysfunction have been reported and this disorder is classically divided into sinonasal and non-sinonasal-related olfactory dysfunction. The aims of this study were firstly, to evaluate the frequency of the various aetiologies of olfactory dysfunction in a population of patients with non-sinonasal-related olfactory dysfunction and secondly, to evaluate the degree of olfactory impairment associated with these various aetiologies. MATERIAL AND METHODS We retrospectively reviewed a cohort of 496 patients with non-sinonasal-related olfactory dysfunction. The aetiology of the olfactory dysfunction was recorded for each patient. The aetiology was determined by a complete clinical assessment, including medical history, complete otorhinolaryngological examination, psychophysical testing of olfactory function, recording of olfactory event-related potentials and brain magnetic resonance imaging. Six groups of patients were defined on the basis of the aetiology of the disease and orthonasal and retronasal psychophysical olfactory performances were evaluated in each group. RESULTS Post-infectious and post-traumatic aetiologies were the most common causes, representing 37.9% and 33.1% of patients, respectively, followed by idiopathic (16.3%), congenital (5.9%), toxic (3.4%) and neurological (3.4%) olfactory dysfunction. Anosmia was significantly more frequent in congenital (93.1%) and post-traumatic (62.8%) olfactory dysfunction, whereas hyposmia was more frequent in the post-infectious group (59.6%). Orthonasal and retronasal olfactory function tests were significantly correlated in all groups except for the congenital group. CONCLUSIONS The data of this study confirm that the most common causes of non-sinonasal-related olfactory dysfunction are post-infectious and post-traumatic. Post-infectious olfactory dysfunction is mainly observed in middle-aged women and is mainly associated with hyposmia, whereas post-traumatic olfactory dysfunction is mainly observed in young men and is associated with a high rate of anosmia.

Presence of olfactory event-related potentials predicts recovery in patients with olfactory loss following upper respiratory tract infection.

The aim of the present study was to evaluate the course of olfactory dysfunction in patients with olfactory loss following infections of the upper respiratory tract.

Effect of three‐drug delivery modalities on olfactory function in chronic sinusitis

Olfactory dysfunction is deemed to be a significant contributor to poor quality of life in chronic rhinosinusitis (CRS).

Olfaction in Chronic Rhinosinusitis

Olfactory dysfunction is a frequent complaint in chronic rhinosinusitis patients and has a significant impact on quality of life. Therefore, it is essential that clinicians are aware of the importance of olfactory dysfunction in chronic rhinosinusitis (CRS) patients and know how to deal with it. Notably, the evaluation of olfactory function (i.e., using psychophysical testing) and imagery of olfactory bulb play an important role in the evaluation of patients and give essential information about the “baseline” olfactory function. Because the high impact of olfactory function on quality of life and medical and/or surgical treatment should be proposed to patients. However, it remains difficult to predict the outcome of treatment as well as long-term efficacy. The first section of this review is dedicated to the assessment of olfactory function. Secondly, we will discuss the etiopathology of olfactory dysfunction in CRS with and without nasal polyps. Finally, we will review literature findings about the efficacy of different treatments on olfactory function.

Clinical usefulness and feasibility of time-frequency analysis of chemosensory event-related potentials*

BACKGROUND The clinical usefulness of olfactory event-related brain potentials (OERPs) to assess olfactory function is limited by the relatively low signal-to-noise ratio of the responses identified using conventional time-domain averaging. Recently, it was shown that time-frequency analysis of the obtained EEG signals can markedly improve the signal-to-noise ratio of OERPs in healthy controls, because it enhances both phase-locked and non phase-locked EEG responses. The aim of the present study was to investigate the clinical usefulness of this approach and evaluate its feasibility in a clinical setting. METHODOLOGY We retrospectively analysed EEG recordings obtained from 45 patients (15 anosmic, 15 hyposmic and 15 normos- mic). The responses to olfactory stimulation were analysed using conventional time-domain analysis and joint time-frequency analysis. The ability of the two methods to discriminate between anosmic, hyposmic and normosmic patients was assessed using a Receiver Operating Characteristic analysis. RESULTS The discrimination performance of OERPs identified using conventional time-domain averaging was poor. In contrast, the discrimination performance of the EEG response identified in the time-frequency domain was relatively high. Furthermore, we found a significant correlation between the magnitude of this response and the psychophysical olfactory score. CONCLUSION Time-frequency analysis of the EEG responses to olfactory stimulation could be used as an effective and reliable diagnostic tool for the objective clinical evaluation of olfactory function in patients.