Reduced olfactory bulb volume in post-traumatic and post-infectious olfactory dysfunction Antje Mueller, 1 Antje Rodewald, 2 Jens Reden, 1 Johannes Gerber, 2 Ruediger von Kummer 2 and Thomas Hummel 1,CA 1 Departments of Otorhinolaryngology; 2 Radiology, University of Dresden Medical School, Fetscherstr, 74, 01307 Dresden, Germany CA Corresponding Author: thummel@rcs.urz.tu-dresden.de Received 7 January 2005; accepted 3 February 2005 The olfactory bulb is a highly plastic structure the volume of which partly re£ects the degree of a¡erent neural activity. In this study, 22 patients with post-infectious olfactory de¢cit, nine participants with post-traumatic olfactory de¢cit, and 17 healthy controls un- derwent magnetic resonance volumetry of the olfactory bulb. Pa- tients presented with signi¢cantly smaller olfactory bulb volumes than controls; signi¢cant correlations between olfactory function and bulb volume were observed. Patients with parosmia exhibited smaller olfactory bulb volumes than those without parosmia. Find- ings indicate that smell de¢cits leading to a reduced sensory input to the olfactory bulb result in structural changes at the level of the bulb. Reduced olfactory bulb volumes may also be considered to be characteristic of parosmia. NeuroReport 16:475^ 478  c 2005 Lippincott Williams & Wilkins. Key words: Anosmia; Dysosmia; Parosmia; Plasticity INTRODUCTION The olfactory bulb is considered to be the first important relay station in odor processing, providing the link between the peripheral olfactory system and brain structures. With a continuing synaptogenesis, the olfactory bulb remains highly plastic throughout adult life, reflecting to some degree the level of afferent neural activity. In animals, one of the most pronounced effects of olfactory deprivation is the reduction in olfactory bulb size [1,2] as a result of a decreased number of cells. These changes are based on the fact that the olfactory bulb is one of the few brain areas to continuously replace its neuronal populations. The bulbar neurogenesis presents with a high sensitivity to the activity level of sensory inputs from the olfactory epithelium [3]. By maintaining a constitutive neurogenesis sensitive to envir- onmental influences, this neuronal recruitment may, in turn, lead to an improvement of sensory abilities [4]. In addition to the continuous replacement of GABAergic neurons, plasticity of mitral/tufted cells in the olfactory bulb has been reported recently [5]. Further, apoptosis may be an important mechanism of plasticity by which the olfactory system is able to adjust the number of neurons in the olfactory bulb [6]. Because of the continuous improvement of imaging techniques, volumetric magnetic resonance analysis offers a means to reliably evaluate the morphology of the olfactory bulb; considering the plasticity of this structure, bulb volume may also reflect the functional state of the human olfactory system. Accordingly, olfactory bulb size has been previously studied in patients with post-traumatic olfactory deficits [7,8], congenital anosmia [9,10], neurodegenerative diseases [11], and in participants with a normal sense of smell [12]. However, it has not been demonstrated whether reduced olfactory bulb volumes are present in the majority of patients with damaged peripheral olfactory structures because of head trauma or infections of the upper respiratory tract (post-URTI). The latter presents with a damage of the most peripheral part of the olfactory system, the olfactory mucosa [13,14]. In patients who experienced head trauma, the injured parts of the olfactory system are often the fila olfactoria [15,16]. The present study was performed to investigate possible morphologic differences of the olfactory bulb in patients with peripheral olfactory deficits, i.e. post-URTI and post- traumatic olfactory dysfunction, and to compare those with healthy controls using volumetric magnetic resonance analysis. MATERIAL AND METHODS Investigations were performed according to the Declaration of Helsinki on Biomedical Studies Involving Human Subjects (WMA, 1997). Participants: A total of 31 patients participated. Twenty- two of them had post-URTI olfactory deficits (mean age, 57 years; range, 30–74 years; 13 women and nine men), nine had post-traumatic olfactory loss (mean age 52 years; range 21–70 years; two women and seven men). Their results were compared with 17 healthy controls (mean age, 49 years; range, 28–62 years; 13 women and four men). All controls indicated that they had a normal sense of smell. Duration of olfactory deficits ranged from 3 months to 9 years. Eleven patients with post-URTI smell deficits and one patient with post-traumatic olfactory dysfunction reported parosmia (mean age, 59 years; range, 48–74 years; nine women and three men; mean duration of olfactory deficits, 10 months; range, 3 months–3 years). CHEMICAL, SENSES NEUROREPORT 0959-4965  c Lippincott Williams & Wilkins Vol16 No 5 4 April 2005 475 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.