Olfaction can be defined as the sense of smell. Asnomia is a chronic olfactory disorder characterized by partial or complete loss of the sense of smell. The prevalence of chronic olfactory disorders has risen over the past few years with current estimates being at 19%. Infections are the major causes of olfactory loss though sinusitis, drugs/toxins, traumatic head injuries and congenital anomalies have also been identified as possible etiologies.

In this study, voxel-based morphometry (VBM) in magnetic resonance imaging (MRI) was used to check for changes on the gray matter of the brain in participants suffering from asnomia. Voxel-based morphometry allows measurement of such changes over the whole brain as opposed to marginal segmentation techniques which restricts measurement to a region of the brain.

Test results revealed significant atrophy of olfactory parts of the brain. These included:

  • The medial prefrontal cortex (MPC) which consists of the subcallosal gyrus (SCG), nucleus accumbens (NAc) and the dorsolateral prefrontal cortex (dlPFC) and the nucleus accumbens (NAc). These are responsible for food cravings, appetite and memory of tastes of food.
  • The occipital gyrus and the precuneus (Prec) responsible for smell memory and taste matching.
  • The cerebellum, superior occipital gyrus (SOG), piriform cortex (PC), anterior insular cortex (IC), orbital frontal cortex (OFC), supramarginal gyrus (SMG), hippocampus and the parahippocampal which are also involved in olfaction.

Study participants who had suffered loss of smell for a period exceeding 2 years also showed marked atrophy in the above areas compared to those who were normal or had been sick for less than two years. Compensatory mechanisms common with loss of function in other senses e.g. eyesight or hearing were not evident. On the contrary there was a marked decrease in size in the areas of the brain that control taste and chewing.

Similar volume brain changes have also been observed in patients suffering from both neurological and neurodegenerative conditions e.g. Parkinson's disease (PD), Alzheimer's disease (AD), depression etc. It is yet to be established if asnomia precedes occurrence of these conditions or if both conditions occur concurrently. Another school of thought suggests that neurological degeneration in itself leads to a decrease in brain volume including the areas involved in olfaction resulting to cases of asnomia. More studies are needed to investigate evidence of causal and temporal relationships in these conditions.

In conclusion, loss of the sense of smell function causes a decrease in stimulation in the areas of the brain responsible for olfaction. This leads to their eventual reduction in volume or atrophy.

Thomas Bitter, Hilmar Gudziol, Hartmut Peter Burmeister, Hans-Joachim Mentzel, Orlando Guntinas-Lichius, Christian Gaser; Anosmia Leads to a Loss of Gray Matter in Cortical Brain Areas, Chemical Senses, Volume 35, Issue 5, 1 June 2010, Pages 407–415, https://doi.org/10.1093/chemse/bjq028