Michael Leon ^1* Emily Troscianko ² Cynthia Woo ¹

• ¹ University of California, Irvine, Irvine, United States
• ² University of Oxford, Oxford, England, United Kingdom

Olfactory loss accompanies at least 139 neurological, somatic, and congenital/hereditary conditions. This observation leads to the question of whether these associations are correlations or whether they are ever causal. Temporal precedence and prospective predictive power suggest that olfactory loss is causally implicated in many medical conditions. The causal relationship between olfaction with memory dysfunction deserves particular attention because this sensory system has the only direct projection to memory centers. Mechanisms that may underlie the connections between medical conditions and olfactory loss include inflammation, as well as neuroanatomical and environmental factors and all 139 of the medical conditions listed here are also associated with inflammation. Olfactory enrichment shows efficacy for both prevention and treatment, potentially mediated by decreasing inflammation. 1.0 Introduction 1.1 Observations linking olfactory loss and medical conditions: Correlation, precedence, and prediction 1.2.1 Olfactory loss is associated with many medical conditions First, a strikingly large number of medical conditions are accompanied by olfactory dysfunction (Tables 123). The remarkably long and diverse list of medical conditions that co-occur with olfactory loss raises the possibility that there is something deeper to these relationships. Many of the associations between olfactory loss and medical conditions are supported by a single study, however, there are several conditions that have been studied creating a tighter fit between olfactory loss and memory loss specifically, we can proceed to ask whether specific life experiences may activate such connections. There are indeed experiences that are known to cause both loss of olfactory ability and loss of memory, the latter sometimes also referred to as "brain fog".