Does The Occurrence of Sleep Disorder or Deficits in Olfaction Provide an Early Indication of Neurodegeneration and Subsequent Dementia?

It is now well recognized that impairments in olfaction often provide the first sign of neurodegenerative disorders such as Alzheimer’s (AD) and Parkinson’s disease (PD) [1-5]. Recent studies have also revealed that olfactory deficits are observed in individuals living with psychiatric disorders including depression [6], and schizophrenia [7]. Accordingly, it is hypothesized that abnormalities in brain function may account for difficulties in the detection odor or identification of specific classes of odor. The first indications of an impairment in olfaction are often manifested as a difficulty in the recognition of specific odors whereas the detection threshold for odor does not differ from age-matched controls [5]. A subsequent progression to anosmia involves further impairment of the classification of odors, and finally an inability to detect the presence of an odor [5]. The ability to identify a specific odor is quite complex, involving as it does the capacity to encode and store in memory past perceptions of a unique volatile chemical signature, which can then be and recalled and categorized by a single name. Olfactory memory encoding and storage involves neural activity within the hippocampus and parahipppocampus, and recall and recognition of the memory of a specific odor engages a neural network linking between amygdala-hippocampus and orbitofrontal cortex [8,9]. As with other forms of prospective memory function, the process by which specific odors are identified may be referred to as “travelling back to the future.”