Anosmia in Neurodegenerative disorders

Abstract

Since the discovery of olfactory dysfunction in Parkinson’s disease (PD) and Alzheimer’s disease (AD) just over three decades ago, research into olfactory dysfunction in PD and AD has grown vastly. It appeared that olfactory dysfunction is similar in PD and AD. However, recent studies show that the severity of olfactory dysfunction differs between PD and AD. Olfactory dysfunction in PD appears to be stable during disease progression and severity except for odour discrimination. In AD however, olfactory dysfunction in identification, discrimination and threshold is associated with severity and the degree of dementia. It has been found that both PD an AD are preceded at approximately five years by olfactory dysfunction before the presence of typical symptoms. Typical symptoms for PD are bradykinesia, rest tremor and rigidity. For AD typical symptoms are cognitive impairment and memory deficits. Also, olfactory dysfunction can identify future PD or AD developers from an at risk group. However, the predictive value of olfactory dysfunction in a general population is unknown. Another difference can be found in treatment response. Olfactory dysfunction improves in AD with acetylcholinesterase inhibitors (ChEI), where in PD, olfaction is unaffected by typical dopaminergic treatment. There are a number of explanations for this difference, among which is that the underlying mechanism in PD is more complex and comprises multiple system, e.g. dopaminergic system and acetylcholinesterase (AChE) activity. It has recently been suggested that the AChE is more important in olfactory dysfunction in PD than the dopaminergic system. Olfactory testing appears very useful in early detection and differential diagnosis and could be very useful in clinical settings, but further research is needed to investigate the underlying mechanisms and possibilities for inclusion in treatment. Also, further research could focus on the link between movement and olfaction as seen in animals. Another possible line of research is that of olfaction and emotion or lack thereof as seen in both PD and AD.