Studies of Olfactory Dysfunction in Major Neurological Disorders

Decreased ability to smell results from a number of medical conditions, ranging from simple congestion of the nose to serious neurological disturbances. In this chapter I provide an overview of the olfactory dysfunctions present in several major neurological disorders and discuss current theories as to their cause. In some disorders, such as Alzheimer's disease and idiopathic Parkinson's disease, hyposmia or anosmia is widespread and present early in the disease process. In others, such as amyotrophic lateral sclerosis, essential tremor, multiple sclerosis, multiple system atrophy, progressive supranuclear palsy, and schizophrenia, such dysfunction, when present at all, is much more sporadic. Given the differences in olfactory test scores among some closely related neurological disorders, standardized olfactory testing may be of considerable value in their early differential diagnosis, as well as in better understanding etiologic factors.