[The forgotten cranial nerve--clinical importance of olfaction].

Hyposmia is often undiagnosed despite the known negative effect on taste, appetite and life quality. However, a new focus on the first cranial nerve has emerged as a consequence of a discovered connection between neurodegenerative disorders and hyposmia. In Parkinson's disease and Alzheimer's disease hyposmia is not only one of the earliest clinical presentations, the degree of hyposmia also correlates with the later progression of these two conditions. Hyposmia should not be ignored nor accepted; instead it should be integrated in any neurological examination, especially in elderly patients.