[Olfaction and neurological diseases: a review of the literature].

Olfactory disorders are often misjudged and rarely rated in the clinical setting. They are nevertheless described in a wide range of neurological disorders, and their evaluation can be useful for diagnosis. Usually irreversible olfactory dysfunction is a well-known complication after head trauma. Severe changes in olfactory tests are observed in Parkinson's disease. Dysfunction is present at disease onset and evidenced with all behavioral tests. Regarding other parkinsonian syndromes, olfactory performances are severely impaired in Lewy body disease, less pronounced in multiple system atrophy and usually preserved in corticobasal degeneration. Olfactory deficits are an early feature in Alzheimer's disease and worsen with disease progression. Rarely reported by patients, they must be searched for with olfactory tests. Though epilepsy is mainly known for its olfactory hallucinatory disorders, alterations of olfactory abilities are also described, especially in mesial temporal epilepsy. Disorders of olfactory perception are finally reported in patients with multiple sclerosis and migraine. After a reminder of anatomical data on the olfactory system, and the different methods of testing used to rate olfactory performances, the current review focuses on the type of olfactory dysfunction and damaged brain areas of the olfactory system encountered in the main neurological diseases.