Hyposmia is a reduced ability to smell and to detect odors. A related condition is anosmia, in which no odors can be detected. Some of the causes of olfaction problems are allergies, nasal polyps, viral infections and head trauma. It is estimated that up to 4 million people in the United States have hyposmia or the related anosmia.
Hyposmia might be a very early sign of Parkinson's disease. Hyposmia is also an early and almost universal finding in Alzheimer's disease and dementia with Lewy bodies. Lifelong hyposmia could be caused by Kallmann syndrome.
Kallmann syndrome (KS), a rare genetic disorder, refers to the association between hypogonadotropic hypogonadism and anosmia or hyposmia due to abnormal migration of olfactory axons and gonadotropin-releasing hormone producing neurons.