The most common cause of permanent hyposmia and anosmia are upper respiratory infections. Qualitative alterations differ from quantitative alterations, which include anosmia and hyposmia. This frequently leads to peripheral polyneuropathy, cerebellar ataxia, retinitis pigmentosa, anosmia, and hearing loss. Destruction to the olfactory tract results in ipsilateral anosmia. Destruction to the olfactory bulb results in ipsilateral anosmia. Many cases of congenital anosmia remain unreported and undiagnosed. Anosmia can also be caused by nasal polyps. Often, chronic sinusitis can lead to anosmia, the inability to smell objects. Some people may be anosmic for one particular odor, a condition known as "specific anosmia". The absence of the sense of smell from birth is known as congenital anosmia. Most conditions of nasal congestion also cause a loss of the sense of smell (anosmia). The ability of olfactory neurons to replace themselves decreases with age, leading to age-related anosmia. Kallmann syndrome is also associated with a lack of sense of smell (anosmia). This demonstrated that unilateral anosmia is not uncommon in anosmia patients. Loss of smell is known as anosmia.