A decreased sense of smell can have many different etiologies. Over time, all individuals suffer from gradual decreases in their sense of smell just as their eyesight or hearing may diminish with age. However, severe loss of smell or taste may indicate other pathologies.
Patients with nasal allergies or chronic sinusitis will often have fluctuating or chronic loss of smell from inflammation and blockage of the olfactory cleft, the space in the nasal cavity containing the majority of smell receptors. Patients with nasal polyps also typically have this complaint, and while treatment of their polyps medically or surgically often improves their sense of smell, chronic polyps and inflammation can erode the smell nerves and lead to permanent smell loss.
Less commonly, tumors of the sinuses and skull base can also lead to smell loss. Head trauma can cause shearing forces on the smell nerves and also cause temporary and sometimes permanent smell loss.
Unfortunately, another frequent cause of smell loss is called idiopathic smell loss, which means that there is not an obvious medical cause. This type of smell loss is thought to be viral related, but this has not been proven yet.