- Blockage of the nasal passages (due to nasal polyps, allergies, sinusitis, physical damage to the nose, etc.)
Sometimes smell function is impaired when the nasal passages are blocked and odors can’t reach the sensory cells (called olfactory sensory neurons or OSNs) that sit deep within the nasal cavity. Nasal polyps, damage to the structure of the nose, inflammation and severe sinusitis can create impediments to odors getting to where they need to go to be detected by the olfactory system.
- Damage to the olfactory nerve (due to head trauma, viral infection, inhaled toxins, etc.)
Cells within the nasal cavity called olfactory sensory neurons (or OSNs) detect odors and send that information through the skull into the brain. These OSNs are regularly subjected to environmental damage from viruses or inhaled toxins. They can also be physically damaged, such as with certain head traumas. Normally, the nose is constantly generating replacement OSNs that are able to detect odors in the nasal cavity and make new connections to the brain to communicate odor information. However, sometimes the damage is too severe for this replacement process to occur efficiently, if at all. If that is the case, a partial or complete loss of smell function can occur.
- Damage to the olfactory brain (due to head trauma, tumors, neurodegenerative disease)
Regions of the brain that receive olfactory information from the nose can be damaged, resulting in smell loss. These brain areas – including the olfactory bulb and olfactory cortex – can be damaged by physical trauma, neurodegenerative diseases, or tumors.
- Congenital smell loss (syndromic, genetic, etc.)
Some people are born without a sense of smell. This is known as congenital (“from birth”) anosmia. In many cases, congenital anosmia is thought to result from gene changes (“mutations”) that impact the structure or function of key parts of the olfactory system. Several genetic syndromes that affect multiple body systems (for example, Bardet-Beidel syndrome) can have a smell loss component.
- Age-related smell loss
While aging itself is not a cause of smell loss, the prevalence of a smell impairment does increase with age. One study found that nearly 25% of people from 53 - 97 years old have a smell impairment, and that the prevalence increased with age.
- Idiopathic (no known cause)
Sometimes, it is impossible to determine the cause of a smell disorder, though some potential causes (such as nasal polyps or severe sinusitis) can be eliminated.