Anosmia is complete loss of smell. Hyposmia is partial loss of smell. Most people with anosmia can taste salty, sweet, sour, and bitter substances but cannot tell the difference between specific flavors. The ability to tell the difference between flavors actually depends on smell, not the taste receptors on the tongue. Therefore, people with anosmia often complain of losing their sense of taste and of not enjoying food.
A loss of smell receptors due to aging causes a decreased ability to smell in older people. People typically notice changes in smell by age 60. After age 70, changes are substantial.
Causes of Anosmia
Anosmia that is not the result of aging occurs when swelling or another blockage of the nasal passages prevents odors from reaching the olfactory area or when parts of the olfactory area or its connections to the brain are destroyed (see table ). The olfactory area, where odors are detected, is located high in the nose (see ).
Common causes
The most common causes include
Head injury Head Injuries (young adults)
Viral infections
A common cause of permanent loss of smell is a head injury, as may occur in a car accident. Head injury can damage or destroy fibers of the olfactory nerves (the pair of cranial nerves that connect smell receptors to the brain) where they pass through the roof of the nasal cavity. Sometimes the injury involves a fracture of the bone (cribriform plate) that separates the brain from the nasal cavity. Damage to the olfactory nerves can also result from infections (such as abscesses) or tumors near the cribriform plate.
Another common cause is an upper respiratory infection, especially influenza Influenza (Flu) Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more (flu). Flu may be the cause in up to one quarter of people with hyposmia or anosmia. Alzheimer disease Alzheimer Disease Alzheimer disease is a progressive loss of mental function, characterized by degeneration of brain tissue, including loss of nerve cells, the accumulation of an abnormal protein called beta-amyloid... read more and some other degenerative brain disorders (such as multiple sclerosis Multiple Sclerosis (MS) In multiple sclerosis, patches of myelin (the substance that covers most nerve fibers) and underlying nerve fibers in the brain, optic nerves, and spinal cord are damaged or destroyed. The cause... read more ) can damage the olfactory nerves, commonly causing loss of smell.
Less common causes
Medications can contribute to anosmia in susceptible people. Polyps, tumors, other infections in the nose, and seasonal allergies (allergic rhinitis Allergic Rhinitis Rhinitis is inflammation and swelling of the mucous membrane of the nose, characterized by a runny nose and stuffiness and usually caused by the common cold or a seasonal allergy. Colds and... read more 
) may interfere with the ability to smell. Occasionally, serious infections of the nasal sinuses or radiation therapy for cancer causes a loss of smell or taste that lasts for months or even becomes permanent. These conditions can damage or destroy smell receptors. The role of tobacco is uncertain.
A very few people are born without a sense of smell.
Anosmia or hyposmia may be an early symptom of COVID-19 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more , an acute respiratory illness that can be severe. COVID-19 is caused by the coronavirus called SARS-CoV2.
Evaluation of Anosmia
The following information can help people decide whether a doctor's evaluation is needed and help them know what to expect during the evaluation.
Warning signs
The following findings are of particular concern:
Recent head injury
Symptoms of nervous system dysfunction, such as weakness, trouble with balance, or difficulty seeing, speaking, or swallowing
Sudden start of symptoms
Local or global outbreak of COVID-19
When to see a doctor
People who have warning signs should see a doctor right away. Other people should see a doctor when possible.
What the doctor does
Doctors first ask questions about the person's symptoms and medical history and then do a physical examination. What doctors find during the history and physical examination often suggests a cause and the tests that may need to be done (see table ).
Doctors ask when and how anosmia started and how long it has lasted. They also ask whether it started before or shortly after a cold, bout of flu, or head injury. They note other symptoms such as a runny or stuffy nose and whether any nasal discharge is watery, bloody, thick, or foul-smelling. Doctors check for neurologic symptoms, especially those that involve a change in mental status (for example, difficulty with short-term memory) or the cranial nerves (for example, double vision or difficulty speaking or swallowing).
Questions about the person's medical history involve sinus disorders, head injury or surgery, allergies, medications and illicit drugs used, and exposure to chemicals or fumes.
During the physical examination, doctors inspect the nasal passages for swelling, inflammation, discharge, and polyps. Doctors also do a complete neurologic examination Neurologic Examination When a neurologic disorder is suspected, doctors usually evaluate all of the body systems during the physical examination, but they focus on the nervous system. Examination of the nervous system—the... read more that is particularly focused on mental status and the cranial nerves.
Testing
To test smell, doctors hold common fragrant substances (such as soap, a vanilla bean, coffee, and cloves) under the person's nose, one nostril at a time. The person is then asked to identify the smell. Smell can be tested more formally with standardized commercial smell test kits. One kit requires the person to scratch and sniff many different smell samples and try to identify them. Another kit contains diluted samples of a smelly chemical. Doctors see how dilute the sample can be before the person can no longer smell the chemical.
If COVID-19 is suspected, viral testing is done, and the person is managed according to local protocols, including isolation guidelines.
If there is no clear cause of anosmia, computed tomography Computed Tomography (CT) In computed tomography (CT), which used to be called computed axial tomography (CAT), an x-ray source and x-ray detector rotate around a person. In modern scanners, the x-ray detector usually... read more 
(CT) or magnetic resonance imaging Magnetic Resonance Imaging (MRI) In magnetic resonance imaging (MRI), a strong magnetic field and very high frequency radio waves are used to produce highly detailed images. MRI does not use x-rays and is usually very safe... read more 
(MRI) of the head (including the sinuses) is done to look for structural abnormalities (such as a tumor, an abscess, or a fracture).
Treatment of Anosmia
Doctors treat the cause of the anosmia. For example, people with sinus infections and irritation may be treated with steam inhalation, nasal sprays, antibiotics, and sometimes surgery. However, the sense of smell does not always return even after successful treatment of sinusitis. Tumors are surgically removed or treated with radiation, but such treatment usually does not restore the sense of smell. Polyps in the nose are removed, sometimes restoring the ability to smell. People who smoke tobacco should stop.
There are no treatments for anosmia itself. People who retain some sense of smell may find that adding concentrated flavoring agents to food improves their enjoyment of eating. Smoke alarms, important in all homes, are even more essential for people with anosmia because they cannot smell smoke. Doctors recommend that people with anosmia use caution before consuming stored food and using natural gas for cooking or heating because they may have difficulty detecting food spoilage and gas leaks.
Key Points
A loss of smell may be part of normal aging.
Common causes include upper respiratory infections, sinusitis, and head injury.
An imaging test such as CT or MRI is typically needed unless the cause is obvious to the doctor.
