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Loss of Smell

(Anosmia)

by Marvin P. Fried, MD

Anosmia is complete loss of smell. Hyposmia is partial loss of smell. Most people with anosmia can recognize 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 anosomia often complain of losing their sense of taste and of not enjoying food.

Causes

Anosmia 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 Some Causes and Features of Anosmia). The olfactory area is where odors are detected and is located high in the nose (see How People Sense Flavors).

Common causes

The most common causes include

  • Head injury (young adults)

  • Viral infections and Alzheimer disease (older adults)

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 (flu). Flu may be the cause in up to one quarter of people with hyposmia or anosmia. Alzheimer disease and some other degenerative brain disorders (such as multiple sclerosis) can damage the olfactory nerves, commonly causing loss of smell.


Less common causes

Drugs can contribute to anosmia in susceptible people. Polyps, tumors, other infections in the nose, and seasonal allergies (allergic rhinitis) 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.


Evaluation

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


When to see a doctor

People who have warning signs should see a doctor within a day or two. Other people should see a doctor when possible, but a doctor's evaluation is not urgent unless other symptoms develop.


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 Some Causes and Features of Anosmia).

Doctors ask about onset and duration of anosmia and its relation to any 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 seek out any 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, 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 that is particularly focused on mental status and the cranial nerves.

Some Causes and Features of Anosmia

Cause

Common Features *

Tests

Blockage within the nose

Nasal allergies

In people who have chronic allergy symptoms (such as nasal congestion and a clear discharge)

No pain

Symptoms that often occur during certain seasons or after exposure to specific substances

A doctor's examination

Nasal polyps

Polyps that are usually seen during the examination

A doctor's examination

Destruction of smell receptors

Chronic sinusitis

A thick, foul-smelling nasal discharge most or all of the time

Previous sinus infections

A doctor's examination

Usually CT

Some viral upper respiratory infections (such as influenza)

Loss of smell that occurs after an infection

A doctor's examination

Tumors (a rare cause)

Possibly vision problems or only loss of smell

CT

MRI

Drugs (such as amphetamines, enalapril, estrogen, naphazoline, phenothiazines, and reserpine or use of decongestants for a long time)

Usually in people who report taking such drugs

A doctor's examination

Toxins (such as cadmium and manganese)

Usually in people who report exposure to such toxins

A doctor's examination

Destruction of olfactory pathways in the brain

Alzheimer disease

Progressive confusion and loss of recent memory

MRI

Sequential memory tests

Head injuries

In people who have had a head injury

CT

Degenerative neurologic disorders (such as multiple sclerosis)

Intermittent episodes of other symptoms of nervous system dysfunction, such as weakness, numbness, or difficulty speaking, seeing, or swallowing

MRI

Sometimes a spinal tap

Brain surgery or infection

In people who have had brain surgery or a brain infection

CT or MRI

Brain tumors

Sometimes headache and/or symptoms of nervous system dysfunction

CT or MRI

* Features include symptoms and results of the doctor's examination. Features mentioned are typical but not always present.

CT = computed tomography; MRI = magnetic resonance imaging.


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 also be tested more formally using 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 there is no clear cause of anosmia, computed tomography (CT) or magnetic resonance imaging (MRI) of the head (including the sinuses) is done to look for structural abnormalities (such as a tumor, an abscess, or a fracture).


Treatment

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 or gas leaks.

Essentials for Older People

The ability to smell decreases as people age. The decrease is caused by a loss of smell receptors. People typically notice changes in smell by age 60. After age 70, changes are substantial.

Key Points

  • Anosmia may be part of normal aging.

  • Common causes include upper respiratory infection, sinusitis, and head injury.

  • An imaging test such as CT or MRI is typically needed unless the cause is obvious to the doctor.

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Drugs Mentioned In This Article

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  • VASOTEC
  • ALBALON