Olfaction, the ability to smell, occurs when specialized sensory neurons high inside the nose detect odor molecules and transmit this information to the brain. This sense is important for both the perception of flavor and personal safety. When this function is completely lost, the condition is called anosmia, which can range from a temporary inconvenience to a persistent health concern.
Defining Anosmia and Hyposmia
Anosmia is defined as the complete inability to detect odors. A related but distinct condition is hyposmia, which involves a measurable reduction in the sense of smell, where scents are detected but are fainter than normal. Both anosmia and hyposmia are considered quantitative disorders, reflecting a change in the intensity of the sense.
Smell loss can also manifest as qualitative disorders, such as parosmia and phantosmia. Parosmia is a distortion of a familiar smell, where a pleasant odor is now perceived as foul or unpleasant. Phantosmia, sometimes called an olfactory hallucination, is the sensation of smelling an odor that is not present. These qualitative changes often represent misfiring signals as the olfactory nerve system attempts to repair itself following damage.
Common Reasons for Losing Your Sense of Smell
The causes of smell loss are broadly categorized based on where the problem occurs along the olfactory pathway: conductive loss or sensorineural loss. Conductive loss happens when a physical obstruction prevents odor molecules from reaching the sensory neurons in the nasal cavity. This type is often temporary and can be caused by conditions that swell the nasal lining, such as allergies, acute sinus infections, or the common cold.
Another frequent conductive cause is the presence of nasal polyps, which are noncancerous growths that physically block the nasal passage. Once the obstruction or inflammation is resolved, whether through medication or surgery, the sense of smell often returns. This type of damage is generally considered more straightforward to treat.
Sensorineural loss involves damage to the olfactory nerve cells themselves or the brain areas that process smell. Viral infections, including influenza and coronaviruses like SARS-CoV-2, are a common cause because they can directly injure supporting cells within the olfactory epithelium, leading to the death of sensory neurons. Head trauma can also cause sensorineural loss if the injury severs the fine nerve filaments passing from the nose to the brain.
Other causes include the natural process of aging, known as presbyosmia, which involves a gradual decline in the regenerative capacity of olfactory cells. Exposure to toxic chemicals, certain medications, and neurodegenerative conditions like Parkinson’s or Alzheimer’s disease can also damage the neural tissue necessary for olfaction. Chronic rhinosinusitis often leads to a mixed loss, featuring both obstruction and sensorineural damage.
Diagnosing the Root Cause
Seeking professional medical evaluation is important, especially if smell loss is sudden, persistent after a cold clears, or accompanied by other symptoms. An otolaryngologist, an ear, nose, and throat specialist, typically begins the diagnostic process with a detailed medical history to determine when the problem started. A physical examination of the nasal passages is performed using a speculum or nasal endoscopy to look for visible obstructions like polyps or signs of inflammation.
Specific smell tests are used to objectively measure the severity of the loss. These tests help determine if the patient is experiencing anosmia or hyposmia and can sometimes distinguish between different types of smell loss. If a structural issue, tumor, or damage to the brain is suspected, the physician may order imaging tests like a CT scan or an MRI. These scans provide a detailed view of the nasal anatomy and the brain’s olfactory pathways, helping rule out serious underlying causes.
Treatment and Management Strategies
Treatment is always tailored to the identified cause. For inflammation-related conductive loss, such as that caused by allergies or chronic sinusitis, treatments often include oral or topical intranasal corticosteroids to reduce swelling. If the loss is due to a physical block like a nasal polyp or a deviated septum, surgical intervention may be recommended to clear the airway and allow odorants to reach the olfactory receptors.
For sensorineural loss, often seen after viral infections, a primary management strategy is olfactory training. This non-surgical technique involves repeatedly sniffing a set of four distinct odors, typically rose, lemon, eucalyptus, and clove, twice daily for several months. The goal is to encourage the regeneration of nerve fibers and strengthen the neural connections between the nose and the brain.
For individuals whose smell loss is permanent, management shifts to coping and safety measures. Since the inability to smell removes an important warning system, it is necessary to install and regularly check smoke detectors and carbon monoxide alarms in the home. Attention must also be paid to food safety, which includes strictly labeling and monitoring expiration dates to avoid consuming spoiled items.