Anosmia is complete loss of smell. Hyposmia is partial loss of smell. If unilateral, anosmia is often unrecognized.
Most patients with anosmia have normal perception of salty, sweet, sour, and bitter substances but lack flavor discrimination, which largely depends on olfaction. Therefore, they often complain of losing the sense of taste (ageusia) and of not enjoying food.
Pathophysiology of Anosmia
Anosmia occurs when
Intranasal swelling or other obstruction prevents odors from gaining access to the olfactory area.
The olfactory neuroepithelium is destroyed.
The olfactory nerve fila, bulbs, tracts, or central connections are destroyed (see table Some Causes of Anosmia).
Some Causes of Anosmia
Cause | Suggestive Findings | Diagnostic Approach* |
|---|---|---|
Intranasal obstruction | ||
History of chronic allergic symptoms (eg, congestion, clear rhinorrhea); no pain | Clinical examination alone | |
Polyps usually visible during examination | Clinical examination alone | |
Destruction of olfactory neuroepithelium | ||
Chronic rhinitis with atrophic and sclerotic mucous membranes, patency of nasal passages, crust formation, a foul odor | Clinical examination alone Sometimes biopsy, which shows the normal ciliated columnar epithelium converted to stratified squamous (squamous metaplasia) and the lamina propria reduced in amount and vascularity | |
Chronic mucopurulent drainage, documented infections | Clinical examination alone Sometimes CT | |
Onset often precedes other symptoms of infection (eg, fever, cough) | Viral testing when available | |
Onset after clinical infection | Clinical examination alone | |
Usually, an apparent history of use | Clinical examination alone | |
Usually, an apparent history of exposure | Clinical examination alone | |
Tumors (eg, meningioma, olfactory neuroblastoma)‡ | Only anosmia or possibly visual difficulty (if the tumor impinges on the optic chiasm or olfactory groove) | CT MRI |
Destruction of central pathways | ||
Alzheimer disease | Progressive confusion and loss of recent memory | MRI Sequential memory tests |
Degenerative neurologic disorders (eg, multiple sclerosis) | Intermittent episodes of other neurologic symptoms (eg, weakness;, numbness; difficulty speaking, seeing, or swallowing | MRI Sometimes lumbar puncture |
Apparent by history | CT or MRI | |
Surgery and CNS infection apparent by history Tumors with or without other neurologic symptoms | CT or MRI | |
* Clinical examination is always done but is mentioned in this column only when it can be the sole means of diagnosis. | ||
† Destruction of olfactory epithelium has not yet been confirmed as the mechanism for anosmia. | ||
‡ Tumors are rare causes of anosmia. | ||
CNS = central nervous system; COVID-19 = coronavirus disease 2019; CT = computed tomography; MRI = magnetic resonance imaging; URI = upper respiratory infection. | ||
Major causes of anosmia include
Head trauma (young adults)
Alzheimer disease (older adults)
Prior upper respiratory infection (URI), especially influenza, is implicated in 14 to 26% of all patients that present with hyposmia or anosmia.
Medications can contribute to anosmia in susceptible patients. Other causes include prior head and neck radiation, recent nasal or sinus surgery, nasal and brain tumors, and toxins. The role of tobacco is uncertain.
Anosmia may be an early symptom and thus a clue to COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
Pearls & Pitfalls
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Evaluation of Anosmia
History
History of present illness should assess the time course of symptoms and their relation to any URI or head injury. Important associated symptoms are nasal congestion, rhinorrhea, or both. The nature of rhinorrhea should be assessed (eg, watery, mucoid, purulent, bloody).
Review of systems should assess neurologic symptoms, particularly those involving mental status (eg, difficulty with recent memory) and cranial nerves (eg, diplopia, difficulty speaking or swallowing, tinnitus, vertigo).
Past medical history should include history of sinus disorders, cranial trauma or surgery, allergies, use of drugs (prescription, over-the-counter, illicit), and exposure to chemicals or fumes.
Physical examination
The nasal passages should be inspected for swelling, inflammation, discharge, and polyps. Having the patient breathe through each nostril sequentially (while the other is manually occluded) may help identify obstruction.
A complete neurologic examination, particularly of mental status and cranial nerves, is done.
Red flags
The following findings in patients with anosmia are of particular concern:
Head injury
Neurologic symptoms or signs
Sudden onset
Local or global epidemic of COVID-19
Interpretation of findings
Sudden onset after significant head trauma or toxin exposure strongly implicates that event as the cause.
A history of chronic rhinosinusitis suggests that anosmia is caused by chronic rhinosinusitis, particularly when significant congestion, polyps, or both are visible during examination. However, because these findings are common in the population, the physician should be alert to the possibility of another cause (eg, tumor, infection).
Progressive confusion and recent memory loss in an older patient suggest Alzheimer disease as the cause.
Waxing and waning neurologic symptoms affecting multiple areas suggest a neurodegenerative disease such as multiple sclerosis.
Slowly progressive anosmia in an older patient with no other symptoms or findings suggests normal aging as the cause.
Testing
An in-office test of olfaction can help confirm olfactory dysfunction. Commonly, one nostril is pressed shut, and a pungent odor such as from a vial containing coffee, cinnamon, or tobacco is placed under the open nostril; if the patient can identify the substance, olfaction is presumed intact. The test is repeated on the other nostril to determine whether the response is bilateral. Unfortunately, the test is crude and unreliable.
If anosmia is present and no cause is readily apparent during clinical evaluation (see table Some Causes of Anosmia), patients should have CT of the head (including sinuses) with contrast to rule out a tumor or unsuspected fracture of the floor of the anterior cranial fossa. MRI is also used to evaluate intracranial disease and may be needed as well, particularly in patients with no nasal or sinus pathology seen on CT scans.
A psychophysical assessment of odor and taste identification and threshold detection may help confirm or exclude the presence of anosmia and characterize the type and degree of subjective loss of smell. This assessment commonly involves use of one or several commercially available testing kits. One kit uses a scratch-and-sniff battery of odors; another kit involves sequential dilutions of an odorous chemical.
If COVID-19 is suspected, patients should be tested and managed according to local protocols.
Treatment of Anosmia
Specific causes are treated; however, smell is not always recovered even after successful treatment of sinusitis.
There are no specific treatments for anosmia, except rarely, when the cause is an infection or a medication.
Patients who retain some sense of smell may find adding concentrated flavoring agents to food improves their enjoyment of eating.
Smoke alarms, important in all homes, are even more essential for patients with anosmia. Patients should be cautioned about consumption of stored food and use of natural gas for cooking or heating because they have difficulty detecting food spoilage and gas leaks.
Geriatrics Essentials: Anosmia
There is a significant loss of olfactory receptor neurons with normal aging, leading to a marked diminution of the sense of smell. Changes are usually noticeable by age 60 and can be marked after age 70.
Key Points
Anosmia may be part of normal aging.
Common causes include URIs, sinusitis, and head trauma.
Cranial imaging is typically required unless the cause is obvious.
