Anosmia is the inability to perceive smell/odor. It can be temporary or permanent and acquired or congenital. There are many causes. Neurological causes can include disturbances to the sensory nerves that make up the olfactory bulb or anywhere along the path in which the signal of smell is transferred to the brain. From the olfactory bulb, the signal is further processed by several other structures of the brain, including the piriform cortex, entorhinal cortex, amygdala, and hippocampus. Any blockage or destruction of the pathway along which smell is transferred and processed may result in anosmia
What are smell disorders?
People who have a smell disorder either have a decrease in their ability to smell or changes in the way they perceive odors.
- Hyposmia [high-POSE-mee-ah] is a reduced ability to detect odors.
- Anosmia [ah-NOSE-mee-ah] is the complete inability to detect odors. In rare cases, someone may be born without a sense of smell, a condition called congenital anosmia.
- Parosmia [pahr-OZE-mee-ah] is a change in the normal perception of odors, such as when the smell of something familiar is distorted, or when something that normally smells pleasant now smells foul.
- Phantosmia [fan-TOES-mee-ah] is the sensation of an odor that isn’t there.
Causes
Smell disorders have many causes, with some more obvious than others. Most people who develop a smell disorder have experienced a recent illness or injury. Common causes of smell disorders are:
- Aging
- Sinus and other upper respiratory infections
- Smoking
- Growths in the nasal cavities
- Head injury
- Hormonal disturbances
- Dental problems
- Exposure to certain chemicals, such as insecticides and solvents
- Numerous medications, including some common antibiotics and antihistamines
- Radiation for treatment of head and neck cancers
- Conditions that affect the nervous system, such as Parkinson’s disease or Alzheimer’s disease.
As stated in the introduction, any problems that cause a disturbance in the pathway that leads to the perception of smell, whether mechanical or along the olfactory neural pathway can lead to anosmia.[rx][rx][rx]
Inflammatory and Obstructive Disorders (50% to 70% of cases of anosmia)
These are the most common causes of anosmia, and these include nasal and paranasal sinus disease (rhino-sinusitis, rhinitis, and nasal polyps). These disorders cause anosmia through inflammation of the mucosa as well as through direct obstruction.
Head Trauma
Head trauma is another common cause of anosmia as trauma to the head can cause damage to the nose or sinuses leading to mechanical blockage and obstruction. Other ways an injury can cause anosmia is by trauma or destruction to the olfactory axons that are present at the cribriform plate, damage to the olfactory bulb, or direct injury to the olfactory areas of the cerebral cortex. The central (CNS) nervous system trauma leading to anosmia can be temporary or permanent depending on the area and extent of the injury. Olfactory neurons have regenerative capabilities that other CNS nerves in the body do not. This unique ability is the center of much current stem cell-related research.
Aging and Neurodegenerative Processes
These processes are associated with the loss of smell which can eventually result in anosmia. Normal aging is associated with decreased sensitivity to smell. As individuals age, they lose the number of cells in the olfactory bulb as well as the olfactory epithelium surface area which is important in sensing smell. Interestingly, there have been studies that associate the impairment of the ability to smell with neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and Lewy Body dementia. Studies linked a low ability to perceive smell associated with an increased risk of developing neurodegenerative diseases. The highest association is between anosmia and later development of alpha-synucleinopathy including Parkinson’s disease, diffuse Lewy body disease, and multisystem atrophy.
Congenital Conditions
Congenital conditions that are associated with anosmia include Kallmann syndrome and Turner syndrome.
Infective Conditions
Anosmia is said to be one of the early symptoms of COVID-19 infection.[rx]
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Post-infectious olfactory dysfunction: This is one of the most common causes of olfactory decline in clinical practice and one of the most common indications for olfactory training.[rx][rx][rx] Following mild to severe upper respiratory tract infection, particularly virally-mediated, post-infectious olfactory dysfunction may occur. Olfactory testing in these patients shows diminished odor threshold and odor discrimination but normal odor identification.[rx] Many patients with COVID-19 infection and a decline of the chemical senses have this type of olfactory dysfunction.[rx] The likelihood of improvement with olfactory training for patients with post-infectious olfactory dysfunction is comparatively good. A study involving more than 100 patients showed that 71% of patients with post-infectious olfactory dysfunction improved with olfactory training over one year, while 37% of patients without olfactory training spontaneously recovered over the same period.[rx] Neither abstinence from alcohol intake nor patient gender appears to affect the chance of improvement with olfactory training in patients with post-infectious olfactory dysfunction.[rx] Olfactory training may be used in combination with corticosteroid treatment in selected patients with post-infectious olfactory dysfunction.[rx][rx][rx]
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Post-traumatic olfactory dysfunction: This type of olfactory dysfunction may occur suddenly or with a delay after a brain or nasal injury.[rx] Olfactory tests reveal diminished odor threshold and odor discrimination ability, while odor identification is normal.[rx] Olfactory training has been reported to work for post-traumatic olfactory dysfunction, but results may be less dramatic than for post-infectious olfactory dysfunction. Jiang et al. reported that 23% of patients with post-traumatic olfactory dysfunction had improvement in olfactory thresholds when training with phenyl ethyl alcohol, while 5% of patients improved when training with mineral oil control.[rx]
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Parkinson’s disease: Patients with Parkinson’s disease have a severe impairment of olfactory functioning, which starts many years before the onset of motor symptoms such as tremors, rigidity, and bradykinesia gait.[rx][rx][rx] Olfactory testing typically reveals impairment of odor discrimination and odor identification abilities, while the odor threshold is relatively normal.[rx] Medication prescribed for Parkinson’s disease does not improve olfactory functioning, and in light of the progressive, neurodegenerative process that underlies Parkinson’s disease, olfactory training may be an important treatment option for patients who request rehabilitation of sense of smell.[rx] The success rate with olfactory training in patients with Parkinson’s disease is lower than in patients with post-infectious olfactory dysfunction; about 20% of patients with Parkinson’s disease benefit from olfactory training, while 10% recover olfactory function spontaneously.[rx]
Other Traumatic or Obstructive Conditions
Other causes of anosmia include toxic agents such as tobacco, drugs, and vapors that can cause olfactory dysfunction, post-viral olfactory dysfunction, facial traumas involving nasal or sinus deformity, neoplasms in the nasal cavity or brain that prohibits the olfactory signal pathway, and subarachnoid hemorrhages. Olfactory groove meningioma can present with slowly worsening impaired olfaction.
Common conditions that can uncommonly cause a decreased sense of smell or anosmia include diabetes mellitus and hypothyroidism.
Medications can sometimes lead to olfactory defects as an unwanted side effect. These medications include beta-blockers, anti-thyroid drugs, dihydropyridine, ACE inhibitors, and intranasal zinc.
Symptoms
Symptoms can range from not being able to smell or taste at all to the reduced ability to smell or taste specific things that are sweet, sour, bitter, or salty. In some cases, normally pleasant tastes or smells may become unpleasant.
Diagnosis
Both smell and taste disorders are treated by an otolaryngologist, a doctor who specializes in diseases of the ear, nose, throat, head, and neck (sometimes called an ENT). An accurate assessment of a smell disorder will include, among other things, a physical examination of the ears, nose, and throat; a review of your health history, such as exposure to toxic chemicals or injury; and a smell test supervised by a health care professional.
There are two common ways to test smell. Some tests are designed to measure the smallest amount of odor that someone can detect. Another common test consists of a paper booklet of pages that contain tiny beads filled with specific odors. People are asked to scratch each page and identify the odor. If they can’t smell the odor, or identify it incorrectly, it could indicate a smell disorder or an impaired ability to smell.
History and Physical
When taking a history of the possible causes of anosmia, a clinician must keep the possible etiologies (listed above) in mind when asking relevant questions.
Sudden smell loss is often associated with head injuries or viral infections, while gradual loss is more associated with allergic rhinitis, nasal polyps, and neoplasms. An intermittent loss is often common in allergic rhinitis and with the use of topical drugs.
It is important to ask about preceding events and the patient’s medical history, as the most common causes of anosmia are chronic rhinitis and head trauma.
The patient’s age can be helpful because if the patient is very young and has other symptoms, the clinician might investigate congenital causes such as Kallmann syndrome. Under such circumstances, careful examination of the gonads and neurological exams are very important. If the patient is elderly, the clinician may investigate whether the sense of smell is due to normal aging or if there are other symptoms to suggest an early stage of a neurodegenerative disorder like Parkinson’s disease.
Social history is also important in assessing occupation-associated exposures to toxins or allergens that can lead to anosmia. Medication history is always important, and sometimes the causal relationship can only be established by stopping the suspected offending agent.
Clinicians should pay attention to associated symptoms as anosmia is a symptom and not a diagnosis. Headaches and behavior disturbances may indicate problems with the CNS.
During the physical examination, clinicians should closely examine the nasal cavity and paranasal sinuses. Findings may be important depending on information retrieved from the patient’s history.
A neurological examination may be useful in revealing other neurological deficits that can suggest a larger neurological problem causing the loss of smell. Fundoscopy for evidence of raised intracranial pressure will help to pave the way for neuroimaging testing.
Examination and skin testing by an allergist might play an important role to evaluate whether rhinitis (if the cause) is allergic or non-allergic.
Simple office testing of smell with chocolates or coffee is sometimes conducted informally by a primary care provider. This test is subjective. If the clinician is concerned about any findings, detailed smell testing can be conducted at the smell centers. Tests include chemosensory testing, and butanol threshold tests, among others. These formal tests can give a more accurate level of “loss of smell” in that a minimum concentration of a chemical at the patient can detect can be given and compared to the average threshold for that patient’s age group. UPSIT, the University of Pennsylvania Small Identification Test (Sensonics, Inc., Haddon Heights, NJ) is the most widely used odor identification test which can be administered in about 10 minutes.[rx][rx][rx][rx][rx]
Other evaluations can be performed depending on the clinician’s suspicion of the underlying cause of the patient’s anosmia. Based on the history and physical examination, if the clinician is suspicious of head trauma, sinus disease, or neoplasm, they may order a magnetic resonance imaging (MRI) or computed tomogram (CT).
If there is concern about allergic rhinitis, a referral to an allergist and subsequent allergen skin testing might be revealed. If the patient has other symptoms that are suggestive of inflammatory diseases, a sedimentation rate might be helpful. Other labs that can be considered depending on the suspected etiology include complete blood count (CBC), plasma creatinine, liver function, thyroid profile, ANA, and measurements of heavy metals, lead, and other toxins.
It is important to note that imaging (MRI) in those with idiopathic olfactory loss is often unrevealing. In a study of 839 patients with olfactory loss, MRI was used to evaluate idiopathic olfactory loss 55% of the time, but only successfully found an imaging abnormality that would explain the loss 0.8% of the time.[rx]
Diagnosis by a doctor is important to identify and treat the underlying cause of a potential smell disorder.
Treatment
The treatment and management depend on the etiology as anosmia is not a diagnosis but a symptom.
As stated above, inflammatory and obstructive diseases are the most common cause of anosmia (para-nasal and nasal sinus diseases), and intranasal glucocorticoids can often manage these causes. Other medications that can be given include antihistamines and systemic glucocorticoids. Antibiotics such as ampicillin can be prescribed for bacterial sinus infections. Surgery can be an opinion for those with chronic sinus problems and nasal polyps that fail conservative medical management.
If your problem is caused by medications, talk to your doctor to see if lowering the dosage or changing the medicine could reduce its effect on your sense of smell. If nasal obstructions such as polyps are restricting the airflow in your nose, you might need surgery to remove them and restore your sense of smell.
Medications such as corticosteroids for chronic rhinosinusitis and other inflammatory conditions may provide some relief from olfactory decline.[rx][rx] Other options include sodium citrate, zinc, and vitamins, but their efficacy has not been definitively proven to date.[rx][rx] A non-surgical and non-pharmacological approach to improve olfactory function is olfactory training, wherein patients expose themselves twice daily to different odors over several months.[rx][rx]
Some people recover their ability to smell when they recover from the illness causing their loss of smell. Some people recover their sense of smell spontaneously, for no obvious reason. If your smell disorder can’t be successfully treated, you might want to seek counseling to help you adjust.
How common are smell disorders?
Your sense of smell helps you enjoy life. You may delight in the aromas of your favorite foods or the fragrance of flowers. Your sense of smell is also a warning system, alerting you to danger signals such as a gas leak, spoiled food, or a fire. Any loss in your sense of smell can hurt your quality of life. It can also be a sign of more serious health problems.
One to two percent of North Americans report problems with their sense of smell. Problems with the sense of smell increase as people get older, and they are more common in men than women. In one study, nearly one-quarter of men ages 60–69 had a smell disorder, while about 11 percent of women in that age range reported a problem.
Many people who have smell disorders also notice problems with their sense of taste.
How does your sense of smell work?
Your sense of smell—like your sense of taste—is part of your chemosensory system or the chemical senses.
Your ability to smell comes from specialized sensory cells, called olfactory sensory neurons, which are found in a small patch of tissue high inside the nose. These cells connect directly to the brain. Each olfactory neuron has one odor receptor. Microscopic molecules released by substances around us—whether it’s coffee brewing or pine trees in a forest—stimulate these receptors. Once the neurons detect the molecules, they send messages to your brain, which identifies the smell. There are more smells in the environment than there are receptors, and any given molecule may stimulate a combination of receptors, creating a unique representation in the brain. These representations are registered by the brain as a particular smell.
Smells reach the olfactory sensory neurons through two pathways. The first pathway is through your nostrils. The second pathway is through a channel that connects the roof of the throat to the nose. Chewing food releases aromas that access the olfactory sensory neurons through the second channel. If the channel is blocked, such as when your nose is stuffed up by a cold or flu, odors can’t reach the sensory cells that are stimulated by smells. As a result, you lose much of your ability to enjoy a food’s flavor. In this way, your senses of smell and taste work closely together.
Without the olfactory sensory neurons, familiar flavors such as chocolate or oranges would be hard to distinguish. Without smell, foods tend to taste bland and have little or no flavor. Some people who go to the doctor because they think they’ve lost their sense of taste are surprised to learn that they’ve lost their sense of smell instead.
Your sense of smell is also influenced by something called common chemical sense. This sense involves thousands of nerve endings, especially on the moist surfaces of the eyes, nose, mouth, and throat. These nerve endings help you sense irritating substances—such as the tear-inducing power of an onion—or the refreshing coolness of menthol.
Are smell disorders serious?
Like all of your senses, your sense of smell plays an important part in your life. Your sense of smell often serves as a first warning signal, alerting you to the smoke of a fire, spoiled food, or the odor of a natural gas leak or dangerous fumes.
When their smell is impaired, some people change their eating habits. Some may eat too little and lose weight while others may eat too much and gain weight. As food becomes less enjoyable, you might use too much salt to improve the taste. This can be a problem if you have or are at risk for certain medical conditions, such as high blood pressure or kidney disease. In severe cases, loss of smell can lead to depression.
Problems with your chemical senses may be a sign of other serious health conditions. A smell disorder can be an early sign of Parkinson’s disease, Alzheimer’s disease, or multiple sclerosis. It can also be related to other medical conditions, such as obesity, diabetes, hypertension, and malnutrition. If you are experiencing a smell disorder, talk with your doctor.
What research is being done on smell disorders?
The NIDCD supports basic and clinical research of smell and taste disorders at its laboratories in Bethesda, Maryland, and universities and chemosensory research centers across the country. These chemosensory scientists are exploring how to:
- Promote the regeneration of sensory nerve cells.
- Understand the effects of the environment (such as gasoline fumes, chemicals, and extremes of humidity and temperature) on smell and taste.
- Prevent the effects of aging on smell and taste.
- Develop new diagnostic tests for taste and smell disorders.
- Understand associations between smell disorders and changes in diet and food preferences in the elderly or among people with chronic illnesses.
References
