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Taste and smell disorders

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Related Terms
  • Alzheimer's disease, anosmia, aromas, Bell's palsy, bitter, chemical sensing system, chemosensation, chemosensory dysfunction, chemosensory mechanism, chemosenses, drug-induced, dysgeusia, flavor, gustatory, gustatory nerve, head injury, head trauma, hypoguesia, hyposmia, laryngectomy, larynx, nerve cells, neurodegenerative disorder, odors, olfactory, olfactory sense, Parkinson's disease, parosmia, salty, radiation therapy, respiratory tract infection, scratch-and-sniff-test, smell, smell and taste deficiency, smell and taste disorder, smell deficiency, smell disorder, sour, sweet, taste, taste and smell deficiency, taste buds, taste deficiency, taste disorder, taste nerve, upper respiratory tract infection, vitamin B12, vitamin deficiency, voice box, zinc.

Background
  • Taste and smell disorders are complete or partial loss of either or both senses. Some individuals may experience distorted taste or smell, such as smelling odors that are not present or perceiving unusual scents or tastes.
  • Taste and smell disorders can be caused by many different factors including heredity, infection, exposure to toxins, and certain medications. Depending on the factors involved, these disorders can be either temporary or permanent.
  • Smell and taste belong to the chemical sensing system, called chemosensation. Individuals are able to taste and smell because specific nerves in the nose, mouth, and throat are stimulated when they come into contact with a stimulus, such as odors or food. Once these cells are stimulated, they transmit messages to the brain, where specific tastes or smells are identified.
  • Olfactory cells are the nerve cells involved with smell. They are located inside a small patch of tissue high in the nose and they connect directly to the brain. Odors stimulate the olfactory nerve cells.
  • Gustatory cells are the nerve cells involved with taste. These cells are grouped together in clusters inside the taste buds in the mouth (especially on the tongue) and throat. When individuals eat or drink, these surface cells are stimulated to send messages through nearby fibers to the brain where the specific tastes are identified.
  • The terms "flavor" and "taste" are often used synonymously, but they are actually two separate senses. Taste refers to sweet, sour, salty, and bitter. Flavor, on the other hand, involves taste and smell, as well as texture, color, and temperature. In fact, smell contributes to about 75% of flavor sensations. This is why individuals who have stuffy noses are often unable to taste their food.
  • Sense of taste and smell allow individuals to enjoy the aromas and flavors of foods and drinks. These senses also allow individuals to identify spoiled foods, which deters them from ingestion and prevents food poisoning. They can also warn individuals of dangers, such as fire, polluted air, and toxic chemicals.
  • More than 200,000 Americans visit physicians each year with complaints of taste and smell disorders, according to researchers. While many patients report a decrease in both smell and taste, about 80% of cases involve smell loss only. This is because the sense of smell contributes to 75% of flavor sensations.

Author information
  • This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

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Causes
  • Age: Researchers have shown that sense of smell is most accurate between the ages of 30 and 60 years old. The sense of smell begins to decline after age 60. Several studies suggest that gradual loss of smell and taste is a normal part of aging. Down's syndrome patients typically experience a more drastic decline in smell as they age than individuals who do not have the disorder.
  • Disease: Neurodegenerative diseases like Parkinson's disease and Alzheimer's disease may impair smell. Since there are currently no cures for these neurological diseases, smell impairment is permanent.
  • Exposure to toxins: Prolonged exposure to toxins, including insecticides and tobacco smoke, may inhibit an individual's ability to taste and/or smell. Smoking tobacco impairs the ability to identify odors and diminishes the sense of taste. Patients who quit smoking may regain some or all sense of smell and/or taste. There have also been reports of decreased sense of smell after exposure to a heavy metal called cadmium, which is found in soils and rocks.
  • Herbs and supplements: The zinc nasal cold gel Zicam® may cause anosmia (total loss of smell). Also, traditional healers observed that chewing the leaves of gymnema resulted in a reversible loss of sweet-taste perception.
  • Heredity: A minority of patients are born with a poor or nonexistent sense of taste and/or smell. When individuals are born without the ability to smell, the condition is called congenital anosmia. Most cases of congenital anosmia are caused by a condition called Kallman's syndrome. This genetic disorder is characterized by a lack of smell sensation and hormone irregularities that prevent the development of sexual characteristics.
  • Infections: Upper respiratory tract infections and nasal allergies have been shown to impair both taste and smell temporarily. These infections cause inflammation of the mouth, which inhibits taste sensations. Oral infections (like oral thrush) can lead to a temporary loss of taste because the creamy white lesions cover the taste buds in the mouth. Infections may also cause increased mucus secretions, which clog the sinuses and prevent odors from entering the nose and stimulating olfactory cells. Once infections or allergies are treated, smell and/or taste sensations return to normal.
  • Injury: Injury to the head and Bell's palsy (inflammation of the nerves in the face) can also impair taste and smell. This is because the brain is responsible for processing the signals of the sensory cells. If the brain is injured, it may not be able to interpret the signal correctly, and the patient may be unable to experience the sensations. Researchers estimate that 5-30% of head trauma patients lose their sense of smell and about 0.5% lose their sense of taste.
  • Laryngectomy: Patients who have had their larynx (voice box) surgically removed (laryngectomy) usually experience a loss of taste and smell. A larynx bypass tube has been used to allow patients to breathe through the nose again. While this has shown to increase smell and taste sensation, the procedure is cumbersome and not considered standard treatment.
  • Medication: Some medicines, including antibiotics like azithromycin (Zithromax®), ciprofloxacin (Cipro®), and clarithromycin (Biaxin®), as well as blood pressure drugs like captopril (Capoten®, Capozide®), diltiazem (Cardizem®), or acetazolamide (Diamox®), and chemotherapy drugs, may temporarily impair taste and smell senses. Research has shown that one chemotherapy drug called cisplatin (Platinol®-AQ) has minimal effects on smell when taken in small doses.
  • Mental illness: Several studies have found an association between schizophrenia and smelling disorders. One study suggests that genetic factors linked to smell disorders may contribute to the development of schizophrenia.
  • A depressive disorder called seasonal affective disorder (SAD) has also been associated with a decrease in the sense of smell.
  • Migraine: Individuals who suffer from migraines may have an increased sensitivity to smell. One study found that during migraine attacks, patients were able to detect lower levels of odors than individuals who did not have migraines.
  • Oral inflammation: If the mouth becomes inflamed, taste may be impaired. For instance, anemia can cause tongue inflammation that interferes with taste. Once inflammation subsides, taste sensations return to normal.
  • Radiation therapy: Cancer patients who undergo radiation therapy of the head and neck often experience a loss of taste and smell. This is because the treatment kills healthy cells, including sensory cells, in addition to cancer. However, once treatment is completed, patients typically regain their ability to taste and smell.
  • Sinus problems: Sinus problems, including polyps (small, abnormal growths) in the nasal or sinus cavities and inflammation of the sinuses, may also lead to smell disorders. These conditions block or reduce airflow in the nose, which impairs the sense of smell. Sensory perception is restored once the sinus condition is resolved.
  • Vagus nerve damage: The vagus nerve has been shown to play an important role in the sense of smell. If the vagus nerve is damaged, patients may experience a decrease or loss of sense of smell.
  • Vitamin deficiencies: Certain vitamin deficiencies, including vitamin B12 and zinc, can also lead to reduced taste and smell senses. The sensory cells involved in taste and smell rely on these vitamins for their growth and maintenance.
  • Other: Endocrine disorders, such as hypothyroidism, hypoadrenalism, and diabetes mellitus, and dental problems, such as infected teeth and gums, have also been associated with taste and smell disorders.

Symptoms
  • General: The severity and duration of symptoms vary depending on the underlying cause.
  • Anosmia: Some patients may experience anosmia, which is the total loss of smell. Some patients may be anosmic for a certain odor. This condition is called specific anosmia. Patients who are born with no sense of smell have congenital anosmia.
  • Hyposmia: Some patients may experience hyposmia, which is the partial loss of smell.
  • Parosmia: Some patients may experience parosmia, which is a distorted sense of smell. Patients may perceive smells when no odor is present or they may perceive familiar odors as smelling strange.
  • Hypoguesia: Hypoguesia occurs when the patient experiences a diminished sense of taste. Taste loss occurs when the individual is unable to identify sweet, sour, salty, or bitter.
  • Dysgeusia: Some patients experience dysgeusia, which is a persistent taste that is usually unpleasant.
  • Flavor loss: Patients who experience both taste and smell impairments may experience diminished flavor sensations. Flavor is a combination of taste (sweet, sour, salty, and bitter) and smell, as well as texture, color, and temperature. In fact, smell contributes to about 75% of flavor sensations.

Diagnosis
  • General: The extent of sensory loss can be measured using tests that determine the lowest concentration of a chemical that a person can detect. A diagnosis may lead to treatment of the underlying cause of the disorder.
  • Taste: During a taste test, patients are asked to identify specific tastes (sweet, salty, bitter, or sweet) after tasting different prepared solutions. The test measures the lowest concentration of a prepared solution that a patient can detect. A healthcare provider will ask a patient to note how the intensity of a taste grows or changes when the concentration is altered.
  • During sip, spit, and rinse tests, patients swish prepared solutions in their mouths. Then the solution is spat out. Patients rinse their mouths before they are tested with a new solution. Alternatively, the solutions may be applied to the corresponding taste buds on the tongue. Sweet and salty taste buds are located at the tip of the tongue. Sour taste buds are on the side of the tongue and bitter taste buds are on the back of the tongue.
  • Smell: Scientists have developed a smell identification test, also called the "scratch-and-sniff" test, to evaluate the sense of smell. During the test, the patient scratches and then smells pieces of papers that have been treated to release different odors. Then, the patient is asked to identify each odor from a list of possibilities. A healthcare provider may also ask a patient to note how the intensity of the smell changes when the concentration of the chemical odor is altered.

Treatment
  • General: Treatment depends on the underlying cause of the disorder. If the condition is drug-induced or a side effect of radiation therapy, symptoms will resolve once the treatment has been completed.
  • Avoiding toxins: If the condition is the result of exposure to toxins, including tobacco smoke, patients may regain some sense of smell and taste once they avoid exposure.
  • Brain stimulation: It has been suggested that deep brain stimulation may improve the sense of smell in patients with Parkinson's disease.
  • Removing nasal obstructions: Nasal obstructions, such as polyps or tumors, may be surgically removed to restore airflow through the nose and correct taste and smell senses.
  • Medications: If an infection is causing the disorder, most patients will experience a complete recovery once the infection has been treated and eradicated.
  • Over-the-counter decongestants like pseudoephedrine (Sudafed®, Actifed®) help relieve congestion (stuffy nose), which often causes impaired smell.
  • A topical steroid nasal steroid spray called budesonide (Rhinocort® Aqua Nasal Spray) has been used to treat stuffy nose in hay fever patients, and it has been shown to increase sense of smell in these patients.
  • Most patients that experience sensory impairment as a result of Bells' palsy recover without treatment. Anti-inflammatories like prednisone may decrease swelling associated with the condition. Antivirals, such as acyclovir (Zovirax®) and famciclovir (Famvir®), may reduce nerve damage caused by viruses.
  • Researchers have suggested that rinsing the nasal cavity with an antiseptic may help improve infections and swelling in the nose. Researchers rinsed patients' noses with 10-20 milliliters of N-Chlorotaurine (NCT) solution to kill disease-causing microorganisms in the nose. Although treatment was well tolerated, is unclear whether it was effective. Further research is warranted.
  • Nasal dilators: According to one study, patients who wore nasal dilators (nasal strips) had an increased sense of smell compared to patients who did wear nasal dilators.
  • Vitamins: Patients who have deficient levels of vitamin B12 or zinc may take supplements to correct the deficiencies. Administering vitamin B12 orally, intramuscularly, or intranasally is effective for preventing and treating dietary vitamin B12 deficiency. An oral dose of 100-250 micrograms a day is usually adequate, although patients with absorption difficulties may need 1,000 micrograms per day of vitamin B12. Oral preparations should not be used in patients with diarrhea, vomiting, severe neurological involvement, or in patients likely to be noncompliant to therapy.
  • 15 milligrams or 100 milligrams of zinc daily for three months has been studied in healthy, elderly people. Patients should avoid the zinc nasal cold gel Zicam® because it has been linked to anosmia (total loss of smell).

Integrative therapies
  • Strong scientific evidence:
  • Vitamin B12: Administering vitamin B12 orally, intramuscularly, or intranasally is effective for preventing and treating dietary vitamin B12 deficiency.
  • Avoid vitamin B12 supplements if allergic or hypersensitive to cobalamin, cobalt, or any other product ingredients. Avoid with coronary stents (mesh tube that holds clogged arteries open) and Leber's disease. Use cautiously if undergoing angioplasty. Vitamin B12 is generally considered safe when taken in amounts that are not higher than the recommended dietary allowance (RDA). The highest dose of vitamin B12 that is safe for pregnant women is 2.6 micrograms daily and 2.8 micrograms during breastfeeding. There is not enough scientific data available about the safety of larger amounts of vitamin B12 during pregnancy.
  • Unclear or conflicting scientific evidence:
  • Zinc: Results from studies investigating the potential role of zinc in treating taste and smell disorders are contradictory. Well-designed research is needed to determine if zinc contributes to the treatment of taste and smell disorders.
  • Patients should avoid the zinc nasal cold gel Zicam® because it has been linked to anosmia (total loss of smell). Zinc is generally considered safe when taken at the recommended dosages. Avoid zinc chloride since studies have not been done on its safety or effectiveness. While zinc appears safe during pregnancy in amounts lower than the established upper intake level, caution should be used since studies cannot rule out the possibility of harm to the fetus.
  • Historical or theoretical uses lacking sufficient evidence:
  • Niacin: There is limited evidence suggesting that Niacin in combination with vitamin A may ameliorate dysgeusia (loss of taste/metallic taste). Additional clinical research is needed before a recommendation can be made.

Prevention
  • Individuals should consume adequate amounts of vitamin B12 and zinc because deficiencies of these substances have been associated with taste and smell disorders.
  • Avoid exposure to toxins, especially tobacco smoke, because they have been associated with taste and smell disorders.
  • Individuals should regularly wash their hands with soap and water to reduce the chance of acquiring infections that may diminish the sense of taste or smell.

Complications
  • Individuals who cannot taste or smell food may not feel satisfied after eating. This may cause some patients to eat too much and gain weight or they may eat too little and lose weight. Others may use too much sugar and/or salt in an effort to make food taste better. This can be a potential problem if the individual has diabetes or high blood pressure because it may exacerbate their conditions.

Copyright © 2011 Natural Standard (www.naturalstandard.com)


The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.


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