Smell and Taste Disorders in Primary Care

Dillon J. Savard; Francesca G. Ursua; Heidi L. Gaddey

DILLON J. SAVARD, MD, FRANCESCA G. URSUA, MD, AND HEIDI L. GADDEY, MD

Am Fam Physician. 2023;108(3):240-248

Patient information: See related handout on smell and taste disorders.

This clinical content conforms to AAFP criteria for CME.

Author disclosure: No relevant financial relationships.

Disorders of smell and taste are reported by approximately one-fifth of people 40 years and older, and one-third of people 80 years and older. These disorders affect quality of life and the ability to identify smoke and toxins. Smell and taste disorders can be early signs of dementia or Parkinson disease and are associated with increased mortality. Dysfunction may be apparent or may develop insidiously. Screening questionnaires are available, but many patients are unaware of their disorder. Most smell and taste disorders are due to sinonasal disease but also could be caused by smoking, medications, head trauma, neurodegenerative disease, alcohol dependence, or less common conditions. The differential diagnosis should guide the evaluation and include anterior rhinoscopy and an examination of the oral cavity, head, and cranial nerves. Further investigation is often unnecessary, but nasal endoscopy and computed tomography of the sinuses may be helpful. Magnetic resonance imaging of the head with contrast should be performed if there is an abnormal neurologic examination finding or if trauma or a tumor is suspected. Olfactory testing is indicated in refractory cases or for patients with poor quality of life and disease associated with smell or taste dysfunction. Smell and taste disorders may resolve when reversible causes are treated, but improvement is less likely when they are due to trauma, age, or neurodegenerative disease. Olfactory training is a self-administered mindful exposure therapy that may improve olfactory function. Physicians should encourage patients to ensure that smoke and other alarms are operational and to adhere to food expiration dates.

Disorders of smell and taste affect a person's quality of life, health, and safety. The COVID-19 pandemic renewed attention to these disorders, and one of the Healthy People 2030 objectives is to increase the proportion of adults with smell or taste disorders who discuss the problem with a clinician.¹ Olfactory dysfunction is reported by 23% of people in the United States 40 years and older and 39% of those 80 years and older.^2,3 Altered taste is reported by 19% of people 40 years and older, and 27% of those 80 years and older.² Smell and taste disorders can affect food consumption habits, which may lead to chronic disease³ (Table 1⁴). Depression, poor sleep, dementia, unintentional weight loss, and malnutrition are associated with disordered smell or taste.^5–15 The ability to detect sources of danger, such as smoke or poison, depends on smell or taste.^10,11 When directly tested, 20.3% of a nationally representative sample of adults 70 years and older without known dysfunction misidentified smoke, and 31.3% misidentified natural gas.³

Clinical recommendation	Evidence rating	Comments
Patients should be referred to otolaryngology for objective testing of olfactory or gustatory function in refractory cases or if they have depression, poor sleep, dementia, unintentional weight loss, or malnutrition.^3,20,38–40	B	Case series of 602 patients, multiple observational studies
Olfactory training should be offered to patients with persistently poor quality of life due to olfactory dysfunction.^63–65	A	Consistent evidence from multiple systematic reviews and meta-analyses, including randomized controlled trials and prospective cohort studies, showing clinically significant improvement after viruses and other etiologies
Physicians should counsel patients with olfactory dysfunction about the risks of occupational chemical exposure, adherence to food expiration dates, signs of food spoilage, and home fire and explosive gas monitoring.^3,40	C	Expert opinion and a cross-sectional study

Disorder	Description
Taste
Ageusia	Complete loss of ability to taste
Dysgeusia	Distorted taste perception (triggered by a taste stimulus)
Hypogeusia	Reduced ability to taste
Phantogeusia	Gustatory hallucination (occurs without a stimulus)
Smell
Anosmia	Complete loss of ability to smell
Hyposmia	Reduced ability to smell
Parosmia	Distorted odor perception (triggered by a stimulus)
Phantosmia	Olfactory hallucination (occurs without a stimulus)

There are many clinical presentations and causes for smell and taste disorders (Table 2).^16–19 Etiologies range from sinonasal disease to less common conditions such as Sjögren syndrome (Table 3).⁴ When only taste disorder is perceived by the patient, it is often due to olfactory dysfunction.^20–26 Therefore, without an apparent taste insult, physicians should focus their evaluation on the olfactory system. Figure 1 provides an approach to evaluating smell and taste disorders in primary care.

Cause	Pathology	History
Age-related (presbyosmia)	Loss or replacement of olfactory neurons with respiratory epithelium Decreased basal cell proliferation Decrease of interneurons in olfactory bulb with reduced activity in olfactory cortex	Insidious onset; unlikely to see improvement Olfactory dysfunction predicts five-year mortality
Neurodegenerative causes (e.g., Alzheimer disease, Parkinson disease)	Alzheimer disease: neurofibril changes in the olfactory bulb and higher olfactory network Parkinson disease: Lewy bodies deposited in the olfactory tract, olfactory bulb, and anterior olfactory nucleus¹⁷	Insidious onset; unlikely to see improvement Onset can be early in Alzheimer and Parkinson diseases
Other medication/toxin exposure/iatrogenic	Altered receptor function by binding G-protein coupling or affecting calcium or sodium channel activity	Sudden onset; mostly resolves with ceasing exposure; can persist in some cases requiring treatment; can have medicolegal implications¹⁸
Postviral olfactory dysfunction	Viral common cold pathogens, COVID-19, influenza, respiratory syncytial virus, parvovirus, HIV Long-term inflammation causes neuroepithelial remodeling to respiratory epithelium	Sudden onset with little fluctuation; often associated with parosmia; one in three recovers in the 14 months after presentation for care¹⁹
Posttraumatic olfactory dysfunction	Severing of olfactory nerve filament or central nervous system injury	Sudden or delayed onset; fluctuation is uncommon; phantosmia and parosmia are common 10% recover by 13 months, but up to 35% can recover depending on severity¹⁹
Sinonasal (e.g., chronic rhinosinusitis with polyps)	Obstruction of the olfactory cleft; temporary interference with olfactory receptor binding due to inflammation; eventual neuroepithelial remodeling/central nervous system changes	Gradual onset and fluctuation over time; typically improves with nasal/systemic corticosteroids Not commonly associated with parosmia

Cause	Leading etiologies^*
Most common
Sinonasal conditions	Upper respiratory tract infection (especially viral, including COVID-19), allergic rhinitis, chronic rhinosinusitis, nasal polyps
More common
Head trauma	Damage to cribriform plate, shearing forces, and intracranial damage; facial trauma
Neurodegenerative disorders	Parkinson disease, parkinsonism, Alzheimer disease, mild cognitive impairment, multiple sclerosis
Less common
Medications	Chemotherapy, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, dihydropyridine calcium channel blockers, diuretics, intranasal zinc, antimicrobials (macrolides, terbinafine, fluoroquinolones, protease inhibitors, griseofulvin, penicillins, tetracyclines, nitroimidazoles [metronidazole (Flagyl)]), antiarrhythmics, antithyroid agents, antidepressants, anticonvulsants, lipid-lowering agents
Intoxicants or illicit substances	Alcohol dependence, cocaine use
Toxins	Ammonia, hairdressing chemicals, gasoline, formaldehyde, paint solvents, welding agents, benzene, sulfuric acids, cadmium, acrylates, iron, lead, chromium
Chronic medical conditions	Renal or hepatic failure, complicated type 2 diabetes mellitus, cancer, HIV
Structural or mechanical conditions	Ischemic stroke, subarachnoid or intracranial hemorrhage, brain or sinonasal tumor
Nutritional deficiencies	Malnutrition, pernicious anemia or vitamin B₁₂ deficiency, deficiencies in vitamins B₆ or A, niacin, zinc, or copper
Postsurgical state	Nasal surgery (septal or sinus)
Postradiation	Especially to head and neck
Congenital conditions	Kallmann syndrome, anosmia
Psychiatric conditions	Anorexia nervosa (not bulimia), major depressive disorder, bipolar disorder, schizophrenia
Endocrine conditions	Pregnancy, hypothyroidism, Addison disease, Cushing syndrome
Autoimmune/inflammatory conditions	Sjögren syndrome, systemic lupus erythematosus, sarcoidosis, herpes encephalitis

Anatomy and Physiology

Chemicals that cause smells (i.e., odorants) are absorbed through the mucosa of the nose, nasopharynx, and oropharynx. Odorants stimulate olfactory neurons in the neuroepithelium over the cribriform plate. Olfactory neurons project directly to the olfactory bulb, which communicates with the olfactory cerebral cortex. Olfactory receptors regenerate every eight to 10 days, followed by five days for the maturation of cilia.^27–29 Dysfunction along these pathways can result in a smell disorder.^30–32

Odorants and tastants (i.e., particles and liquids that cause the taste sensation) dissolve in saliva and directly contact gustatory receptors in dedicated cells for each of the five tastes (bitterness, saltiness, sourness, sweetness, and umami [savory]). These cells are in taste buds on the tongue, soft palate, pharynx, larynx, epiglottis, and the proximal one-third of the esophagus. Different nerves innervate the anterior two-thirds of the tongue, posterior one-third of the tongue, palate, pharynx, and larynx. The trigeminal nerve also mediates certain characteristics of smell and taste, such as irritation and temperature. Taste is mediated by several different nerves depending on anatomic location; therefore, gustatory loss is less common than olfactory loss.^9,33,34

Clinical Assessment

HISTORY

Validated questionnaires help clinicians screen for subjective olfactory dysfunction (Table 4).^11,35,36 A National Health and Nutrition Examination Survey used three questions that were not formally validated but are a reasonable approach to screen patients for olfactory dysfunction.^2,3 There are no validated subjective tools to assess taste.¹⁶

U.S. Department of Health and Human Services. Increase the proportion of adults with smell or taste disorders who discuss the problem with a provider – HOSCD-12. Accessed October 24, 2022. https://health.gov/healthypeople/objectives-and-data/browse-objectives/sensory-or-communication-disorders/increase-proportion-adults-smell-or-taste-disorders-who-discuss-problem-provider-hoscd-12

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