Approach
In many cases, a full history and clinical exam is sufficient to diagnose and manage the problem. However, additional investigations may be necessary to exclude or confirm underlying etiologies. Isolated taste dysfunction is rare. Olfactory function has a significant role in "flavor," so if there is concomitant olfactory loss or disturbance, its underlying cause should be ascertained.
History
It is important to determine whether patients can distinguish salt, sour, sweet, bitter, and umami, and whether the disturbance is quantitative (hypogeusia or ageusia) or qualitative (parageusia or phantogeusia). Dysgeusia includes complaints such as a metallic taste or a permanent bitter, sour, salty, or, less commonly, sweet taste. These may be triggered or relieved by eating.
It is important to ascertain whether the reported taste disturbance was of acute onset or was gradual in nature. Acute taste loss may be due to iatrogenic or toxic causes, whereas a more insidious history may suggest an underlying neurologic or neoplastic process. Any concomitant loss of olfactory function should be ascertained.
Full medical history including medications, when they were started, and how this relates to the onset of the taste disorder, is essential. Many common drugs may induce dysgeusia directly or indirectly, through salivary qualitative and/or quantitative changes and by direct secretion in saliva or gingival crevice fluid.[18] ACE inhibitors and antibiotics are commonly implicated.[1][56] Mouth rinses (e.g., chlorhexidine gluconate) may alter taste sensation by interfering with cell membranes.[64]
Assessment of existing medical conditions (e.g., endocrinopathy or neurologic disease) may allow the taste dysfunction to be placed in context. A history of any precipitating event, such as recent trauma, medical procedure, medication, or cancer therapy (regional head and neck radiation therapy, systemic chemotherapy, or targeted agents) is required.
Physical examination
A complete examination of the oral cavity and nasopharynx is recommended, with particular attention to signs of previous surgery (e.g., absent wisdom teeth or tonsils). Evidence of oral mucosal lesions or gingival and periodontal inflammation may suggest a nutritional deficiency or inflammatory process. The quantity of oral salivary flow is important because hyposalivation secondary to medications (common), Sjogren syndrome, or past cancer therapies (including radiation therapy) may impact saliva production. Signs of hyposalivation include inflamed mucosa, presence of mucus threads or frothy or bubbly saliva, and tooth decay or loss.
The ears should be examined for evidence of serous otitis (acute and chronic otitis media) or previous surgery. The eyes should be examined for evidence of dryness (or keratoconjunctivitis sicca), which may suggest Sjogren syndrome, while nodularity of these glands may suggest neoplasia. The neck should be assessed for any evidence of previous neck surgery or thyroid enlargement. Parotid and/or submandibular gland enlargement may suggest Sjogren syndrome. Depending on the clinical history, a neurologic exam, including assessment of cognitive function, is indicated. This should include a careful evaluation of cranial nerve function. Examination of the respiratory system and assessment of vital signs should be performed if coronavirus disease 2019 (COVID-19) is suspected.
Chemosensory gustatory testing
It is difficult for patients to measure taste disturbance objectively. Chemical gustometry or electrogustometry testing of the anterior two-thirds and posterior tongue should be performed.[1]
Chemical gustometry may be applied in drops to the tongue, by oral rinsing, or by new technologies based on rapidly dissolving taste strips:[65][66]
Solutions in increasing concentrations are usually applied to the surface of the tongue (with a dropper or a taste strip) with the fundamental tastes: citric acid (sour), glucose (sweet), sodium chloride (salty), quinine (bitter), and umami.
Results are reported as any decrease of taste sensation on the area of the tongue tested.
Electrogustometry
An electro-anodic stimulation with a continuous current causes hydrolysis of the saliva, stimulating the gustative chemoreceptors and giving a sour, sometimes metallic, taste recognized by the patient.
Results are reported as nondetectable or increased thresholds on the area of the tongue tested.
Olfactory testing
In most patients with taste disorders, olfactory function should be evaluated.[11]
As a general rule, testing should be carried out rather than just asking patients about their olfactory performance. There are a number of validated olfactory testing methods, all based on similar principles.[67] Odors are presented and the patient has to identify the smelled odor from a choice of 4 possibilities. According to the number of correctly identified odors, the patient can be categorized as normosmic, hyposmic, or anosmic.
The threshold test gives better results with respect to detection of subtle decreases in olfactory function.[68]
Laboratory investigations
Laboratory tests may be considered if:
Suggested by the medical history (although the underlying condition may have been established during past medical exams).
No alternative etiologies for taste dysfunction are apparent.
Tests may include:
CBC and peripheral blood smear, serum ferritin, vitamin B12, and folate to exclude an underlying anemia.
Serum zinc.
Serum fasting morning glucose, or an oral glucose tolerance test to exclude diabetes mellitus.
Thyroid-stimulating hormone and thyroxine assays to exclude hypothyroidism.
Renal function (BUN, creatinine, and serum electrolytes) and liver function to exclude renal/hepatic insufficiency.
Testing for the presence of anti-Ro/SSA and/or anti-La/SSB antibodies, if Sjogren syndrome is suspected.
Acetylcholine receptor antibody assay may be indicated in myasthenia gravis.
Sialometry and sialochemistry (analysis of saliva) to define the microenvironment of the taste buds.[17]
Real-time reverse transcription polymerase chain reaction, if COVID-19 is suspected. Diagnostic tests should be performed according to guidance issued by local health authorities and should adhere to appropriate biosafety practices.
Imaging studies
Neurologic imaging
If the history and clinical exam suggest any significant unidentified intracranial pathology, then cranial CT and MRI exam may be indicated.
Cranial CT may show a skull fracture, intracranial bleeds, or subdural hematoma, and areas of ischemia. In Alzheimer disease, imaging may show hippocampal volume loss, atrophy of the medial temporal lobe, and posterior cortical atrophy. In multiple sclerosis, demyelination perpendicular to the lateral ventricles and corpus callosum may be evident. Intracranial tumors may be seen as an area of hypodensity or hyperdensity on cranial computed tomography (CT), and magnetic resonance imaging (MRI) may help identify an acoustic neuroma or meningioma. Cranial CT may exclude glomus tumor or other mastoid process abnormalities (e.g., cholesteatoma). Salivary gland tumors leading to hyposalivation may also be detected on cranial radiographic studies.[69]
Other imaging
Rarely, an underlying malignancy may manifest as a paraneoplastic syndrome in which dysgeusia is a feature. Most commonly, small cell lung cancer or thymoma is the underlying malignancy.[57] Therefore, a chest CT scan may be of value to identify any pulmonary malignancy.
A chest radiograph should be considered in suspected cases of COVID-19.
Specialized investigations
May be useful adjuncts to investigate neurologic conditions.
On EEG, a slowing of background rhythm is a frequent finding in Alzheimer disease, and abnormalities in temporal lobe electrical rhythm may be seen in epilepsy. If amyotrophic lateral sclerosis is suspected, electromyography may show evidence of diffuse, ongoing, chronic denervation.
In myasthenia gravis, an edrophonium test may show progressive weakening with repetitive muscle stimulation. In Guillain-Barre syndrome (GBS), nerve conduction studies will usually show evidence of demyelination and, less commonly, evidence of significant axon loss.
In select patients, lumbar puncture with cerebrospinal fluid (CSF) exam may be indicated. In suspected GBS, an acellular CSF with increased protein and normal glucose will help exclude other diagnoses. In multiple sclerosis, oligoclonal bands may be present.
In patients with Bell palsy, testing of the stapedial reflex will usually show an absent reflex along the efferent limb of the reflex arc; electromyography may show reduced compound muscle action potential. In Ramsay Hunt syndrome, demonstration of herpes zoster virus by polymerase chain reaction or Tzanck testing in vesicular fluid may be used when clinical diagnosis is uncertain. Audiometry may also be appropriate in patients with Bell palsy, Ramsay Hunt syndrome, or middle ear infection.
Additional investigation for Sjogren syndrome may include Schirmer testing (which may show reduced tear production), slit-lamp ophthalmoscopy, where the presence of a punctate keratopathy may indicate keratoconjunctivitis sicca, and saliva volume measurement (which may show reduced saliva production). Labial salivary gland biopsy is an adjunctive test in suspected Sjogren syndrome; a focal lymphocytic infiltrate within a gland supports the diagnosis.
Therapeutic trial
Therapeutic trials should be considered if there is an underlying risk factor, such as dry mouth, or nutritional deficiency. Where hyposalivation is present (e.g., with specific medications or in Sjogren syndrome) a therapeutic trial with a sialogogue may lead to clinical improvement. A trial with zinc supplementation may be beneficial, and should be considered in patients with a history of gradual-onset taste loss if there are no clinical features to suggest another cause. Medications that may increase taste include megestrol, and cannabinoids.[70]
Management includes dietary counseling with guidance in food choices, food preparation and seasoning (increase spice, if tolerated; increase umami foods and umami flavoring), and avoiding unpleasant foods.[71]
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