Unilateral gustatory impairment can be identified by withdrawing the tongue with a gauze sponge and using a moistened applicator to place a few crystals of salt, sugar, lemon (sour), and quinine (bitter) on discrete parts of the tongue; the tongue is then wiped clean and the subject is asked to report what was sensed. One use of such testing is to corroborate the existence of Bell palsy by comparing taste sensation on each side of the anterior tongue (see Chap. 44). A stimulus that has been used as a surrogate for sour sensation is a low-voltage direct current, the electrodes of which can be accurately placed on the tongue surface. If the taste loss is bilateral, mouthwashes with a dilute solution of sucrose, sodium chloride, citric acid, and quinine may be used. After swishing, the test fluid is spit out and the mouth rinsed with water. The patient indicates whether a taste was detected and then is asked to try to identify it. Special types of apparatus (electrogustometers) have been devised for the measurement of taste intensity and for determining the detection and recognition thresholds of taste and olfactory stimuli (Krarup; Henkin et al), but these are beyond the needs of the usual clinical examination.
Ageusia, or Loss of the Sense of Taste (Table 11-2)
Apart from the loss of taste sensation that accompanies normal aging, smoking is probably the most common cause of impairment of taste sensation. Extreme drying of the tongue from any cause may lead to temporary loss or reduction of the sense of taste (ageusia or hypogeusia), as saliva is essential for normal taste function. Saliva acts as a solvent for chemical substances in food and for conveying them to taste receptors. Dryness of the mouth (xerostomia) from inadequate saliva, as occurs in Sjögren syndrome; hyperviscosity of saliva, as in cystic fibrosis; irradiation of head and neck; and pandysautonomia all interfere with taste. Also, in familial dysautonomia (Riley-Day syndrome), the number of circumvallate and fungiform papillae is reduced, accounting for a diminished ability to taste sweet and salty foods. If unilateral, ageusia is seldom the source of complaint. Taste is frequently lost over the anterior two-thirds of one side of the tongue in cases of mundane Bell palsy, as indicated previously and in Chap. 44.
A permanent decrease in the acuity of taste and smell (hypogeusia and hyposmia), sometimes associated with perversions of these sensory functions (dysgeusia and dysosmia), may follow influenza-like illnesses. These abnormalities have been associated with pathologic changes in the taste buds as well as in the nasal mucous membranes. In a group of 143 patients who presented with hypogeusia and hyposmia, 87 were of this postinfluenza type, as determined by Henkin and colleagues; the remainder developed their symptoms in association with scleroderma, acute hepatitis, viral encephalitis, myxedema, adrenal insufficiency, malignancy, deficiency of vitamins B and A, and the administration of a wide variety of drugs. Also, according to Schiffman, more than 250 drugs have been implicated in the alteration of taste sensation, making it necessary to consider virtually all drugs as a cause of taste loss. Lipid-lowering drugs, antihistamines, antimicrobials, antineoplastics, bronchodilators, antidepressants, and antiepileptics are the main offenders, but little is known about the mechanisms by which drugs induce these effects. More obvious is altered taste because of nasally and orally administered inhalant drugs, including the “triptans” for migraine and a variety of antiallergy and antiasthmatic medications.