Distortions of taste and loss of taste are sources of complaint in patients with certain local malignant tumors. Oropharyngeal tumors may, of course, abolish taste by invading the chorda tympani and lingual nerves or the skull base foramina through which these nerves pass. Malnutrition because of neoplasm or radiation therapy may also cause ageusia. Some patients with certain cancers remark on a reduced perception for bitter foods, and some who have been radiated for breast cancer or sublingual or oropharyngeal tumors find sour foods intolerable. The loss of taste from radiation of the oropharynx is usually recovered within a few weeks or months; the reduced turnover of taste buds caused by radiation therapy usually recovers.
An interesting syndrome of idiopathic hypogeusia—in which decreased taste acuity is associated with dysgeusia, hyposmia, and dysosmia—has been described by Henkin, Schechter and colleagues. Food has an unpleasant taste and aroma, to the point of being revolting (cacogeusia and cacosmia); the persistence of these symptoms may lead to a loss of weight, anxiety, and depression.
Unilateral lesions of the medulla oblongata have not been reported to cause ageusia, perhaps because the nucleus of the tractus solitarius is outside the zone of infarction or because there is representation from both sides of the tongue in each nucleus. Unilateral thalamic and parietal lobe lesions, however, have both been associated with contralateral impairment of taste sensation in rare cases.
As indicated previously, a gustatory aura occasionally marks the beginning of a seizure originating in the frontoparietal (supra-sylvian) cortex or in the uncal region. Gustatory hallucinations are much less frequent than olfactory ones. Nevertheless, gustatory sensations were reported in 30 of 718 cases of intractable epilepsy (Hausser-Hauw and Bancaud). During surgery, these investigators produced an aura of disagreeable taste by electrical stimulation of the parietal and frontal opercula, and also by stimulation of the hippocampus and amygdala (uncinate seizures). In their view, the low-threshold seizure focus for taste in the temporal lobe is secondary to functional disorganization of the opercular gustatory cortex by the seizure. Gustatory hallucinations were more frequent with right hemisphere lesions, and in half of the cases the gustatory aura was followed by a convulsion.
Zinc supplements are contained in over-the-counter and complementary medical products aimed at improving smell and appetite and for the treatment of incipient colds. We have had no opportunity to confirm the often-cited benefits of zinc on any of these conditions, and the supporting evidence is sparse; however, the continued administration of zinc in high doses has been associated with the development of copper deficiency and a myeloneuropathy (see Chaps. 38 and 42).
Burning mouth syndrome Another poorly defined disorder is the burning mouth syndrome, which occurs mainly in postmenopausal women and is characterized by persistent, severe intraoral pain (particularly of the tongue). We have seen what we believe to be fragmentary forms of the syndrome in which pain and burning are isolated to the alveolar ridge or gingival mucosa. The oral mucosa appears normal and some patients may report a diminution of taste sensation. A small number of such patients prove to have diabetes, Sjögren syndrome, or vitamins B2 or B12 deficiency (causing glossitis), but in most no systemic illness or local abnormality can be found. Many such patients that we have encountered appeared to have a depressive illness, but they responded only inconsistently to administration of antidepressants. A few patients have this oral complaint as a component of a small fiber neuropathy or ganglionopathy (see Chap. 43). Clonazepam may be useful, and capsaicin has been tried with uncertain results. This disorder and others in which burning is a prominent feature is commented on in Chap. 7.