Ageusia or hypogeusia is a loss or impairment of the sense of taste (gustation). This may be tested by application to each half of the protruded tongue the four fundamental tastes (sweet, sour, bitter, and salt).
Isolated ageusia is most commonly encountered as a transient feature associated with coryzal illnesses of the upper respiratory tract, as with anosmia. Indeed, many complaints of loss of taste are in fact due to anosmia, since olfactory sense is responsible for the discrimination of many flavors.
Neurological disorders may also account for ageusia. Afferent taste fibers run in the facial (VII) and glossopharyngeal (IX) cranial nerves, from taste buds in the anterior two-thirds and posterior one-third of the tongue respectively. Central processes run in the solitary tract in the brainstem and terminate in its nucleus (nucleus tractus solitarius), the rostral part of which is sometimes called the gustatory nucleus. Fibers then run to the ventral posterior nucleus of the thalamus, hence to the cortical area for taste adjacent to the general sensory area for the tongue (insular region).
Lesions of the facial nerve proximal to the departure of the chorda tympani branch in the mastoid (vertical) segment of the nerve (i.e., proximal to the emergence of the facial nerve from the stylomastoid foramen), can lead to ipsilateral impairment of taste sensation over the anterior two-thirds of the tongue, along with ipsilateral lower motor neurone facial weakness (e.g., in Bell’s palsy), with or without hyperacusis. Lesions of the glossopharyngeal nerve causing impaired taste over the posterior one-third of the tongue usually occur in association with ipsilateral lesions of the other lower cranial nerves (X, XI, XII; jugular foramen syndrome) and hence may be associated with dysphonia, dysphagia, depressed gag reflex, vocal cord paresis, anesthesia of the soft palate, uvula, pharynx and larynx, and weakness of trapezius and sternocleidomastoid.
Ageusia as an isolated symptom of neurological disease is extremely rare, but has been described with focal central nervous system lesions (infarct, tumor, demyelination) affecting the nucleus of the tractus solitarius (gustatory nucleus) and/or thalamus, and with bilateral insular lesions.
Anosmia and dysgeusia have also been reported following acute zinc loss.
Finelli PF, Mair RG. Disturbances of taste and smell. In: Bradley WG, Daroff RB, Fenichel GM, Marsden CD (eds.). Neurology in clinicalpractice (3rd edition). Boston: Butterworth Heinemann, 2000: 263-269 Hepburn AL, Lanham JG. Sudden-onset ageusia in the antiphospholipid syndrome. Journal of the Royal Society of Medicine 1998; 91: 640-641