Ageusia

Ageusia (pronounced ay-GOO-see-uh) is the loss of taste functions of the tongue, particularly the inability to detect sweetness, sourness, bitterness, saltiness, and umami (the taste of monosodium glutamate). It is sometimes confused for anosmia - a loss of the sense of smell. Because the tongue can only indicate texture and differentiate between sweet, sour, bitter, salty, and umami most of what is perceived as the sense of taste is actually derived from smell. True aguesia is relatively rare compared to the milder forms of taste loss: hypogeusia and dysgeusia. Hypogeusia represents a partial loss of taste whereas dysgeusia denotes a distortion or alteration of taste.

Diagnosis
In order to discover the extent of the ageusia, a scientist attempts to discern the minimum level of a chemical that a patient can detect by taste. Patients may also be asked to compare various concentrations of chemicals in order that the doctor may ascertain what level of intensity that the patient can differentiate. Various methods are used, including the "sip, spit, and rinse" test as well as direct application of chemicals to the tongue.

Neurological Damage
Tissue damage to the nerves that support the tongue can cause ageusia, especially damage to the lingual nerve and the glossopharyngeal nerve. The lingual nerve passes taste for the front two-thirds of the tongue and the glossopharyngeal nerve passes taste for the back third of the tongue. Neurological disorders such as Bell's Palsy, Familial Dysautonomia, and Multiple Sclerosis will cause similar problems to nerve damage, as will certain infectious conditions like primary amoeboid meningoencephalopathy. The lingual nerve (who is a branch of the trigeminal V3 nerve, but carries taste sensation back to the chorda tympani nerve to the geniculate ganglion of the facial nerve) can also be damaged during otologic surgery giving place to a feeling of metal taste.

Problems with the Endocrine System
Vitamin deficiency, namely vitamins B3 and Zinc, could lead to problems with the Endocrine system, which may in turn lead to taste loss or alteration. Disorders of the Endocrine System such as Cushing's Syndrome, Hypothyroidism and Diabetes Mellitus could lead to similar problems. Ageusia can also be caused by medicinal side-effects from Antirheumatic Drugs such as Penicillamine, Antiproliferative drugs such as Cisplatin, ACE Inhibitors, and other drugs including Azelastine, Clarithromycin and Zopiclone.

Other Causes
Local damage and inflammation that interferes with the taste buds or local nervous system such as that stemming from radiation therapy, glossitis, tobacco abuse, and denture use will also cause ageusia. Other known causes of ageusia include loss of taste sensitivity from aging (resulting in a difficulty detecting salty or bitter taste), Anxiety Disorder, Cancer, Renal Failure and Hepatic failure.