Chapter 030. Disorders of Smell, Taste, and Hearing (Part 5) Disorders of the Sense of Taste Disorders of the sense of taste are caused by conditions that interfere with the access of the tastant to the receptor cells in the taste bud (transport loss), injure receptor cells (sensory loss), or damage gustatory afferent nerves and central gustatory pathways (neural loss) (Table 30-2). Transport gustatory losses result from xerostomia due to many causes, including Sjögren's syndrome, radiation therapy, heavy-metal intoxication, and bacterial colonization of the taste pore. Sensory gustatory losses are caused by inflammatory and degenerative diseases in the oral cavity; a vast number of drugs, particularly those that interfere with cell turnover such as antithyroid and antineoplastic agents; radiation therapy to the oral cavity and pharynx; viral infections; endocrine disorders; neoplasms; and aging. Neural gustatory losses occur with neoplasms, trauma, and surgical procedures in which the gustatory afferents are injured. Taste buds degenerate when their gustatory afferents are transected but remain when their somatosensory afferents are severed. Patients with renal disease have increased thresholds for sweet and sour tastes, which resolves with dialysis. Table 30-2 Causes of Gustatory Dysfunction Transport Gustatory Losses Drugs Heavy-metal intoxication Radiation therapy Sjögren's syndrome Neural Gustatory Losses Diabetes mellitus Hypothyroidism Oral neoplasms Oral surgery Xerostomia Sensory Gustatory Losses Aging Candidiasis Drugs (antithyroid and antineoplastic) Endocrine disorders Oral neoplasms Pemphigus Radiation therapy Viral infections (especially with herpes viruses) Radiation therapy Renal disease Stroke and other CNS disorders Trauma Upper respiratory tract infections A side effect of medication is the single most common cause of taste dysfunction in clinical practice. Xerostomia, regardless of the etiology, can be associated with taste dysfunction. It is associated with poor oral clearance and poor dental hygiene and can adversely affect the oral mucosa, all leading to dysgeusia. However, severe salivary gland failure does not necessarily lead to taste complaints. Xerostomia, the use of antibiotics or glucocorticoids, or immunodeficiency can lead to overgrowth of Candida; overgrowth alone, without thrush or overt signs of infection, can be associated with bad taste or hypogeusia. When taste dysfunction occurs in a patient at risk for fungal overgrowth, a trial of nystatin or other antifungal medication is warranted. Upper respiratory infections and head trauma can lead to both smell and taste dysfunction; taste is more likely to improve than smell. The mechanism of taste disturbance in these situations is not well understood. Trauma to the chorda tympani branch of the facial nerve during middle ear surgery or third molar extractions is relatively common and can cause dysgeusia. Bilateral chorda tympani injuries are usually associated with hypogeusia, whereas unilateral lesions produce only limited symptoms. As noted above, aging itself may be associated with reduced taste sensitivity. The taste dysfunction may be limited to a single compound and may be mild. . Chapter 030. Disorders of Smell, Taste, and Hearing (Part 5) Disorders of the Sense of Taste Disorders of the sense of taste are caused by conditions that interfere with the access of. heavy-metal intoxication, and bacterial colonization of the taste pore. Sensory gustatory losses are caused by inflammatory and degenerative diseases in the oral cavity; a vast number of drugs, particularly. cell turnover such as antithyroid and antineoplastic agents; radiation therapy to the oral cavity and pharynx; viral infections; endocrine disorders; neoplasms; and aging. Neural gustatory losses