38 Neurological Dysfunction (Including Taste and Dysarthria)

The oral cavity is responsible for both the chemosensory functions of taste and sensation, as well as the motor functions of speech and swallowing. Neurological dysfunction leads to problems with taste disturbance, dysarthria, and dysphagia (see Chapter 48 for causes of dysphagia).

Taste Disturbance

Disorders of taste must be distinguished from disorders of olfaction. There are four genuine taste qualities: salty, sweet, sour, and bitter. Recently a fifth taste quality, “umani,” the taste of glutamate, aspartate, and certain ribonucleotides, has been described. The overall taste perception is dependent on not only the true taste sensation but also the smell, texture, and temperature. In addition, taste is dependent on saliva, which is required to dissolve the tastants so they may be detected by the taste receptors. During mastication, odorants are released from food and detected in the olfactory cleft through retronasal olfaction via the nasopharynx. The combination of smell and taste contributes to flavor, which is often mistakenly called taste. The smell or aroma of food and drink is the most important contributor to flavor.

Taste buds are located in the fungiform, foliate, and circumvallate papillae, but not the filiform papillae. Fungiform papillae are located at the tip and lateral edge of the anterior two thirds of the tongue, whereas foliate papillae are located at the posterolateral tongue. Circumvallate papillae are arranged in a V shape at the junction of the anterior two thirds and posterior one third of the tongue. Filiform papillae are located throughout the tongue. Despite the historical perception that there is a topographic mapping of taste sensation on the tongue, this is incorrect. All five taste sensations are perceived by each taste receptor, although it may be more sensitive to one type of stimulus. Taste to the oral tongue (chorda tympani) and the palate (greater superficial petrosal) is mediated by CN VII, whereas CN IX mediates taste to the tongue base, the vallecula, and the pharynx. Taste buds on the laryngeal surface of the epiglottis are innervated by CN X, but their role in everyday taste perception is unknown. All taste afferents converge in the nucleus solitarius in the medulla. Higher neural pathways are less well understood.

Taste disturbances can be categorized as taste loss, taste intensification, or taste phantoms. Taste loss may either be complete (ageusia) or partial (hypogeusia). Taste intensifications, or hypergeusias, are much less common than taste losses. Phantoms, or dysgeusias/parageusias, are tastes experienced in the absence of overt stimulation.

Common causes of taste disturbance include the following (Table 38.1):

Primary smell disorder: Eighty percent of taste disorders are actually primary smell disorders. The three most common causes of smell disturbances are obstructive nasal and sinus disease, upper respiratory infection, and head trauma. The history and physical, as well as the subsequent workup, should be directed toward these causes.

Upper respiratory infection: Probably the second most common cause of taste alteration, independent of olfactory loss. It is hypothesized that this is due to damage to the chorda tympani nerve through the path from the eustachian tube to the middle ear.

Poor oral health: Oral infections, including candidiasis, herpes simplex, periodontitis, and sialadenitis can all lead to taste alteration. This may be due to overgrowth or colonization of the taste pore, or due to foul chemicals released as a result of the infection. Tooth loss, replacement with a removable prosthesis, and a subsequent decline in masticatory function may result from poor oral hygiene. The decreased mastication leads to decreased release of tastants from the food, preventing access to the taste buds. Furthermore, mastication leads to release of odorants from the food, allowing for retronasal olfaction that contributes to the sensation of flavor.

Chorda tympani injury: Can occur secondary to ear surgery, or infections such as chronic otitis media, Bell palsy, Ramsay Hunt syndrome, or Lyme disease. Patients typically complain of a metallic taste rather than a taste loss. Dysgeusia is frequently temporary due to bilateral innervation.

Medications: Over 250 medications affect the chemical senses. Although the exact mechanism by which various drugs alter taste sensation may be unknown, common effects of medications include xerostomia, secretion of the drug into saliva causing an adverse taste sensation or interference with the taste conduction pathway, or alteration of cell turnover.

ACE-inhibitors: Captopril is among the most commonly noted medications associated with taste disturbance. Frequent complaints include hypogeusia and a strongly metallic, bitter, or sweet taste.

Anticholinergics: These medications cause excessive drying of saliva. Decreased saliva prevents tastants from dissolving, leading to hypogeusia and dysgeusia. Xerostomia may also cause the taste buds to close, preventing access of the tastants. Other medications that cause drying of saliva include antidepressants and antihistamines.

Other: Antibacterial mouthwash, aspirin, antiparkinson drugs, acetazolamide, lithium, lipid-lowering drugs, antibiotics, antineoplastic drugs, and penicillamine

Age: Taste perception, like olfactory function, becomes somewhat impaired with normal aging, although the effects of aging on taste are less pronounced. Elderly tend to complain of tastes being less intense. In addition to diminished taste perception found with aging, the elderly population is susceptible to many other factors that alter taste. Age-related disease states, surgery, poor dentition, multiple medications, or cumulative exposure to toxins are all contributing factors.

Less common causes of taste disturbance include the following:

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