Xerostomia

Xerostomia, also known as dry mouth, is a subjective complaint of dryness in the mouth, which may be associated with a change in the composition of saliva, reduced salivary flow, or have no identifiable cause.
This symptom is very common and is often seen as a side effect of many types of medication. It is more common in older people and in people who breathe through their mouths. Dehydration, radiotherapy involving the salivary glands, chemotherapy and several diseases can cause reduced salivation, or a change in saliva consistency and hence a complaint of xerostomia. Sometimes there is no identifiable cause, and there may sometimes be a psychogenic reason for the complaint.

Definition

Xerostomia is the subjective sensation of dry mouth, which is often associated with hypofunction of the salivary glands. The term is derived from the Greek words ξηρός meaning "dry" and στόμα meaning "mouth". A drug or substance that increases the rate of salivary flow is termed a sialogogue.
Hyposalivation is a clinical diagnosis that is made based on the history and examination, but reduced salivary flow rates have been given objective definitions. Salivary gland hypofunction has been defined as any objectively demonstrable reduction in whole and/or individual gland flow rates. An unstimulated whole saliva flow rate in a normal person is 0.3–0.4 ml per minute, and below 0.1 ml per minute is significantly abnormal. A stimulated saliva flow rate less than 0.5 ml per gland in 5 minutes or less than 1 ml per gland in 10 minutes is decreased. The term subjective xerostomia is sometimes used to describe the symptom in the absence of any clinical evidence of dryness. Xerostomia may also result from a change in composition of saliva. Salivary gland dysfunction is an umbrella term for the presence of xerostomia, salivary gland hyposalivation, and hypersalivation.

Signs and symptoms

Hyposalivation may give the following signs and symptoms:

Dental caries – Without the buffering effects of saliva, tooth decay becomes a common feature and may progress much more aggressively than it would otherwise. It may affect tooth surfaces that are normally spared, e.g., cervical caries and root surface caries. This is often seen in patients who have had radiotherapy involving the major salivary glands, termed radiation-induced caries. Therefore, it is important that any products used in managing dry mouth symptoms are sugar-free, as the presence of sugars in the mouth support the growth of oral bacteria, resulting in acid production and development of dental caries. In addition, reduced salivary flow and altered composition can result in a lowered buffering capacity and pH, which contributes to dental caries and tooth surface demineralization. Research shows that enamel demineralization typically begins when oral pH drops below about 5.5, while root dentin, being more vulnerable, can start to erode at a higher pH of around 6.7. Many saliva substitutes and oral moisturizers are formulated with a pH below 6.7, which can pose a risk to dentin over time.
Acid erosion. Saliva acts as a buffer and helps to prevent demineralization of teeth.
Oral candidiasis – A loss of the antimicrobial actions of saliva may also lead to opportunistic infection with Candida species.
Ascending sialadenitis – an infection of the major salivary glands that may be recurrent. It is associated with hyposalivation, as bacteria are able to enter the ductal system against the diminished flow of saliva. There may be swollen salivary glands even without acute infection, possibly caused by autoimmune involvement.
Dysgeusia – altered taste sensation and dysosmia, altered sense of smell.
Intraoral halitosis – possibly due to increased activity of halitogenic biofilm on the posterior dorsal tongue.
Burning mouth syndrome – a burning or tingling sensation in the mouth.
Saliva that appears thick or ropey.
Mucosa that appears dry.
A lack of saliva pooling in the floor of the mouth during examination.
Dysphagia – difficulty swallowing and chewing, especially when eating dry foods. Food may stick to the tissues during eating.
The tongue may stick to the palate, causing a clicking noise during speech, or the lips may stick together.
Gloves or a dental mirror may stick to the tissues.
Fissured tongue with atrophy of the filiform papillae and a lobulated, erythematous appearance of the tongue.
Saliva cannot be "milked" from the parotid duct.
Difficulty wearing dentures, e.g., when swallowing or speaking. There may be generalized mucosal soreness and ulceration of the areas covered by the denture.
Mouth soreness and oral mucositis.
Lipstick or food may stick to the teeth.
A need to sip drinks frequently while talking or eating.
Dry, sore, and cracked lips and angles of mouth.
Thirst.

However, sometimes the clinical findings do not correlate with the symptoms experienced. For example, a person with signs of hyposalivation may not complain of xerostomia. Conversely a person who reports experiencing xerostomia may not show signs of reduced salivary secretions. In the latter scenario, there are often other oral symptoms suggestive of oral dysesthesia. Some symptoms outside the mouth may occur together with xerostomia.
These include:

Xerophthalmia.
Inability to cry.
Blurred vision.
Photophobia.
Dryness of other mucosae, e.g., nasal, laryngeal, and/or genital.
Burning sensation.
Itching or grittiness.
Dysphonia.

There may also be other systemic signs and symptoms if there is an underlying cause such as Sjögren's syndrome, for example, joint pain due to associated rheumatoid arthritis.

Cause

The differential of hyposalivation significantly overlaps with that of xerostomia. A reduction in saliva production to about 50% of the normal unstimulated level will usually result in the sensation of dry mouth. Altered saliva composition may also be responsible for xerostomia.

Physiological

Salivary flow rate is decreased during sleep, which may lead to a transient sensation of dry mouth upon waking. This disappears with eating or drinking or with oral hygiene. When associated with halitosis, this is sometimes termed "morning breath". Dry mouth is also a common sensation during periods of anxiety, probably owing to enhanced sympathetic drive. During periods of stress, our body responds in a 'fight or flight' state that will interfere with the saliva flow in the mouth. Dehydration is known to cause hyposalivation, the result of the body trying to conserve fluid. Physiologic age-related changes in salivary gland tissues may lead to a modest reduction in salivary output and partially explain the increased prevalence of xerostomia in older people. However, polypharmacy is thought to be the major cause in this group, with no significant decreases in salivary flow rate being likely to occur through aging alone.

Drug induced xerostomia

Aside from physiological causes of xerostomia, iatrogenic effects of medications are the most common cause. A medication which is known to cause xerostomia may be termed xerogenic. Over 400 medications are associated with xerostomia. Although drug induced xerostomia is commonly reversible, the conditions for which these medications are prescribed are frequently chronic. The likelihood of xerostomia increases in relation to the total number of medications taken, whether the individual medications are xerogenic or not. The sensation of dryness usually starts shortly after starting the offending medication or after increasing the dose. Anticholinergic, sympathomimetic, or diuretic drugs are usually responsible.

Sjögren's syndrome

Xerostomia may be caused by autoimmune conditions which damage saliva-producing cells. Sjögren's syndrome is one such disease, and it is associated with symptoms including fatigue, myalgia and arthralgia. The disease is characterised by inflammatory changes in the moisture-producing glands throughout the body, leading to reduced secretions from glands that produce saliva, tears and other secretions throughout the body. Primary Sjögren's syndrome is the combination of dry eyes and xerostomia. Secondary Sjögren's syndrome is identical to primary form but with the addition of a combination of other connective tissue disorders such as systemic lupus erythematosus or rheumatoid arthritis.

Celiac disease

Xerostomia may be the only symptom of celiac disease, especially in adults, who often have no obvious digestive symptoms.

Radiation therapy

Radiation therapy for cancers of the head and neck where the salivary glands are close to or within the field irradiated is another major cause of xerostomia. A radiation dose of 52 Gy is sufficient to cause severe salivary dysfunction. Radiotherapy for oral cancers usually involves up to 70 Gy of radiation, often given along with chemotherapy which may also have a damaging effect on saliva production. This side effect is a result of radiation damage of the parasympathetic nerves. Formation of salivary gland ducts depends on the secretion of a neuropeptide from the parasympathetic nerves, while development of the end buds of the salivary gland depends on acetylcholine from the parasympathetic nerves.

Sicca syndrome

"Sicca" simply means dryness. Sicca syndrome is not a specific condition, and there are varying definitions, but the term can describe oral and eye dryness that is not caused by autoimmune diseases.

Other causes

Oral dryness may also be caused by mouth breathing, usually caused by partial obstruction of the upper respiratory tract. Examples include hemorrhage, vomiting, diarrhea, and fever.
Alcohol may be involved in the cause of salivary gland disease, liver disease, or dehydration.
Smoking is another possible cause. Other recreational drugs, such as methamphetamine, cannabis, hallucinogens, or heroin, may be implicated.
Hormonal disorders, such as poorly controlled diabetes, chronic graft versus host disease, or low fluid intake in people undergoing hemodialysis for renal impairment, may also result in xerostomia due to dehydration.
Nerve damage can be a cause of oral dryness. An injury to the face or surgery can cause nerve damage to the head and neck area which can effect the nerves that are associated with the salivary flow.
Xerostomia may be a consequence of infection with hepatitis C virus.
A rare cause of salivary gland dysfunction may be sarcoidosis.
Infection with Human Immunodeficiency Virus/Acquired immunodeficiency Syndrome can cause a related salivary gland disease known as Diffuse Infiltrative Lymphocytosis Syndrome.
Similar to taste dysfunction, xerostomia is one of the most prevalent and persistent oral symptoms associated with COVID-19. Despite a close association with COVID-19, xerostomia, dry mouth and hyposalivation tend to be overlooked in COVID-19 patients and survivors, unlike ageusia, dysgeusia and hypogeusia.