Functions of Saliva
Saliva has various functions:
Digestive function: Starch is broken down into maltose by the function of amylase.
Lubricating function: By moistening the oral mucosa, saliva facilitates mastication, swallowing, and speech.
Food bolus-forming function: By moistening food, saliva helps with crushing the food and forming a food bolus.
Dissolving function: By dissolving gustatory substances within food, saliva promotes the sensation of taste.
Autopurificatory function: Saliva washes out food from the oral cavity.
Antimicrobial function: Saliva contains antimicrobial substances that confer resistance to microbes.
Buffering function: Saliva neutralizes acids in the oral cavity.
Tooth-protecting function: Salivary protein protects surfaces of the teeth; saliva elevates intra-oral calcium and phosphate ion levels and prevents teeth enamel from dissolving.
Salivary secretion mechanism
Humans secrete approximately 1 to 1.5 L of saliva daily.
Salivary secretion increases during mastication and talking.
There are two types of saliva: stimulated saliva and resting saliva.
Stimulated saliva is increased in response to stimulation such as by mastication, talking, and taste.
Resting saliva is released into the oral cavity in small amounts when there is no stimulation and moistens the oral mucosa.
Saliva is secreted from 3 major glands (parotid, submandibular, and sublingual glands) and 5 minor glands (labial, buccal, lingual, palatal, and molar glands).
Serous saliva containing amylase is secreted from the parotid gland, and highly viscous saliva is secreted from the sublingual gland.
Hyposalivation
Hyposalivation refers to a condition where there are reduced salivary flow rates, from reduced or inhibited salivary secretion.
It can be caused by the following factors:
1. Ageing
There are many recent theories about why salivary secretion changes with age. Salivary gland tissue has been shown to change with age, and this is suspected to have an effect. However, some researchers argue that age has little effect on stimulated saliva, even when age-related decreases have been observed in resting saliva.
2. Disease
These factors include the salivary gland disease called Sjogren's syndrome, hormonal and/or metabolic disorders such as diabetes mellitus and hyperthyroidism, body fluid and electrolyte disorders due to dehydration or renal disease, and neurogenic factors due to stress.
3. Adverse drug reactions
4. Other
Xerostomia
Xerostomia is the term for dry oral mucosa or oral mucosa with reduced moisture. Oral dryness can occur even in patients with normal salivary secretion, due to respiratory and other factors.
Causes of xerostomia
Disease factors
Collagen diseases such as Sjögren's syndrome
Organ-specific autoimmune diseases targeting the lacrimal and salivary glands, with dry eye and oral dryness as the cardinal symptoms
Diseases common in women aged 30-60 years
Primary disease localized to the exocrine glands, concentrated in the salivary and lacrimal glands, and comorbidities with collagen diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA); generally classified as secondary disease
Diabetes mellitus
With diabetes mellitus, sugar-containing urine has a high osmotic pressure and is excreted in large quantities (polyuria) because sugar is excreted in urine
Manifestations include dehydration and xerostomia
Diabetes insipidus
Diabetes insipidus is characterized by abnormally large quantities of urine due to the lack of response to renal hormones and insufficiency of antidiuretic hormone secreted from the posterior pituitary gland
Excessive urination leads to dehydration and thirst
Hyperthyroidism and hypothyroidism
In hyperthyroidism, patients show palpitation, excessive sweating (hyperhidrosis), fatigue, and thirst with metabolic activation, elevated cardiac function, and sympathetic nerve excitation
In hypothyroidism, patients show whole-body edema and thirst, with decreased metabolism
Salivary gland disease
Sarcoidosis of the parotid gland (can present as granulomatous lesions), tuberculosis and syphilis are rarely seen as granulomatous lesions
Systemic amyloidosis is condition in which extensive amyloid deposits build up around ducts and in the lumina of acini, causing pressure atrophy of the parenchyma
Viral diseases include epidemic parotitis (mumps), and cytomegalic inclusion disease due to cytomegalovirus
Fever and febrile disease
Patients with fever are prone to thirst due to dehydration
Depression and similar conditions
Oral dryness can occur as an adverse reaction to an anti-depression drug and frequently as a somatic symptom due to the depression itself
Factors related to dysfunction
Dysfunction due to ill-fitting artificial teeth (dentures)
Ill-fitting dentures can lead to incomplete swallowing and mastication, which in turn causes a reduction in stimulated saliva
Paralysis-related dysfunction
With paralysis of the oral cavity, structures on the affected side do not move, resulting in functional decline and a reduction in stimulated saliva
Respiratory conditions
The patient may habitually keep their mouth open to ease breathing, which causes oral dryness
Treatment-related factors
Radiotherapy
Degeneration and loss of salivary gland cells have been seen at some doses of radioactivity
Exposure to radiation at 30-60 Gy generally reduces salivary glands by 30%-50%
The parotid gland-the source of serous saliva-is particularly prone to being affected
Drug-induced xerostomia
Oral dryness occurs as an adverse drug reaction to antihypertensives and sleeping medication, which are widely taken by older people
Adverse drug reactions can occur with long-term drug use
Salivary gland surgery
Salivary secretion is reduced when salivary glands are removed due to oral cancer
Other factors
Lifestyle disease and living environment
Smoking, alcohol consumption, and eating between meals are among the potential causes of oral dryness
Drugs and salivary secretion
Salivary secretion can be affected by a wide range of drugs.
Actions of the central nervous system and peripheral nervous system drugs and their receptors
1) Anticholinergic drugs
Antispasmodic drugs (atropine and scopolamine)
Antiparkinsonian drugs (biperiden and trihexyphenidyl)
Antiulcer drugs (scopolamine, propantheline, timepidium, and ethylpiperidine)
2) Psychoneurotic drugs
Antipsychotics (chlorpromazine, fentazin, haloperidol, and sulpiride)
Anti-depressants (imipramine, amitriptyline, maprotiline, and trazodone)
Anti-anxiety drugs (triazolam, chlordiazepoxide, diazepam, cloxazolam, oxazolam, prazepam, etc.)
3) Tranquilizers and sleeping medicine (phenobarbital)
4) Antihistamines (H1 antagonists: diphenhydramine, dimenhydrinate, diphenylpyraline, homochlorcyclizine, and chlorpheniramine; H2 antagonists: famotidine, and nizatidine)
Drugs and salivary secretion
Salivary secretion can be affected by a wide range of drugs.
2. Drugs involved in electrolyte or water transfer
1) Antihypertensives
Diuretics (furosemide, spironolactone, triamterene, acetazolamide, and d-mannitol)
Calcium antagonists (nifedipine, nicardipine, verapamil, and diltiazem)
2) Bronchodilators (ephedrine, salbutamol, and tulobuterol)
Salivary examination in practice
Salivary examinations cover the following:
1. Subjective symptoms
2. Clinical diagnostic criteria (next slide)
3. Evaluation of oral dryness
Salivary moisture paper test
Oral moisture measurement
Measurements of resting saliva volume: spit test and wet test (cotton roll test)
Measurements of stimulated saliva volume: gum test, paraffin test, and Saxon test
Clinical diagnostic criteria (Kakinoki et al., Research Grants for Longevity Science, Ministry of Health, Labour and Welfare, 1999)
Since elderly and disabled individuals may often not complain about oral dryness, simple criteria related to the clinical findings in salivary examinations is needed.
Clinical diagnostic criteria (Kakinoki et al., Research Grants for Longevity Science, Ministry of Health, Labour and Welfare, 1999)
Grade 0: Normal; no oral dryness or viscosity of saliva
Grade 1: Mild xerostomia; saliva shows some viscosity or is reduced; sticky or ropey saliva
Grade 2: Moderate xerostomia; very little saliva and shows fine bubbles
Grade 3: Severe xerostomia; dry tongue, with little or no saliva observed
1) Moisture paper test
The moisture paper test was developed to assess water and other fluid contents. In this test, we measure the amount of saliva absorbed by a test paper applied to the oral mucosa for a set period of time.
3) Measurements of resting saliva volume
The patient spits out the saliva secreted while sitting in a resting position (spit test)
A dental cotton roll is placed on the anterior portion of the floor of the mouth, and the change in the weight of the cotton roll soaked the saliva is measured (cotton roll test)
4) Measurements of stimulated saliva volume
The patient chews a small piece of gum or paraffin fragment for a fixed time, and the secreted salivary volume is measured (gum test or paraffin test).
The patient is asked to chew a piece of gauze, and the amount of saliva absorbed by the gauze is measured.
The photograph here shows the paraffin test.
Is salivary volume reduced?
Let's look now at what effects a reduction in saliva has on swallowing.
Food debris accumulates on the oral and pharyngeal mucosa due to a decline in autopurification
The oral mucosa becomes prone to injury from dentures and other factors
Masticatory function declines with inability to moisten the food bolus
The dried mucous membranes become sticky, restricting tongue mobility
The sense of taste is impaired
Tooth decay occurs with a build-up of plaque that becomes difficult to remove
When dysphagia occurs
Saliva aspiration can occur because saliva can't be swallowed
The patient becomes dehydrated and salivary secretion is suppressed due to limited water intake
Treatments of the cause
Prevention or curbing of adverse drug reactions (for adverse drug reactions)
Drugs to improve secretion of saliva (for decreased salivary secretion)
Hydration (for dehydration and insufficient water intake)
Artificial saliva (for decreased salivary secretion)
Oral rehabilitation (for oral dysfunction)
Improve lifestyle and physical condition (for poor lifestyle and physical condition)
Treatments of symptoms
Oral moisture: Use moisturizing agents and artificial saliva
Oral care: Important to do before eating food as well
Response to painful or uncomfortable oral mucosa: Drug treatments or moisture retention for pain relief
Response to oral dysfunction: Optimization of diet form for the patients with difficulty chewing or swallowing
Recommended readings
- Ansai T., Kakinoki Y. (ed): "Let's Start Today! Clinical Xerostomia - Cardinal Symptoms and Approach (in Japanese). Ishiyaku, Tokyo, 2008.
- Japanese Society of Gerodontology. Compilation of Terminology for Gerodontology (in Japanese). Ishiyaku, Tokyo, 2008.
