«The persistent dry mouth Martin M. Ferguson (New Zealand Family Physician, Vol 29, No 4, August 2002) While the full extent of individuals in the ...»
Continuing Medical Education
The persistent dry mouth
Martin M. Ferguson
(New Zealand Family Physician, Vol 29, No 4, August 2002)
While the full extent of individuals in the adult population with a persistent dry mouth is not
accurately known, epidemiological data would suggest that it may affect at least 5% of adults. In
many instances the diagnosis and attendant problems are not well recognised, or are merely accepted.
Source of saliva
Saliva is produced by the three pairs of major salivary glands, the parotids, submandibulars and sublinguals. In addition, there is a significant contribution from the many minor glands that line the oral cavity (Figure 1).
The nature of saliva from each varies, with a serous secretion from the parotids and a mucous secretion from the sublinguals and minor glands. That from the submandibular is mixed.
Saliva is a complex fluid, varying in composition from moment to moment, under the control of both cholinergic and adrenergic branches of the autonomic nervous system as well as humoral factors (Figure 2). The rationale for this fine control is not well understood.
Functions of saliva Saliva has numerous functions and these control the environment of the mouth.
Cleansing By virtue of its water content, saliva cleanses the dentition and mucosa, washing off debris towards the back of the mouth, where it is swallowed.
Lubrication Mucins lubricate the hard and soft tissues, enabling them to move freely against one another.
Antimicrobials Saliva contains several antimicrobial systems including secretory-IgA, peroxidase, lysozyme, lactoferrin, histatins and cystatins. Their role is not to sterilise the oral cavity but collectively to generate a healthy commensal flora.
Mucosal integrity The hydration of the mucosa is maintained by virtue of the water and mucin content. In addition, there are growth factors in saliva, such as epidermal growth factor, that may also contribute to mucosal integrity.
Buffering Enamel begins to lose surface calcium and phosphate below pH 5.7. These conditions regularly occur with ingestion of dietary acids or in plaque where carbohydrate is metabolised. The principal buffer in saliva is bicarbonate but there is also a smaller amount of phosphate and protein buffers.
Remineralisation If the loss of calcium and phosphate from the enamel was an irreversible process, then in a relatively short time the dentition would be seriously eroded or else destroyed by caries. Saliva is supersaturated with calcium and phosphate, with these high concentrations being achieved through binding to peptides, such as statherin and proline-rich-peptides. At neutral pH, these peptides convey the minerals to the enamel surface where they are incorporated back into the hydroxyapatite. Fluoride is also incorporated into the crystal lattice of enamel at this time, which promotes resistance to acid dissolution.
Taste All substances require to be in solution for tasting.
Digestion Saliva has long been known to contain amylase that will digest carbohydrates but the importance of this in general digestion is unlikely. There are also several other digestive enzymes in saliva and their role may simply be to break down retained food particles.
Aetiology of salivary hypofunction Numerous disorders can result in salivary hypofunction, either functional or structural in aetiology.
- The sensation of a dry mouth is an experience with which many are familiar when anxious but it may be a feature of anxiety neurosis or depression (Figure 3).
- While a degree of xerostomia has been recognised to be a side effect of a multitude of medications, this is particularly troublesome with tricyclic antidepressants, antipsychotics, antihistamines and decongestants (Figure 4).
- Individuals who are dehydrated rarely present complaining primarily of a dry mouth, although this can be the case in diabetes insipidus or with excessive use of diuretics.
- Aplasia or hypoplasia of the salivary glands is exceedingly rare but results in degrees of dryness according to the extent of the defect.
- The salivary glands are sensitive to radiation and are frequently damaged consequent to radiotherapy to the head and neck region. While there is usually some resolution over the subsequent six months, the effects tend to be permanent. The discomfort due to dryness in the mouth may be compounded particularly by mucosal damage as well as loss of taste.
- Sjögren's syndrome is an inflammatory exocrinopathy that may co-exist with one of the connective tissue disorders including rheumatoid arthritis, SLE, progressive systemic sclerosis, dermatomyositis and polymyositis (Figures 5 and 6). The exocrine glands are affected by an inflammatory infiltrate, the most prominent being salivary and lacrimal glands (Figures 7-9). However, other exocrine glands may be involved resulting in nasal crusting and dyspareunia. Some individuals develop a lymphoma within the glands (Figure 10).
- Patients receiving bone marrow transplant may develop graft-versus-host disease. In the mouth, this presents as ulceration and lichen planus along with a glandular inflammatory infiltrate, to an extent similar in gross appearance to Sjögren's syndrome. Salivary gland swelling and persistent dryness occurs.
- Salivary glands may be affected for a limited duration by infections such as mumps, leading to temporary dryness. But individuals infected with HIV tend to develop an inflammatory infiltrate, with swelling and tenderness along with xerostomia.
- Diabetes mellitus is a disorder that traditionally was considered to cause a persistent dry mouth due to dehydration. While there is no doubt that dryness can be a feature of hyperglycaemia, the consensus now points to the salivary hypofunction being due to autonomic neuropathy, in a similar way to the cardiovascular changes. This would be consistent with the non-inflammatory enlargement (sialosis) that occurs in some diabetics (Figure 11).
- Cirrhosis may likewise lead to autonomic neuropathy with a consequential dry mouth along with sialosis but the evidence is as yet less clear (Figure 12).
Problems of salivary hypofunction
- The commonest complaint is of a persistent dry mouth (Figures 13-15). This is regularly present throughout the day and usually necessitates taking frequent drinks. Individuals also waken during the night and therefore tend to take a glass of water with them to bed. It is appropriate to exclude those who complain only of being dry at night and first thing in the morning as they are more likely to be snoring or mouth breathing during sleep.
- In addition to being dry, some people experience a burning or scalded sensation, which probably reflects the surface changes in the moist lubricating layer of mucin. A drink of water only provides temporary relief. Hot or pungent foods cause discomfort and even standard toothpastes sting. There are other non-salivary causes of a burning mouth that should be excluded in these patients including lichen planus, nutritional deficiencies, allergies, infections, and neurological and psychological disturbances.
- During speech, there is effectively mouth breathing. This further aggravates the existing dryness, hindering movement of the oral soft tissues. This necessitates the need for frequent sips of water, which can be particularly inconvenient in many occupations.
- With the alteration in salivary composition and quantity, plaque accumulates more readily on the teeth. This initiates an inflammatory reaction in the gums, resulting in gingivitis and periodontal disease. In the longer term there can be gingival recession and eventually tooth loss.
- Similarly, dental caries has an increased incidence. In any adult who presents with a significant rise in caries experience it is improbable that their diet has changed markedly or oral hygiene practices have altered. It is far more likely that there is a change in saliva, even in the absence of feeling their mouth to be dry.
- With a change in the oral microbial flora, candidiasis is found more commonly. There is frequently denture stomatitis, with the palatal mucosa becoming inflamed or even hyperplastic due to the irritant substances released by candida colonising the fitting surface of dentures. Old or ill-fitting dentures contribute to this.
- Dentures normally sit on a thin film of mucin above the mucosa. This enables there to be minor movement during function. In the absence of sufficient lubrication, both complete and partial dentures tend to place undue stress on the underlying mucosa regardless of their mechanical adequacy. Some individuals abandon their dentures or wear them only for 'social occasions'.
- Ascending bacterial infections, up the excretory ducts, are more common in xerostomia.
Sialadenitis presents as painful swelling over a parotid or submandibular gland. Thick mucoid material or even pus can be expressed from the duct orifice and, rarely, an abscess may drain through the overlying skin.
- The symptoms of persistent salivary hypofunction might be viewed as being analogous to chronic pain. Discomfort is present all or most of the time and it interferes with their life on a daily basis.
Investigations The detailed investigation of salivary hypofunction is outwith the scope of this article and it should be appreciated that a number of the procedures do have limited value.
While a careful history is not properly an investigation, most specialists place much weight on the patient's symptoms. Regardless of whether or not the mouth looks dry, the sensation and complaints from an individual are important and might over-ride laboratory results. Another relevant factor is that a number of the investigative procedures have been developed and analysed statistically under research circumstances, using patients who have clearly been diagnosed. In clinical practice, this is not always as straightforward.
Examination of the mouth may reveal no obvious clinical change, with the mucosa appearing moist and healthy despite the patient's complaints (Figure 13). The first visible change in saliva texture is frothiness, with the normally transparent, watery saliva assuming a frothy or a white and frothy appearance (Figure 14). Less commonly the mucosa is patently dry and thick (Figure 15); tenacious mucus may be evident on the posterior pharyngeal wall (Figure 8).
Serology is useful in screening for diabetes mellitus (fasting glucose; glycated haemoglobin), Sjögren's syndrome (ESR; CRP; ANA; ENA; Rheumatoid Factor), HIV infection and sarcoid (ESR;
Serum ACE). Further tests may be indicated depending upon the initial results.
For decades, great emphasis has been placed on salivary flow rates, using resting or stimulated flows; whole or mixed saliva. Commercial adaptations have been marketed. While these findings are widely published in research papers, there are limitations in their usefulness in routine practice. Values fluctuate for any one individual but more importantly there are not clear dividing lines between health and disease in many instances. What it a putative low value may be consistent with an absence of any disease. Some centres use flow rates to monitor progress in an individual patient.
In addition to salivary volume, there may be compositional changes that lead to symptoms.
However, as yet these have not been adopted in routine practice.
Biopsy of the salivary glands can contribute to the diagnosis of Sjögren's syndrome or sarcoid.
Usually, several small lobules of minor salivary glands are excised from under the mucosa inside the lower lip, and the appropriate histology sought (Figure 6).
Straight radiographs are of no value in diagnosing salivary hypofunction - radio-opaque calculi do not cause dryness of the mouth. In sialography, a radio-opaque liquid is infused up to the gland duct from its orifice in the mouth. Radiographs outline the entire ductal system and punctate globules (sialectasis) may be evident as well as ductal strictures and dilatations.
Several tissues in the body accumulate iodide in addition to the thyroid but do not organify it to produce thyroxine. The ducts within the salivary glands accumulate iodide and use of this property is made to assess a parameter of gland function. For scintiscanning, another ion is used (Tc99mpertechnetate) as it is not retained in the thyroid and causes much less radiation to the patient than do isotopes of iodine but is similarly accumulated in tissues.
Management An energetic and co-ordinated approach to salivary hypofunction has much to offer for quality of life. Unfortunately this is often overlooked when some more pressing systemic problem is being addressed. Some patients are elderly or infirm due to associated disease and require further support in this area.
The obvious substitute for inadequate saliva is water and xerostomic patients do tend to drink more. Some find that sparkling mineral water affords greater relief. However, there have been many attempts to find saliva substitutes. Glycerine and lemon has long been tried but glycerine is hygroscopic and can result in a feeling of dryness. Most preparations have been based on a slightly viscous solution of carboxymethylcellulose to which have been added buffers, calcium and fluoride.
Alternatively, a solution is marketed based on animal mucin; a point to consider with some religious groups or vegetarians. While all of these substitutes offer some immediate relief, so too does water, and a number of centres have largely abandoned using them. A central problem if fluid delivery.
Normally saliva is being secreted into the mouth continuously albeit at variable flow rates. To attempt to duplicate this requires frequent applications of whatever substitute is being tried.
For extreme cases of dryness, an oil and water emulsion has been developed as a moisturiser but no commercial preparation is currently available.