A10 Xerostomia

Topic

Xerostomia is the subjective sensation of dry mouth and is frequently associated with hyposalivation—an objectively measurable reduction in salivary flow; hyposalivation is defined as a resting salivary flow rate falling below approximately one-tenth of a milliliter per minute. Although xerostomia and hyposalivation often occur together, the former is what the individual perceives, while the latter is what is directly measured at the salivary gland.

Saliva performs several critical functions within the mouth: it lubricates soft tissues and teeth; protects against microorganisms through components such as secretory immunoglobulin A, lysozyme, and lactoferrin; buffers oral pH to prevent excessive acidity; promotes tooth enamel remineralization; and clears cariogenic food substrates from the mouth.

When hyposalivation occurs, the various balances maintained by saliva are simultaneously disrupted, leading to several consequences: rampant caries develop and progress much more rapidly than typical dental cavities; periodontal disease is promoted; oral candidiasis easily takes hold; and the individual begins to experience difficulty swallowing and speaking due to the lack of lubrication in oral tissues.

Basal salivary flow—the flow maintained at rest, outside of episodes of acute hyposalivation—depends on two factors: glandular acinar function (i.e., the amount of saliva the salivary gland acini are capable of producing) and the autonomic nerve stimulus that triggers these acini to secrete. When either of these factors diminishes, basal salivary flow decreases as well.

In cases of xerostomia caused by head and neck radiotherapy, glandular acinar function is lost because the glandular parenchyma containing the acini is irreversibly destroyed by the radiation. This destruction of the glandular parenchyma does not increase simply in proportion to the radiation dose received; rather, it combines a component that grows in direct proportion to the dose with another that grows in proportion to the square of the dose, meaning that increasingly higher radiation doses destroy the glandular parenchyma with disproportionately greater intensity.

In addition to a reduction in quantity, the saliva still produced in cases of xerostomia changes in quality: its viscosity increases drastically, resulting in a thick saliva. The greater this viscosity, the lower the saliva's ability to spread throughout the mouth and fulfill the lubricating function described earlier.

This thick saliva, with its reduced spreading capacity, also clears away cariogenic substrates remaining on the teeth much more slowly; the time required for the mouth to clear these substrates—normally ten to fifteen minutes—extends to over sixty minutes in cases of xerostomia. This slow clearance, combined with the loss of the other protective salivary functions mentioned earlier, explains why the dry mouth associated with xerostomia ultimately leads to rampant caries and the other consequences previously described for hyposalivation.

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