An injury to the face or surgery can cause nerve damage to your 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 (HCV) and a rare cause of salivary gland dysfunction may be sarcoidosis.Infection with Human Immunodeficiency Virus/Acquired immunodeficiency Syndrome (AIDS) can cause a related salivary gland disease known as Diffuse Infiltrative Lymphocytosis Syndrome (DILS).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. Diagnostic approach
A diagnosis of hyposalivation is based predominantly on the clinical signs and symptoms. The Challacombe scale maybe used to classify the extent of dryness. The rate of the salivary flow in an individuals mouth can also be measured. There is little correlation between symptoms and objective tests of salivary flow, such as sialometry. This test is simple and noninvasive, and involves measurement of all the saliva a patient can produce during a certain time, achieved by dribbling into a container. Sialometery can yield measures of stimulated salivary flow or unstimulated salivary flow. Stimulated salivary flow rate is calculated using a stimulant such as 10% citric acid dropped onto the tongue, and collection of all the saliva that flows from one of the parotid papillae over five or ten minutes.
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 (subjective xerostomia). In the latter scenario, there are often other oral symptoms suggestive of oral dysesthesia ("burning mouth syndrome"). Some symptoms outside the mouth may occur together with xerostomia. These include:

Xerophthalmia (dry eyes). Inability to cry. Blurred vision. Photophobia (light intolerance). Dryness of other mucosae, e.g., nasal, laryngeal, and/or genital. Burning sensation. Itching or grittiness. Dysphonia (voice changes).There may also be other systemic signs and symptoms if there is an underlying cause such as Sjögrens 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. Physiologic
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. 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.