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R. Poon, N. Su, V. Ching, M. Darling and M. Grushka British Dental Journal 2014; 217: E14

Editor's summary

Colloquially and somewhat dismissively referred to as either spit or dribble, I have long thought that saliva is one of the most underrated of bodily fluids. Having written that, from the viewpoint of operative dentistry it is something of a nuisance in its capacity to flow into all the places that you don't want it to go. But we will let that pass just now.

There has been much research on saliva in recent years: its constituents, its healing properties and its possible uses as a diagnostic tool. Indeed a lot of product development investment was made some years ago and the humble secretion looked as if it was poised to make major inroads into diagnostic health and medicine. Some of that development has come to pass and I suspect that in due course the value of this approach will gain greater currency partly because of its ease of non-invasive collection. We have seen this in practice in the collection and analysis of salivary bacteria in relation to caries prediction and progression as well as its capacity to remineralise pre-carious lesions. Refining and harnessing this technology still holds great promise.

While one diagnostic aspect which mitigates against this is when the flow of saliva is low this in itself may provide further clues to oral and general health. This research exemplifies my favourite statement in authors' starting points as they state 'Clinically a pattern was noted: patients with Sjogren's syndrome had decreased unstimulated and stimulated salivary flow. However, patients with burning mouth syndrome (BMS) had normal stimulated flows but below normal unstimulated flows.' Observation prompting curiosity and creating the opportunity for the exploration of its causes.

Interestingly, the enquiry has led to the conclusion that BMS patients have statistically significantly decreased unstimulated salivary flow rate with non-statistically significantly decreased stimulated flow rate. This in turn has stirred further thoughts that low unstimulated flow may be secondary to changes in autonomic function, which is itself an extended area of research which may ultimately serve to benefit patients with both xerostomia and BMS.

Another practical and important point arising from this paper, and the accompanying Commentary, is that so few of us as clinicians regularly measure salivary flow rates in our patients. As background information this provides valuable baseline data from which subsequent measures can be compared and is something that we may consider valuable given the ageing populations that we increasingly examine and treat.

The full paper can be accessed from the BDJ website ( www.bdj.co.uk ), under 'Research' in the table of contents for Volume 217 issue 7.

Stephen Hancocks

Editor-in-Chief

Author questions and answers

1. Why did you undertake this research?

Salivary flows are taken as a preliminary test for every patient that presents with dry mouth, burning mouth pain or Sjögrens syndrome. Clinically, a pattern was noted: patients with Sjögrens syndrome had decreased unstimulated and stimulated salivary flow. However, patients with burning mouth syndrome (BMS) had normal stimulated flows but below normal unstimulated flows. We were interested in finding if there were significant differences between the salivary flows in these two patient populations that often report dry mouth, which led us to investigate whether a simple salivary flow test could differentiate between BMS and Sjögren's syndrome.

2. What would you like to do next in this area to follow on from this work?

The pattern that is seen consistently in the BMS patient population (ie normal stimulated and low unstimulated flows) indicates that salivary flow rate measurement may be useful as a clinical diagnostic tool. It would also be of interest to determine if there is any change in salivary flows even after BMS patients are effectively treated with medication, such as low-dose clonazepam, which is helpful for BMS. In some cases, after clonazepam is introduced to a patient, complaints of dry mouth may still require medications to increase saliva (eg Salagen) suggesting that low unstimulated flow may be secondary to changes in autonomic function.

Commentary

A major problem in carrying out studies on the effects of different diseases on salivary flow rates is the very wide range of normal values. Typically, in any group of individuals, the standard deviation for salivary flow rate is a very high fraction of the mean, as seen in Tables 1 and 2 of this article by Grushka and colleagues. Large numbers of subjects are required in order to show significant differences among groups, since the variance decreases only as the square root of the number of subjects. In Dr Grushka's study these numbers, especially the controls (15), are actually quite low for determining whether patients with burning mouth syndrome (BMS) have reduced salivary flow rates, but this is understandable for the BMS patients, who were all from her oral medicine practice.

Hyposalivation is usually considered to be present when the unstimulated salivary flow rate (USF) is <0.1 mL/min1 and, surprisingly, none of the mean values in Table 1 is less than this, even in the patients with Sjögren's syndrome. However, xerostomia (the subjective symptom of dry mouth) has been reported in patients with salivary flow rates in the normal range.2 Without having data on the USF of Dr Grushka's patients before they developed BMS, it is not possible to tell whether their USFs have decreased since they developed BMS or whether their USF was low before the onset of BMS. In other words, they may always have had relatively low salivary flow rates!

In medical practice it is common to maintain records of a patient's blood pressure etc, beginning with the patient's first visit. Unfortunately, it is probably only in Sweden where the missing data for a study like Dr Grushka's would be available, since dentists there are apparently routinely taught to measure each patient's salivary flow rates at their first visit and periodically thereafter. Such baseline data are invaluable if the patient subsequently complains of xerostomia. One hopes that the dental profession in other countries will soon institute salivary flow rate measurements, which take little time and can be delegated to auxiliary personnel.