There is insufficient evidence to describe normative values for SSF in either healthy populations or clinically relevant groups. It is also not possible to determine whether SSF is a clinically useful screening tool for dysphagia at present. It was clear from the analysis that data on swallow frequency from different centres, using different technologies or different methodologies, cannot reliably be compared. However, from the intervention studies in table 3, it appears that differences in a swallow index measured in the same centre using the same study design are sensitive to clinical condition or to intervention. This seems reasonable since in this situation the major methodological differences are controlled. Nevertheless the relation of any measure of SSF with a physiologically meaningful reference standard cannot be assumed.
Effect of age and gender on spontaneous swallow frequency
Despite high heterogeneity, there is a negative correlation of SSF with age. Healthy young (< 60 years) have a mean SSF of 0.98 while healthy old have a mean SSF of 0.21. Simply, older people swallow less. This is in agreement with the works of Crary et al.  and Tanaka et al. , both of whom report significantly higher SSF in young normal (healthy) subjects when compared to elderly normal (healthy) subjects using the same swallow detection technology (see Table 2). However, note that mean SSFs across even the healthy young group are highly variable and do not give a reliable indication of what a true ‘normal’ SSF is.
As with all correlation studies, cause and effect is difficult to assess. The reduced SSF with advanced age may be directly implicated in dysphagia. If so, it is surprising that SSF is so similar across the elderly population, irrespective of dysphagia risk. If some other age-related process causes dysphagia then SSF is merely a convoluted method of estimating age.
Finally, one conference abstract reported a higher rate of spontaneous swallow in women . This is an important area to investigate, but no full papers have yet been published assessing the effect of gender on SSF.
The role of spontaneous swallow frequency in clinical practice
Accurate measurement of SSF may be the most appropriate reference for basic research in swallow physiology. If instead we consider the diagnosis of dysphagia as our criterion, then a validated clinical assessment by SALT may be a more appropriate reference standard. Crary et al.  compared SSF to a clinical assessment, reporting that a threshold of 0.4 swallows/min showed high sensitivity (0.96) but low specificity (0.68) for dysphagia. The workers suggest SSF may be a useful screening tool but lacks the specificity for a definitive diagnosis. Our data would agree; those at heightened risk of dysphagia have a much lower SSF than younger healthy participants. With the exception of PD, the mean SSF was in every other case lower than Crary’s threshold. Note however that this is the mean SSF; sensitivity and specificity are defined on a patient-by-patient level.
Secretion management and interventions to increase SSF
Interventions to increase swallowing frequency have been used to aid in secretion management and have increased understanding in conditions where hypersalivation and resultant drooling is common, namely, Parkinson’s [17, 18] and Wilson’s Disease . PD is also associated with increased risk of dysphagia. More than 80% of patients are predicted to develop dysphagia following disease onset . Figure 3 suggests that PD patients have a lower SSF than healthy younger subjects but they appear to swallow more regularly than other at-risk groups. This may be explained by known and unknown cases of hypersalivation in this patient population . Moreover, SSF reported across studies involving PD patients is highly variable, potentially due, in part, to the risk of associated conditions such as dysphagia that have not been diagnosed. Such conflicting variables make it difficult to determine the true value of using SSF in this population.
Interventions to stimulate swallows appeared to be successful though these studies were often not blinded or randomised so were open to bias, and it is difficult to know to what degree any improvements could be considered clinically significant. Four studies were effective at increasing SSF in groups with dysphagia; two found facio-oral tract therapy to significantly increase swallow rate [21, 22] one used gustatory stimulation , and one successfully applied vibrations to stimulation the larynx in a dysphagic group . Studies reported that these interventions have the potential to improve swallow function in those with dysphagia via the measurement of SSF, but no studies report long term outcomes.
It remains unclear what a normative swallow rate is and what a clinically significant change in rate may be. Nevertheless he pre- and post- intervention data reported maybe give useful insight for researchers and clinicians to consider in the future when treating these patient groups.
SSF measurement methodologies
Heterogeneity between studies was high, particularly with regard to methods used to assess SSF. A variety of techniques were used to measure spontaneous swallows, most frequently with the single or combined use of: electromyography (EMG), acoustic/sound recording, respiratory bellows/transducers (around the ribcage), bellows/transducers located around the larynx, or scoring by a trained observer. Other techniques included auscultation, fiberoptic endoscopic evaluation (FEES), and magnetic resonance imaging (MRI).
A clinically useful measure of SSF should have good validity, i.e. should give comparable results to a reference standard. A reference for the measurement of SSF (i.e. the detection of swallows over prolonged periods of time) does not currently exist. Nevertheless any clinical physiological measurement – sphygmomanometry for example – is predicated on the idea that the measurement indicates the underlying blood pressure irrespective of the specific technology. There is general consensus as to what constitutes a swallow, and so there is objectively a ‘correct’ SSF, at least in principle. If measurements disagree to the extent reported here, we cannot avoid the conclusion that some (or possibly all) technologies are generating incorrect data.
To resolve these issues, comparative studies between several measures of SSF are needed. Only one group compared two systems for measuring SSF , the first an acoustic recording technique, the comparator a comprehensive multichannel physiologic recording with surface EMG, swallow apnoea, and cervical auscultation. There was no significant difference in SSF between the two methods (0.85 /min vs. 0.81 /min). Superficially this suggests that a microphone alone might be used to record swallows. Perhaps more importantly, the consistency between two different technologies might suggest that the technology works well and that other methodological factors (for example: the use of long-term ambulatory use versus lab-based investigations with prescribed activities) is responsible for the poor agreement between centres.
FEES is a well-used and reliable method for assessing a single swallow and may be an appropriate reference standard for an event-by-event comparison, a form of criterion validity. Large scale validation studies of SSF instrumentation are required before systematic trials in clinical populations, with data involving known swallows and potential artefacts. In order to ensure high specificity with regards to swallow detection, designs must rule out noise artefacts in addition to reliably identifying all swallows.