Variability of quantitative scintigraphic salivary indices in normal subjects

Several quantitative measures of salivary uptake and discharge have been proposed recently in the scintigraphic evaluation of xerostomia. We investigated the scatter of four time-activity curve C(TAC)-derived indices in a group of volunteer subjects who met extensive inclusionary and exclusionary criteria of salivary normalcy.

Thirty-one adult volunteers underwent dynamic salivary scintigraphy with gustatory stimulation. Any candidates with subjective xerostomia, conditions or medications associated with dry mouth, salivary gland enlargement or pregnancy were excluded from study. All subjects had normal oral exams, xerostomia scores and unstimulated whole-mouth salivary flow rates. After the intravenous administration of 99mTcO-4, scintigraphy was performed with generation of TACs derived from regions of interest centered about the four major salivary glands and the oral cavity. At 45 min postinjection, hard lemon candy was given for 15 min as a gustatory stimulus. The following functional indices were calculated for each gland: partitioned percentage (PP) of total prestimulated activity, maximum net uptake ratio (NUR) and its time of occurrence (TNUR) and percentage stimulated discharge fraction (DF).

The following ranges were observed: parotid PP, 22%-49%; submandibular PP, 4%-31%; parotid NUR, 2.2-16.0; submandibular NUR, 1.4-16.2; parotid TNUR, 8-45 min; submandibular TNUR, 2-45 min; parotid DF, 20%-99%; and submandibular DF, 27%-98%. Every subject except one 91-yr-old man showed frequent periodic unstimulated oral transfer of salivary activity with a rising oral TAC and responded to gustatory stimulation.

So-called quantitative indices may perform poorly in the scintigraphic evaluation of xerostomic patients because the effects of normal simultaneous glandular trapping, uptake, oral discharge and possible vascular washout combine to widen reference limits. Coordinated analysis of oral cavity and glandular activities, glandular index averaging and better temporal resolution may help improve diagnostic performance.