Characteristics of Glutamate-evoked Pain in the Masseter Region: Differences Between Targeted Injections in Subcutis, Muscle and at Bone Surface

Objectives: To investigate whether glutamate-evoked pain in the masseter region would differ between 3 different depths of glutamate injection targeting the subcutaneous, muscle and bone surface regions.
Methods: 16 healthy volunteers participated and in randomized order, received injections of glutamate (1.0 M, 0.2 mL) and isotonic saline (0.9 %, 0.2 mL) in the masseter region targeting the subcutaneous, intramuscular and bone surface regions in randomized order. Following the injection, pain intensity was measured with an electronic visual analog scale (eVAS) and numerical rating scale (NRS) scores of unpleasantness, tiredness, tension, soreness and stiffness were obtained. Pressure pain sensitivity (PPS), pain drawing areas and McGill Pain Questionnaire (MPQ) scores were also assessed. Repeated measures ANOVA and McNemar’s test were used for analyses. Values of P < .05 were considered statistically significant.
Results: Overall, subcutaneous injections (glutamate and isotonic saline) induced significantly more unpleasantness and pain than intramuscular injections. The PPS scores evoked after the glutamate injection at the surface of the bone were significantly higher than after intramuscular glutamate injection. Subcutaneous glutamate injections were more often described as “sharp” and “pinching”.
Conclusions: The subcutaneous injection was more painful and unpleasant than the intramuscular injection. The glutamate injection at the surface of the bone sensitized the deep pain tissues to pressure stimulation. Clinically, it may be difficult to differentiate between the source or site of pain originating from the masseter region but the specific quality and word descriptors could assist in the differential diagnosis.