Effect of pinching-evoked pain on jaw-stretch reflexes and exteroceptive suppression periods in healthy subjects

Objective To investigate the influence of conditioning cutaneous nociceptive inputs by a new “pinch” model on the jaw-stretch reflex and the exteroceptive suppression periods (ES1 and ES2) in jaw muscles. Methods The jaw-stretch reflex was evoked with the use of a custom-made muscle stretcher and electrical stimuli were used to evoke an early and late exteroceptive suppression period (ES1 and ES2) in the jaw-closing muscles. Electromyographic (EMG) activity was recorded bilaterally from the masseter and temporalis muscles. These brainstem reflexes were recorded in 19 healthy men (28.8 ± 1.1 years) during three different conditions: one painful clip applied to the earlobe; one painful clip applied to the nostril, and four painful clips applied simultaneously to the earlobe, nostril, eyebrow, and lower lip. Pain intensity induced by the application of the clips was scored continuously by the subjects on a 100 mm visual analogue scale (VAS). Results The highest VAS pain scores were evoked by placement of four clips (79 ± 0.5 mm). There was no significant modulation of the jaw-stretch reflex (ANOVAs: P = 0.929), the ES1 (P = 0.298) or ES2 (P = 0.082) in any of the three painful conditions. Conclusions Intense and tonic cutaneous pain could be elicited by this new “pinch” pain model; however, there was no significant modulation on either excitatory or inhibitory brainstem reflex responses. Significance The novel observation that high-intensity pinch stimuli applied to the craniofacial region fail to modulate two different brainstem reflexes is in contrast to other experimental pain studies documented facilitation of the jaw-stretch reflexes or inhibition of exteroceptive suppression periods. The clinical implication of the present findings is that only some craniofacial pain conditions could be expected to show perturbation of the brainstem reflex responses.