Reduced response to the thermal grill illusion in chronic pain patients

Sensory illusions may reveal fundamental features of the nervous system. The thermal grill illusion is such a pain illusion, where interlaced warm and cool temperature bars (thermal grill) produce a paradoxical burning sensation. Previous studies have only systematically investigated the thermal grill illusion in pain-free volunteers. The objective of this study was to investigate whether the response to the thermal grill illusion was tolerable in patients with chronic pain and whether the response differed between patients with chronic pain and pain-free volunteers.

Sixteen pain-free participants and 18 chronic pain patients (seven not receiving opioids and 11 receiving opioids).

The thermal grill response was investigated using a custom-built thermal grill. Heat and cold pain thresholds were also determined.

Chronic pain patients reported less intense pain, heat, and unpleasantness to the thermal grill compared with pain-free participants; in particular, there was an overall main effect for significantly less heat from the thermal grill compared with pain-free participants (P = 0.016). At the 22/38°C combination, although the majority of pain-free participants experienced the illusion to some degree, the majority of pain patients in both groups did not (median pain score 0). Although perceived heat from the thermal grill was significantly lower in chronic pain patients, both heat and cold pain thresholds did not differ among the three populations.

This preliminary data suggest that the thermal grill response may provide insights into pain sensitivity that are not detected by conventional thermal quantitative sensory testing.