Increased cold and heat pain thresholds influence the thermal grill illusion in schizophrenia

Background Patients with schizophrenia show decreased sensitivity towards clinical and experimental painful conditions. To date, the exact underlying mechanisms are not completely understood. One method to examine central integrative processes of pain perception is the thermal grill illusion (TGI), in which interlacing cold and warm bars create the illusion of a painful sensation. Methods In 18 unmedicated patients with acute paranoid schizophrenia, cold and heat pain thresholds (CPT/HPT) as well as the perception of the TGI were examined and compared to 18 matched controls. In addition, symptom scales were obtained in order to relate pain perception to psychopathology. Results CPT and HPT were significantly increased in patients compared to controls. In the range of TGI stimuli that were perceived painful by controls, patients did not indicate painful sensations, instead the stimulus response curve of TGI pain perception was shifted towards higher stimulus intensities, i.e., greater temperature differentials between cold and warm bars. This increase was comparable to that seen in CPT and HPT. There was no association with psychopathology for any pain parameter. Conclusions CPT and HPT, as well as temperature differentials for the perception of the TGI were increased in patients with schizophrenia as compared to controls. Similar to visual illusions, in which reduced contrast sensitivity has been shown to alter the perception of illusions, the discriminatory somatosensory deficit, which is reflected in higher CPT and HPT as well as the previously reported increased warmth perception thresholds, might account for the attenuation of TGI in patients.