Introduction: The International Association for the Study of Pain (IAPS), in 1986, defined pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage". Thus, the few studies on this phenomenon conducted on schizophrenic patients did not result in a firm consensus; certain studies showed that such patients seemed to have a higher threshold against pain (hypoalgesia) than healthy subjects, whilst other studies showed that the threshold is the same, but the absence of expressing the pain would be due to the pathology itself (non-expression of the pain, denial). Insensitivity to pain would be the consequence of a complex reaction between a biological sensorial abnormality and the psychopathology of schizophrenia itself (including the affective processes). Hence, various hypotheses referring to biological, psychological and sociological mechanisms have been proposed.
Biological theories: Various other hypotheses based on biological factors have been suggested. One of the interesting biologically-based hypotheses postulates that the insensitivity is due to a dysregulation of N-methyl-d-aspartate (NMDA). The biological factors are still not fully explored and would only explain in part the phenomenon of the apparent insensitivity to pain of individuals with schizophrenia.
Psychological theories: The thresholds of pain and a higher level of tolerance could be explained by an indifference to external stimuli and by inappropriate mental functions for these tests. The deficit is situated, therefore, both in the sensory discrimination of the stimulus (biological function) but also in the interpretation (cognitive and emotional functions). These different hypotheses (biological and psychological) might explain the insensitivity to pain of schizophrenic patients.
Pain and schizophrenia: THE REALITY: Schizophrenic patients have a sensitivity to pain which is identical to that of healthy subjects. The apparent analgesia would be the result of a denial "attitude", a different manner of expressing pain in relation with the non-verbal communication difficulties, and not an alteration in the brain functions nor a biological anomaly. Diverse methodological biases arise from the studies of pain in patients with schizophrenia.