Hypoalgesia in schizophrenia is independent of antipsychotic drugs: A systematic quantitative review of experimental studies (Potvin & Marchand, 2008)

Potvin, S., & Marchand, S. (2008). Hypoalgesia in schizophrenia is independent of antipsychotic drugs: A systematic quantitative review of experimental studies. Pain, 138(1), 70-78.

This meta-analysis study containing 12 studies aims to investigate whether patients of schizophrenia have lower pain response when comparing to the normal control groups. In addition, the authors also attempts to examine the impacts of confounding factors on the results of experimental studies; these factors include the nature of pain stimuli, the use of antipsychotic medications, the nature of the psychiatric condition (e.g., process or acute type) and the validity of diagnosis.

The studies about hypoalgelsia fall into four different groups:

• Clinical case reports
  
• Population studies examining the prevalence of pain in patients with schizophrenia
  
• Population studies examining the prevalence of schizophrenia in patients with chronic pain
  
• Experimental studies
  

While mixed findings are found in existing literature about pain perception, there is also a lack of satisfactory explanation for the observations of pain insensitivity in schizophrenia.

The author provides the following as an explanation for the inconclusive findings: "(T)he heterogeneity of patients and the heterogeneity of experimental conditions (, which) may account for heterogeneity of results.

I absolutely agree with this comment because, personally, I have been diagnosed as having delusion disorder, bipolar disorder, schizophrenia, schizoaffective disorder, depression disorder etc. In addition, the diagnosis of schizophrenia has this garbage bag nature—all else don't fit too well.. let's label it schizophrenia… Moreover, what is the nature of pain and how is pain measured? If the notion of pain insensitivity holds true, is it true for all types of pain (e.g., electric shock, thermal stimuli) or is it truer for certain types of stimuli over the others? For instance, although I seem to have fairly high tolerance or threshold for electrical stimuli, I find myself having very low tolerance for heat or thermal stimuli. In addition, taking a multi-dimensional view of pain and different measures might be applied to assess pain of different dimensions (or even same dimensions)… will the results be consistent and why would results on different dimensions have to be consistent?

The studies retained in the analyses of this study have the following characteristics:

• It contained a group of schizophrenia patients
  
• The study contained a control group of healthy people
  
• It was an experimental study where stimuli were given and pain was measured
  
• The study was published between 1952 and 2007
  

The statistics

The reported mean and standard deviation from each study were used to calculate effect size estimate- Hedges' g, which also adjust for sample size, for the schizophrenia patients and the healthy control groups using D-STAT. In this analysis, positive effect size indicates diminished pain response in schizophrenia patients.

The authors also aggregated effect size estimates for each pain measure to generate a mean effect size estimate for each study, which, then, were pooled together to produce a composite effect size estimates. Additional mean effect size estimates were also calculated (see table).

In addition, this study did not find gender and age differences.

The conclusions

Results of this study provide supporting evidence for the notion that schizophrenia patients might have diminished pain responses when compared to health people.

Since analgesic effects were found both in patients medicated with antipsychotic drugs and those not taking medication, such finding casts doubts on the notion that analgesia is a mere artifacts of antipsychotic medications. In addition, some early report on analgesia in psychotic patients were actually published before the introduction of antipsychotic medications.

Since patients with schizophrenia have been found to have blunt response to more than pain (e.g., basic emotion), it is possible that the blunt response is not pain-specific and is more global.

The results found larger effect size when pain is induced using electric stimuli than using thermal stimuli. In other words, all pains are not equal in their effects on patients with schizophrenia.

The authors further provided a critic/recommendations on the short-comings of existing research:

• Future research should take on a multi-dimensional view about pain and use multiple pain assessments to measure "pain."
  
• More electrophysiological studies examining the spinal, autonomic and cortical components of pain are needed.
  
• More studies needed to investigate the relationship between pain perception and schizophrenia.
  
• More studies needed to examine the relationship between the endogenous pain modulation system in schizophrenia.