Reasoning styles of patients with resistant delusions: a study involving cognitive behavioural therapy

Aims: As delusions have been associated with specific reasoning anomalies and cognitive behavioural therapy (CBT) has been shown effective in modifying delusions, we aimed to assess whether changes in reasoning style mediate response to CBT in patients with delusions resistant to medication. Method: Forty patients with medication resistant delusions were referred from psychiatric outpatient clinics. Seventeen patients were accepted into a short CBT programme that targeted their delusions. Probabilistic reasoning, attributional biases and theory-of-mind (ToM) were assessed pre- and post-treatment. Outcome was measured using ratings of delusional conviction, preoccupation and distress at the beginning of each session. Results: Fourteen patients completed the programme. Delusional conviction was reduced in eleven with eight having reductions of greater than 50%. Younger age, shorter length of illness and better insight predicted outcome. Reasoning biases were present and despite some inconsistent evidence of improvement in verbal ToM tasks with CBT, they did not predict outcome and were largely unchanged by CBT. Conclusion: CBT is helpful in the treatment of resistant delusions. Abnormal reasoning styles in patients with resistant delusions mark a vulnerability that is largely unchanged by CBT.