Should cognitive impairment be included in the diagnostic criteria for schizophrenia?

Neurocognitive impairment is considered a core component of schizophrenia, and is increasingly under investigation as a potential treatment target. On average, cognitive impairment is severe to moderately severe compared to healthy controls, and almost all patients with schizophrenia demonstrate cognitive decrements compared to their expected level if they had not developed the illness. Compared to patients with affective disorders, cognitive impairment in schizophrenia appears earlier, is more severe, and is more independent of clinical symptoms. Although the DSM-IV-TR and ICD-10 descriptions of schizophrenia include several references to cognitive impairment, neither the diagnostic criteria nor the subtypology of schizophrenia include a requirement of cognitive impairment. This paper forwards for consideration a proposal that the diagnostic criteria include a specific criterion of "a level of cognitive functioning suggesting a consistent severe impairment and/or a significant decline from premorbid levels considering the patient's educational, familial, and socioeconomic background". The inclusion of this criterion may increase the "point of rarity" with affective psychoses and may increase clinicians' awareness of cognitive impairment, potentially leading to more accurate prognosis, better treatment outcomes, and a clearer diagnostic signal for genetic and biological studies. Future research will need to address the validity of these possibilities. The reliable determination of cognitive impairment as part of a standard diagnostic evaluation will present challenges to diagnosticians with limited resources or insufficient expertise. Cognitive assessment methods for clinicians, including brief assessments and interview-based assessments, are discussed. Given the current emphasis on the development of cognitive treatments, the evaluation of cognition in schizophrenia is an essential component of mental health education.