Schizophrenia is a mental disorder characterized by delusion, hallucination, formal thought disorder, disorganized or catatonic behavior, negative symptoms (e.g. emotional blunting, decreased initiative, impoverished speech etc.) and cognitive dysfunction. Though not described as diagnostic criteria, cognitive dysfunction is the strongest determinant of functional recovery as it has a gradual deteriorating effect on social and occupational functioning. It precedes coincides and outlasts positive symptoms.
Among different cognitive abilities it affects attention, memory, processing speed, social cognition and executive function most. Recent studies are showing schizophrenia affects those who have compromised cognitive function or low IQ. Moreover different brain imaging techniques have shown structural and functional abnormality in certain areas. Though schizophrenia has been regarded as a psychotic disorder, cognitive under performance precedes the clinical onset, remains throughout life, has a genetic basis and influences functional outcome.
A recent meta-analysis by Khandaker et al. has shown that low IQ increases the risk for developing schizophrenia in a dose-response fashion (effect size 0.43): every point decrease in IQ increases the risk by 3.7%. Another meta-analysis, also found low IQ to increase the risk for schizophrenia, with an effect size of about 0.5. Interestingly, this risk was already evident by age 13 years, many years prior to psychosis onset. Another important aspect is whether impairment starts before clinical illness.
One study compared the population average on childhood scholastic test performance as measured by the Iowa State tests of basic skills and educational development to 70 subjects who later went on to develop schizophrenia. These tests were administered to all children across the state of Iowa in grades 4, 8, and 11 (corresponding to the ages of 9, 13, and 16 years) assessing 5 cognitive domains. Although the (prospective) patients did not differ from the state average at ages 9 and 13 years, they underperformed significantly at age 16 years (with an effect size of around0.35), with the most pronounced deficits in language skills.
Thus, cognitive functioning related to scholastic test performance appears to decline between the ages of 13 and 16 years in the subjects who go on to develop schizophrenia. Though at present we are not in a position to explain the clinical feature as a manifestation of mere cognitive impairment, it is obvious that identifying this impairment at a prepsychotic period may help to start an early intervention.
Find more at: Schizophrenia Research and Treatment