Pattern of cognitive deficits
There is growing awareness that cognitive deficits in schizophrenia represent a core feature of the disorder and cannot simply be dismissed as secondary consequences of psychotic symptoms (Breier 1999). The degree of cognitive impairment is greater in child and adolescent onset than in adult onset patients. A consistent finding in child and adolescent onset patients is a mean IQ of between 80 and 85 (1 SD below the population mean), with about one-third of cases having an IQ below 70 (Jacobsen & Rapoport 1998; Hollis 1999). This represents a mean IQ score about 10 points lower than the mean IQ in adult schizophrenia. These findings raise several important questions.
First, are the cognitive deficits specific or generalized: are some aspects of cognitive functioning affected more than others?
Secondly, which deficits precede the onset of psychosis and could be causal, and which are consequences of psychosis?
Thirdly, is the pattern of deficits specific to schizophrenia or shared with other developmental and psychotic disorders?
Fourthly, are cognitive impairments progressive or static after the onset of psychosis?
Recent research (R. Asarnow et al. 1994, 1995) suggests that children with schizophrenia have specific difficulties with cognitive tasks that make demands on short-term working memory and selective and sustained attention and speed of processing.
These deficits are similar to the deficits reported in adult schizophrenia (Nuechterlain & Dawson 1984; Saykin et al. 1994).
Deficits of attention, short-term and recent long-term memory have also been reported in adolescents with schizophrenia (Friedman et al. 1996). In contrast, well-established ‘overlearned’ rote language and simple perceptual skills are unimpaired in child and adolescent onset schizophrenia. Asarnow et al. (1991, 1995) have shown that children with schizophrenia have impairments on the span of apprehension task (a target stimulus has to be identified from an array of other figures when displayed for 50 ms). Performance on the task deteriorates markedly when increasing demands are made on information processing capacity (e.g. increasing the number of letters in the display from three to 10). Furthermore, event-related potentials on the span of apprehension task in both children and adults with schizophrenia, compared with age-matched controls, show less negative endogenous activity measured between 100 and 300 ms after the stimulus. Similar findings of reduced event-related potentials have been found during the CPT in both childhood and adult onset schizophrenia (Strandburg et al. 1999). These findings indicate a deficit in the allocation of attentional resources to a stimulus (Strandburg et al. 1994; Asarnow et al. 1995). As with adults, children and adolescents with schizophrenia show high basal autonomic activity and less autonomic responsivity than controls (Gordon et al. 1994), with attenuated increases in skin conductance following the presentation of neutral sounds (Zahn et al. 1997). Childhood onset patients, like adults, show increased reaction times with a loss of ipsimodal advantage compared with healthy controls (Zahn et al. 1998). Abnormalities in smooth pursuit eye movements (SPEM) have also been found in adolescent schizophrenics (mean age 14.5), which suggests continuity with the finding of abnormal SPEM in adult schizophrenics (Iacono & Koenig 1983). Children with schizophrenia also show similar impairments to adult patients on tests of frontal lobe executive function such as the Wisconsin Card Sorting Test (WCST; R. Asarnow et al. 1994).
In summary, while basic sensorimotor skills, associative memory and simple language abilities tend to be preserved in children with schizophrenia, deficits are most marked on tasks which require focused and sustained attention, flexible switching of cognitive set, high information processing speed and suppression of prepotent responses (Asarnow et al. 1995). Similar deficits affecting attention, memory and motor skills have been found in children genetically at ‘high risk’ for schizophrenia (Erlenmeyer-Kimling et al. 2000) and non-psychotic relatives of schizophrenic probands (Park et al. 1995). This adds further weight to the argument that cognitive deficits cannot be simply dismissed as non-specific consequences of schizophrenic symptoms, but rather are likely to be indicators of underlying genetic and neurobiological risk.
Executive functions and onset of schizophrenia
A diverse array of cognitive processes has been integrated under the cognitive domain of ‘executive functions’ which are presumed to be mediated by the prefrontal cortical system. Executive function skills are necessary to generate and execute goal-directed behaviour, especially in novel situations. Goal-orientated actions require that information in the form of plans and expectations are held ‘on-line’ in working memory and flexibly changed in response to feedback. Much of social behaviour and social development would appear to depend on these capacities as they involve integration of multiple sources of information, appreciation of others’ mental states, inhibition of inappropriate prepotent responses and rapid shifting of attention.
Any cognitive theory of schizophrenia needs to explain the timing of onset which usually occurs during adolescence or early adulthood. Deficits in executive function and social cognition could be the developmental abnormality that predisposes to schizophrenia as executive function deficits impinge on social skills that usually emerge in early adolescence. This period is associated with a rapid growth in abstract analytical skills, together with the development of the sophisticated social and communication abilities that underlie successful social relationships. It is during this period of development (approximately age 8 - 15 years) that preschizophrenic social impairments become most apparent (Done et al. 1994) and there is also a relative decline in cognitive abilities in preschizophrenic subjects (Jones et al. 1994).
According to this ‘risk’ model of executive function deficit, the onset of psychosis depends on the interaction between social and cognitive capacities and the demands of the environment.
During adolescence, increasing academic and social demands may act as stressors on a ‘high-risk’ subject, pushing them over the threshold for psychosis. The greater the premorbid impairment, the earlier the age that a critical liability threshold will be passed and symptoms emerge. This model predicts that similar executive function deficits are found in non-psychotic genetically ‘high-risk’ relatives. However, executive function deficits are probably not a primary cause of schizophrenia given that they also occur in other neurodevelopmental disorders including autism (Ozonoff et al. 1991; Hughes & Russell 1993) and attention deficit hyperactivity disorder (ADHD) (Welsh et al. 1991; Pennington et al. 1993; Karatekin & Asarnow 1998).
Course of cognitive deficits
Kraepelin’s term ‘dementia praecox’ implied a progressive cognitive decline as part of the disease process. Jones et al. (1994) described how academic performance becomes progressively more deviant during adolescence in those individuals destined to develop schizophrenia in adult life. There is also some tentative evidence for a decline in IQ following the onset of psychosis in childhood onset schizophrenia. In the NIMH study (Alaghband-Rad et al. 1995), the mean postpsychotic IQ was 83.7 (SD 17.3) compared with a mean prepsychotic IQ of 87.7 (SD 25.4). Although a decline in IQ during the early phase of psychosis has been reported in adults with schizophrenia (Bilder et al. 1992), in the NIHM study the decline was in both raw and scaled IQ scores and continued for up to 24 - 48 months after onset (Jacobsen & Rapoport 1998). There was no evidence for a decline in postpsychotic IQ raw scores repeated after 2 years, although scaled (age-adjusted) IQ scores did still decline (Bedwell et al. 1999). Russell et al. (1997) found a small non-significant IQ decline, of only 2 - 3 points, in a 20-year longitudinal followup study of IQ in schizophrenia (about one-third of these cases had first onset of psychosis in adolescence).
In summary, when raw scores, rather than scaled scores, are analysed, there is little evidence for an absolute loss in cognitive ability in the early postpsychotic phase of schizophrenia.
If a true decline does occur it is during, or before, the transition to psychosis. The small drop in IQ after the onset of psychosis could possibly be caused by the effect of psychotic symptoms on performance. Overall, the evidence points more to a premature arrest, or slowing, of normal cognitive development in child and adolescent onset schizophrenia rather than to a dementia.
Chris Hollis is Professor of Child and Adolescent Psychiatry and Head of the Developmental Psychiatry Section, University of Nottingham (University Hospital, Queen's Medical Centre, Nottingham NG7 2UH). His research interests include the developmental psychopathology of early psychoses, attention-deficit hyperactivity disorder and developmental language and communication disorders.
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