Premorbid deficits

The majority of patients with schizophrenia have cognitive impairment.(4) This is not entirely due to 'cognitive decline', but to some extent a lower than average IQ detectable premorbidly. Part of the evidence for this comes from two large population-based cohort studies in the United Kingdom (5,6) and a Swedish conscript cohort.(Z) In these studies individuals were given intelligence tests during childhood or adolescence, at which time, none was manifesting signs of schizophrenia. After several years, some of the cohort developed schizophrenia and, as a group, when compared to the remainder, they showed a lower than average IQ (by the equivalent of about 5-10 points). Similarly, when school reports of children who later developed schizophrenia are compared with their classmates or siblings, or best of all, identical twins, the results show the same trend.(8) These premorbid differences on IQ tests are, by definition, generalized although language and planning deficits may be slightly more obvious.

Getting an accurate picture of premorbid deficits in routine clinical practice is difficult unless school reports over many years have been retained. However, a rough idea can be gained simply by establishing the level of education achieved (if clearly within the premorbid period) and contrasting this to family expectations and social norms. Deviation from an expected level or in comparison to siblings can provide additional comparisons. Tests of reading ability such as the National Adult Reading Test,(9) have been shown to be relatively immune from acquired cognitive decline so may serve as a proxy for premorbid ability.

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