A relative decline in verbal ability between the ages of 13 and 18 years is associated with an increased risk of schizophrenia and other psychoses in adulthood, according to findings from a Swedish longitudinal cohort study involving nearly 11,000 adolescent boys and young men.
For example, compared with the unaffected population, each increase of 1 standard deviation in verbal ability in the 10,717 study subjects was associated with an adjusted hazard ratio for schizophrenia and schizoaffective disorder of 0.59 after adjustment for urbanicity, parental educational level, and family history of psychosis, Dr. James H. MacCabe of King’s College London, and his colleagues reported online Jan. 16 in JAMA Psychiatry, formerly Archives of General Psychiatry.
Similarly, a relative decline in verbal ability between ages 13 and 18 was the strongest predictor (other than family history of psychosis) of other affective and nonaffective psychoses (hazard ratios of 0.45 and 0.67, respectively, in the fully adjusted model), the investigators found. (JAMA Psychiatry 2013 Jan. 16 [doi:10.1001/2013.jamapsychiatry.43]).
"In all diagnostic groups, a clear relative decline was observed in verbal ability, compared with the male general population between ages 13 and 18 years," they said, noting that this was a stronger predictor of later psychosis than poor verbal ability at age 18 years alone, and that verbal decline was "by far the strongest and most consistent finding."
Also, the associations between verbal decline and psychosis persisted – and in fact, were strengthened – even when cases with onset before age 25 years were excluded from the analysis, thereby ruling out the possibility that the findings were confounded by prodromal effects, the investigators said.
"This suggests that relative verbal decline in adolescence and young adulthood reflects a neurodevelopmental process that occurs at fixed age and puts the individual at an increased lifetime risk for psychosis, as opposed to a prodromal process that occurs shortly before the onset of symptoms," the investigators said.
To assess the influence of cognitive developmental trajectories during adolescence and young adulthood on the risk for later affective and nonaffective psychoses, the investigators studied four population-based cohorts of boys and young men born in Sweden in 1953, 1967, 1972, and 1977 and followed through 2006.
Standardized timed verbal, spatial, and inductive ability test scores at baseline and age 18 years were compared, and associations between premorbid cognitive functioning and the risk of schizophrenia and schizoaffective disorder, other nonaffective psychoses, bipolar disorder, and other affective psychoses were assessed.
The declines in verbal ability noted in this study are relative to the general population. They are not likely a reflection of actual deterioration in verbal ability, but probably are an indication that individuals who will develop psychosis do not progress as quickly as their peers, the investigators noted.
"The consistency of the results across all four diagnostic groups and the statistical significance in three diagnostic groups provide a strong indication that these are not chance findings," they added.
Nonetheless, replication in another sample would "add further weight," they said.
Additional large-scale longitudinal studies that examine brain structural and cognitive changes in order to clarify the potential role of reduction in gray matter density in a region of the left motor cortex that is activated by the articulation of speech also are needed, they said, explaining that recent studies have demonstrated a correlation between change in verbal IQ and changes in gray matter density in that brain region.
This study was supported by funding from numerous organizations, including the Swedish Research Council for Working Life and Social Research, the Swedish Council for Research in the Humanities and Social Sciences, and the Swedish Council for Planning and Coordination of Research. The authors reported having no conflicts of interest.