Neuroanatomy of comorbid schizophrenia and learning disability: a controlled study.

BACKGROUND: Reasons for the higher frequency of schizophrenia in learning-disabled populations are uncertain. We investigated the neuroanatomical basis for this phenomenon by structural magnetic resonance imaging (MRI) in patients with learning disability and schizophrenia, learning-disabled patients, and patients with schizophrenia.
METHODS: Age-matched and sex-matched patients with learning disability (20 cases), schizophrenia (25), and both disorders (23) underwent MRI scans of the brain. Whole brain areas and specific regions of interest were examined. 29 normal controls were also scanned.
FINDINGS: The scans of the group with both disorders were closely similar to those of the schizophrenic group, in terms of both general structures and the structure of the amygdala-hippocampus. However, the amygdala-hippocampus was significantly smaller on both sides than that of normal controls (left 4.1 vs 4.5 cm3, p=0.011; right 4.2 vs 4.99 cm3, p<0.0001). The brains of learning-disabled patients were generally smaller than those of the other three groups, but the amygdalohippocampal complexes were larger.
INTERPRETATION: In terms of brain structure, patients with comorbid learning disability and schizophrenia resemble patients with schizophrenia and not those with learning disability. We suggest that the higher frequency of schizophrenia in learning-disabled patients is due to a greater tendency of schizophrenic patients to develop cognitive deficits, and that within the learning-disabled population there may be individuals whose deficits result from undiagnosed schizophrenia.