Amygdala Volume and Verbal Memory Performance in Schizophrenia and Bipolar Disorder

Do euthymic bipolar patients have normal cognitive functioning?

PURPOSE OF REVIEW

Research on the neurocognitive functions of bipolar patients has yielded inconsistent results over recent years. There is a growing need for clarification regarding the magnitude, clinical relevance and confounding variables of cognitive impairment in bipolar patients.

RECENT FINDINGS

Current findings of studies investigating executive functions, psychomotor speed and memory functions suggest heterogeneous cognitive functioning in patients. A significant amount of variance can be attributed to treatment factors or interactions of those factors with the course of illness and individual characteristics. Furthermore, cognitive domains are presumably inter-related. The impact of bipolar illness on cognition can be influenced by age of onset, pharmaceutical treatment approaches, individual response, familial risk factors, and clinical features. Although brain activation patterns appear to be altered, these alternations do not necessarily correlate with impairment in cognitive performance. Without carefully controlling for confounding variables, the actual effect of bipolar disorder on cognitive performance scores cannot be evaluated.

SUMMARY

Cognitive deficits of clinical relevance are documented for a substantial proportion, but not the majority, of bipolar patients. Yet, available data are inconclusive with respect to the origin of these deficits. Future studies on cognitive deficits in bipolar patients need to deliver detailed descriptions of drug treatment and clinical features.

Neuroimaging and neuropathological findings in bipolar disorder

Bipolar disorder (BPD) is increasingly recognized as a neuropathological disorder characterized by reductions in grey matter (GM) volume, as measured by magnetic resonance imaging (MRI) and neuronal and postmortem glial cell changes. Here, we use an anatomical framework to discuss the neurobiology of BPD, focusing on individual components of the "visceromotor network" that regulates bodily homeostasis along with neurophysiological and neuroendocrine responses to stress. MRI-defined reductions in GM volume, combined with neuronal changes, are observed in the perigenual anterior cingulate cortex (ACC) of individuals with BPD, while postmortem glial cell loss is also a characteristic of Brodmann's Area 9. Both postmortem neuronal loss and reduced GM volume have been reported in the amygdala and hippocampus. These structural changes to components of the visceromotor network are associated with increased regional cerebral blood flow (rCBF) or blood oxygenated level-dependent (BOLD) activity in response to affective or rewarding stimuli, raising the possibility that the BPD-associated structural changes are secondary to a glutamate-driven excitotoxic process.