Structural brain correlates of IQ changes in bipolar disorder

Background. There is increasing evidence that cognitive deficits are present in bipolar disorder(BP), but their neural correlates have not been fully explored. The aim of this study is to correlatestructural brain abnormalities with cognitive performance in BP and to explore differences betweenclinical subtypes.Method. Thirty-six BP patients (13 men, 23 women) with a mean age of 39 years (range 21–63years) underwent neuropsychological testing and imaging. Twenty-five patients had bipolar disorderI (BP I) and 11 had bipolar disorder II (BP II). Patients with co-morbid psychiatric diagnosis,drug and alcohol abuse or systemic illness were excluded. Correlations between cognitive performanceand structural brain changes were explored using high-resolution anatomical imaging andmagnetization transfer imaging (MTI).Results. In the whole BP group the difference between estimated pre-morbid IQ and current IQ wassignificantly correlated with left-sided reduction of the magnetization transfer ratio (MTR) in thesuperior temporal gyrus, uncus and para-hippocampal gyrus. In BP II patients the areas wherethese correlations were significant extended to the right superior and middle temporal gyri, cingulategyrus, pre-cuneus and adjacent frontal and parietal white matter. The volume of superiortemporal white matter was also correlated with IQ difference in this subgroup.Conclusions. The study highlights the association between fronto-temporal abnormalities and declinein IQ in BP. The more extensive abnormalities present in BP II patients suggest that persistentdepression, rather than mania, may be a key pathophysiological factor or that BP II represents aclinical phenotype with a higher risk of developing cognitive abnormalities.