A systematic review of abnormalities in intracortical myelin across psychiatric illnesses

Background Brain imaging studies have thoroughly investigated brain gray matter abnormalities to assess pathophysiological mechanisms underlying psychiatric illnesses, however white matter has only recently been investigated. Abnormalities in myelination and white matter structures found in the cerebral cortex, known as intracortical myelin (ICM), have been linked with psychiatric illnesses including bipolar disorder (BD) and schizophrenia (SCZ). Here, we provide a comprehensive review of findings that investigate the nature of ICM abnormalities in psychiatric illnesses from neuroimaging studies. Methods This systematic search collected studies that evaluated ICM abnormalities using gray/white matter contrast, cortical magnetization transfer ratios or thickness measurements in SCZ, BD, major depressive disorder (MDD) and obsessive compulsive disorder (OCD). Results 20 studies were included. Evidence suggests that ICM abnormalities in the frontal lobe are common to all studied psychiatric illnesses. Prominent deficits were also identified across the gyri and insular regions in SCZ; and temporal, parietal and occipital cortices in both BD and MDD. In contrast, increases in ICM were identified across the parietal and temporal cortices in SCZ, and parietal cortex in OCD. Limitations This review exclusively used published, peer-reviewed articles which may overlook other available literature. Few studies across each psychiatric illness with non-standardized protocols may explain discrepancies in findings and limit a meta-analysis from being performed Conclusions Overall, studies report that selective ICM abnormalities with prominent changes in the frontal cortices are associated with the aforementioned psychiatric illnesses. Further studies are required to elucidate how ICM alterations may be underpinning symptomatology including cognitive difficulty, emotional dysregulation, and memory impairment.