Findings from a small study suggest that increased mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) in the corpus callosum may be associated with cognitive impairment in patients with Parkinson disease (PD). Findings from the study were published in Neurology.
Patients with PD (n=75) and healthy controls (n=24) who had both diffusion tensor imaging (DTI) and structural magnetic resonance imaging (MRI) brain scans in a prospective study of clinical and neuroimaging markers of PD cognitive impairment were included in the study. The researchers characterized patients as having normal cognition (n=23), mild cognitive impairment (n=35), or dementia (PDD) (n=17), according to the Movement Disorder Society criteria.
In addition, DTI scalar values such as fractional anisotropy (FA), MD, AD, and RD were established for the cortical projections of “5 callosal segments on a midsagittal plane, single slice using a topographically derived parcellation method.” The 5 segments included segment 1 (most anterior), prefrontal cortex; segment 2, premotor and supplementary motor cortex; segment 3, motor cortex; segment 4, sensory cortex; and segment 5 (most posterior), parietal, temporal, and occipital cortex. To examine the associations between cognitive domain z-scores and callosal segment DTI values, the researchers performed linear regression analyses.
Compared with healthy controls, patients with PD had increased AD values in the anterior 3 callosal segments (segment 1: P =.033; segment 2: P =.006; segment 3: P =.002). In group pairwise comparison analyses, patients with PDD had increased MD, AD, and RD values vs patients with PD and normal cognition in segment 1 (all P <.0005) and segment 2 (P =.030, P =.033, and P =.035, respectively). In addition, patients with PDD had increased MD, AD, and RD values vs patients with PD and mild cognitive impairment in segment 1 (P =.014, P =.018, and P =.020, respectively). There was no difference between patients with PD, healthy controls, and PD cognitive groups with regard to FA values.
FA values for the callosal segments correlated with attention/working memory, executive function, and language domain z-scores in segment 1 (P <.0005 for all, except for attention/working memory, P =.004), as well as memory and visuospatial function domain z-scores in segment 5 (P <.0005). There were significant associations between MD values and attention/working memory domain z-scores in callosal segments 1, 4, and 5; memory, executive function, and language domain z-scores in segment 1; and visuospatial function domain z-scores in segments 1 and 4 (all P <.0005). In addition, AD values had a significant association with attention/working memory domain z-scores in callosal segments 1, 4, and 5, as well as memory, executive function, language, and visuospatial function domain z-scores in segments 1 and 4 (all P <.0005).
Study limitations included the small sample size and the reliance on the midsagittal plane, which offers a single view of the corpus callosum.
“These findings suggest that there are abnormalities in the callosal white matter that may be specifically linked to the presence of cognitive impairment in PD,” the researchers wrote. “Our findings uphold classic brain-behavior relationships including the anterior-posterior dissociations that are particularly relevant in understanding cognitive deficits in PD.”
Bledsoe IO, Stebbins GT, Merkitch D, Goldman JG. White matter abnormalities in the corpus callosum with cognitive impairment in Parkinson disease. Neurology. 2018;91(24):e2244-e2255.