Group comparisons: Cross-sectional thickness
The results of the group comparisons are illustrated in and summarized in . There were no significant differences in the thickness of the CC in PHD subjects when the groups were divided by estimated time to onset. In contrast, in HD subjects, the cross-sectional thickness of the CC and of each one of the CC regions was significantly smaller as compared to controls (CC1 p < 10-6, CC2 p < 0.005, CC3 p < 0.01, CC4 p < 0.005, CC5 p< 0.001, adjusted for multiple comparisons). Dividing the group further by disease severity did not reveal significant differences between HD1 and HD2 subjects.
Summary of Diffusion Alterations in HD
Cross-sectional thickness of the CC and its sub-regions.
We evaluated the magnitude of the thinning as a percentage from measured control values (). In the group of HD subjects as a whole, CC5, the most posterior region (CC5), was thinned by 24%; CC4 by almost 21%, CC3 by 13%, CC2 by 17% and CC1 by 21%, C2 by 17%. While the magnitude of the thinning was greatest in CC5, there was no significant difference amongst the CC regions nor between the two HD groups. In PHD subjects, reductions in cross-sectional thickness were reduced by approximately 5% in CC3, CC4 and CC5 in the PHD “far” group; in the PHD “near” group, reductions were more substantial: by 8% in CC3, by 14% in CC4 and by 10% in CC5. The distribution of these results recapitulates cortical changes, which have demonstrated thinning of sensori-motor and occipital areas early in disease.
Percent Reductions in Cross-sectional thickness of CC sub-regions.
Group Comparisons: Diffusion measures
Fractional Anisotropy: Using a linear regression model which included age and sex as covariates, we found significant reductions in FA in every region of the CC in the PHD group (CC1 p < 0.95, CC3 p < 0.005, CC3 p < 0.01, CC4 p < 0.05, CC5 p < 0.01). When the group was stratified according to time to estimated onset, significant reductions in FA in the “far” to estimated onset were present only in CC2; in the PHD “near” group, significant reductions were present in CC2, CC3 and CC5 (CC2 p< 0.001, CC3p < 0.05, CC5 p < 0.01, corrected for multiple comparisons). In HD subjects, FA was significantly lower in all CC regions, (p < 0.0001, corrected for multiple comparisons). Radial Diffusivity: Radial diffusivity was significantly higher in PHD subjects in CC1, CC2, CC3 and CC5. When the group was stratified by estimated time to onset, radial diffusivity in the “far” group was increased only in regions CC2 and CC5 (p< 0.05, p < 0.001, respectively, corrected); in the “near” group, radial diffusivity was increase in regions CC2, CC3, and CC5 (p < 0.005 for all regions, corrected). Radial diffusivity was significantly increased in all CC regions in HD subjects (CC1 p < 0.0001, CC2 p < 0.005, CC3 p < 0.001, CC4 p < 10 -6, CC5 p < 0.0005). Axial Diffusivity: Axial diffusivity was not significantly different in PHD subjects, either in the group as a whole or when the group was stratified by time to estimated onset. In contrast, in HD subjects, axial diffusivity was increased in every region of the CC (CC1 p < 0.005, CC2 p < 0.05, CC3 p < 0.05, CC4 p < 0.001, CC5 p < 0.05). Results are summarized in .
Corpus Callosum DTI measures: FA, Axial Diffusivity, Radial Diffusivity
Thickness and diffusion measures were also calculated along the medial axis skeleton of the corpus callosum at each of 200 points equally spaced along the structure for a point-by-point, more fine-grained, evaluation and are shown in . Using a functional data analysis approach, we found significant thinning extending from points 80 to 165 in PHD subjects closer to expected onset; these points correspond roughly to sub-regions CC3 and CC4, regions that include projections from sensori-motor and parietal cortical regions. The cross-sectional thickness values for both HD1subjects and HD2 subjects were significantly different from controls across each of the 200 points. FA: In PHD subjects closer to expected onset, FA values along the entire skeleton were significantly reduced and overlapped with FA values of HD subjects from points 50 to 190, which included portions of CC2, CC3, CC4 and portions of CC5, consistent with the regional analyses. FA values in HD subjects were significantly reduced throughout the entire skeleton as compared to controls, more so in HD2 subjects; however, the differences in FA values were not significantly different between HD1 and HD2 subjects. Radial Diffusivity: Radial diffusivity was increased throughout the skeleton in PHD subjects, more so in PHD subjects closer to expected onset, where the curve along points 110 to 150 virtually overlapped with that of HD symptomatic subjects. Axial Diffusivity: Axial diffusivity measures were not significantly different from controls in either PHD group; however, there were significant differences in axial diffusivity in HD subjects.
To summarize, changes in diffusion measures, which capture micro-structural changes in the CC in HD were present in subjects more than a decade prior to their expected clinical onset. These alterations appeared to occur in a topologically selective, and temporally predictable, pattern that may reflects early cortical “disconnection”. The cross-sectional thickness of the CC was also reduced, but measurable changes were present only in the PHD “near” group, and only in select regions, as well as in the HD group, suggesting that diffusion changes might precede or provide a more sensitive measure of very early pathological change in HD.
Relationship between regional changes in CC and Clinical Measures
We considered whether changes in the CC FA and thickness were associated with performance on the cognitive tasks of the UHDRS. We performed linear regressions to associate performance on the Symbol Digit, Verbal Fluency and Stroop Color Word with FA and CC thickness for every sub-region for PHD and HD subjects, adjusting for age.
We found significant relationships between the Symbol Digit and the FA (p<0.0001) and radial diffusivity (p<0.0001) with the entire CC, and in the anterior and posterior regions of the CC (p<0.0001 in CC1, CC2, and CC5). We also found significant relationships between Verbal Fluency and FA and radial diffusivity of the entire CC (p<10-4), and with sub-regions CC1 (p<10-4), CC2 (p<10-3), and with CC1 (p<10-3). We found a significant relationship between Stroop Color Word and radial diffusivity only in CC5 (p<0.05). The scatterplots showing these relationships are shown in . These findings implicate disrupted inter-hemispheric information transfer in cognitive dysfunction in HD.
Scatter plots demonstrating the correlation between diffusion parameters and clinical measures
There was an association between FA and thickness in HD subjects in all CC regions (R2 values HD: CC1 0.77, p < 10 −5, CC2 0.71, p < 10 −4, CC3 0.60 p < 0.005, CC4 0.58 p < 0.005, CC5 0.55 p < 0.01). However, in PHD subjects this association was present only in CC1 and CC2 (R2 values PHD: CC1 0. 70, p < 0.0005, CC2 0.42, p < 0.05), suggesting that FA and thickness may be differentially sensitive to subtle damage and may show associations only after more significant damage is apparent. There was no relationship between normalized caudate volume or CAG repeat number and any of the diffusion measures: FA, radial or axial diffusivity.