Loss of Venous Integrity in Cerebral Small Vessel Disease
A 7-T MRI Study in Cerebral Autosomal-Dominant Arteriopathy With Subcortical Infarcts and Leukoencephalopathy (CADASIL)
Background and Purpose—Previous pathological studies in humans or in animal models have shown alterations of small arteries and veins within white matter lesions in cerebral small vessel disease. We aimed to evaluate in vivo, the integrity of the cerebral venous network using high-resolution MRI both within and outside white matter hyperintensities in cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL).
Methods—High-resolution T2*-weighted images were obtained at 7-T in 13 CADASIL patients with no or only mild symptoms and 13 age- and sex-matched controls. Macroscopic veins were automatically counted in the centrum semiovale and compared between patients and controls. In addition, T2* was compared between groups in the normal-appearing white matter.
Results—Vein density was found lower in CADASIL patients compared with that in controls (−14.6% in patients, P<0.001). This was detected both within and outside white matter hyperintensities. Mean T2*, that is presumably inversely related to the venous density, was also found increased in normal-appearing white matter of patients (+7.2%, P=0.006). All results were independent from the extent of white matter hyperintensities.
Conclusions—A significant reduction in the number of visible veins was observed in the centrum semiovale of CADASIL patients both within and outside white matter hyperintensities, together with an increase of T2* in the normal-appearing white matter. Additional studies are needed to decipher the exact implication of such vasculature changes in the appearance of white matter lesions.
Cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a rare hereditary small vessel disease of the brain caused by mutations of the NOTCH3 gene.1 Animal models and pathological studies have shown that both small arteries and small veins are altered within white matter lesions in cerebral small vessel disease.2,3 At ultrahigh field, susceptibility effects cause blooming of the signal void secondary to the presence of deoxygenated blood in veins, so that veins with diameter as small as a few tens of micrometers, far inferior to the voxel size, can actually be detected in vivo.
In the present study, we aimed to investigate whether alterations of the venous vascular network can be detected in CADASIL patients at an early stage of the disorder using 7-T high-resolution MRI. Both venous-related signal changes at the macroscopic level and those possibly related to microvascular changes at lower scale (through T2* measurement) were assessed in CADASIL patients with no or only mild symptomatology compared with age- and sex-matched healthy controls.
Materials and Methods
Patients included were not demented (preserved global cognitive abilities and Mini-Mental State Examination >24) and not disabled (defined by modified Rankin scale ≤1). In the present study, 13 patients with high-quality 7-T MRI of the whole brain were included for analyses, by comparison with 13 age- and sex-matched healthy controls. A local ethics committee validated the protocol, and all subjects gave their written consent for participating in the study.
MRI Protocol and Image Processing
Subjects underwent both 3 and 7-T MRI evaluations on the same day, comprising 3-dimensional (3D) T1-weighted images at 3 T with 1-mm isotropic resolution and 2D T2* acquisitions with 0.7-mm isotropic resolution covering the whole hemispheres at 7 T. A specific postprocessing pipeline was used to obtain a 3D whole brain high-resolution T2* image from different 2D blocks obtained at 7 T.4 Details of the MRI protocol and image processing are included in the online-only Data Supplement.
An axial block of interest with 6-mm thickness was defined on the Montreal Neurological Institute template in the centrum semiovale, parallel to anterior commissure–posterior commissure plane with midplane tangent to the corpus callosum (Figure I, in the online-only Data Supplement). Minimum intensity projection along the direction perpendicular to the block was derived for each subject in its native space resulting in a single 2D image where macroscopic veins appear hypointense because of local susceptibility effects caused by deoxyhemoglobin. Given the length L and width W of the brain on that image, the middle of a 60-mm segment was positioned at L/2 and W/3 or −W/3 for both hemispheres. Gray levels along both segments were then extracted, from which vein density within or outside white matter hyperintensities (WMH) was estimated.
To evaluate differences of T2* between patients and controls in normal-appearing white matter (NAWM), 4 regions of interest (ROI) were manually defined on T2*-reconstructed volumes in the NAWM of each patient. Each patient was matched to a control subject (same sex, age as close as possible [mean absolute difference, 2.8 years]). Mean signal and standard deviation were computed to estimate the mean T2* in each ROI.
Statistical analyses were made using the R software (http://www.r-project.org/). For categorical variables, χ2 tests were used. As samples were limited, the Wilcoxon rank-sum test was used to compare subject’s characteristics between groups. For the ROI analysis, a pairwise Wilcoxon test was used as each ROI in each patient corresponded to an ROI in a control subject. For the vein density results, ANOVA models adjusted for age, sex, and brain length L were used.
Characteristics of the 13 patients and 13 controls included in the present study are presented in the Table. The 2 groups did not differ in terms of age, sex, or Mini-Mental State Examination.
At the macroscopic level, fewer veins were detected in CADASIL patients compared with control subjects (0.176 versus 0.206 veins/mm [−14.6%], P<0.001; Table). This reduction was visually obvious on minimum intensity projection images (Figure). The effect was stronger within WMH (mean vein density in patients, 0.137 veins/mm). Outside WMH, vein density was still inferior compared with controls (0.189 versus 0.206 veins/mm [−8.3%], P<0.04; Table). Vein density was not correlated with age (r=0.05, P=0.85 in patients; r=−0.008, P=0.98 in controls). No significant correlation was detected between vein density and WMH volume (r=−0.22, P=0.46) or between mean T2* and WMH volume (r=0.15, P=0.63).
A significant T2* increase was observed in the NAWM of CADASIL patients compared with age- and sex-matched controls (29.9 versus 27.9 ms, +7.2%, P=0.006; Table). When each ROI was compared 2-by-2, mean T2* was found higher in CADASIL patient than in controls in 76.9% of regions. No significant correlation was found between vein density and mean T2* in NAWM (r=−0.32; P=0.11).
In the present study, we observed a significant reduction in the number of small visible veins detected using 7-T MRI within the centrum semiovale of CADASIL patients compared with age- and sex-matched controls. Although venous density was dramatically reduced within WMH, it was also found significantly decreased in NAWM. In addition, T2* was increased in the NAWM of patients, a finding that may be related to the reduction of vascular density although other contributing factors cannot be excluded.5 Altogether, these results suggest that the loss of integrity in the global microvasculature may precede the appearance of WMH, but the cross-sectional nature of our study did not allow confirming this hypothesis.
In this study, we did not assess the relationships between the reduction of venous structure and clinical data, given the limited sample of patients with no or only mild symptoms. Small sample size and the lack of pathological data are the main limitations. However, the present results were highly significant and visually obvious.
Finally, using 7-T in vivo MRI, these data support a reduction of the density of visible white matter venous vasculature in CADASIL, both within and outside WMH and at the early stage of clinical manifestations. Further investigations are needed to understand whether these alterations reflect causal changes in the development of white matter lesions in this disorder.
Sources of Funding
This work was funded by a Network of European Funding for Neuroscience Research grant (01EW1207) under the Seventh Framework Programme and the European Research Area Net, with the support of the French-cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy association, the PLANIOL foundation, the NRJ foundation, and the Leducq foundation.
The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.114.005726/-/DC1.
- Received April 4, 2014.
- Revision received April 25, 2014.
- Accepted April 28, 2014.
- © 2014 American Heart Association, Inc.