In this study, we found that the severity of both PWMHs and DWMHs was correlated with the DMV score. In addition, quantitative analysis showed that the WMH volume was also correlated with the DMV score. Therefore, we believe the DMV score is correlated with the severity of WMHs, and DMV dysfunction may be involved in the occurrence and progression of WMHs.
SWI is an MRI imaging technology that is particularly sensitive to paramagnetic materials. Venous blood in humans contains a high concentration of paramagnetic deoxyhemoglobin, so veins can be clearly displayed on SWI images. Veins with smooth walls, unobstructed drainage and a high flow rate can be displayed clearly and continuously on SWI images; otherwise, the signals are discontinuous or even disappear. Therefore, SWI is considered a reliable indicator of venous hemodynamic changes[14]. The finding in the present study that DMV dysfunction was closely related to WMH burden might be explained as follows.
Histopathology studies indicate that collagen deposition in the DMVs may be the cause of venous stenosis and occlusion. Periventricular venous collagenosis (PVC) is a degenerative disease. With vascular high-risk factors such as aging and hypertension, collagen is gradually deposited in the vascular wall in a concentric circle, leading to stenosis or even occlusion of the vascular lumen[10]. Moody et al. found abnormal tortuosity of small veins in white matter lesions and collagen deposition in the venous wall on pathology examinations [15], and they inferred that venous collagen disease might be the cause of WMHs. Other authors have agreed that PVC plays an important role in the pathogenesis of white matter injury [6, 16–17]. In this situation, DMV signal interruption or even disappearance is expected on SWI images, with corresponding increases in the DMV score.
The changes in venous hemodynamics may also contribute to WMHs. Normal cerebral blood supply is stable and pulseless, and this regulation depends on the coordination of cerebrospinal fluid and the intracerebral venous system[18]. However, vascular regulation declines with increasing age, which causes increased blood flow fluctuation. This long-term fluctuation gradually increases the pressure in the DMVs, resulting in collagen deposition in the venous wall[19]. The long-term increase in venous pressure can also lead to disordered venous reflux with a subsequent decrease in cerebral blood flow and eventually chronic hypoperfusion injury of the white matter[20–21].Moreover, increasing venous pressure leads to increased vascular permeability, which causes blood components to leak into the extravascular white matter, aggravating white matter injury[22–23]. Excessive extracellular fluid accumulation will also lead to abnormal remodeling of microveins[24], thus increasing the pressure of the venous circulation and further aggravating white matter injury. These changes in venous hemodynamics contribute to inadequate blood supply in a narrowing DMV and to relatively low oxygen extraction, which presents as decreased visibility of the DMV on imaging.
Therefore, changes related to both venous collagen disease-related DMV stenosis and occlusion, and venous hemodynamic changes that increase venous pressure and block reflux ultimately result in insufficient oxygen extraction, leading to higher DMV scores, which verifies the hypothesis that the DMV score reflects the pathogenesis of WMHs and can be correlated with the severity of the WMH burden.
We found very few cases of low DMV scores with large WMH volumes in our study. This also reminds us that WMH may be caused not only by venous dysfunction, but also by reduced cerebral blood flow, infiltration of inflammatory factors and vascular permeability changes. Therefore, a multi-aspect comprehensive analysis can better elucidate the pathogenesis of WMHs.
There are several limitations in our study. Firstly, the patients in our study only came from one center. In the future, we will select a larger number of patients from multiple centers for research. Additionally, perfusion images were lacking, and would be useful to clarify the association between WMH perfusion and DMV dysfunction. Thirdly, we did not have long-term follow-up and could not understand the dynamic changes of DMV.
In conclusion, the DMV score was highly correlated with the severity of WMHs, and DMV dysfunction may be involved in the occurrence and progression of WMHs, providing a new direction for the prevention and treatment of WMH-induced diseases.