Bateman Grant Alexander, Bateman Alexander Robert
Department of Medical Imaging, John Hunter Hospital, Locked Bag 1 Newcastle Region Mail Center, Newcastle, NSW, 2310, Australia.
School of Medicine and Public Health, College of Health, Medicine and Wellbeing, Newcastle University, Callaghan Campus, Newcastle, NSW, Australia.
Fluids Barriers CNS. 2025 Mar 19;22(1):29. doi: 10.1186/s12987-025-00640-1.
In a recent study of normal pressure hydrocephalus published by Ohmura et al., there was a progressive reduction in the cerebral blood volume within the cortex, as measured by near-infrared spectroscopy, following an increase in the intracranial pressure from an infusion study. The authors contend that this reduction in blood volume occurred due to the collapse of the venous structures, starting from the smallest veins adjacent to the capillaries and involving the entire venous outflow tract. We wish to outline some problems with this interpretation.
It has been previously shown that venous collapse secondary to an increase in intracranial pressure always starts at the most distal point in the veins. The critical buckling pressure for a tube depends on the cube of the ratio of the wall thickness and the internal diameter. The smallest veins have ratios which are larger than the distal cortical veins, so the latter are the ones to collapse first. The collapse of the distal venous outflow cuff always leads to an increase in the transmural pressure of the veins upstream from it, leading to venous dilatation and not a reduction in venous volume. Only a simultaneous arteriolar constriction of a greater volume than the venous volume increase can account for the progressive reduction in blood volume, which occurs once the ICP is greater than the sinus outflow pressure in normal pressure hydrocephalus.
The reduction in cerebral blood volume which occurs in the cortex in normal pressure hydrocephalus cannot be due to widespread venous collapse. Therefore, there must be a large component of arteriolar constriction accompanying this disease.
在大村等人最近发表的一项关于正常压力脑积水的研究中，通过近红外光谱法测量发现，在输液研究导致颅内压升高后，皮质内的脑血容量逐渐减少。作者认为，血容量的减少是由于静脉结构的塌陷所致，从与毛细血管相邻的最小静脉开始，并累及整个静脉流出道。我们希望概述这种解释存在的一些问题。
先前已经表明，颅内压升高继发的静脉塌陷总是始于静脉的最远端。管子的临界屈曲压力取决于壁厚与内径之比的立方。最小的静脉其该比值大于远端皮质静脉，所以后者是首先塌陷的。远端静脉流出袖套的塌陷总是导致其上游静脉的跨壁压力增加，导致静脉扩张而非静脉容量减少。只有当小动脉收缩的体积大于静脉容量增加时，才能解释一旦颅内压高于正常压力脑积水中的窦流出压力时血容量的逐渐减少。
正常压力脑积水中皮质出现的脑血容量减少并非由于广泛的静脉塌陷。因此，这种疾病必然伴有大量的小动脉收缩。
