Is the ischemia found in normal pressure hydrocephalus secondary to venous compression or arterial constriction? A comment on Ohmura et al

Abstract

Background: 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.

Main body: 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.

Conclusions: 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.

Keywords: Cerebral blood flow; Cerebral blood volume; Ischemia; Normal pressure hydrocephalus.

Fig. 1

Results of modelling. a. Depicts the normal findings. The red segment is the arterial, orange is the capillary, yellow is the veins, green is the outflow cuff and blue is the venous sinus. The vascular pressures are shown within the vessels. The blue numbers are the transmural pressures at each site. The resistances and volumes for each segment are shown below the vessel. b. Shows the findings in hypertension with the red area indicating an increase in resistance in the arteries. The perfusion pressure is increased and the CBV is reduced.c. Shows the findings in hypotension with the green area highlighting a reduction in resistance in the arteries. The perfusion pressure is reduced and the CBV is increased. d. Shows the findings in raised ICP with increased resistance in the outflow cuff and reduced elsewhere. Note the major resistance moves from the arteries to the outflow cuff and a backup of pressure dilates the veins and capillaries. The perfusion pressure is reduced and the CBV is increased. e. Shows the global findings in normal pressure hydrocephalus. It should be noted that the changes are a mixture of 1b and 1d combined, hence the CBV is normal. f. Shows the findings following shunt insertion with decreased resistance in the outflow cuff. Note the reduction in the arterial resistance as compared to NPH suggesting reversibility. The perfusion pressure is increased and the CBV is reduced