Delayed ipsilateral intraparenchymal haemorrhage is a recently recognised complication after endovascular flow diversion for intracranial aneurysms. Although the mechanism of this phenomenon is not understood, one proposed explanation (the windkessel hypothesis) is that removal of aneurysmal compliance increases distal pulse pressure.

We present a case of delayed haemorrhage after placement of a Pipeline stent, discuss the proposed mechanisms, and describe a novel electrical analogue model that was used to evaluate the likely haemodynamic effect of stent placement.

Model-based analysis suggests that stenting is not likely to produce a significant change in distal pulse pressure. Moreover, basic fluid dynamics principles suggest that a local reduction in disturbed flow in the region of the aneurysm could produce only a minor increase in distal pressure (a few mmHg), which is unlikely to be the main cause of the observed haemorrhage.

The windkessel hypothesis is unlikely to explain the occurrence of delayed ipsilateral intraparenchymal haemorrhage after flow diversion; however, other mechanisms involving altered haemodynamics distal to the treated aneurysm may play a role. Further studies involving the assessment of haemodynamic changes after flow diversion would be useful to understand, and eventually mitigate, this currently unpredictable risk.