The hemodynamic complexities underlying transient ischemic attacks in early-stage Moyamoya disease: an exploratory CFD study.

Moyamoya disease (MMD) is a rare cerebro-occlusive disease with unknown etiology that can cause both ischemic and hemorrhagic stroke. MMD is characterized by progressive stenosis of the terminal internal carotid artery (ICA) and development of basal brain collaterals. Early-stage MMD is known to cause hemodynamic insufficiency despite mild or moderate stenosis of the intracranial arteries, but the exact mechanism underlying this pathophysiological condition is undetermined. We used high-resolution Large Eddy Simulations to investigate multiple complex hemodynamic phenomena that led to cerebral ischemia in five patients with early-stage MMD. The effects of transitional flow, coherent flow structures and blood shear-thinning properties through regions of tortuous and stenosed arteries were explored and linked to symptomatology. It is evidently shown that in some cases complex vortex structures, such as Rankine-type vortices, redirects blood flow away from some arteries causing significant reduction in blood flow. Moreover, partial blood hammer (PBH) phenomenon was detected in some cases and led to significant hemodynamic insufficiency. PBH events were attributed to the interaction between shear-thinning properties, transitional flow structures and loss of upstream pressure-velocity phase lag. We clearly show that the hemodynamic complexities in early-stage MMD could induce ischemia and explain the non-responsiveness to antiplatelet therapy.