Bulk flow of cerebrospinal fluid observed in periarterial spaces is not an artifact of injection

The flow of cerebrospinal fluid (CSF) through perivascular spaces surrounding the vasculature is thought to be an important part of the brain’s mechanism for clearing metabolic waste products, including those associated with disorders such as Alzheimer’s disease. Experiments that track tracer particles injected into the cisterna magna of mouse brains have shown evidence of pulsatile CSF flow in perivascular spaces around pial (surface) arteries, with a bulk (average) flow in the same direction as the blood flow. Although measurements are consistent with the idea that this flow is driven primarily by arterial pulsations, the driving mechanism is still not completely understood, and several published articles have suggested that the bulk flow might be an artifact, driven by the injection itself. Here we address this hypothesis with new in vivo experiments in which the injection of suspended tracer particles into the cisterna magna is done with a dual-syringe system, with simultaneous injection and withdrawal of equal amounts of fluid. This method produces no net increase in CSF volume and no significant increase in intracranial pressure, and yet particle-tracking reveals flows in the pial periarterial spaces that are completely consistent with the flows observed in earlier experiments with single-syringe injection.