Cerebrospinal fluid and blood flow patterns in idiopathic normal pressure hydrocephalus

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Phase‐contrast magnetic resonance imaging (PC‐MRI) is a promising diagnostic and predictive method to be used in idiopathic normal pressure hydrocephalus (INPH) as it measures flow of blood and cerebrospinal fluid (CSF) in mL/min and is non‐invasive. The most consistent finding of previous PC‐MRI studies in INPH has been increased cardiac‐related CSF pulsatility in the aqueduct.1 This has been suggested, but not confirmed, as an efficient predictive test for outcome after surgery. It is also unclear whether the increase in pulsatility is caused by a reduced intracranial compliance7 or by an increased ventricular volume.8
A historic sign of good shunt response was the accumulation of tracer in the ventricles after lumbar injection (radionuclide cisternography).9 A modern equivalent of this has been measurement of direction of the mean flow in the aqueduct: It has been reported that this direction is usually reversed in INPH, that is, flowing into the ventricles.11 These findings have been interpreted as disturbances of the CSF dynamics in INPH and suggested to normalize after CSF diversion. However, few of the PC‐MRI studies feature control groups with healthy elderly1 and of those several have employed relatively small and/or non‐age‐matched control groups or a mix of idiopathic and secondary NPH.
Better predictive tests are needed in the clinical practice of INPH.9 But before putting resources into evaluating new predictive models and tests, such as aqueduct flow and aqueduct CSF pulsatility, it is of importance to understand the pathophysiology behind the new biomarkers. Because disturbance of the CSF flow could be caused by changes in intracranial blood flow, these disturbances may be best understood by taking a comprehensive approach to studying the flow dynamics of INPH, including both the net and pulsatile components of blood flow as well as CSF flow. The aim of this study was therefore to describe the dynamic pattern of CSF and cerebral blood flow in typical INPH, classified using the INPH guidelines,15 as compared to age‐matched healthy elderly.
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