Aqueductal CSF Stroke Volume Is Increased in Patients with Idiopathic Normal Pressure Hydrocephalus and Decreases after Shunt Surgery

Abstract

BACKGROUND AND PURPOSE: Increased CSF stroke volume through the cerebral aqueduct has been proposed as a possible indicator of positive surgical outcome in patients with idiopathic normal pressure hydrocephalus; however, consensus is lacking. In this prospective study, we aimed to compare CSF flow parameters in patients with idiopathic normal pressure hydrocephalus with those in healthy controls and change after shunt surgery and to investigate whether any parameter could predict surgical outcome. MATERIALS AND METHODS: Twenty-one patients with idiopathic normal pressure hydrocephalus and 21 age-and sex-matched healthy controls were prospectively included and examined clinically and with MR imaging of the brain. Eighteen patients were treated with shunt implantation and were reexamined clinically and with MR imaging the day before the operation and 3 months postoperatively. All MR imaging scans included a phase-contrast sequence. RESULTS: The median aqueductal CSF stroke volume was significantly larger in patients compared with healthy controls (103.5 L; interquartile range, 69.8-142.8 L) compared with 62.5 L (interquartile range, 58.3-73.8 L; P Ͻ .01) and was significantly reduced 3 months after shunt surgery from 94.8 L (interquartile range, 81-241 L) to 88 L (interquartile range, 51.8-173.3 L; P Ͻ .05). Net flow in the caudocranial direction (retrograde) was present in 11/21 patients and in 10/21 controls. Peak flow and net flow did not differ between patients and controls. There were no correlations between any CSF flow parameters and surgical outcomes. CONCLUSIONS: Aqueductal CSF stroke volume was increased in patients with idiopathic normal pressure hydrocephalus and decreased after shunt surgery, whereas retrograde aqueductal net flow did not seem to be specific for patients with idiopathic normal pressure hydrocephalus. On the basis of the results, the usefulness of CSF flow parameters to predict outcome after shunt surgery seem to be limited. ABBREVIATIONS: ACSV ϭ aqueductal CSF stroke volume; iNPH ϭ idiopathic normal pressure hydrocephalus; IQR ϭ interquartile range; MMSE ϭ Mini-Mental State Examination; NPH ϭ normal pressure hydrocephalus; PC ϭ phase-contrast; TUG ϭ Timed Up and Go Test

A model for the oscillatory flow in the cerebral aqueduct

“…2018; Shanks etal. 2019), corresponding to larger values of the parameters and .

Figure 3.The variation of with obtained from the simplified flow model for , and different values of .

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“…The figure covers the range of conditions typically found in healthy subjects, characterized by values of L s /L of order unity and Womersley numbers ranging from α 2 for the respiratory cycle to α 4 for the cardiac cycle. The plot can also be used in connection with iNPH patients, who typically show enlarged aqueducts and higher tidal volumes (Ringstad et al 2015;Markenroth Bloch et al 2018;Shanks et al 2019), corresponding to larger values of the parameters α and L s /L.…”

“…2015; Markenroth Bloch, Töger & Ståhlberg 2018; Shanks etal. 2019). Correspondingly, the characteristic stroke length is comparable to the aqueduct length .…”

“…The variation corresponding to the aqueduct of figure 1(c), obtained after smoothing the segmented contour, is shown in figure 1(e). The corresponding aqueduct volume L 0 πa 2 ds can be equated to that of a circular cylinder with the same length L to define the characteristic aqueduct radius a c from πa 2 c L = L 0 πa 2 ds, yielding typical values of order a c ∼ 1-1.5 mm L. We address the pulsating motion induced by the periodic pressure difference Δp(t) p 3 − p 4 , resulting in a periodic volumetric flow rate Q(t) with the same period T. The corresponding stroke volume V s = T 0 |Q| dt/2 has been measured to be comparable to the aqueduct volume πa 2 c L (Ringstad et al 2015;Markenroth Bloch, Töger & Ståhlberg 2018;Shanks et al 2019). Correspondingly, the characteristic stroke length L s = V s /(πa 2 c ) is comparable to the aqueduct length L. Since the temporal variations of the aqueduct volume, associated with the deformation of the bounding tissue, are much smaller than the aqueduct volume itself (Kurtcuoglu et al 2007), the aqueduct can be assumed to be rigid for the analysis of the flow, as done below in our analysis.…”

“…2015; Shanks etal. 2019), must account for the effects of respiration. The present analysis is general, in that the parametric ranges investigated include conditions corresponding to both cardiac- and respiratory-driven motion.…”