Pulsatility in CSF dynamics: pathophysiology of idiopathic normal pressure hydrocephalus.

BACKGROUND It is suggested that disturbed CSF dynamics are involved in the pathophysiology of idiopathic normal pressure hydrocephalus (INPH). The pulsatility curve describes the relationship between intracranial pressure (ICP) and the amplitude of cardiac related ICP pulsations. The position of baseline ICP on the curve provides information about the physiological state of the CSF dynamic system. The objective of the study was to investigate if shunt surgery modifies the pulsatility curve and the baseline position on the curve, and how this relates to gait improvement in INPH. METHODS 51 INPH patients were investigated with lumbar CSF dynamic investigations preoperatively and 5 months after shunt surgery. During the investigation, ICP was measured at baseline, and then a CSF sample was removed, resulting in pressure reduction. After this, ICP was regulated with an automated infusion protocol, with a maximum increase of 24 mm Hg above baseline. The pulsatility curve was thus determined in a wide range of ICP values. Gait improvement was defined as a gait speed increase ≥0.1 m/s. RESULTS The pulsatility curve was unaltered by shunting. Baseline ICP and amplitude were reduced (-3.0±2.9 mm Hg; -1.1±1.5 mm Hg; p<0.05, n=51). Amplitude reduction was larger for gait improvers (-1.2±1.6 mm Hg, n=42) than non-improvers (-0.2±0.5 mm Hg, n=9) (p<0.05) although mean ICP reduction did not differ. CONCLUSIONS The pulsatility curve was not modified by shunt surgery, while the baseline position was shifted along the curve. Observed differences between gait improvers and non-improvers support cardiac related ICP pulsations as a component of INPH pathophysiology.