Deformation of brain in normal pressure hydrocephalus is more readily associated with slow vasomotion rather than heartbeat related pulsations of intracranial pressure.

Objective: Enlarged brain ventricles, compressed parasagittal cerebrospinal fluid spaces, steep callosal angle, dilated sylvian fissures and focal cortical sulcal dilatation are typical imaging features of idiopathic normal pressure hydrocephalus (iNPH). The pathophysiological mechanisms behind these morphological changes are poorly understood, but the hydrodynamic concepts of communicating hydrocephalus suggest that increased heartbeat related intracranial pulsations are involved in ventricular enlargement. In this cross-sectional study we analysed the association between the radiological findings of iNPH and the physiological intracranial pressure (ICP) waveform components.

Methods: 117 patients with suspected iNPH underwent computerised overnight ICP monitoring with calculation of heartbeat related ICP pulse wave amplitude (calculated in the frequency domain, AMP, and time domain, MWA), amplitude of respiration induced ICP waves (RESP), power of slow vasogenic waves (SLOW), and index of cerebrospinal compensatory reserve (RAP). Radiological morphological data was recorded from computed tomography using Evans Index (EI), frontal occipital horn ratio (FOHR), and disproportionately enlarged subarachnoid space hydrocephalus (DESH) score.

Results: The strongest correlation was observed between SLOW and DESH (r = 0.44, p < 0.012). SLOW also correlated with ventricular size as measured with EI (r = 0.23, p = 0.045) and FOHR (r = 0.26, p = 0.037). ICP and RESP also correlated with DESH (r = 0.25, p = 0.037 and r = 0.25, p = 0.038, respectively). AMP and MWA were not correlated with the radiological data.

Conclusions: Mainly SLOW showed correlations with the morphological imaging features of iNPH. SLOW is influenced by vasomotion and intracranial compliance. This study suggests that the magnitude of ICP slow wave activity, but not ICP pulse component is related to the size of brain ventricles and DESH in iNPH.