The Implications of Shunt Surgery in the Intracranial Hydrodynamics of Normal Pressure Hydrocephalus (NPH)

Abstract

In order to elucidate the mechanics of shunt functioning in normal pressure hydrocephalus (NPH), intracranial pressure (ICP), pressure volume index (PVI) and outflow resistance (Ro) measured by Marmarou’s bolus injection technique (Marmarou et al. 1975), and other ICP parameters were measured. Out of 27 patients who underwent shunt surgery, symptomatic improvement was observed in 24. Twenty-eight shunt operations in these 24 patients were analyzed. The mean ICP (mICP) dropped to 6.0 ± 2.2 mmHg from the pre-opening value of 10.3 ± 2.0 mmHg. The peak pressure of Bwaves (BPp) decreased from 20.7 ± 5.6 to 11.9 ± 3.7 mmHg. Ro decreased from 5.82 ± 4.67 to 1.40 ± 0.78 mmHg/ml/min and PVI increased from 26.9 ± 10.4 to 42.6 ± 20.2 ml. The post-operative mICP did not correlate with the shunt valve closing pressure at all, but the reduction of mICP and BPp significantly correlated with the reduction of Ro. The Ro(gross), the postoperative Ro, significantly correlated with the Ro(gross) calculated by the following equation: 1/Ro(gross) = 1/Ro(h) + 1/Ro(shunt), where Ro(h): Ro of the patient before shunt and Ro(shunt): Ro of the shunt system. The shunt installation lowers mICP and BPp by reducing Ro and increases intracranial buffering capacity, as noted by the increase in PVI. The reduction in Ro seems to play a key role in alleviating the abnormal hydrodynamic situation in NPH patients.