CSF outflow resistance as predictor of shunt function. A long-term study

Little is known about the long-term impact of a CSF shunt on the human CSF hydrodynamic system. In patients with communicating hydrocephalus, patency of the shunt system is not regularly assessed. In order to reveal postoperative changes in the CSF hydrodynamic system, we prospectively investigated the features of the system in shunted patients with idiopathic adult hydrocephalus syndrome (IAHS) over a 3-year period.

Thirty-two patients with IAHS were studied at baseline and at 3, 9, 18 and 36 months postoperatively. All patients were operated on with a Hakim standard valve system and a ventriculo-peritoneal approach. At each visit, the patients were investigated with computed tomography/magnetic resonance imaging, video recording of gait and a lumbar constant pressure infusion method. Six brand-new Hakim valves were investigated in a bench test, and these results were compared with the in vivo results.

After shunt insertion, the CSF outflow resistance was significantly decreased (13.6 vs 3.8 mmHg/ml/min). The mean outflow resistance of the six in vitro tested valves corresponded to the postoperative values. The variation in resistance in the functioning shunts at different postoperative investigations was negligible. The mean intracranial pressure in the supine position was 13.8 mmHg at the baseline and 14.3, 14.5, 14.8 and 15.7 mmHg at the follow-up visits, respectively. Postoperatively, the CSF pressure after sitting for 10 min (i.e. 'siphoning effect') decreased significantly (mean decrease -5.3, -5.4, -4.7 and -5.3 mmHg at each visit, respectively). Shunt-related complications occurred in seven patients (underdrainage four, overdrainage three). Despite a functioning shunt, eight patients never improved and another nine patients first improved but later deteriorated.

The CSF outflow resistance is much decreased postoperatively and does not alter over time in patients with functioning shunts. We consider CSF outflow resistance to be a reliable indicator of shunt function and of fundamental importance to distinguish a dysfunctioning shunt from an aggravation of the primary condition in patients with communicating hydrocephalus. The unaltered intracranial pressure together with the in vitro model results, suggests that the intra-abdominal pressure might be a major determinant of the postoperative intracranial pressure.