Idiopathic normal pressure hydrocephalus (INPH) is characterized by a clinical triad of symptoms: gait disturbance, urinary incontinence and cognitive impairment. INPH typically develops among the elderly and shares some symptoms with other neurological disorders. In general, 60-70% of patients with the complete clinical triad and signs of hydrocephalus on CT/MRI will improve after cerebrospinal fluid (CSF) shunting. Implantation of a shunt, however, carries complications such as infection, obstruction, underdrainage, and overdrainage. The cumulative complication rate and revision rate is estimated to be 35-80% among adults. The England shunt registry described an overall incidence of underdrainage of 52%, and studies have resulted in incidences of overdrainage between 2 and 21%. Børgesen claims that 80% of complications of shunting for INPH are related to overdrainage. Overdrainage may lead to low intracranial pressure syndrome and subdural effusions (SDE). Low intracranial pressure syndrome is characterized by orthostatic headaches and sometimes nausea, vomiting, drowsiness, diplopia, upward gaze palsy and visual defects. As INPH patients are often elderly patients with concomitant brain atrophy and normal CSF pressure they are especially at risk for developing signs and symptoms of overdrainage.

Overdrainage can be prevented or treated by implanting an anti-siphon device (ASD) or changing the opening pressure of the valve (OPV). Different ASDs have been developed since the first publication in 1973. They have in common that the lumen of the catheter is closed under influence of a negative hydrostatic pressure at the level of the ASD. The optimal valve setting for treating INPH is still the subject of controversy. The dilemma in INPH patients is that a low OPV might lead to overdrainage-related symptoms. A high valve setting might lead to underdrainage and the patient misses the chance of maximum improvement.

Studies examining whether to use a low or high OPV have been contradictory so far.

After randomization, baseline primary and secondary outcome measure assessments are conducted preoperatively. Within 24 hours of shunting, skull, thoracic, and abdominal X-rays are done to verify device connections and position. Follow-up visits at 1, 3, and 9 months include all primary and secondary outcome assessments. All measurements are done by a research assistant who visits the participating hospitals to prevent inter-observer variation. Study end for each patient is defined as completing the 9 month visit or death, detection of SDE on CT, or shunt removal or revision due to infection or dysfunction.

The PL in Group 1 (control group) is allowed to be increased to a higher setting when an SDE is detected. The patient thereby reaches study end, and the treating physician is free to decide how to manage the PL. The 9-month follow-up is still obtained. The PL in Group 2 is preset at the highest PL and eventually lowered 1 step at each follow-up visit when the modified mini mental state (3MS) or Gait score is not increased ≥ 15% provided that no SDE is visible on a new CT scan. The 15% is considered the minimum percentage to reliably determine improvement.