Idiopathic normal pressure hydrocephalus in neurological practice : A study of epidemiology and methods for selection of patients for surgery

Background: We have previously seen that idiopathic normal pressure hydrocephalus (iNPH) patients having elevated intracranial pressure (ICP) pulse amplitude consistently respond to shunt surgery. In this study we explored how the cerebrospinal fluid pressure (CSFP) pulse amplitude determined during lumbar infusion testing, correlates with ICP pulse amplitude determined during over-night ICP monitoring and with response to shunt surgery. Our goal was to establish a more reliable screening procedure for selecting iNPH patients for shunt surgery using lumbar intrathecal infusion. Methods: The study population consisted of all iNPH patients undergoing both diagnostic lumbar infusion testing and continuous over-night ICP monitoring during the period 2002-2007. The severity of iNPH was assessed using our NPH grading scale before surgery and 12 months after shunting. The CSFP pulse was characterized from the amplitude of single pressure waves. Results: Totally 62 iNPH patients were included, 45 of them underwent shunt surgery, in whom 78% were shunt responders. Among the 45 shunted patients, resistance to CSF outflow (Rout) was elevated (≥ 12 mmHg/ml/min) in 44. The ICP pulse amplitude recorded over-night was elevated (i.e. mean ICP wave amplitude ≥ 4 mmHg) in 68% of patients; 92% of these were shunt responders. In those with elevated overnight ICP pulse amplitude, we found also elevated CSFP pulse amplitude recorded during lumbar infusion testing, both during the opening phase following lumbar puncture and during a standardized period of lumbar infusion (15 ml Ringer over 10 min). The clinical response to shunting after 1 year strongly associated with the over-night ICP pulse amplitude, and also with the pulsatile CSFP during the period of lumbar infusion. Elevated CSFP pulse amplitude during lumbar infusion thus predicted shunt response with sensitivity of 88 and specificity of 60 (positive and negative predictive values of 89 and 60, respectively). Conclusions: In iNPH patients, shunt response can be anticipated in 9/10 patients with elevated overnight ICP pulse amplitude, while in only 1/10 with low ICP pulse amplitude. Additionally, the CSFP pulse amplitude during lumbar infusion testing was elevated in patients with elevated over-night ICP pulse amplitude. In particular, measurement of CSFP pulse amplitude during a standardized infusion of 15 ml Ringer over 10 min was useful in predicting response to shunt surgery and can be used as a screening procedure for selection of iNPH patients for shunting. * Correspondence: per.kristian. eide@rikshospitalet.no Department of Neurosurgery, Rikshospitalet University Hospital, N-0027 Oslo, Norway Eide and Brean Cerebrospinal Fluid Research 2010, 7:5 http://www.cerebrospinalfluidresearch. com/content/7/1/5 CEREBROSPINAL FLUID RESEARCH © 2010 Eide and Brean; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Background The clinical condition normal pressure hydrocephalus (NPH) incorporates gait disturbance, mental deterioration and urinary incontinence, combined with enlarged cerebral ventricles and a normal lumbar cerebrospinal fluid pressure (CSFP) [1]. Usually no cause is identified, in which case the condition is denoted idiopathic NPH (iNPH). Although the pathophysiology of iNPH is disputed [2], previous studies have shown that shunt surgery can be effective, and that clinical improvement can be sustained for years [3-5]. Hydrodynamic tests, in particular the lumbar infusion test, have been used for selecting patients for surgery, although the literature is very divergent concerning its role in iNPH [6]. During lumbar infusion testing, the static CSFP can either be monitored during constant flow infusion, constant pressure infusion or during bolus infusion to the thecal sac. In our practice, we have for many years used a modification of the constant rate infusion test originally described by Katzman [7] for determination of resistance to CSF outflow (Rout). However, the utility of Rout in selecting iNPH patients for surgery is controversial [8-13]. On the other hand, we have found that the ICP pulse (that is the amplitude of the single cardiac-beat induced ICP waves) during over-night ICP monitoring is very useful for predicting shunt response in iNPH [14]. Thus, in our previous series of 130 shunted iNPH patients, shunt response was seen in 9 of 10 patients with elevated ICP wave amplitudes but only in 1 of 10 with low ICP wave amplitudes [15]. With regard to lumbar infusion testing, the various approaches (e.g. constant flow, constant pressure or bolus infusion methods) consistently assess the static and not the pulsatile CSFP. Others [16] and our group [8,11,17] have reported experiences from assessing the pulsatile CSFP during lumbar infusion testing. Based on these experiences, it could be anticipated that determining the CSFP pulse during lumbar infusion testing might better characterize the pressure-volume reserve capacity than the static CSFP. Moreover, successful assessment of the pulsatile CSFP during lumbar puncture might represent an advantage, given that lumbar puncture is a low-risk procedure, and more useful in a clinical neurological setting than continuous ICP monitoring. The pulsatile CSFP can be measured during the opening phase of lumbar puncture, as well as during lumbar infusion. Thus, our goal with the present study was to establish a more reliable screening procedure for selection of iNPH patients for shunt surgery, based on determining the CSFP pulse amplitude during lumbar infusion testing. For this purpose, in the present study we explored how measurement of the pulsatile CSFP during lumbar infusion testing correlated with the ICP pulse monitored over-night and with the response to shunting. To do this we retrieved all lumbar infusion tests done during diagnostic work-up for iNPH in this department during the time period 2002-2007. These infusion tests were stored as continuous CSFP raw data (originally sampled at 100-200 Hz). In the present study, these raw data files were re-analyzed; the CSFP pulse amplitude was determined during the opening phase after lumbar puncture and also during a period of lumbar infusion (standardized as 15 ml infusion over 10 min). All patients had their ICP monitored over-night; therefore the infusion test results could be related to the pulsatile ICP recorded over-night, and with the clinical response to shunting. Methods Patient material The patient material consisted of all patients being assessed for iNPH at the Department of Neurosurgery, Rikshospitalet University Hospital, during the 6-years period 2002-2007, in whom both over-night ICP monitoring and lumbar infusion testing had been done during the diagnostic pre-operative work-up. The patients were referred from local neurological departments based on their symptoms of gait disturbance, incontinence, and dementia, combined with radiological ventriculomegaly. For diagnostic work-up the patients were hospitalized for 3 days. Following clinical and radiological assessment on day 1 (day of admittance), ICP monitoring was done from day 2 to day 3. The lumbar infusion test was done on day 3. After discharge from the department on day 3, they returned 1-3 weeks later for surgical treatment provided this was advocated. This study was approved by the hospital authority of Rikshospitalet University Hospital and by the Norwegian Social Science Data Services. The Regional Committee for Research Ethics was informed in writing, and had no objections to the study. Clinical and radiological assessment Our diagnostic work-up for iNPH patients has previously been described [14,15]. In short, based on findings at neurological examination, the severity of clinical iNPH was graded using our NPH grading scale (scores ranging from 3-15), which assesses the combined severity of gait disturbance, urinary incontinence and dementia. The size of the ventricles was assessed using the linear measure Evan’s index [14]. Diagnostic ICP monitoring and lumbar infusion testing Diagnostic continuous ICP monitoring was done through a frontal burr hole prepared under local Eide and Brean Cerebrospinal Fluid Research 2010, 7:5 http://www.cerebrospinalfluidresearch. com/content/7/1/5 Page 2 of 11