CSF outflow resistance and pressure-volume index determined by steady-state and bolus infusions

Assessment of Pressure-Volume Index During Lumbar Infusion Study: What Is the Optimal Method?

INTRODUCTION

Assessment of the pressure-volume index (PVI) during lumbar infusion study (LIS) has been proposed to evaluate the overall compliance of the cranio-spinal system. It is calculated from the measurement of CSF pressure changes, ΔP from Pb to Pp, in response to repeated bolus injections of a volume (ΔV) within the lumbar subarachnoid space.

MATERIAL AND METHODS

We retrospectively analyzed 18 patients who underwent LIS for suspicion of normal pressure hydrocephalus, including a series of three fast bolus injections of 3 mL of saline at different levels of CSF pressure. We compared two methods for PVI calculation: (a) PVI_slope using the slope α of a linear fit ΔP = α(Pb - P ₀), PVI = ΔV/log₁₀(α + 1); (b) PVI_mean using the PVI calculated independently for each bolus injection assuming P ₀ = 0, PVI = mean(ΔV/log₁₀(Pp_i/Pb_i))_i=1.3.

RESULTS

We found a significant discrepancy between the two methods: the average difference (PVI_slope - PVI_mean) was -3.93 mL (95% confidence interval [8.77; -16.64]). In the PVI_slope, method, the mean P ₀ was 2.12 mmHg (±3.41 mmHg).

DISCUSSION

The clinical reliability of PVI_mean (assuming P ₀ = 0) depends on the value of P ₀. PVI_slope provides results, independent of P ₀. Future studies should focus on determining pathological PVI range rather than fixed cut-off values.

CSF Dynamics for Shunt Prognostication and Revision in Normal Pressure Hydrocephalus

BACKGROUND

Despite the quantitative information derived from testing of the CSF circulation, there is still no consensus on what the best approach could be in defining criteria for shunting and predicting response to CSF diversion in normal pressure hydrocephalus (NPH).

OBJECTIVE

We aimed to review the lessons learned from assessment of CSF dynamics in our center and summarize our findings to date. We have focused on reporting the objective perspective of CSF dynamics testing, without further inferences to individual patient management.

DISCUSSION

No single parameter from the CSF infusion study has so far been able to serve as an unquestionable outcome predictor. Resistance to CSF outflow (Rout) is an important biological marker of CSF circulation. It should not, however, be used as a single predictor for improvement after shunting. Testing of CSF dynamics provides information on hydrodynamic properties of the cerebrospinal compartment: the system which is being modified by a shunt. Our experience of nearly 30 years of studying CSF dynamics in patients requiring shunting and/or shunt revision, combined with all the recent progress made in producing evidence on the clinical utility of CSF dynamics, has led to reconsidering the relationship between CSF circulation testing and clinical improvement.

CONCLUSIONS

Despite many open questions and limitations, testing of CSF dynamics provides unique perspectives for the clinician. We have found value in understanding shunt function and potentially shunt response through shunt testing in vivo. In the absence of infusion tests, further methods that provide a clear description of the pre and post-shunting CSF circulation, and potentially cerebral blood flow, should be developed and adapted to the bed-space.

Spinal Compliance Curves: Preliminary Experience with a New Tool for Evaluating Suspected CSF Venous Fistulas on CT Myelography in Patients with Spontaneous Intracranial Hypotension

BACKGROUND AND PURPOSE

Craniospinal space compliance reflects the distensibility of the spinal and intracranial CSF spaces as a system. Craniospinal space compliance has been studied in intracranial pathologies, but data are limited in assessing it in spinal CSF leak. This study describes a method to estimate craniospinal space compliance using saline infusion during CT myelography and explores the use of craniospinal space compliance and pressure-volume curves in patients with suspected cerebrospinal-venous fistula.

MATERIALS AND METHODS

Patients with suspected cerebrospinal-venous fistula underwent dynamic CT myelography. During the procedure, 1- to 5-mL boluses of saline were infused, and incremental changes in CSF pressure were recorded. These data were used to plot craniospinal space compliance curves. We calculated 3 quantitative craniospinal space compliance parameters: overall compliance, compliance at opening pressure, and the pressure volume index. These variables were compared between patients with confirmed cerebrospinal-venous fistula and those with no confirmed source of CSF leak.

RESULTS

Thirty-four CT myelograms in 22 patients were analyzed. Eight of 22 (36.4%) patients had confirmed cerebrospinal-venous fistulas. Bolus infusion was well-tolerated with no complications and transient headache in 2/34 (5.8%). Patients with confirmed cerebrospinal-venous fistulas had higher compliance at opening pressure and overall compliance (2.6 versus 1.8 mL/cm H₂0, P < .01). There was no difference in the pressure volume index (77.5 versus 54.3 mL, P = .13) between groups.

CONCLUSIONS

A method of deriving craniospinal space compliance curves using saline intrathecal infusion is described. Preliminary analysis of craniospinal space compliance curves provides qualitative and quantitative information about pressure-volume dynamics and may serve as a diagnostic tool in patients with known or suspected cerebrospinal-venous fistulas.

Timing of intraventricular infusion test for diagnosing idiopathic normal pressure hydrocephalus

BACKGROUND

Infusion tests, which measure resistance to outflow (R_out), are used in selecting patients suspected for idiopathic normal pressure hydrocephalus (iNPH) for shunt surgery. Infusion tests can be performed through an external ventricular drain (EVD). A 24-hour time gap from EVD insertion to an infusion test is a routine practice at our department due to concerns that the surgical procedure might influence the test results in the immediate postoperative period. The objective of the study was to investigate if timing of an intraventricular infusion test influences the results of the test in patients suspected for iNPH.

METHODS

Ten patients scheduled for an intraventricular infusion test were included. Measurements of baseline intracranial pressure (ICP) and plateau ICP were obtained during constant rate intraventricular infusion test performed at two time points (1 and 24 h after EVD insertion) and R_out was calculated from these measures and compared within patients.

RESULTS

Eight patients completed both infusion tests. In one of the 18 infusion tests performed, it was not possible to define an ICP plateau and this infusion test was excluded, leaving 7 paired infusion tests. Median R_out was 12.9 mmHg/ml/min (range 7.0-22.0) 1 h after EVD insertion and 11.3 mmHg/ml/min (range 7.8-18.1) after 24 h. Overall, there were no statistically significant differences in R_out (P = 0.83), baseline ICP (P = 0.70), or plateau ICP (P = 0.81) between the recordings performed 1 h and 24 h after EVD insertion. For two of the seven patients with paired infusion tests, there was poor agreement between R_out values at 1 and 24 h.

CONCLUSION

Overall, R_out estimates do not change significantly between 1 and 24 h after EVD insertion. We therefore propose that infusion tests can be performed shortly after surgery to reduce the period of indwelling EVD and duration of hospitalization.

The role of perfusion and diffusion MRI in the assessment of patients affected by probable idiopathic normal pressure hydrocephalus. A cohort-prospective preliminary study

Background

Invasive tests measuring resistance to cerebral spinal fluid (CSF) outflow and the effect of temporary drainage of CSF are used to select candidates affected by idiopathic normal pressure hydrocephalus (iNPH) for shunt surgery. Neither test, however, completely excludes patients from treatment. Perfusion and diffusion magnetic resonance imaging (MRI) are non-invasive techniques that might be of value in selecting patients for surgical treatment and understanding brain changes in iNPH patients. The aim of this study was to understand the role of perfusion and diffusion MRI in selecting candidates for shunt surgery and to investigate the relationship between cerebral perfusion and possible microstructural changes in brain tissue before and after invasive tests, and after ventricular-peritoneal (VP) shunt implantation, to better clarify pathophysiological mechanisms underlying iNPH.

Methods

Twenty-three consecutive patients with probable iNPH were included in this study. Patients underwent a clinical and neuroradiological evaluation before and after invasive tests, and after surgery. Only patients who showed a positive result in at least one of the invasive tests were submitted for VP shunt implantation. Perfusion and diffusion magnetic resonance imaging (MRI) was performed before and after invasive tests and after shunt surgery.

Results

Thirteen patients underwent surgery and all showed clinical improvement after VP shunt implantation and a significant increase in perfusion in both periventricular white matter (PVWM) and basal ganglia (BG) regions. The 10 patients that did not have surgery showed after invasive tests, a significant reduction in perfusion in both PVWM and BG regions. Comparing the changes in perfusion with those of diffusion in positive patients we found a significant positive correlation in BG and a significant inverse correlation in PVWM area.

Conclusions

Perfusion MRI is a non-invasive technique that could be useful together with invasive tests in selecting patients for surgical treatment. Furthermore, the relationship between perfusion and diffusion data could better clarify pathophysiological mechanisms underlying iNPH. In PVWM area we suggest that interstitial edema could reduce microvascular blood flow and interfere with the blood supply to these regions. In BG regions we suggest that a chronic hypoxic insult caused by blood hypo-perfusion produces a chronic cytotoxic edema. Both in PVWM and in BG regions, pathophysiological mechanisms could be modified after VP-shunt implantation.

Repeatability of cerebrospinal fluid constant rate infusion study

OBJECTIVE

Infusion tests are important tools to assess cerebrospinal fluid (CSF)dynamics used in the preoperative selection of patients for shunt surgery, or to predict the scope of improvement from shunt revision. The aim of this study was to assess the repeatability of the key quantitative parameters describing CSF dynamics that are determined with infusion testing.

MATERIALS AND METHODS

Eighteen patients in whom a constant infusion test was repeated within 102 days, without any intermediate surgical intervention, were studied. From each test baseline ICP, baseline pulse amplitude, outflow resistance, elastance coefficient and slope of the amplitude-pressure line were calculated and investigated with a regression and Bland-Altman analysis.

RESULTS

Significant correlations (P < 0.01) were found for the outflow resistance (R = 0.96), the elastance coefficient (R = 0.778) and the slope of the amplitude-pressure line (R = 0.876). The estimated 95% confidence level for outflow resistance was 3 mmHg/ml min. Likewise, the elastance coefficient lay within a range of 0.16/ml and the slope of the amplitude-pressure line within 0.25. The most inconsistent parameter found were baseline ICP (R = 0.272) and baseline pulse amplitude (R = 0.171).

CONCLUSION

The results of this study imply that the parameters resulting from an infusion study have to be considered within a range rather than as an absolute value.

Measurement of CSF dynamics with oscillating pressure infusion

INTRODUCTION

Infusion tests are used to diagnose and select patients with idiopathic normal pressure hydrocephalus (INPH) for shunt surgery. The test characterizes cerebrospinal fluid dynamics and estimates parameters of the cerebrospinal fluid system, the pressure-volume index (PVI) and the outflow conductance (Cout). The Oscillating Pressure Infusion (OPI) method was developed to improve the test and reduce the investigation time. The aim of this study was to evaluate the new OPI method by comparing it with an established reference method.

METHODS

Forty-seven patients (age 71.2 ± 8.9 years) with communicating hydrocephalus underwent a preoperative lumbar infusion investigation with two consecutive infusion protocols, reference (42 min) and new (20 min), that is, 94 infusion tests in total. The OPI method estimated Cout and PVI simultaneously. A real-time analysis of reliability was applied to investigate the possibility of infusion time reduction.

RESULTS

The difference in Cout between the methods was 1.2 ± 1.8 μl/s/kPa (ΔRout = -0.8 ± 3.5 mmHg/ml/min), P < 0.05, n = 47. With the reliability analysis, the preset 20 min of active infusion could have been even further reduced for 19 patients to between 10 and 19 min. PVI was estimated to 16.1 ± 6.9 ml, n = 47.

CONCLUSIONS

The novel Oscillating Pressure Infusion method produced real-time estimates of Cout including estimates of reliability that was in good agreement with the reference method and allows for a reduced and individualized investigation time.

One-year outcome in the European multicentre study on iNPH

OBJECTIVES

To assess the 1-year outcome after shunt surgery in patients with idiopathic normal pressure hydrocephalus (iNPH).

METHODS

Patients (n = 142) were prospectively included in the European multicentre study by 13 centres. Diagnoses were based solely on clinical and radiological findings. All received a programmable ventriculoperitoneal shunt. Re-examinations, 12 months after surgery, were performed in 115 patients, and the outcome was assessed by the modified Rankin scale (mRs) and a new iNPH grading scale. Improvement was defined as ≥1 step on the mRs and ≥5 points on the iNPH scale.

RESULTS

The scores on both scales were significantly improved after 1 year of shunt treatment (Ps < 0.001). Sixty-nine per cent of the patients were improved according to the mRs and 84% according to the iNPH scale. The proportion able to live independently (scores 0-2 on the mRs) was increased from 53% before to 82% 12 months after surgery (P < 0.001). Neither classification (typical or questionable) nor comorbidity affected the level of improvement. Patients not completing the study were worse off with regard to their clinical condition at entry than completers. Twenty-eight per cent of the patients experienced complications and were either conservatively (13%) or surgically (15%) treated.

CONCLUSION

The results of this prospective multicentre study on patients with iNPH diagnosed solely on clinical and radiological criteria support shunt surgery in patients presenting with symptoms and signs and MRI findings suggestive of iNPH.

Intracranial pressure and ventricular expansion in hydrocephalus: have we been asking the wrong question?

The force that enlarges the cerebral ventricles and deforms the brain in hydrocephalus remains unclear. It is still widely thought to be elevated intraventricular pressure developing behind an obstruction to the flow of CSF. This view has led to the prediction that a large pressure difference should exist between the ventricles proximal to the obstruction and the subarachnoid space of the cerebral convexity distal to the obstruction. Yet measurements have shown consistently that such transmantle pressure differences are either small or absent. We propose a theory that reconciles the view that hydrocephalus is caused by obstruction to the flow of CSF with the observed absence of large pressure gradients across the cerebral mantle. Obstruction to CSF flow produces only a small pressure gradient -- usually less than 1 mm Hg -- that is sufficient to overcome the added resistance to flow and thereby to balance the absorption of CSF with its production. This mini-gradient is the effective force that initiates and sustains ventricular enlargement. It can coexist either with high or with normal intracranial pressure. The level of intracranial pressure is determined by the efficiency with which increments of ventricular pressure are transmitted through the parenchyma to the outer surface of the brain. In the presence of a rigid skull some transmission is required by basic laws of Newtonian mechanics. The efficiency of transmission depends primarily on the elastic properties of the brain. If the brain is relatively incompressible, transmission is efficient and high intracranial pressure is required to maintain the mini-gradient between the ventricles and the subarachnoid space, resulting in tension hydrocephalus. If the brain is more compressible, the parenchyma attenuates any increase of intraventricular pressure, reducing transmission to the outer surface. Intracranial pressure need not rise above normal levels to maintain the mini-gradient, leading to normal pressure hydrocephalus. The theory explains why tests measuring CSF resistance have limited diagnostic usefulness in hydrocephalus. It also predicts that very small stresses are sufficient to produce large deformations of the brain if these are allowed to occur slowly.

Assessment of cerebrospinal fluid outflow resistance

The brain and the spinal cord are contained in a cavity and are surrounded by cerebrospinal fluid (CSF), which provides physical support for the brain and a cushion against external pressure. Hydrocephalus is a disease, associated with disturbances in the CSF dynamics, which can be surgically treated by inserting a shunt or third ventriculostomy. This review describes the physiological background, modeling and mathematics, and the investigational methods for determining the CSF dynamic properties, with specific focus on the CSF outflow resistance, R out. A model of the cerebrospinal fluid dynamic system, with a pressure-independent R out, a pressure-dependent compliance and a constant formation rate of CSF is widely accepted. Using mathematical expressions calculated from the model, along with active infusion of artificial CSF and observation of corresponding change in ICP allows measurements of CSF dynamics. Distinction between normal pressure hydrocephalus and differential diagnoses, prediction of clinical response to shunting and the possibility of assessment of shunt function in vivo are the three most important applications of infusion studies in clinical practice.

Adaptive method for assessment of cerebrospinal fluid outflow conductance

Outflow conductance (C(out)) is important for predicting shunt responsiveness in patients with suspected idiopathic adult hydrocephalus syndrome (IAHS). C (out) is determined by performing an infusion test into the cerebrospinal fluid system, and the reliability of the test is dependent on the measurement time. The objective of this study was to develop an adaptive signal analysis method to reduce the investigation time, by taking the individual intracranial pressure variations of the patient into consideration. The method was evaluated on 28 patients with suspected IAHS. The results from full time investigations (60 min) were compared to the results of the new algorithm. Applying the new adaptive method resulted in a reduction of mean investigation time by 14.3 +/- 5.9 min (mean +/- SD), p<0.01. The reduction of reliability in the C(out) estimation was found clinically negligible. We thus recommend this adaptive method to be used when performing constant pressure infusion tests.

Assessment of cerebrospinal fluid outflow conductance using constant-pressure infusion—a method with real time estimation of reliability

The outflow conductance (C(out)) of the cerebrospinal fluid (CSF) system is a parameter considered to be predictive in selection for hydrocephalus surgery. C(out) can be determined through an infusion test. A new apparatus for performing infusion tests in a standardized and automated way was developed. The objective was to evaluate repetitiveness as well as to propose and evaluate a method for real time estimation of the reliability of individual C(out) investigations. Repeated investigations were performed on an experimental model simulating the CSF system, and on 14 patients with hydrocephalus. DeltaC(out), calculated as the 95% confidence interval of C(out), was introduced as an estimate of the reliability of individual C(out) investigations. On the model, no significant difference was found between DeltaC(out) and the actual C(out) variation in repeated investigations (p = 0.135). The correlation between the first and the second patient investigation was high (R = 0.99, p < 0.05), although there was a significant difference between the investigations (p < 0.05). The standard deviation of difference was 2.60 microl (s kPa)(-1). The repetitiveness of C(out) with the new apparatus was high, and DeltaC(out) reflected the reliability of each investigation. This feature has to be taken into account in every individual case, before making a decision or performing research based on measurements of C(out) in the future.

Cerebrospinal fluid pulse pressure method: a possible substitute for the examination of B waves

OBJECT

The appearance of numerous B waves during intracranial pressure (ICP) registration in patients with idiopathic adult hydrocephalus syndrome (IAHS) is considered to predict good outcome after shunt surgery. The aim of this study was to describe which physical parameters of the cerebrospinal fluid (CSF) system B-waves reflect and to find a method that could replace long-term B-wave analysis.

METHODS

Ten patients with IAHS were subjected to long-term registration of ICP and a lumbar constant-pressure infusion test. The B-wave presence, CSF outflow resistance (R(out)), and relative pulse pressure coefficient (RPPC) were assessed using computerized analysis. The RPPC was introduced as a parameter reflecting the joint effect of elastance and pulsatory volume changes on ICP and was determined by relating ICP pulse amplitudes to mean ICP.

CONCLUSIONS

The B-wave presence on ICP registration correlates strongly with RPPC (r = 0.91, p < 0.001, 10 patients) but not with CSF R(out). This correlation indicates that B waves-like RPPC-primarily reflect the ability of the CSF system to reallocate and store liquid rather than absorb it. The RPPC-assessing lumbar short-term CSF pulse pressure method could replace the intracranial long-term B-wave analysis.

Postural changes in intracranial pressure in chronically shunted patients

A subset of hydrocephalic patients with indwelling shunts become symptomatic when they are upright and active. Intracranial pressure (ICP) measurements in these patients have shown a significant drop in pressure when the patient is upright with return to normal levels when the patient is supine. In 20 chronically shunted hydrocephalic patients who previously had no siphon protection devices, ICP changes in supine and upright position were studied at the time when the patient had external ventriculostomy for treatment of shunt infection. Our hypothesis was that these patients might display rapid changes in ICP from fluid shifts occurring in non-CSF compartments. To minimize the effects of hysteresis, drift and zero-point error, measurements were made using a fluid manometer rather than a strain gauge pressure transducer. The pressure-volume index was estimated using the standard technique of bolus injection. Intracranial CSF volume was estimated on CT scans. The fluid shift was calculated using a mathematical model of the CSF compartment that incorporates negative pressure and volume components that permits simulation of siphoning. Sixteen patients had small, slit ventricles; 3 patients had moderate-sized ventricles and in 1 patient the ventricular size was normal. The average intracranial CSF volume estimated on CT scan was 12 cm(3). There was a mean drop in ICP in the upright position of 159 mm H(2)O. The mean PVI of 42 ml suggested a volume displacement out of proportion to the available intracranial CSF volume. Based on these findings, we conclude that even in the absence of drainage through the shunt, chronically shunted patients still display a fall in ICP when assuming the upright position. This raises the possibility of fluid shifts other than of CSF through nonshunt pathways. Possible mechanisms involving altered CSF-venous system interaction are discussed.

Evaluation of a method for noninvasive intracranial pressure assessment during infusion studies in patients with hydrocephalus

OBJECT

A mathematical model previously introduced by the authors allowed noninvasive intracranial pressure (nICP) assessment. In the present study the authors investigated this model as an aid in predicting the time course of raised ICP during infusion tests in patients with hydrocephalus and its suitability for estimating the resistance to outflow of cerebrospinal fluid (Rcsf).

METHODS

Twenty-one patients with hydrocephalus were studied. The nICP was calculated from the arterial blood pressure (ABP) waveform by using a linear signal transformation, which was dynamically modified by the relationship between ABP and cerebral blood flow velocity. This model was verified by comparison of nICP with "real" ICP measured during lumbar infusion tests. In all simulations, parallel increases in real ICP and nICP were evident. The simulated Rcsf was computed using nICP and then compared with Rcsf computed from real ICP. The mean absolute error between real and simulated Rcsf was 4.1 +/- 2.2 mm Hg minute/ml. By the construction of simulations specific to different subtypes of hydrocephalus arising from various causes, the mean error decreased to 2.7 +/- 1.7 mm Hg minute/ml, whereas the correlation between real and simulated Rcsf increased from R = 0.73 to R = 0.89 (p < 0.001).

CONCLUSIONS

The validity of the mathematical model was confirmed in this study. The creation of type-specific simulations resulted in substantial improvements in the accuracy of ICP assessment. Improvement strategies could be important because of a potential clinical benefit from this method.

The Dutch normal-pressure hydrocephalus study. How to select patients for shunting? An analysis of four diagnostic criteria

BACKGROUND

Comparison of the predictive value of four "diagnostic tests" for the outcome of shunting in patients with normal-pressure hydrocephalus (NPH).

METHODS

Ninety-five NPH patients who received shunts were followed for 1 year. Gait disturbance and dementia were quantified by an NPH scale and handicap by a modified Rankin scale. Primary outcome measures were differences between the preoperative and last scores on both the NPH scale and the modified Rankin scale. Clinical and computed tomographic (CT) findings typical of NPH, absence of cerebrovascular disease, and a resistance to outflow of cerebrospinal fluid (CSF) >/= 18 mmHg/ml/minute were designated as a positive test outcome; clinical and CT findings compatible with NPH, presence of cerebrovascular disease, and an outflow resistance < 18 mmHg/ml/minute as a negative test outcome.

RESULTS

For each of the four tests the percentage of patients classified as improved was significantly greater for those with positive than with negative test results. Measurement of CSF outflow resistance was the only significant prognostic factor for the improvement ratio in NPH scale and CT in the modified Rankin scale according to multivariate logistic regression analysis. The accurate predictive value of the combination of typical clinical and CT findings was 0.65, that of the positive test results of outflow resistance, clinical and CT findings was 0.74.

CONCLUSION

The best strategy is to shunt NPH patients if their outflow resistance is >/= 18 mmHg/ml/minute or, when the outflow resistance is lower, if their clinical as well as their CT findings are typical of NPH.

Dutch normal-pressure hydrocephalus study: prediction of outcome after shunting by resistance to outflow of cerebrospinal fluid

The authors examined whether measurement of resistance to outflow of cerebrospinal fluid (Rcsf) predicts outcome after shunting for patients with normal-pressure hydrocephalus (NPH). In four centers 101 patients (most of whom had idiopathic NPH) who fulfilled strict entry criteria underwent shunt placement irrespective of their level of Rcsf obtained by lumbar constant flow infusion. Gait disturbance and dementia were quantified by using an NPH scale and the patient's level of disability was assessed by using the modified Rankin scale (mRS). In addition the Modified Mini-Mental State Examination was performed. Patients were assessed prior to and 1, 3, 6, 9, and 12 months after surgery. Primary outcome measures were based on differences between the preoperative and last NPH scale scores and mRS grades. Improvement was defined as a change measuring at least 15% in the NPH scale score and at least one mRS grade. Intention-to-treat analysis of all patients at 1 year yielded improvement for 57% in NPH scale score and 59% in mRS grade. Efficacy analysis, excluding serious events and deaths that were unrelated to NPH, was performed for 95 patients. Improvement rose to 76% in NPH scale score and 69% in mRS grade. Six cut-off levels of Rcsf were related to improvement in NPH scale score using two-by-two tables. Positive predictive values were approximately 80% for an Rcsf of 10, 12, or 15 mm Hg/ml/minute, 92% for an Rcsf of 18 mm Hg/ml/minute, and 100% for an Rcsf of 24 mm Hg/ml/minute. Negative predictive values were low. More important was the highest likelihood ratio of 3.5 for an Rcsf of 18 mm Hg/ml/minute. Extensive comorbidity was a major prognostic factor. Measurement of Rcsf reliably predicts outcome if the limit for shunting is raised to 18 mm Hg/ml/minute. At lower Rcsf values the decision depends mainly on the extent to which clinical and computerized tomography findings are typical of NPH.

Relationship between compliance and resistance to outflow of CSF in adult hydrocephalus

Resistance to outflow of cerebrospinal fluid (Rcsf) was determined by constant flow infusions and pressure-volume index (PVI) using bolus infusions in 114 patients with various types of hydrocephalus. A clear correlation was found between PVI and Rcsf and, to a lesser degree, between these two parameters and baseline pressure. The PVI was not related to patient's age, duration of disease, type of hydrocephalus, or ventricular size, indicating that the relationship between PVI and Rcsf was genuine and not caused by patient selection. It is concluded that, in adult hydrocephalus, compliance is not an independent parameter but chiefly determined by Rcsf.

Reduction of ventricular size after shunting for normal pressure hydrocephalus related to CSF dynamics before shunting

Reduction of ventricular size was determined by repeated computed tomography in 30 adult patients shunted for normal pressure hydrocephalus (NPH) and related to the pressure-volume index (PVI) and resistance to outflow of cerebrospinal fluid (Rcsf) measured before shunting. Rapid and marked reduction of ventricular size (n = 10) was associated with a significantly lower PVI than slow and moderate to marked (n = 13) or minimal to mild reduction (n = 7). Otherwise no relationship could be found between the reduction of ventricular size and PVI or Rcsf. It is concluded that both rate and magnitude of reduction of ventricular size after shunting for NPH are extremely variable. High brain elasticity seems to be the best predictor of rapid and marked reduction.