ICP dependent changes of CSF outflow resistance

Effect of resting pressure on the estimate of cerebrospinal fluid outflow conductance

Background

A lumbar infusion test is commonly used as a predictive test for patients with normal pressure hydrocephalus and for evaluation of cerebrospinal fluid (CSF) shunt function. Different infusion protocols can be used to estimate the outflow conductance (C_out) or its reciprocal the outflow resistance (R_out), with or without using the baseline resting pressure, P_r. Both from a basic physiological research and a clinical perspective, it is important to understand the limitations of the model on which infusion tests are based. By estimating C_out using two different analyses, with or without P_r, the limitations could be explored. The aim of this study was to compare the C_out estimates, and investigate what effect P_rhad on the results.

Methods

Sixty-three patients that underwent a constant pressure infusion protocol as part of their preoperative evaluation for normal pressure hydrocephalus, were included (age 70.3 ± 10.8 years (mean ± SD)). The analysis was performed without (C_{excl Pr}) and with (C_{incl Pr}) P_r. The estimates were compared using Bland-Altman plots and paired sample t-tests (p < 0.05 considered significant).

Results

Mean C_out for the 63 patients was: C_{excl Pr} = 7.0 ± 4.0 (mean ± SD) μl/(s kPa) and C_{incl Pr} = 9.1 ± 4.3 μl/(s kPa) and R_out was 19.0 ± 9.2 and 17.7 ± 11.3 mmHg/ml/min, respectively. There was a positive correlation between methods (r = 0.79, n = 63, p < 0.01). The difference, ΔC_out= -2.1 ± 2.7 μl/(s kPa) between methods was significant (p < 0.01) and ΔR_out was 1.2 ± 8.8 mmHg/ml/min). The Bland-Altman plot visualized that the variation around the mean difference was similar all through the range of measured values and there was no correlation between ΔC_out and C_out.

Conclusions

The difference between C_out estimates, obtained from analyses with or without P_r, needs to be taken into consideration when comparing results from studies using different infusion test protocols. The study suggests variation in CSF formation rate, variation in venous pressure or a pressure dependent C_out as possible causes for the deviation from the CSF absorption model seen in some patients.

Dependency of cerebrospinal fluid outflow resistance on intracranial pressure

OBJECT

The outflow resistance (Rout) of the cerebrospinal fluid (CSF) system has generally been accepted by most investigators as independent of intracranial pressure (ICP), but there are also those claiming that it is not. The general belief is that this question has been investigated numerous times in the past, but few studies have actually been specifically aimed at looking at this relationship, and no study has been able to provide scientific evidence to elucidate fully this fundamental and important issue. The objective of this study was to investigate the relationship between ICP and CSF outflow in 30 patients investigated for idiopathic normal-pressure hydrocephalus.

METHODS

Lumbar infusion tests with constant pressure levels were performed, and ICP and corresponding flow were measured on 6 pressure levels for each patient. All data were standardized for comparison.

RESULTS

In the range of moderate increases from baseline pressure (approximately 5-12 mm Hg, mean baseline pressure 11.7 mm Hg), the assumption of a pressure-independent Rout was confirmed (p = 0.5). However, when the pressure increment from baseline pressure was larger (approximately 15-22 mm Hg), the relationship had a nonlinear tendency (p < 0.05).

CONCLUSIONS

The results of this study support the classic textbook theory of a pressure-independent Rout in the normal ICP range, where the CSF system is commonly operating. However, the theory might have to be questioned in regions where ICP exceeds baseline pressure by too much.

Predictors of outcome in patients with normal-pressure hydrocephalus

Despite knowledge emerging over the last 40 years, the postoperative results after shunt implantation in patients diagnosed with normal-pressure hydrocephalus (NPH) have not improved significantly over the last decade. For that reason predictors have to be identified in order to preoperatively predict outcome. From 1982 to 2000 we prospectively studied 200 patients diagnosed with NPH. From the patients, who were surgically treated by a shunt implantation we reexamined 155 (78%) postoperatively a mean time interval of 7 months. The NPH was graded according to the results of the intrathecal infusion test in an early stage NPH (without brain atrophy) and late stage NPH (with brain atrophy). In our study, we focussed attention on the possible predictors: patients age, length of disease, clinical signs (gait ataxia, dementia and bladder incontinence), aetiology idiopathic/secondary as well as implanted valve type and the value of resistance to cerebrospinal fluid outflow. To measure the outcome we used the NPH recovery rate and as the statistical test the chi(2) according to Pearson. In 80 patients with an early stage NPH (without cerebral atrophy), a short course of disease (<1 year), a slight degree of dementia and an implanted Miethke-Dual-Switch valve were significant predictors for a positive postoperative outcome. The outflow resistance measured in the intrathecal infusion test showed only a minimal relevance to outcome. Those 75 patients with a late state NPH (with cerebral atrophy) had a better outcome when dementia was not present, the outflow resistance was above 20mm Hg/min/ml, the CSF tap-test was positive and a Miethke-Dual-Switch valve was implanted.

The importance of the intrathecal infusion test in the diagnosis of normal pressure hydrocephalus

The intrathecal infusion test is a reliable method in diagnosing normal pressure hydrocephalus. Between May 1982 and January 2000 we investigated 300 patients suspected of having normal pressure hydrocephalus (NPH) by carrying out an intrathecal infusion test in a constant flow technique. The resistance to cerebrospinal fluid outflow (R(out)) in the intrathecal infusion test was the main criterion of grouping patients into those with normal pressure hydrocephalus or those with cerebral atrophy. A further differentiation into early stage and late stage was made by measuring the compliance (C(p)) - this being the secondary criterion. In 162 patients (54%) the diagnosis of NPH could be confirmed. Of these, 154 patients (95%) underwent a shunt operation. Graduation of NPH and cerebral atrophy following the results of the infusion test at an early stage and an advanced stage allow the conclusion of prognostic evaluations about the course of disease to be made. Patients with NPH in an early stage report in the follow up an improvement of their symptoms after a shunt operation in 65 percent of cases, and those with advanced stage NPH, in 50 percent of cases. The computer aided infusion test allows a reliable differentiation between patients with NPH and those with cerebral atrophy.

The importance of the intrathecal infusion test in the diagnostics of normal pressure hydrocephalus

OBJECTIVE

The intrathecal infusion test is a reliable method in diagnosing normal pressure hydrocephalus.

METHODS

Between May 1982 and January 1997 we investigated 200 patients suspected for a normal pressure hydrocephalus (NPH) by carrying out an intrathecal infusion test in a constant flow technique. The resistance to cerebrospinal fluid outflow (Rout) in the intrathecal infusion test was the main criterion for grouping patients into these with normal pressure hydrocephalus or those with cerebral atrophy. A further differentiation into early stage and late stage was made by measuring the compliance (Cp)--this being the secondary criterion.

RESULTS

In 107 patients (54%) the diagnosis of a NPH could be confirmed. Of these, 102 patients (95%) underwent a shunt operation. Graduation of NPH and cerebral atrophy following the results of the infusion test in an early stage and an advanced stage allows the conclusion of prognostic evaluations about the course of disease to be made. Patients with a NPH in an early stage are reporting in the follow up about an improvement of their symptoms after a shunt operation in 65 percent of cases and those with an advanced stage NPH in 50 percent.

CONCLUSION

The computer aided infusion test allows a reliable differentiation between patients with NPH and those with cerebral atrophy.

Effects of increased intracranial pressure in brain surface microcirculation in rats

The effects of increased intracranial pressure (ICP) on pial vessel diameters were investigated in rats using a cranial window and fluorescence microscopy. The brain surface was superfused with mock cerebro-spinal fluid (CSF) at a constant rate (5 ml/h), and the ICP was raised up to 20 mmHg by elevating a CSF-reservoir connected to the CSF-outlet of the window. Arterioles dilated as the ICP increased (+ 12% dilation at 20 mmHg ICP). Following a rapid reduction of the raised ICP to normal, arteriolar diameters did not return to control values (+ 7% dilatation), while venules dilated (+ 3%), indicating reactive hyperaemia. At this time, CO2 inhalation induced a low response in the arterioles (+ 0.4%/mmHg PaCO2 increase) and an over-response in the venules (+ 0.3%/mmHg). The CO2 response decreased in smaller arterioles (< or = 30-40 microns phi). In addition, a few animals showed extravasation of Na(+)-fluorescein administered intravenously. Our results indicate that reactive hyperaemia can take place following a rapid return from an increased ICP to an normal level, even in cases of mild intracranial hypertension; a disruption of the blood-brain barrier may follow.