Cortical metabolic changes and clinical outcome in normal pressure hydrocephalus after ventriculoperitoneal shunt: Our preliminary results

INTRODUCTION

Our objective was to evaluate the cortical metabolic changes and clinical outcome in patients affected by idiopathic normal pressure hydrocephalus (iNPH) after a placement of ventriculoperitoneal (VP) shunt.

MATERIALS AND METHODS

10 patients affected by suspected iNPH underwent a CSF hydrodynamics evaluation based on a lumbar infusion test (LIT). The main selection criterion for surgery was based on intracranial elasticity (IE)>0.30. All subjects with an IE>0.30 underwent a PET scan with 18 fluorodeoxiglucose (¹⁸F-FDG) at baseline (PET1) and 1 month after surgery (PET2). Furthermore, the same patients were submitted to clinical evaluation before and 1 month after surgery through neuropsychological tests and gait analysis.

RESULTS

An overall number of 20 18F-FDG PET scans were performed in all the enrolled patients. As compared to PET1, PET2 showed an increase in glucose consumption in the left frontal and left parietal lobe in PET2 as compared to PET1 (P<.001). All the enrolled patients presented a significant increase in neuropsychological scores (i.e Frontal Assessment Battery and Montreal Cognitive Assessment) and have clinically improved at gait analysis. A significant correlation was found between the increase of cortical glucose consumption in the left parietal area and the cognitive improvement as detectable by neuropsychological assessment.

CONCLUSIONS

Improvement in 18F FDG PET glucose metabolism could be considered a useful imaging marker for the assessment of iNPH response to VP shunting.

The role of ICP overnight monitoring (ONM) in children with suspected craniostenosis

PURPOSE

Secondary craniostenosis is a relevant problem pediatric neurosurgeons are confronted with and poses challenges regarding reliable diagnosis of raised ICP, especially in case of absent or questionable papilledema. How to identify children with elevated ICP is still controversial and diagnostics vary. We report on our experience with computerized ICP ONM in relation to imaging derived parameters.

METHODS

Thirty-four children with primary or secondary craniostenosis and clinical suspicion of raised ICP were investigated. We compared clinical signs, history, and radiographic assessment with the results of computerized ICP ONM. Differences were significant at a p < 0.05.

RESULTS

Baseline ICP was significantly higher in patients with combined suture synostosis, who also had a higher rate of questionable papilledema. Children with narrowed external CSF spaces in MRI had significantly higher ICP levels during REM sleep. Mean RAP was significantly elevated in patients with multi-suture synostosis, indicating poor intracranial compensatory reserve. Syndromal craniostenosis was associated with elevated ICP, RAP was significantly lower, and skull X-rays showed more impressions (copper beaten skull). RAP increased with more severe impressions only to decline in most severe abnormalities, indicating exhaustion of cerebrovascular reserve at an upper ICP breakpoint of 23.9 mmHg. Headaches correlated to lower ICP and were not associated with more severe X-ray abnormalities.

CONCLUSION

Narrowed external CSF spaces in MRI seem to be associated to elevated ICP. Skull X-rays can help to identify patients at risk for chronically elevated ICP. Severe X-ray changes correlate with exhausted cerebrovascular reserve as indicated by RAP decline. Only ICP monitoring clearly identifies raised ICP and low brain compliance. Thus, in cases with ambiguous imaging, ONM constitutes an effective tool to acquire objective data for identification of surgical candidates.

ICP Monitoring and Phase-Contrast MRI to Investigate Intracranial Compliance

OBJECTIVE

The amplitude of intracranial pressure (ICP) can be measured by ICP monitoring. Phase-contrast magnetic resonance imaging (PCMRI) can quantify blood and cerebrospinal fluid (CSF) flows. The aim of this work was to investigate intracranial compliance at rest by combining baseline ICP monitoring and PCMRI in hydrocephalus patients.

MATERIALS AND METHODS

ICP monitoring was performed before infusion testing to quantify ΔICP_rest at the basal condition in 33 suspected hydrocephalus patients (74 years). The day before, patients had had a PCMRI to assess total cerebral blood flow (tCBF), intracranial blood volume change (stroke volume SVblood), and cervical CSF volume change (the stroke volume CSV). Global (blood and CSF) intracranial volume change (ΔIVC) during each cardiac cycle (CC) was calculated. Finally, Compliance: C_rest = ΔIVC/ΔICP_rest was calculated. The data set was postprocessed by two operators according to blind analysis.

RESULTS

Bland-Altman plots showed that measurements presented no significant difference between the two operators. ΔICP_rest = 2.41 ± 1.21 mmHg, tCBF = 469.89 ± 127.54 mL/min, SVblood = 0.82 ± 0.32 mL/cc, CSV = 0.50 ± 0.22 mL/cc, ΔIVC = 0.44 ± 0.22 mL, and C_rest = 0.23 ± 0.15 mL/mmHg. There are significant relations between SVblood and CSV and also SVblood and tCBF.

CONCLUSIONS

During "basal" condition, the compliance amplitude of the intracranial compartment is heterogeneous in suspected hydrocephalus patients, and its value is lower than expected! This new parameter could represent new information, complementary to conventional infusion tests. We hope that this information can be applied to improve the selection of patients for shunt surgery.

Comparison of ventricular drain location and infusion test in hydrocephalus

OBJECTIVES

Suspected cerebrospinal fluid shunt (CSF) dysfunction in hydrocephalic patients poses a diagnostic uncertainty. The clinical picture can be non-specific and CT imaging alone is not always pathognomonic. Infusion tests are an increasingly used investigation for real-time hydrodynamic assessment of shunt patency. We report the correlation between infusion test results with the quality of ventricular drain placement on CT scans in a large retrospective group of hydrocephalic patients.

MATERIALS & METHODS

Three hundred and six infusion test results performed in 200 patients were correlated with 306 corresponding CT head scans. Nominal logistic regression was used to correlate shunt catheter position on CT imaging to patency of ventricular drain as determined by infusion tests.

RESULTS

Infusion test results of shunt patency are statistically congruent with the analysis of shunt catheter position on CT head scans. Catheter tips completely surrounded by either parenchyma or CSF on CT imaging are strongly associated with evidence of occlusion or patency from infusion tests, respectively (χ² = 51.68, P < 0.0001, n = 306 and χ² = 31.04, P < 0.0001, n = 306).

CONCLUSIONS

The most important anatomical factor for shunt patency is the catheter tip being completely surrounded by CSF. Infusion tests provide functional and reliable assessment of shunt patency in vivo and are strongly correlated with the position of the ventricular catheter on CT imaging.

Intracranial pressure, its components and cerebrospinal fluid pressure-volume compensation

Clinical measurement of intracranial pressure (ICP) is often performed to aid diagnosis of hydrocephalus. This review discusses analysis of ICP and its components' for the investigation of cerebrospinal fluid (CSF) dynamics. The role of pulse, slow and respiratory waveforms of ICP in diagnosis, prognostication and management of hydrocephalus is presented. Two methods related to ICP measurement are listed: an overnight monitoring of ICP and a constant-rate infusion study. Due to the dynamic nature of ICP, a 'snapshot' manometric measurement of ICP is of limited use as it might lead to unreliable results. Therefore, monitoring of ICP over longer time combined with analysis of its waveforms provides more detailed information on the state of pressure-volume compensation. The infusion study implements ICP signal processing and CSF circulation model analysis in order to assess the cerebrospinal dynamics variables, such as CSF outflow resistance, compliance of CSF space, pressure amplitude, reference pressure, and CSF formation. These parameters act as an aid tool in diagnosis and prognostication of hydrocephalus and can be helpful in the assessment of a shunt malfunction.

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.

Clinical assessment of cerebrospinal fluid dynamics in hydrocephalus. Guide to interpretation based on observational study

OBJECTIVES

The term hydrocephalus encompasses a range of disorders characterised by clinical symptoms, abnormal brain imaging and derangement of cerebrospinal fluid (CSF) dynamics. The ability to elucidate which patients would benefit from CSF diversion (a shunt or third ventriculostomy) is often unclear. Similar difficulties are often encountered in shunted patients to predict the scope for improvement by shunt re-adjustment or revision. In this study we aimed to update our knowledge of how key quantitative parameters describing CSF dynamics may be used in diagnosis of shunt-responsive hydrocephalus and in the assessment of shunt function.

METHODS

A number of quantitative parameters [including resistance to CSF outflow (Rcsf), pulse amplitude of intracranial pressure waveform (AMP), RAP index and slow vasogenic waves] were studies in 1423 patients with 2665 CSF infusion tests and 305 overnight intracranial pressure (ICP)-monitoring sessions over a 17 year period.

OBSERVATIONS

We demonstrate our observations for typical values of Pb, Rcsf, AMP, slow vasogenic waves derived from infusion studies or overnight ICP monitoring in differentiating atrophy from shunt-responsive normal pressure hydrocephalus or acute hydrocephalus. From the same variables tested on shunted patients we demonstrate a standardised approach to help differentiate a properly-functioning shunt from underdrainage or overdrainage.

CONCLUSIONS

Quantitative variables derived from CSF dynamics allow differentiation between clinically overlapping entities such as shunt-responsive normal pressure hydrocephalus and brain atrophy (not shunt responsive) as well as allowing the detection of shunt malfunction (partial or complete blockage) or overdrainage. This observational study is intended to serve as an update for our understanding of quantitative testing of CSF dynamics.

Impact of duration of symptoms on CSF dynamics in idiopathic normal pressure hydrocephalus

OBJECTIVE

Cerebrospinal fluid (CSF) pressure-volume compensation may change over time as part of normal ageing, where the resistance to CSF outflow increases and the formation of CSF decreases with age. Is CSF compensation dependent on duration of symptoms in idiopathic normal pressure hydrocephalus (iNPH)?

METHODS

We investigated 92 patients presenting with iNPH. Mean age was 73 (range 47-86). There were 60 men and 32 women. They all presented with gait disturbance and ventricular dilatation. Memory deficit occurred in 72% and urinary incontinence in 52% of patients. All patients underwent computerized CSF infusion tests. Sixty-four shunted patients were available for follow-up, and their improvement was expressed using the NPH score.

RESULTS

Mean intracranial pressure (ICP) was 10.1±5.1 mmHg, and mean resistance to CSF outflow was 17.3±5.2 mmHg/(ml/min). Mean duration of symptoms was 24±19 months (range from 2 weeks to 86 months). Baseline ICP, magnitude of ICP pulse waveform, brain compliance and improvement after shunting (72% of patients improved) did not exhibit any dependency on the duration of symptoms. The resistance to CSF outflow showed a strong tendency to decrease in time with the duration of symptoms beyond 2 years (R= -0.702; P<0.005).

CONCLUSION

This is a preliminary observation, and it suggests that for patients with duration of symptoms longer than 2-3 years, the threshold for normal resistance to CSF outflow should be duration-adjusted.

Evaluation of the cerebrovascular pressure reactivity index using non-invasive finapres arterial blood pressure

A pressure reactivity index (PRx) can be assessed in patients with continuous monitoring of arterial blood pressure (ABP) and intracranial pressure (ICP) as a moving correlation coefficient between slow fluctuations of these two signals within a low frequency bandwidth. The study aimed to investigate whether the invasive ABP monitoring can be replaced with non-invasive measurement of ABP using a Finapres plethysmograph (fABP) to calculate the fPRx. There is a well-defined group of patients, suffering from hydrocephalus and undergoing CSF pressure monitoring, which may benefit from such a measurement. 41 simultaneous day-by-day monitoring of ICP, ABP and fABP were performed for about 30 min in 10 head injury patients. A Bland-Altman assessment for agreement was used to compare PRx and fPRx calculations. Performance metrics and the McNemary test were used to determine whether fPRx is sensitive enough to distinguish between functioning and disturbed cerebrovascular pressure reactivity. The fPRx correlated with PRx (R(Spearman) = 0.92, p < 0.001; bias = -0.04; lower and upper limits of agreement: -0.26 and 0.17, respectively). The fPRx distinguished between active and passive reactivity in more than 89% cases. The fPRx can be used with care for assessment of cerebrovascular reactivity in patients for whom invasive ABP measurement is not feasible. The fPRx is sensitive enough to distinguish between functional and deranged reactivity.

Link between vasogenic waves of intracranial pressure and cerebrospinal fluid outflow resistance in normal pressure hydrocephalus

Recent studies on normal pressure hydrocephalus (NPH) have pointed to a possible link between the disturbance in CSF circulation and cerebrovascular factors. We investigated the quantitative relationship between the resistance to CSF outflow (Rcsf) and vasogenic waves of ICP in patients with normal pressure hydrocephalus. Forty-five patients with NPH were investigated by an infusion study. The magnitudes of vasogenic ICP components: pulse, respiratory and slow vasogenic waves were assessed, and compared with Rcsf. Both baseline respiratory and slow waves of ICP were positively correlated with Rcsf. The respiratory wave at baseline was a single independent predictor of Rcsf (r = 0.66, p < 0.0002). All vasogenic components increased significantly during the infusion test. The magnitude of the increase was positively correlated with Rcsf. The vasogenic ICP waves, notably the respiratory wave of ICP, correlate with the resistance to CSF outflow.

How does CSF dynamics change after shunting?

OBJECTIVE

Hydrocephalus is much more complex than a simple disorder of cerebrospinal fluid (CSF) circulation. Shunting primarily corrects disturbed fluid flow which may have an impact on cerebral blood flow and metabolism. We studied hydrocephalic patients before and after shunting to characterize changes in their CSF compensatory parameters.

MATERIAL AND METHODS

We selected 25 patients and studied them retrospectively. All patients had ventriculomegaly and clinical symptoms of normal pressure hydrocephalus. After shunting, they were still presenting with some adverse symptoms, mainly headaches, slow improvement or no improvement of ventriculomegaly. Therefore, they underwent further infusion studies to assess shunt function. In all cases, the shunts were confirmed to be draining CSF adequately. Parameters of CSF dynamics: baseline intracranial pressure (ICP), resistance to CSF outflow, cerebrospinal elasticity, content of vasogenic pressure waves (pulse, respiratory and B waves) and compensatory reserve assessed as moving correlation coefficient between mean CSF pressure and pulse amplitude (RAP), were compared before and after shunting.

RESULTS

Mean ICP and resistance to CSF outflow decreased (P < 0.003) after shunting. All vasogenic pressure waves decreased (P < 0.005). Compensatory reserve (RAP) significantly improved (P < 0.005).

CONCLUSION

A functioning shunt has an important impact on CSF circulation and pressure-volume compensation. Infusion studies can demonstrate the return of disturbed CSF dynamics to normal values even if clinical or radiological changes are not dramatic.

Pulse amplitude of intracranial pressure waveform in hydrocephalus

BACKGROUND

There is increasing interest in evaluation of the pulse amplitude of intracranial pressure (AMP) in explaining dynamic aspects of hydrocephalus. We reviewed a large number of ICP recordings in a group of hydrocephalic patients to assess utility of AMP.

MATERIALS AND METHODS

From a database including approximately 2,100 cases of infusion studies (either lumbar or intraventricular) and overnight ICP monitoring in patients suffering from hydrocephalus of various types (both communicating and non-communicating), etiology and stage of management (non-shunted or shunted) pressure recordings were evaluated. For subgroup analysis we selected 60 patients with idiopathic NPH with full follow-up after shunting. In 29 patients we compared pulse amplitude during an infusion study performed before and after shunting with a properly functioning shunt. Amplitude was calculated from ICP waveforms using spectral analysis methodology.

FINDINGS

A large amplitude was associated with good outcome after shunting (positive predictive value of clinical improvement for AMP above 2.5 mmHg was 95%). However, low amplitude did not predict poor outcome (for AMP below 2.5 mmHg 52% of patients improved). Correlations of AMP with ICP and Rcsf were positive and statistically significant (N = 131 with idiopathic NPH; R = 0.21 for correlation with mean ICP and 0.22 with Rcsf; p< 0.01). Correlation with the brain elastance coefficient (or PVI) was not significant. There was also no significant correlation between pulse amplitude and width of the ventricles. The pulse amplitude decreased (p < 0.005) after shunting.

CONCLUSIONS

Interpretation of the ICP pulse waveform may be clinically useful in patients suffering from hydrocephalus. Elevated amplitude seems to be a positive predictor for clinical improvement after shunting. A properly functioning shunt reduces the pulse amplitude.

Intracranial baroreflex yielding an early cushing response in human

The Cushing response is a pre-terminal sympatho-adrenal systemic response to very high ICP. Animal studies have demonstrated that a moderate rise of ICP yields a reversible pressure-mediated systemic response. Infusion studies are routine procedures to investigate, by infusing CSF space with saline, the cerebrospinal fluid (CSF) biophysics in patients suspected of hydrocephalus. Our study aims at assessing systemic and cerebral haemodynamic changes during moderate rise of ICP in human. Infusion studies were performed in 34 patients. This is a routine test perform in patients presenting with symptoms of NPH during their pre-shunting assessment. Arterial blood pressure (ABP) and cerebral blood flow velocity (FV) were non-invasively monitored with photoplethysmography and transcranial Doppler. The rise in ICP (8.2 +/- 5.1 mmHg to 25 +/- 8.3 mmHg) was followed by a significant rise in ABP (106.6 +/- 29.7 mmHg to 115.2 +/- 30.1 mmHg), drop in CPP (98.3 +/- 29 mmHg to 90.2 +/- 30.7 mmHg) and decrease in FV (55.6 +/- 17 cm/s to 51.1 +/- 16.3 cm/s). Increasing ICP did not alter heart rate (70.4 +/- 10.4/min to 70.3 +/- 9.1/min) but augmented the heart rate variance (0.046 +/- 0.058 to 0.067 +/- 0.075/min). In a population suspected of hydrocephalus, our study demonstrated that a moderate rise of ICP yields a reversible pressure-mediated systemic response, demonstrating an early Cushing response in human and a putative intracranial baroreflex.

The relationship between CSF circulation and cerebrovascular pressure-reactivity in normal pressure hydrocephalus

OBJECTIVE

Previously, we documented association between CSF circulation and transcranial-Doppler derived autoregulation in non-shunted patients suffering from hydrocephalus. In the present study we sought to investigate the relationship between the resistance to CSF outflow and pressure-reactivity both in shunted and non-shunted NPH patients.

MATERIAL AND METHODS

Sixty-eight patients (47 non-shunted and 21 shunted) with NPH have been examined as a part of routine diagnostic procedure. Resistance to CSF outflow (Rcsf) was measured using a ventricular constant rate infusion test. Cerebrovascular pressure-reactivity was assessed as a moving correlation coefficient (PRx) between coherent 'slow waves' of ICP and arterial blood pressure (ABP). This variable has previously been demonstrated to correlate with the autoregulation of CBF in patients following head injury. Results. In non-shunted patients cerebrovascular pressure-reactivity (PRx) was negatively correlated with Rcsf (R = -0.5; p < 0.0005). This relationship was inverted in shunted patients: a positive correlation between PRx and Rcsf was found (R = 0.51; p < 0.03).

CONCLUSION

Cerebrovascular pressure-reactivity is disturbed in patients with normal resistance to CSF outflow, suggesting underlying cerebrovascular disease. This result confirms our previous finding where transcranial Doppler autoregulation was investigated. After shunting the pressure-reactivity strongly depends on shunt functioning and deteriorates when the shunt is blocked.

Clinical testing of CSF circulation in hydrocephalus

INTRODUCTION

Recent 'NPH Dutch trial' has re-emphasised the importance of the resistance to cerebrospinal fluid (CSF) outflow (Rcsf) in the diagnosis of hydrocephalus. We re-evaluated the clinical utility of the physiological measurements revealing CSF dynamics. The results were summarized from our previous publications. The Computerised Infusion Test was designed to perform quick and low-invasive assessment of CSF dynamics described by parameters as Rcsf, brain compliance, elasticity coefficient, estimated sagittal sinus pressure, CSF formation rate and other variables. Overnight ICP monitoring with quantitative analysis of CSF dynamics was used in those cases where infusion study was unreliable or producing results close to the borderline. We found that the threshold of normal and increased Rcsf should be age-matched because in patients older than 55 Rcsf increases 0.2 mm Hg/(ml/min) per year (p < 0.04: N = 56). Rcsf was positively correlated with cerebral autoregulation (R = 0.41; p < 0.03; N = 36) indicating that in patients with symptoms of NPH but normal Rcsf underlying cerebrovascular disease is more frequent. Computerized infusion tests and overnight ICP monitoring are useful diagnostic technique alone or in conjunction with other forms of physiological measurement.

Factors determining mean ICP in hydrocephalic patients with Hakim-programmable valve: implications of the parallel arrangement of the CSF outflow resistance and shunt

OBJECTIVE

Measurement of CSF pressure is used clinically to test shunt function in vivo in hydrocephalic patients. Criteria for appropriate shunt function have never been validated.

METHOD

Hakim-Programmable valve was tested in a model of CSF circulation with variable resistance to CSF outflow (from 12 to 50 mm Hg/ml/min), increased hydrodynamic compliance (> 1.5 ml/mm Hg), and constant perfusion of a rate of 0.4 ml/min, i.e. conditions typical for hydrocephalus. The main question was how the simulated CSF pressure was influenced by the shunt setting and the residual resistance to CSF outflow.

RESULTS

Measured baseline CSF pressure correlated well with shunt operating pressure only when high resistance to CSF outflow (50 mm Hg/(ml/min)) was used. For the medium resistance (20 mm Hg/(ml/min)) operating pressure was strongly affected by system's absorption capacity. For low resistance (12 mm Hg/(ml/min)) operating pressure through the valve was independent on valve's settings and no fluid drainage through the valve was recorded.

CONCLUSION

Patients with moderately elevated resistance to CSF outflow (12-18 mm Hg/(ml/min)) cannot possibly react to changes of the valve's settings above 100 mm H2O. Mean CSF pressure results both from shunt setting and patient's own re-absorption capacity.

Hysteresis of the cerebrospinal pressure-volume curve in hydrocephalus

The objective was to study the displacement of the cerebrospinal fluid pressure-volume curve during the descent relative to the ascent of intracranial pressure recorded during the cerebrospinal fluid constant rate infusion test. This phenomenon can be interpreted as the hysteresis of the pressure-volume curve. The cerebrospinal fluid dynamics were tested in fifty-eight patients with clinical symptoms of hydrocephalus. After finished infusion, ICP was recorded until it returned to steady state level. Pressure-volume curves were plotted separately for ascending and descending phases of the test. The parameters of CSF compensation were estimated on the basis of mathematical mono-exponential model of CSF circulation. The pressure-volume curve post-infusion was visibly shifted upward in 69% of tests. Those who demonstrated the upward shift of the pressure-volume curve had greater an elastance coefficient of the cerebrospinal space (with shift: E1 = 0.26 +/- 0.14; without shift: E1 = 0.17 +/- 0.06; p < 0.05). Magnitude of the shift was positively correlated with pulse amplitude of ICP (r = -0.763; p < 0.0001). The accuracy of clinical examination of the pressure-volume compensatory reserve, which take into account both compression and decompression phase of the study, may be affected by this phenomenon.

Continuous assessment of cerebral autoregulation: clinical and laboratory experience

The method for the continuous assessment of cerebral autoregulation using slow waves of MCA blood flow velocity (FV) and cerebral perfusion pressure (CPP) or arterial pressure (ABP) has been introduced seven years ago. We intend to review its clinical applications in various scenarios. Moving correlation coefficient (3-6 min window), named Mx, is calculated between low-pass filtered (0.05 Hz) signals of FV and CPP or ABP (when ICP is not measured directly). Data from ventilated 243 head injuries and 15 patients after poor grade subarachnoid haemorrhage, 38 patients with Carotid Artery stenosis, 35 patients with hydrocephalus and fourteen healthy volunteers is presented. Good agreement between the leg-cuff test and Mx has been confirmed in healthy volunteers (r = 0.81). Mx also correlated significantly with the static rate of autoregulation and transient hyperaemic response test. Autoregulation was disturbed (p < 0.021) by vasospasm after SAH and worse in patients with hydrocephalus in whom CSF circulation was normal (p < 0.02). In head injury, Mx indicated disturbed autoregulation with low CPP (< 55 mmHg) and too high CPP (> 95 mmHg). Mx strongly discriminated between patients with favourable and unfavourable outcome (p < 0.00002). This method can be used in many clinical scenarios for continuous monitoring of cerebral autoregulation, predicting outcome and optimising treatment strategies.

Laboratory testing of hydrocephalus shunts -- conclusion of the U.K. Shunt evaluation programme

16 models of valves, currently in use in the U.K., have been tested long-term in the U.K. Shunt Evaluation Laboratory according to the protocol based on the new ISO 7197 standard. Valves tested were: Medtronic PS Medical: Delta Valve, Flow Control and Lumbo-Peritoneal Shunt, Heyer-Schulte Nero-Care: In-line, Low Profile and Pudenz Flushing Valve, Codman: Codman-Hakim Programmable, Hakim Precision, Accu-Flo, Holter, Uni-Shunt, and siphon-preventing device -- SiphonGuard, NMT: Orbis-Sigma Valve, Omni-Shunt and Hakim Valve, Sophysa: Sophy Programmable Valve, Radionics: Contour-Flex Valve. The majority of the valves had a non-physiologically low hydrodynamic resistance (with the exception of Orbis-Sigma, PS Lumbo-Peritoneal and Heyer-Schulte In-Line). This may result in overdrainage both related to posture and during nocturnal cerebral vasogenic waves. A long distal catheter increases the resistance of these valves by 100-200%. Drainage through valves without siphon-preventing mechanism is very sensitive to body posture. This may produce grossly negative intracranial pressure after implantation. A few shunts (Delta, Low Profile and Pudenz-Flushing with Anti-Siphon Devices) offer a reasonable resistance to negative outlet pressure, and hence potentially might prevent complications related to overdrainage. On the other hand, valves with siphon-preventing devices may be blocked by raised subcutaneous pressure (exception: SiphonGuard, but this device may block the drainage because of its faulty design). In most of the silicone-diaphragm valves, closing pressure varied and reached values lower than that specified by the manufacturer (exception: Heyer-Schulte Pudenz Flushing Valve). All programmable valves are susceptible to overdrainage in the upright body position. Programmed settings may be changed by external magnetic fields. Most shunts are very sensitive to the presence of small particles in the drained fluid. The behavior of a valve revealed during such testing is of immediate relevance to the surgeon and may not be adequately described in the manufacturer's product information. These results are also relevant to the assessment of shunt function in-vivo using an infusion test.

A laboratory model of testing shunt performance after implantation

Constant rate infusion tests are used clinically to test shunt function in vivo in hydrocephalic patients. The criteria for appropriate shunt function have never been validated in the laboratory. Nine of the most commonly used types of hydrocephalus valves construction were selected and tested in a model of the CSF circulation incorporating increased resistance to CSF outflow [24 mmHg/(ml/min)] and decreased hydrodynamic compliance (<2 ml/mmHg), that are typical conditions in hydrocephalus. The aim was to document the pressure response to constant rate infusion of a model of CSF circulation with different valves and to define which measures are useful in shunt testing in vivo. The pressure-course of simulated CSF pressure was established and proved to be equivalent to clinical results. The baseline CSF pressure failed to correlate with shunt operating pressure for medium pressure valves (R = 0.14, p > 0.05). End-equilibrium pressure recorded during infusion correlated strongly with the opening pressure (R = 0.94, p = 0.0001) and the shunt's resistance (R = 0.86, p = 0.0026). The infusion test is able to assess shunt function. End-equilibrium pressure recorded during the test has been confirmed to correlate with the shunt's performance.

Vascular components of cerebrospinal fluid compensation

OBJECT

The aim of the study was to assess how cerebrospinal fluid (CSF) pressure-volume compensation depends on cerebrovascular tone.

METHODS

In 26 New Zealand White rabbits, intracranial pressure (ICP), arterial blood pressure, and basilar artery blood flow velocity were measured continuously. Saline was infused into the cranial subarachnoid space to assess CSF compensatory parameters: the resistance to CSF outflow, the elastance coefficient, and the amplitude of the ICP pulsatile waveform. Infusions were repeated on two different levels of CO2 concentration in the arterial blood (PaCO2), at normotension and hypotension, and after the death of the animal. An increase in PaCO2 from a mean of 27 to 48 mm Hg was accompanied by an 18% increase in the resistance to CSF outflow (p<0.005) and a 64% increase (p<0.05) in the elastance coefficient. A decrease in arterial blood pressure from a mean of 100 to 51 mm Hg caused a 25% decrease in CSF outflow resistance (p<0.01) but did not affect the elastance coefficient. Postmortem, a 23% decrease in the CSF outflow resistance was associated with a 102% decrease in the elastance coefficient.

CONCLUSIONS

Cerebrovascular parameters have a limited but significant impact on CSF infusion studies. The vascular component of ICP may be identified as a significant factor contributing to this phenomenon. During infusion studies, physiological parameters influencing vascular conditions should be maintained as stable as possible.

Hydrodynamic properties of hydrocephalus shunts

Hydrodynamic properties of hydrocephalus shunts are not always properly characterized by the manufacturer. Therefore, the choice of the shunt should be made, by matching performance of the shunt to the disturbed profile of CSF circulation of a given patient. The aim of the present shunt evaluation study is to evaluates all types of shunts presently in use in the U.K. and make this information available to neurosurgeons. Ten most common models of valves have been tested to date: Medtronik PS Medical: Delta Valve, Flow Control Valves and Lumbo-Peritoneal Shunt, Heyer-Schulte: In-line, Low Profile and Pudenz Flushing Valve, Codman: Medos-Programmable, Hakim-Precision, Sophy Programmable Valve, Cordis Orbis-Sigma. Our results show the majority of valves have low hydrodynamic resistance (exception: PS Lumboperitoneal, Orbis-Sigma), which increase by 100-200% after connection of a long distal catheter. A few shunts with siphon-preventing mechanism (Delta, Hayer-Schulte Low Profile, Pudenz-Flushing) offer reasonable resistance to negative outlet pressures, however, these valves may be blocked by raised subcutaneous pressure. All programmable valves are susceptible to siphoning. Programmed settings may be changed by external magnetic field.

Posture-related overdrainage: comparison of the performance of 10 hydrocephalus shunts in vitro

OBJECTIVE

Approximately 10 to 30% of shunt revisions may be attributed to posture-related overdrainage. The susceptibility of various hydrocephalus shunts to overdrainage of cerebrospinal fluid requires independent laboratory evaluation.

METHODS

Shunts were tested in vitro by using precise computer-controlled equipment that was able to evaluate pressure-flow performance curves under various conditions. Hydrodynamic resistance and opening, closing, and operational pressures were evaluated for at least 28 days with normal (atmospheric) and decreased (-23 mm Hg, based on the International Standard Organization/Draft International Standard 7197 standard, which simulates conditions in upright body positions) outlet pressures.

RESULTS

Ten different models of valves have been tested to date (Medtronic PS Medical Delta valve, flow-control valve, and lumboperitoneal shunt, Heyer-Schulte in-line, low-profile, and Pudenz flushing valves, Codman-Medos programmable and nonprogrammable valves, Sophy programmable valve, and Cordis Orbis-Sigma valve). The majority of these valves produced significantly negative (less than -10 mm Hg) average intracranial pressures in vertical body positions. In conjunction with nonphysiologically low hydrodynamic resistance (with the exception of the Orbis-Sigma valve, Medtronic PS Medical lumboperitoneal shunt, and Heyer-Schulte in-line valve), this may result in overdrainage related to body posture. The clinically reported rate of complications related to overdrainage is probably reduced by the long distal catheter, which increases the resistance of these valves by 100 to 200%. A few shunts (the Delta valve, low-profile valve, and Pudenz flushing valve with anti-siphon devices) offer reasonable resistance to negative outlet pressure, preventing complications related to overdrainage, but all valves with siphon-preventing devices may be blocked by increased subcutaneous pressure.

CONCLUSION

Shunts without mechanisms preventing very low intracranial pressure in vertical body positions should be identified and avoided for patients likely to develop complications related to cerebrospinal fluid overdrainage.

Laboratory testing of the Spiegelberg brain pressure monitor: a technical report

OBJECTIVES

The Spiegelberg brain pressure monitor is a low cost intracranial pressure monitoring system that has been used clinically for some time, mainly in Germany. To provide a rigorous bench comparison of the Spiegelberg monitor with the Camino pressure monitor an evaluation programme has been carried out in the UK Shunt Evaluation Laboratory.

DESIGN

Drift over 72 hours and with temperature, a frequency response, and the accuracy of measurement of both static and pulsatile pressures have been tested simultanously in Camino and Spiegelberg transducers using a computerised rig.

RESULTS

Long term zero drift was less than 0.7 mm Hg in both transducers. The Spiegelberg monitor showed no temperature drift whereas the Camino monitor had a drift of around 0.3 mm Hg/degrees C. The Spiegelberg monitor underread mean pressures <40 mm Hg by <1 mm Hg, but the error increased to 4.7 mm Hg at 100 mm Hg. The frequency bandwidth of the Spiegelberg monitor was 4 Hz at a low pressure. Underreading of the amplitude increased with the mean pressure, with a delay of about 0.1 s in the detection of the peaks of pulse waveform.

CONCLUSION

The Spiegelberg transducer had excellent accuracy for static intracranial pressure measurement, but complex waveform analysis may be biased by its limited dynamic response.

Hydrodynamic properties of hydrocephalus shunts: United Kingdom Shunt Evaluation Laboratory

BACKGROUND

Although about 80% of properly diagnosed patients with hydrocephalus improve after implantation of any model of shunt, the remaining 20% may develop further complications because of inadequate shunt performance. Therefore, hydrocephalus shunts require careful independent laboratory evaluation.

METHOD

Computer supported shunt testing, based on the new International Standard Organisation directives, characterises various aspects of pressure-flow performance of shunts such as variability with time, susceptibility to reflux, siphoning, temperature related behaviour, external pressure, the influence of a strong magnetic field (for example, MRI), presence of pulsation in differential pressure, particles in drained fluid, etc.

RESULTS

Seven different models of valves, representing most common constructions, have been tested so far. Most contemporary valves have a hydrodynamic resistance which is too low. This may result in overdrainage both related to posture and during nocturnal cerebral vasogenic waves. A long distal catheter increases the resistance of these valves by 100%-200%. Most shunts are very sensitive to the presence of air bubbles and small particles in drained fluid. Few shunt models offer reasonable resistance to negative outlet pressure, preventing complications related to overdrainage. Valves with an antisiphon device may be blocked by raised subcutaneous pressure. All programmable valves are susceptible to overdrainage in an upright position.

CONCLUSION

The behaviour of a valve during such testing is of immediate relevance to the surgeon and may not be adequately described in the manufacturer's product information.

Systematic overestimation of intracranial pressure measured using a Camino pressure monitor

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Significance of intracranial pressure waveform analysis after head injury

The authors have investigated the relationships between the amplitude of the ICP pulse wave, the mean values of ICP and CPP, and the outcome of 56 head injured ventilated patients. The ICP was monitored continuously using a Camino transducer (35 patients) or subdural catheter (21 patients). The mean Glasgow Coma Score was 6 (range 3-13; 5 patients had a GCS > 8 after resuscitation). Patients were grouped according to their Glasgow Outcome Score assessed at 12 months after injury. The amplitude of ICP pulse waveform was assessed using the fundamental harmonic of the pulse waveform (AMP) to avoid distortion caused by different frequency responses of the pressure transducers used in the study. Statistical analysis revealed that in patients with fatal outcome the ICP pulse amplitude increased when the mean ICP increased to 25 mmHg and then began to decrease. The upper breakpoint of the AMP-ICP relationship was not present in patients with good/moderate outcome. The moving correlation coefficient between the fundamental harmonic of ICP pulse wave and the mean ICP (RAP: R-symbol of correlation between A-amplitude and P-pressure) was introduced to describe the time-dependent changes in correlation between amplitude and mean ICP. The RAP was significantly lower in patients who died or remained in the vegetative state. In 7 patients who died from uncontrollable intracranial hypertension RAP was oscillating or decreased to 0 or negative values well before brain-stem herniation. The combination of an ICP above 20 mmHg for a period longer than 6 hours with low correlation between the amplitude and pressure (RAP < 0.5) was described as an predictive index of an unfavourable outcome.

Laboratory testing of three intracranial pressure microtransducers: technical report

Three comparatively priced intracranial pressure (ICP) microtransducers are now available, each characterized by the manufacturer as having very low zero drift over long periods, an excellent frequency response, and a low measurement error. The three microtransducers, coded Transducer A (Camino OLM ICP monitor; Camino Laboratories, San Diego, CA), Transducer B (Codman Microsensor ICP Transducer; Codman & Shurtlef Inc., Randolph, MA), and Transducer C (ICP Monitoring Catheter Kit OPX-SD [4F]; InnerSpace Medical, Irvine, CA), were examined in a pressure-flow test rig designed for assessment of hydrocephalus shunts. All three microtransducers compiled with the manufacturers' specifications and gave high-quality readings under test conditions. However, some differences were noted; Transducer C had the lowest 24-hour zero drift (drifts in all transducers were < 0.8 mm Hg). The temperature drift was very low in Transducer B and C, but Transducer A had a significantly higher drift (0.27 mm Hg/degrees C). Transducer A had a static error < 0.3 mm Hg, Transducer B < 2 mm Hg, and Transducer C < 8 mm Hg. Frequency detection in Transducers A and B were very good (bandwidth, > 30 Hz), whereas Transducer C had a limited bandwidth of 20 Hz. Transducer B scored the best overall, but all three scored satisfactorily during bench testing.

Comparison between classic-differential and automatic shunt functioning on the basis of infusion tests

Infusion tests were performed in order to examine cerebrospinal compensatory ability in two groups of patients with impaired compensation, subjected to shunt implantation. The functioning of the classic differential shunts was compared to the automatically (Orbis-Sigma) functioning shunts. The reference group, with intact compensatory parameters is also presented. The influence of different types of shunts on the intracranial compensation ability was compared. Automatic shunts produce less decrease in the resorption resistance of CSF in the post-shunted examination than the classic differential shunts. Classic differential shunts disturb the mono-exponential character of the pressure-volume relationship in a higher degree than automatic shunts. Shunt functioning models were proposed as well as the method of detection of eventual recovery to the normal resorption reserve. This method can be applied only to patients with automatic shunts.