Observational study of intracranial pressure instability in patients with pseudotumour cerebri syndrome.

Introduction: A fixed CSF pressure (CSFp) of 25 cmH2O (18 mmHg) has been utilised to date to define and classify pseudotumour cerebri syndrome (PTCS). Furthermore, ICP monitoring, and CSF infusion tests have not been frequently performed in this group of patients. Research question: We aimed to report typical, unusual and unstable patterns of ICP in patients with PTCS. Material and methods: We reviewed the recordings of CSF infusion tests and overnight ICP monitoring of patients with suspected or confirmed IIH between January 2003-December 2020.We excluded all patients with a shunt in situ and selected recordings that represented unstable patterns of ICP changes in PTCS. Results: 463 CSF infusion tests and 26 ICP monitorings of PTCS patients had been performed in this timeframe. We divided results of observed pattern into two group: those with known venous sinus measurements (Group A) and those without (Group B). Observed recordings formed a total of 5 and 4 different patterns respectively, based on the behaviour of ICP and slow waves at rest, overnight, and during infusion as well as in relationship to the clinical presentation of each patient. Discussion and conclusion: Accurate monitoring of ICP in PTCS is quintessential. Full understanding of each element of its pathophysiology and their interaction would be essential and include quantification of the CSF pressure not only as a number, but also with consideration of its dynamic contents. Cerebral venous pressure measurements and/or monitoring may be useful. Consideration of the presence or absence of papilloedema in the context of disturbed CSF dynamics could reveal further diagnostic and therapeutic insights.

Development of normal pressure hydrocephalus following post-traumatic external hydrocephalus in an adult patient.

We report the case of 74-year-old patient suspected of post-traumatic external hydrocephalus (EH) following a mild traumatic brain injury with a progressive neurological decline and a concomitant enlargement of subarachnoid spaces without ventriculomegaly on CT scan. A lumbar puncture revealed raised ICP and a careful CSF withdrawal was performed, resulting in an immediate neurological improvement, confirming the diagnosis of EH. During the 20-month follow-up, the patient presented progressive signs of normal pressure hydrocephalus (NPH): gait and cognitive decline, ventriculomegaly and the lumbar infusion study confirmed disturbed CSF dynamics. The patient underwent a ventriculoperitoneal shunt surgery, resulting in a long-lasting improvement.

External Hydrocephalus After Traumatic Brain Injury: Retrospective Study of 102 Patients.

INTRODUCTION: External hydrocephalus (EH) refers to impairment of extra-axial cerebrospinal fluid flow with enlargement of the subarachnoid space (SAS) and concomitant raised intracranial pressure (ICP). It is often confused with a subdural hygroma and overlooked, particularly when there is no ventricular enlargement. In this study, we aimed to describe the epidemiology of EH in a large population of adults with traumatic brain injury (TBI). METHODS: This observational, retrospective cohort study was conducted in adult patients who were admitted with TBI to the Department of Clinical Neuroscience at Addenbrooke's Hospital (Cambridge, UK) over a period of 3 years (2014-2017). Patients were included in the study if they had ICP monitoring and at least three CT scans within the first 21 days to assess SAS evolution. Patients who underwent a decompressive craniectomy were excluded. SAS was assessed individually on each CT scan by two independent investigators. ICP data were analysed with ICM+ software (Cambridge Enterprise Ltd., Cambridge, UK). Short-term and 6-month outcomes were examined. The groups of patients with and without EH were compared. RESULTS: Of the 102 patients included in the study, 30.4% developed EH after a delay of 2.98 ± 2.4 days. The initial Glasgow Coma Scale (GCS) scores did not differ between patients with and without EH. Subarachnoid haemorrhage was found to be the main risk factor for EH. Patients with EH required a significantly longer period of mechanical ventilation (+6.9 days), were more likely to have a tracheostomy (55% versus 33%), and had a longer stay in the intensive care unit (+8.5 days). ICP was higher during the 48 h after diagnosis of EH than during the previous 48 h. EH survivors had a lower mean Glasgow Outcome Scale Extended (GOS-E) score (4.6 versus 5.9, P = 0.031) and were more likely to receive a permanent shunt for secondary hydrocephalus (17.4% versus 1.8%, odds ratio 7.1). CONCLUSION: In adults with TBI, EH remains insufficiently understood and probably underdiagnosed. This study showed that it is a frequent complication of TBI, with significant clinical consequences.

Analysis of Intracranial Pressure Pulse-Pressure Relationship: Experimental Validation.

The slope of linear relationship between the amplitude of pulsations in intracranial pressure (ICP) versus mean ICP has recently been suggested as a useful guide for selecting patients for shunt surgery in normal pressure hydrocephalus (NPH). To better understand how the pathophysiology of cerebral circulation influences this parameter, we aimed to study the relationship between mean pressure and pulsation amplitude in a wide range of conditions affecting cerebrovascular tone and ICP in experimental conditions.We retrospectively analysed experimental material collected previously. Three physiological manoeuvres were studied in 29 New Zealand white rabbits: lumbar infusion with an infusion rate ≤0.2 mL/min to induce mild intracranial hypertension (n = 43), sympathetic blockade to induce arterial hypotension (n = 19), and modulation of the ventilator tidal volume, simultaneously influencing arterial carbon dioxide partial pressure (PaCO2) to induce hypocapnia or hypercapnia (n = 17). We investigated whether the slope of the pulse amplitude (AMP)-ICP line depended on PaCO2 and arterial blood pressure (ABP) changes.We found a linear correlation between AMP-ICP and ICP with positive slope. Regression of slope against mean ABP showed a negative dependence (p = 0.03). In contrast, the relationship between slope and PaCO2 was positive, although not reaching statistical significance (p = 0.18).The slope of amplitude-pressure line is strongly modulated by systemic vascular variables and therefore should be taken as a descriptor of cerebrospinal fluid dynamics with great care.

Lower Breakpoint of Intracranial Amplitude-Pressure Relationship in Normal Pressure Hydrocephalus.

The relationship between intracranial pulse amplitude (AMP) and mean intracranial pressure (ICP) has been previously described. Generally, AMP increases proportionally to rises in ICP. However, at low ICP a lower breakpoint (LB) of amplitude-pressure relationship can be observed, below which pulse amplitude stays constant when ICP varies. Theoretically, below this breakpoint, the pressure-volume relationship is linear (good compensatory reserve, brain compliance stays constant); above the breakpoint, it is exponential (brain compliance decreases with rising ICP).Infusion tests performed in 169 patients diagnosed for idiopathic normal pressure hydrocephalus (iNPH) during the period 2004-2013 were available for analysis. A lower breakpoint was observed in 62 patients diagnosed for iNPH. Improvement after shunt surgery in patients in whom LB was recorded was 77% versus 90% in patients where LB was absent (p < 0.02). There was no correlation between improvement and slope of amplitude-pressure line above LB.The detection of a lower breakpoint is associated with less frequent improvement after shunting in NPH. It may be interpreted that cerebrospinal fluid dynamics of patients working on the flat part of the pressure-volume curve and having a 'luxurious' compensatory reserve, are more frequently caused by brain atrophy, which is obviously not responding to shunting.

Single Center Experience in Cerebrospinal Fluid Dynamics Testing.

Normal pressure hydrocephalus is more complex than a simple disturbance of the cerebrospinal fluid (CSF) circulation. Nevertheless, an assessment of CSF dynamics is key to making decisions about shunt insertion, shunt malfunction, and for further management if a patient fails to improve. We summarize our 25 years of single center experience in CSF dynamics assessment using pressure measurement and analysis. 4473 computerized infusion tests have been performed. We have shown that CSF infusion studies are safe, with incidence of infection at less than 1%. Raised resistance to CSF outflow positively correlates (p < 0.014) with improvement after shunting and is associated with disturbance of cerebral blood flow and its autoregulation (p < 0.02). CSF infusion studies are valuable in assessing possible shunt malfunction in vivo and for avoiding unnecessary revisions. Infusion tests are safe and provide useful information for clinical decision-making for the management of patients suffering from hydrocephalus.

Differences in Cerebrospinal Fluid Dynamics in Posttraumatic Hydrocephalus Versus Atrophy, Including Effect of Decompression and Cranioplasty.

INTRODUCTION: Challenges in diagnosing post-traumatic hydrocephalus (PTH) have created a need for an accurate diagnostic tool. We aim to report CSF dynamics in PTH and atrophy, along with differences before and after cranioplasty. METHODS: We retrospectively analyzed traumatic brain injury patients with ventriculomegaly who had infusion studies. We divided patients depending on CSF dynamics into two groups: 'likely PTH' (A) and 'likely atrophy' (B). A group of idiopathic normal pressure hydrocephalus shunt-responsive patients was used for comparison (C). RESULTS: Group A consisted of 36 patients who were non-decompressed or had a cranioplasty in situ for over 1 month. Group B included 16 patients with low Rout, AMP, and dAMP, 9 of whom were decompressed. Rout and dAMP were significantly higher in Group A than B, but significantly lower than Group C (45 iNPH patients). RAP change during infusion in group A indicated depleted compensatory reserve compared to ample reserve in group B. Repeat studies in five decompressed patients post-cranioplasty showed all parameters increased. CONCLUSIONS: Infusion tests are not useful in decompressed patients, whilst cranioplasty allowed differentiation between possible PTH and atrophy. Rout and AMP were significantly lower in PTH compared to iNPH and did not always reflect the degree of hydrocephalus reported on imaging.

Global Cerebral Autoregulation, Resistance to Cerebrospinal Fluid Outflow and Cerebrovascular Burden in Normal Pressure Hydrocephalus.

INTRODUCTION: We previously examined the relationship between global autoregulation pressure reactivity index (PRx), mean arterial blood pressure (ABP), Resistance to cerebral spinal fluid (CSF) outflow (Rout) and their possible effects on outcome after surgery on 83 shunted patients. In this study, we aimed to quantify the relationship between all parameters that influence Rout, their interaction with the cerebral vasculature, and their role in shunt prognostication. METHODS: From 423 patients having undergone infusion tests for possible NPH, we selected those with monitored ABP and calculated its mean and PRx. After shunting, 6 months patients' outcome was marked using a simple scale (improvement, temporary improvement, and no improvement). We explored the relationship between age, different CSF dynamics variables, and vascular parameters using multivariable models. RESULTS: Rout had a weaker predictive value than ABP (Fisher Discrimination Ratio of 0.02 versus 0.42). ABP > 98 was an independent predictor of shunt outcome with odd ratio 6.4, 95% CI: 1.8-23.4 and p-value = 0.004. There was a strong and significant relationship between the interaction of age, PRx, ABP, and Rout (R = 0.53 with p = 7.28 × 10-0.5). Using our linear model, we achieved an AUC 86.4% (95% CI: 80.5-92.3%) in detecting shunt respondents. The overall sensitivity was 94%, specificity 75%, positive predictive value (PPV) of 54%, and negative predictive value of 97%. CONCLUSION: In patients with low Rout and high cerebrovascular burden, as described by high ABP and disturbed global autoregulation, response to shunting is less likely. The low PPV of high resistance, preserved autoregulation and absence of hypertension could merit further exploration.

Comparison of Assessment for Shunting with Infusion Studies Versus Extended Lumbar Drainage in Suspected Normal Pressure Hydrocephalus.

INTRODUCTION: Tools available for diagnosis of normal pressure hydrocephalus (NPH) and prediction of shunt-response are overnight ICP monitoring, infusion studies, and extended lumbar drainage (ELD). We investigated the shunt-response predictive value by infusion tests versus ELD. MATERIAL AND METHODS: We retrospectively recruited 83 patients who had undergone both infusion study and ELD assessments and compared infusion study hydrodynamics with improvement at clinic follow-up after ELD and after shunting. RESULTS: 62 patients had Rout >11 mmHg/mL/min. 28 Showed physiotherapy-documented improvement following ELD, and were selected for shunting, of which 21 were shunted. Of these, 19 showed improvement. Eight patients with Rout >20 mmHg/mL/min showed no response to ELD and were not shunted.There were 21 patients with Rout <11 mmHg/mL/min: five were shunted, showed improvement at follow-up, and had Rout >6 mmHg/mL/min. ICP amplitude did not differ at baseline or plateau between responders and non-responders. CONCLUSIONS: ELD response and CSF dynamics differed remarkably. All patients with Rout <6 mmHg/mL/min showed no improvement with ELD, indicating that ELD and shunting might be contraindicated in these subjects. High Rout patients with no response to ELD could merit further consideration.

The Role of Cerebrospinal Fluid Dynamics in Normal Pressure Hydrocephalus Diagnosis and Shunt Prognostication.

BACKGROUND: Over the years, there have been several reports and trials of the resistance to cerebrospinal fluid (CSF) outflow (Rout) in normal pressure hydrocephalus (NPH). This work aimed to revisit the utility of testing CSF circulation in a large population of patients clinically presenting with NPH. MATERIALS AND METHODS: We retrospectively analyzed the data of 369 NPH patients-either shunted or with endoscopic third ventriculostomy (ETV)-in Cambridge between 1992 and 2018. We determined the patients' outcomes (improvement versus no improvement at 6 months) by applying a threshold on R out values and compared our results with those of existing literature. We also conducted a correlation analysis between all variables and calculated Chi-Statistics (as a measure of separability between improvement and no improvement outcomes) to determine a subset of variables which achieved the highest accuracy in prediction of outcome. RESULTS: In our dataset, R out of 18 mmHg*min/mL achieved the highest Chi-statistics of 9.7 with p-value <0.01 when adjusted for age. In addition to R out, intracranial pressure (ICP) values at the baseline and plateau, CSF production rate and ICP amplitude to slope ratio showed significant Chi-Statistics values (more than 5). Using these variables, an overall accuracy of 0.70 ± 0.09 was achieved for prediction of the shunt outcome. CONCLUSION: Rout can be used for selecting patients for shunt surgery but not for excluding patients from treatment. Critical, multivariable approaches are required to comprehend CSF dynamics and pressure-volume compensation in NPH. Outcome definition and assessment could also be brought to question.

Correlation between the total number of features of paediatric pseudotumour cerebri syndrome and cerebrospinal fluid pressure.

PURPOSE: Accurate diagnosis of pseudotumour cerebri syndrome (PTCS) in children is challenging. We aimed to see if the clinical and radiological assessment that is carried out before lumbar puncture could predict subsequently recorded CSF pressures, and thus whether it could be used to increase diagnostic certainty of paediatric PTCS. METHODS: We used internationally recognised diagnostic criteria to derive a list of clinical, brain neuroimaging and venography features that were accepted to be associated with a diagnosis of PTCS. We performed a retrospective cohort study of children referred to our centre with suspected PTCS, identifying the presence or absence of those features for each child at initial presentation. The sum total scores of the features that were present were correlated with the child's recorded CSF pressure. RESULTS: The sum total scores were significantly positively correlated with recorded CSF pressures. The positive correlation was seen when clinical and brain neuroimaging features were included alone, and the correlation was slightly stronger when venography features were included in addition. CONCLUSION: Calculating the sum total of clinical, brain neuroimaging and venography features (where venography is performed) present at initial presentation can help in the management of children under investigation for PTCS. Children with high scores are more likely to have severely raised CSF pressures and thus may warrant more urgent LP investigations. By contrast, in children with subtle abnormalities in optic disc appearance such that disc oedema cannot be ruled out, a low score may add further reassurance and less urgency to proceed to LP.

Cerebrospinal fluid dynamics in pediatric pseudotumor cerebri syndrome.

PURPOSE: There is a growing body of evidence highlighting the importance of comprehensive intracranial pressure (ICP) values in pseudotumor cerebri syndrome (PTCS). Due to the highly dynamic nature of ICP, several methods of ICP monitoring have been established, including the CSF infusion study. We have performed a retrospective review of the CSF dynamics measurements for all pediatric patients investigated for PTCS in our center and examined their diagnostic value compared with clinical classification. METHODS: We retrospectively recruited 31 patients under 16 years of age investigated for PTCS by CSF infusion test. We used the clinically provided Friedman classification 13/31 patients with definite PTCS (group A), 13/31 with probable PTCS (group B), and 5/31 not PTCS (group C), to compare CSF dynamics in the 3 groups. RESULTS: CSF pressure (CSFp) was significantly increased in group A (29.18 ± 7.72 mmHg) compared with B (15.31 ± 3.47 mmHg; p = 1.644e-05) and C (17.51 ± 5.87; p = 0.01368). The amplitude (AMP) was higher in the definite (2.18 ± 2.06 mmHg) than in group B (0.68 ± 0.37; p = 0.01382). There was no in either CSFp or AMP between groups B and C. No lower breakpoint of the AMP-P line was observed in group A but was present in 2/13 and 2/5 patients in groups B and C. In group A, sagittal sinus pressure (SSp) and elasticity were the only parameters above threshold (p = 4.2e-06 and p = 0.001953, respectively), In group B, only the elasticity was significantly higher than the threshold (p = 004257). Group C did not have any of the parameters raised. The AUC of CSFp, elasticity, and SSp for the 3 groups was 93.8% (84.8-100% CI). CONCLUSIONS: Monitoring of CSFp and its dynamics, besides providing a more precise methodology for measuring CSFp, could yield information on the dynamic parameters of CSFp that cannot be derived from CSFp as a number, accurately differentiating between the clinically and radiologically derived entities of PTCS.

Coupling of CSF and sagittal sinus pressure in adult patients with pseudotumour cerebri.

OBJECTIVE: Pseudotumour cerebri syndrome (PTCS including idiopathic intracranial hypertension) is characterised by the symptoms and signs of raised cerebrospinal fluid pressure (CSFp) in the absence of ventricular dilatation or an intracranial mass lesion. Its aetiology is unknown in the majority of cases but there is much evidence for impaired CSF absorption. Traditionally, sagittal sinus pressure has been considered to be independent of CSF pressure in adults. However, the discovery of stenoses of intracranial venous sinuses and introduction of venous sinus stenting has highlighted the importance of the venous drainage in PTCS. In this study, we have explored the relationship between CSFp and SSp before and during a CSF infusion test and during CSF drainage. MATERIALS AND METHODS: Ten patients (9 females:1 male) with PTCS underwent infusion studies in parallel with direct retrograde cerebral venography. Both SSp and CSFp were recorded at a baseline and during CSFp elevation in a course of a CSF infusion test. The drainage of CSF after the CSF infusion was performed in 7 patients. In 5 cases, jugular venous pressure was also measured. RESULTS: CSFp and SSp including their amplitudes correlated significantly and strongly both at baseline (R = 0.96; p = 0.001) and during infusion (R = 0.92; p = 0.0026). During drainage, this correlation was maintained until SSp reached a stable value, whereas CSFp continued to decrease. CONCLUSIONS: In this series of ten patients with PTCS, CSFp and SSp were coupled, both at baseline and during infusion. The implications of such coupling for the calculation of CSF outflow resistance are discussed.

Cerebral autoregulation, cerebrospinal fluid outflow resistance, and outcome following cerebrospinal fluid diversion in normal pressure hydrocephalus.

OBJECTIVE: Normal pressure hydrocephalus is not simply the result of a disturbance in CSF circulation, but often includes cardiovascular comorbidity and abnormalities within the cerebral mantle. In this study, the authors have examined the relationship between the global autoregulation pressure reactivity index (PRx), the profile of disturbed CSF circulation and pressure-volume compensation, and their possible effects on outcome after surgery. METHODS: The authors studied a cohort of 131 patients in whom a clinical suspicion of normal pressure hydrocephalus was investigated. Parameters describing CSF compensation and circulation were calculated during the CSF infusion test, and PRx was calculated from CSF pressure and mean arterial blood pressure (MAP) recordings. A simple scale was used to mark the patients' outcome 6 months after surgery (improvement, temporary improvement, and no improvement). RESULTS: The PRx was negatively correlated with resistance to CSF outflow (R = -0.18; p = 0.044); patients with normal CSF circulation tended to have worse autoregulation. The correlation for patients who were surgically treated (n = 83) was R = -0.28; p = 0.01, and it was stronger in patients who experienced sustained improvement after surgery (n = 48, R = -0.43; p = 0.002). In patients who did not improve, the correlation was not significantly different from zero (n = 19, R = -0.07; p = 0.97). There was a trend toward higher values for PRx in nonresponders than in responders (0.16 ± 0.04 vs 0.09 ± 0.02, respectively; p = 0.061), associated with higher MAP values (107.2 ± 8.2 in nonresponders vs 89.5 ± 3.5 in responders; p = 0.195). The product of MAP × (1 + PRx), which was proposed as a measure of combined arterial hypertension and deranged autoregulation, showed a significant association with outcome (greater value in nonresponders; p = 0.013). CONCLUSION: Autoregulation proves to associate with CSF circulation and appears strongest in shunt responders. Outcome following CSF diversion is possibly most favorable when CSF outflow resistance is increased and global cerebral autoregulation is intact, in combination with arterial normotension.

Waveform Analysis of Intraspinal Pressure After Traumatic Spinal Cord Injury: An Observational Study (O-64).

Following a traumatic brain injury (TBI), intracranial pressure (ICP) increases, often resulting in secondary brain insults. After a spinal cord injury, here the cord may be swollen, leading to a local increase in intraspinal pressure (ISP). We hypothesised that waveform analysis methodology similar to that used for ICP after TBI may be applicable for the monitoring of patients with spinal cord injury.An initial cohort of 10 patients with spinal cord injury, as presented by the first author at a meeting in Cambridge in May 2012, were included in this observational study. The whole group (18 patients) was recently presented in the context of clinically oriented findings (Werndle et al., Crit Care Med, 42(3):646-655, 2014, PMID: 24231762). Mean pressure, pulse and respiratory waveform were analysed along slow vasogenic waves.Slow, respiratory and pulse components of ISP were characterised in the time and frequency domains. Mean ISP was 22.5 ± 5.1, mean pulse amplitude 1.57 ± 0.97, mean respiratory amplitude 0.65 ± 0.45 and mean magnitude of slow waves (a 20-s to 3-min period) was 3.97 ± 3.1 (all in millimetres of mercury). With increasing mean ISP, the pulse amplitude increased in all cases. This suggests that the ISP signal is of a similar character to ICP recorded after TBI. Therefore, the methods of ICP analysis can be helpful in ISP analysis.

Shunt Testing In Vivo: Observational Study of Problems with Ventricular Catheter.

Most shunt obstructions happen at the inlet of the ventricular catheter. Three hundred six infusion studies from 2007 to 2011 were classified as having a typical pattern of either proximal occlusion or patency. We describe different patterns of shunt ventricular obstruction.Solid block: Cerebrospinal fluid (CSF) aspiration was impossible. Baseline pressure was without pulse waveform (respiratory waveform may be visible). A quick increase of pressure to a level compatible with the shunt's setting was recorded in response to infusion. Distal occlusion of the shunt via transcutaneous compression resulted in a rapid increase in pressure to levels above 50 mmHg. This pattern was attributed to a solid ventricular block.Slit ventricles: At baseline, a pattern similar to that of the solid block was observed. After compression, the pressure increases, the pulse waveform appears, and the intracranial pressure is often stabilized at 25-40 mmHg. It is probable that previously slit ventricles were opened during the test.Partial block: In a partial block of the ventricular catheter by an in-growing choroid plexus, the pulse waveform at baseline was observed and CSF aspiration was possible. During infusion, the pressure increased, but the pulse amplitude disappeared. During the increase in the pressure in the shunt prechamber, the connection with the ventricles is disturbed by repositioning of the plexus.Infusion study via the shunt prechamber is able to visualize ventricular obstruction of the hydrocephalus shunt.

Intraspinal pressure and spinal cord perfusion pressure after spinal cord injury: an observational study.

OBJECT: In contrast to intracranial pressure (ICP) in traumatic brain injury (TBI), intraspinal pressure (ISP) after traumatic spinal cord injury (TSCI) has not received the same attention in terms of waveform analysis. Based on a recently introduced technique for continuous monitoring of ISP, here the morphological characteristics of ISP are observationally described. It was hypothesized that the waveform analysis method used to assess ICP could be similarly applied to ISP. METHODS: Data included continuous recordings of ISP and arterial blood pressure (ABP) in 18 patients with severe TSCI. RESULTS: The morphology of the ISP pulse waveform resembled the ICP waveform shape and was composed of 3 peaks representing percussion, tidal, and dicrotic waves. Spectral analysis demonstrated the presence of slow, respiratory, and pulse waves at different frequencies. The pulse amplitude of ISP was proportional to the mean ISP, suggesting a similar exponential pressure-volume relationship as in the intracerebral space. The interaction between the slow waves of ISP and ABP is capable of characterizing the spinal autoregulatory capacity. CONCLUSIONS: This preliminary observational study confirms morphological and spectral similarities between ISP in TSCI and ICP. Therefore, the known methods used for ICP waveform analysis could be transferred to ISP analysis and, upon verification, potentially used for monitoring TSCI patients.

Hydrocephalus shunt technology: 20 years of experience from the Cambridge Shunt Evaluation Laboratory.

OBJECT: The Cambridge Shunt Evaluation Laboratory was established 20 years ago. This paper summarizes the findings of that laboratory for the clinician. METHODS: Twenty-six models of valves have been tested long-term in the shunt laboratory according to the expanded International Organization for Standardization 7197 standard protocol. RESULTS: The majority of the valves had a nonphysiologically low hydrodynamic resistance (from 1.5 to 3 mm Hg/[ml/min]), which may result in overdrainage 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 a siphon-preventing mechanism is very sensitive to body posture, which may result in grossly negative intracranial pressure. Siphon-preventing accessories offer a reasonable resistance to negative outlet pressure; however, accessories with membrane devices may be blocked by raised subcutaneous pressure. In adjustable valves, the settings may be changed by external magnetic fields of intensity above 40 mT (exceptions: ProGAV, Polaris, and Certas). Most of the magnetically adjustable valves produce large distortions on MRI studies. CONCLUSIONS: The behavior of a valve revealed during testing is of relevance to the surgeon and may not be adequately described in the manufacturer's product information. The results of shunt testing are helpful in many circumstances, such as the initial choice of shunt and the evaluation of the shunt when its dysfunction is suspected.