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.

Should we measure cerebral blood flow in head-injured patients?

Inadequate cerebral blood flow (CBF) after head injury is an important cause of secondary ischaemic damage. Rapid identification of episodes of hypo- or hyperperfusion would allow timely intervention and would possibly improve outcome. Despite a large number of methods to estimate CBF, this concept is only marginally implemented in clinical practice. The methods to detect such episodes are limited for technical reasons, but also because the thresholds of ischaemia and hyperaemia are variable after head injury. Furthermore, we are not always able to manipulate CBF in a controlled manner. Accordingly, it is not surprising that attempts to compare a CBF-targeted strategy with another management option have failed to demonstrate a clear benefit. Methods need to be developed that allow either identification of thresholds for critically low or high CBF in individual patients, allow monitoring oxygen extraction fraction, representing circulatory reserve, or alternatively provide a measure of the volume of ischaemic or hyperaemic brain.

Multi-modal monitoring of acute brain injury

OBJECTIVE

To review the scientific basis for and utility of the traditional cerebral monitors used currently in neurointensive care, together with research techniques that are soon likely to become used in managing severe head injury and subarachnoid haemorrhage.

DESIGN AND CONTENT

Firstly, the pathophysiology of acute brain injury including cerebral haemodynamics, oxygen and metabolism and the role of secondary insults are discussed. Secondly, the importance of assessment of cerebrovascular autoregulation and reactivity is reviewed together with methods for its continuous non-invasive measurement using transcranial Doppler and intracranial pressure/arterial pressure recordings. Thirdly, the respective roles of jugular venous oxygen and brain tissue oxygen monitoring are analysed. Fourthly, the use of cerebral microdialysis is described, together with an overview of its utility.

CONCLUSION

Cerebral multimodal monitoring can be helpful for the optimal management of acute brain injury and essential for future exploratory trials of neuroprotective drugs.

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.

Continuous monitoring of cerebrovascular autoregulation: a validation study

BACKGROUND

Continuous monitoring of dynamic cerebral autoregulation, using a moving correlation index of cerebral perfusion pressure and mean middle cerebral artery flow velocity, may be useful in patients with severe traumatic brain injury to guide treatment, and has been shown to be of prognostic value.

OBJECTIVE

To compare an index of dynamic cerebral autoregulation (Mx) with an index of static cerebral autoregulation (sRoR).

METHODS

Mx was validated in a prospective comparative study against sRoR, using 83 testing sessions in 17 patients with traumatic brain injury. sRoR and Mx were calculated simultaneously during pharmacologically induced blood pressure variations.

RESULTS

Mx was significantly correlated with sRoR (R = -0.78, p < 0.05). Nine patients were found to have failure of cerebral autoregulation, with an sRoR value < 50%. If an Mx value of 0.3 was used as the cut off point for failure of cerebral autoregulation, this index had 100% sensitivity and 90% specificity for demonstrating failure of autoregulation compared with the sRoR. An increase in cerebral blood flow velocity correlated significantly with Mx (R = 0.73, p < 0.05) but not with cerebral perfusion pressure (R = 0.41).

CONCLUSIONS

Dynamic and static cerebral autoregulation are significantly correlated in traumatic brain injury. Cerebral autoregulation can be monitored continuously, graded, and reliably assessed using a moving correlation analysis of cerebral perfusion pressure and cerebral blood flow velocity (Mx). The Mx index can be used to monitor cerebral blood flow regulation. It is useful in traumatic brain injury because it does not require any external stimulus.

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.

Bifrontal decompressive craniectomy in the management of posttraumatic intracranial hypertension

Bifrontal decompressive craniectomy has been used on an ad hoc basis for the treatment of post-traumatic intracranial hypertension for more than thirty years. In this observational study we report the clinical outcome and physiological effects of the procedure in a series of 26 patients with refractory intracranial hypertension treated on a protocol driven basis. Bifrontal decompressive craniectomy was associated with significant reductions in mean ICP from 37.5 to 18.1 mmHg (p = 0.003). In addition, craniectomy reduced the amplitude of ICP waves (p < 0.02) and increased compensatory reserve (p < 0.05). A favourable outcome was achieved in 69% of patients; 8% were severely disabled and 23% died. We conclude that this study provides pathophysiological evidence that bifrontal decompressive craniectomy significantly reduces posttraumatic intracranial hypertension and improves pressure dynamics. Our results support the continued use of bifrontal decompressive craniectomy in selected patients after head injury.

Cerebral venous blood outflow: a theoretical model based on laboratory simulation

OBJECTIVE

The cerebrovascular bed and cerebrospinal fluid circulation have been modeled extensively except for the cerebral venous outflow, which is the object of this study.

METHODS

A hydraulic experiment was designed for perfusion of a collapsible tube in a pressurized chamber to simulate the venous outflow from the cranial cavity.

CONCEPT

The laboratory measurements demonstrate that the majority of change in venous flow can be attributed to either inflow pressure when the outflow is open, or the upstream transmural pressure when outflow is collapsed. On this basis, we propose a mathematical model for pressure distribution along the venous outflow pathway depending on cerebral blood flow and intracranial pressure. The model explains the physiological strong coupling between intracranial pressure and venous pressure in the bridging veins, and we discuss the limits of applicability of the Starling resistor formula to the venous flow rates. The model provides a complementary explanation for ventricular collapse and origin of subdural hematomas resulting from overshunting in hydrocephalus. The noncontinuous pressure flow characteristic of the venous outflow is pinpointed as a possible source of the spontaneous generation of intracranial slow waves.

CONCLUSION

A new conceptual mathematical model can be used to explain the relationship between pressures and flow at the venous outflow from the cranium.

Cerebral autoregulation following head injury

OBJECT

The goal of this study was to examine the relationship between cerebral autoregulation, intracranial pressure (ICP), arterial blood pressure (ABP), and cerebral perfusion pressure (CPP) after head injury by using transcranial Doppler (TCD) ultrasonography.

METHODS

Using ICP monitoring and TCD ultrasonography, the authors previously investigated whether the response of flow velocity (FV) in the middle cerebral artery to spontaneous variations in ABP or CPP provides reliable information about cerebral autoregulatory reserve. In the present study, this method was validated in 187 head-injured patients who were sedated and receiving mechanical ventilation. Waveforms of ICP, ABP, and FV were recorded over intervals lasting 20 to 120 minutes. Time-averaged mean FV and CPP were determined. The correlation coefficient index between FV and CPP (the mean index of autoregulation [Mx]) was calculated over 4-minute epochs and averaged for each investigation. The distribution of averaged mean FV values converged with the shape of the autoregulatory curve, indicating lower (CPP < 55 mm Hg) and upper (CPP > 105 mm Hg) thresholds of autoregulation. The relationship between the Mx and either the CPP or ABP was depicted as a U-shaped curve. Autoregulation was disturbed in the presence of intracranial hypertension (ICP > or = 25 mm Hg) and when mean ABP was too low (ABP < 75 mm Hg) or too high (ABP > 125 mm Hg). Disturbed autoregulation (p < 0.005) and higher ICP (p < 0.005) occurred more often in patients with unfavorable outcomes than in those with favorable outcomes.

CONCLUSIONS

Autoregulation not only is impaired when associated with a high ICP or low ABP, but it can also be disturbed by too high a CPP. The Mx can be used to guide intensive care therapy when CPP-oriented protocols are used.

Continuous assessment of cerebral autoregulation--clinical verification of the method in head injured patients

Previously, using transcranial Doppler ultrasonography, we investigated whether the hemodynamic response to spontaneous variations in cerebral perfusion pressure (CPP) provides reliable information about cerebral autoregulatory reserve. In the present study we have verified this method in 166 patients after head trauma. Waveforms of intracranial pressure (ICP), arterial pressure and transcranial Doppler flow velocity (FV) were captured daily over 0.5-2.0 hour periods. Time-averaged mean flow velocity (FV) and CPP were resolved. The correlation coefficient indices between FV and CPP (Mx) were calculated over 3 minutes epochs, and averaged for each investigation. An index of CBF (flow velocity diastolic to mean ratio) was calculated independently for each investigation. Mx depended on CPP (p < 0.0001) increasing to positive values when CPP decreased below 60 mm Hg. This threshold coincided with an averaged breakpoint for autoregulation, expressed by the index of CBF. Mx depended on outcome following head injury stronger than the Glasgow Coma Score on admission (ANOVA, F values 18 and 15 respectively; N = 166). In patients who died, cerebral autoregulation was disturbed during the first two days following injury. These results indicate an important role for the continuous monitoring of autoregulation following head trauma.

Non-invasive cerebral perfusion pressure (nCPP): evaluation of the monitoring methodology in head injured patients

The method of direct calculation of cerebral perfusion pressure (CPP) as the difference between mean arterial pressure and intracranial pressure (ICP) produces a number, which not always adequately expresses brain perfusion. We investigated an alternative non-invasive method, based on waveform analysis of Transcranial Doppler blood flow velocity in Middle Cerebral Arteries (MCA). 25 consecutive head injured patients, paralysed, sedated and ventilated were studied. Intracranial pressure (ICP) arterial blood pressure (ABP) were monitored continuously. The left and right MCAs were insonated daily (116 measurements) using a purpose-built transcranial Doppler monitor (Deltex Ltd, Chichester, U.K.) with software capable of the non-invasive estimation of CPP. Time averaged values of ABP, mean and diastolic flow velocities (FVm, FVd) were calculated and CPPe was computed as: ABP*FVd/FVm + 14. An absolute difference between real CPP and CPPe was less than 10 mm Hg in 82% of measurements and less than 13 mm Hg in 90% of measurements. The method demonstrated a high potential to detect both short-term and long-term changes in CPP. The method is of potential benefit for the intermittent measurement and continuous monitoring of changes in brain perfusion pressure in situations where the direct measurement of CPP is not available or its reliability is in question.

Laboratory evaluation of the phoenix CRx diamond valve

OBJECTIVE

To assess the long-term hydrodynamic properties of a new cerebrospinal fluid flow-regulating hydrocephalus shunt called the CRx Diamond valve (Phoenix Biomedical Corp., Valley Forge, PA).

METHODS

Three samples of a Diamond valve were tested in the United Kingdom Shunt Evaluation Laboratory during a 40-day period. Tests were performed for long-term pressure-flow performance, overdrainage, susceptibility to ambient temperature changes, external pressure, reflux, presence of small particles in the reagent, mechanical durability, and magnetic resonance imaging compatibility.

RESULTS

Tests demonstrated that the Diamond valve stabilized flow within the range of 0.36 to 0.62 ml/min when pressure varied from 14 to 23 mm Hg. Hydrodynamic resistance demonstrated pressure-dependent variability from 20 to 78 mm Hg/(ml/min). The time drift of hydrodynamic parameters was significant (P < 0.001). The valve was insensitive to changes in temperature, external pressure, rapid fluctuations of differential pressure, small particles in fluid, and reflux.

CONCLUSION

The Diamond valve demonstrated the intended variable resistance, which increased as the pressure increased. This property may help it limit overdrainage related to body posture as well as nocturnal vasogenic waves. Flow through the valve stabilizes within a wide range, which may contribute to the prevention of excessive pressure buildup after implantation. However, shunt placement should be avoided in patients who present with normal baseline intracranial pressure but an increased incidence of high vasogenic intracranial pressure waves.

Age dependence of cerebrospinal pressure-volume compensation in patients with hydrocephalus

OBJECT

The dynamics of both drainage and storage capacity become altered during the sequential pathological processes that lead to hydrocephalus. Cerebrospinal fluid (CSF) formation and drainage rate have been reported to be age dependent. The aim of this study was to investigate whether CSF compensatory parameters are dependent on age in patients who have symptoms of hydrocephalus and apparently normal intracranial pressure (ICP).

METHODS

Forty-six patients who presented with ventriculomegaly, the clinical symptoms of hydrocephalus, and normal ICPs underwent a computerized CSF infusion test. Parameters used to describe CSF compensation were calculated and correlated with the age of each patient. The mean ICPs were found to be independent of the age of the patient. Resistance to CSF outflow (Rcsf), however, demonstrated a nonlinear increase with advancing age (r = -0.57; p < 0.0001) and was associated with a decrease in the CSF production rate, which also occurred with increasing age (r = 0.49; p < 0.002). Both the pulse amplitude of the ICP waveform and the slope of the amplitude-ICP regression line increased significantly with advancing age (r = 0.39; p < 0.01 and r = 0.43, p < 0.004, respectively). The nonlinear increase in the elastance coefficient indicated increasing brain stiffness, which acompanies older ages (r = -0.31; p < 0.04).

CONCLUSIONS

In a study of patients with symptoms of hydrocephalus, but normal ICPs, the increase in Rcsf and decrease in CSF production were most pronounced in patients who were older than 56 years of age. This relationship was more significant than previously suggested.

Preliminary experience of the estimation of cerebral perfusion pressure using transcranial Doppler ultrasonography

OBJECTIVE

The direct calculation of cerebral perfusion pressure (CPP) as the difference between mean arterial pressure and intracranial pressure (ICP) produces a number which does not always adequately describe conditions for brain perfusion. A non-invasive method of CPP measurement has previously been reported based on waveform analysis of blood flow velocity measured in the middle cerebral artery (MCA) by transcranial Doppler. This study describes the results of clinical tests of the prototype bilateral transcranial Doppler based apparatus for non-invasive CPP measurement (nCPP).

METHODS

Twenty five consecutive, paralysed, sedated, and ventilated patients with head injury were studied. Intracranial pressure (ICP) and arterial blood pressure (ABP) were monitored continuously. The left and right MCAs were insonated daily (108 measurements) using a purpose built transcranial Doppler monitor (Neuro Q(TM), Deltex Ltd, Chichester, UK) with software capable of the non-invasive estimation of CPP. Time averaged values of mean and diastolic flow velocities (FVm, FVd) and ABP were calculated. nCPP was then computed as: ABPxFVd/FVm+14.

RESULTS

The absolute difference between real CPP and nCPP (daily averages) was less than 10 mm Hg in 89% of measurements and less than 13 mm Hg in 92% of measurements. The 95% confidence range for predictors was no wider than +/-12 mm Hg (n=25) for the CPP, varying from 70 to 95 mm Hg. The absolute value of side to side differences in nCPP was significantly greater (p<0.05) when CT based evidence of brain swelling was present and was also positively correlated (p<0.05) with mean ICP.

CONCLUSION

The device is of potential benefit for intermittent or continuous monitoring of brain perfusion pressure in situations where the direct measurement is not available or its reliability is in question.

A model of the cerebral and cerebrospinal fluid circulations to examine asymmetry in cerebrovascular reactivity

The authors examined the steal phenomenon using a new mathematical model of cerebral blood flow and the cerebrospinal fluid circulation. In this model, the two hemispheres are connected through the circle of Willis by an anterior communicating artery (ACoA) of varying size. The right hemisphere has no cerebrovascular reactivity and the left is normally reactive. The authors studied the asymmetry of hemispheric blood flow in response to simulated changes in arterial blood pressure and carbon dioxide concentration. The hemispheric blood flow was dependent on the local regulatory capacity but not on the size of the ACoA. Flow through the ACoA and carotid artery was strongly dependent on the size of the communicating artery. A global interhemispheric "steal effect" was demonstrated to be unlikely to occur in subjects with nonstenosed carotid arteries. Vasoreactive effects on intracranial pressure had a major influence on the circulation in both hemispheres, provoking additional changes in blood flow on the nonregulating side. A method for the quantification of the crosscirculatory capacity has been proposed.

Predicting delayed ischemic deficits after aneurysmal subarachnoid hemorrhage using a transient hyperemic response test of cerebral autoregulation

OBJECTIVE

To assess whether the development of delayed ischemic deficits (DIDs) after aneurysmal subarachnoid hemorrhage can be predicted using transcranial Doppler ultrasonography and the transient hyperemic response test (THRT).

METHODS

An increase in the middle cerebral artery peak flow velocity (FV) of more than 9% of baseline values after 5 to 9 seconds of carotid artery compression was defined as a normal THRT result, indicating good autoregulatory reserve. The transcranial Doppler criteria for vasospasm were a FV of more than 120 cm/s and a Lindegaard ratio of more than 3. Twenty patients with no immediate postoperative neurological deficits were studied. The FVs at all of the major cerebral arteries were measured daily after surgery, and the THRT results were assessed bilaterally.

RESULTS

Five of six patients with abnormal THRT results in the first examination after surgery (primary THRT impairment) developed DIDs; none of the remaining patients developed DIDs (Fisher exact test, P = 0.0004). All five patients with DIDs initially exhibited low FVs but all subsequently developed increases in FVs to values of more than 150 cm/s and four exhibited FVs of more than 200 cm/s. The time of onset of DIDs corresponded to the time of onset of moderate vasospasm (FV > 150 cm/s). None of the patients with initially normal THRT results developed DIDs, although four patients did exhibit late (secondary) THRT impairment, which was associated with FVs of more than 120 cm/s.

CONCLUSION

When the effects of primarily impaired (after surgery) autoregulation are magnified by vasospasm, the risk of DIDs seems to be very high. Vasospasm alone does not seem to cause DIDs. The development of DIDs could therefore be predicted using the THRT for patients after aneurysm clipping.

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.

Association between arterial and intracranial pressures

Fluctuations of arterial blood pressure (ABP) almost always produce a response in intracranial pressure (ICP). However, the time-relationship between the ABP and ICP waveforms is complex and involves a variety of mechanisms, including those responsible for regulation of cerebrovascular tone and venous blood outflow through collapsible cortical and bridging veins.

Assessment of critical closing pressure in the cerebral circulation as a measure of cerebrovascular tone

Critical closing pressure (CCP) calculated from the blood flow velocity (FV) and arterial blood pressure (ABP) waveforms has been previously reported to be useful in the assessment of the dynamics of cerebral circulation. We investigated the relationship between CCP and intracranial pressure (ICP) and cerebrovascular tone in a model of intracranial hypertension in 22 anaesthetised New Zealand White rabbits during manipulations of arterial CO2, ABP and vasodilatation caused by hypoxia. Recordings were made of FV in the basilar artery, ABP and ICP during subarachnoid infusion of saline. During infusion ICP and CCP were significantly correlated (R=0.68; p<0.001), but the magnitude of increase in ICP and CCP during infusion were not correlated to each other. Linear regression between the difference: CCP-ICP (representing a factor due to vasogenic tone) and cerebral perfusion pressure (CPP=ABP-ICP) was highly significant (R=-0.87; p<0.01). Generally, CCP decreased significantly (p<0.05) with hypercarbia, arterial hypotension and after and post-hypoxia and the difference: CCP-ICP decreased consistently after each vasodilatatory manoeuvre studied. Our data confirmed the linear relationship between CCP and ICP, and between the difference: CCP-ICP and cerebrovascular tone. However, because the magnitude of increase in ICP was not correlated to magnitude of change in CCP, CCP cannot be use for detection of increasing ICP quantitatively.

Specific patterns of cognitive impairment in patients with idiopathic normal pressure hydrocephalus and Alzheimer's disease: a pilot study

OBJECTIVES

Eleven patients with idiopathic normal pressure hydrocephalus (NPH) were selected from an initial cohort of 43 patients. The patients with NPH fell into two distinctive subgroups: preshunt, group 1 (n=5) scored less than 24 on the mini mental state examination (MMSE) and were classified as demented and group 2 (n=6) scored 24 or above on the MMSE and were classified as non-demented.

METHODS

All patients were neuropsychologically assessed on two occasions: preshunt and then again 6 months postshunt. Group 1 completed the mini mental state examination (MMSE) and the Kendrick object learning test (KOLT). In addition to the MMSE and KOLT, group 2 completed further tasks including verbal fluency and memory and attentional tasks from the CANTAB battery. Nine of the 11 patients also underwent postshunt MRI.

RESULTS

Group 1, who, preshunt, performed in the dementing range on both the MMSE and KOLT, showed a significant postoperative recovery, with all patients now scoring within the normal non-demented range. Group 2, although showing no signs of dementia according to the MMSE and KOLT either preshunt or postshunt, did show a specific pattern of impairment on tests sensitive to frontostriatal dysfunction compared with healthy volunteers, and this pattern remained postoperatively. Importantly, this pattern is distinct from that exhibited by patients with mild Alzheimer's disease. Eight of the nine patterns of structural damage corresponded well to cognitive performance.

CONCLUSIONS

These findings are useful for three main reasons: (1) they detail the structural and functional profile of impairment seen in NPH, (2) they demonstrate the heterogeneity found in this population and show how severity of initial cognitive impairment can affect outcome postshunt, and (3) they may inform and provide a means of monitoring the cognitive outcome of new procedures in shunt surgery.