Hydrocephalus. a Practical Guide to Csf Dynamics and Ventriculoperitoneal Shunts Clinical Feature of Inph

H ydrocephalus is defined and the mechanisms of CSF hydrodynamics discussed. Supplementary tests used in the investigation of idiopathic normal pressure hydrocephalus are reviewed with a detailed explanation of constant flow CSF infusion tests. The principles governing valve selection are illustrated. Hydrocephalus is the abnormal accumulation of CSF within the cranium due to defective CSF production, flow or absorption. The CSF usually accumulates within the ventricular system, however 'external hydrocephalus' with widening of the subarachnoid spaces is described. Hydrocephalus can be due to obstructive causes preventing normal CSF flow through the CSF pathways, or due to abnormal absorption of CSF: communicating hydro-cephalus. CSF flow studies frequently show a complex picture with contributions from both mechanisms. Overproduction of CSF that exceeds the absorption capacity of the arachnoid granulations is rare. CSF physiology CSF is mainly produced by passive ultrafiltration of plasma with some active electrolyte transport from the ven-tricular choroid plexus. The rate of CSF production is around 20ml/hour, although MRI evidence suggests that this may increase significantly during sleep. To maintain equilibrium, CSF is normally absorbed into the major venous sinuses by a passive mechanism through the one-way valves of the arachnoid villi. The normal CSF pressure at the reference level (the foramen of Monro) in the recumbent adult is 100-200mm H2O (7-15mmHg) with mean pressures of 20mmHg regarded as elevated. Pressures from 0-7mmHg do not usually signify any pathology. The CSF pressure fluctuates with the arterial pulse wave and respiratory excursions. Symptomatic patients with obstructive hydrocephalus continue producing CSF. CT and MRI scans identify the site of obstruction. The definitive treatment for most of these patients is removal of the obstructive cause. If CSF diversion is required and the outlet of the IIIrd or IVth ventricle obstructed, a IIIrd ventriculostomy is usually the first choice treatment, particularly in newly diagnosed patients with aqueduct stenosis. Communicating hydrocephalus occurs when the lateral , IIIrd and IVth ventricles appear to communicate freely. The absorption capacity of the arachnoid villi is exceeded or obstruction of CSF flow occurs within the subarach-noid space. This condition is usually managed with a ven-triculoperitoneal shunt system and provides the focus for this paper. Normal pressure hydrocephalus In the pre-CT scan era Adams et al. reported three cases of ventriculomegaly associated with gait disturbance, dementia and incontinence. 1 All three patients had normal CSF pressure (140-50, 160 and 175mmH2O) on lum-bar puncture but improved with either a ventriculo-atrial …


Normal pressure hydrocephalus
In the pre-CT scan era Adams et al. reported three cases of ventriculomegaly associated with gait disturbance, dementia and incontinence. 1All three patients had normal CSF pressure (140-50, 160 and 175mmH2O) on lumbar puncture but improved with either a ventriculo-atrial shunt (two cases) or a Torkildsen (lateral ventricle to cisterna magna) diversion (one case).Two cases were idiopathic and one due to a cyst in the IIIrd ventricle.The condition is now classified as (i) Primary or Idiopathic Normal Pressure Hydrocephalus (INPH) and (ii) Secondary Normal Pressure Hydrocephalus.In the latter group of patients a well-established cause is evident (eg.subarachnoid haemorrhage, traumatic brain injury, meningitis).Whilst the primary pathology may increase the certainty of diagnosing hydrocephalus, the results of treatment may be confounded by the original brain insult.
Even in the presence of a classic triad of symptoms the response to treatment is often disappointing.Indeed Black reported that 67.2% of patients with gait, cognitive and urinary symptoms and signs improved with a shunt. 2 The outcome was significantly worse in patients with only dementia and gait disturbance (31.6% improved).Overall, 35.4% of the 62 patients studied suffered complications, including subdural haematomas and fits.The challenge therefore lies in increasing diagnostic accuracy and timely management at a point when symptoms and signs are retrievable.

The Symptomatic Triad
The gait disturbance in INPH includes at least two of the following features: wide based stance, out-turned feet, decreased step height, decreased step length, decreased speed, increased trunk sway, en bloc turning requiring three or more steps for 180 o , and poor heel-toe walking.Cognitive features are wide-ranging and include attention deficits, psychomotor retardation, impaired recall and memory deficits, executive dysfunction, behavioural and personality changes.Such features can be quantified using a summative mental state examination.Urinary dysfunction is characterised by nocturia, urgency, frequency or incontinence reflecting a low capacity neurogenic bladder.
Evidence-based clinical diagnostic criteria for the diagnosis of INPH have only recently been developed.A consensus panel recommends that INPH candidates be categorised into 'probable' and 'possible' groups based upon history, examination, brain imaging and CSF opening pressure. 3

Probable INPH
This requires a gait disturbance and either cognitive and/or urinary disturbances in a patient over 40 years old.In addition the history, imaging and lumbar puncture opening pressures must be consistent with the diagnosis.The imaging findings are characterised by ventriculomegaly not due to atrophy or obstructive hydrocephalus, associated with one or more of the following; temporal horn enlargement, a callosal angle of 40 o or more (due to bowing of the corpus callosum), periventricular lucency not due to ischaemia and a flow void in the aqueduct or IVth ventricle.The accepted range of CSF opening pressure for probable INPH is 70-245mmH2O (5-18mmHg).

Possible INPH
This group may have a more acute history in a younger patient with only one of the triad of symptoms and an opening CSF pressure outside the guidelines above.The imaging findings may appear to be consistent with atrophy.

Correspondence to:
Peter Whitfield Email: Peter.Whitfield@ phnt.swest.nhs.uk are commonly observed in T2-weighted MRI they are also associated with hypertension and cerebrovascular disease and are therefore not pathognomic of hydrocephalus. 4 Post-shunting MRI scans do show an improvement in the frontal horn periventricular changes but such pre-operative features are not required to predict a good outcome. 5Calculation of the 'stroke volume' of CSF moving in a craniocaudal direction during systole using phase contrast CSF velocity MR imaging has shown that a volume greater than 42µL correlated with a favourable response to shunting. 6This is characterised by a signal flow void.PET cerebral blood flow (CBF) studies have shown that pre and post-shunt assessments of haemodynamic reserve using a carbonic anhydrase inhibitor to stimulate increased PaCO2 indicate that shunt responders show an improvement in their cerebrovascular reserve compared with non-responders. 7This suggests that altered CBF dynamics are important in the pathogenesis of INPH and in determining the success of treatment.Unfortunately specific thresholds for low CBF have not been identified as pre-operative predictors of treatment success.
Due to the conundrum and the difficulties in determining shunt responsive cases several other tests have been developed to aid management.These include intracranial pressure monitoring, CSF infusion tests, the tap test and a period of CSF drainage.The main drawback of these tests is the low sensitivity and poor predictive value of some tests (see Table 1).The additional tests that are often used are detailed below.

ICP monitoring
Patients with INPH frequently have normal ICP.However, 24 hour monitoring may reveal several abnormalities that indicate poor cerebral compliance (Figure 1).An ICP recording shows systolic and diastolic pulsations.Plateau (A) waves with elevations exceeding 50mmHg for periods of 5-20 min are not normally seen in patients with idiopathic hydrocephalus.However, careful analysis of the ICP trace -using computer software with threshold filters -reveals low amplitude (commonly 1-5mmHg) superimposed B waves with a period of 30 seconds to 2 minutes. 8The prevalence of B waves appears to increase during normal REM sleep and with rises in intracranial pressure.A recent detailed analysis in patients with communicating and non-communicating hydrocephalus indicates that B waves are commonly observed but have a poor correlation to clinical outcome. 9

Tap test
Many authors have reported the withdrawal of 40-50ml of CSF as a useful test, with responders benefiting from shunt insertion.However the test has a low sensitivity (26-62%) and should not be used to rule out a diagnosis of idiopathic normal pressure hydrocephalus. 10ternal Lumbar CSF Drainage This test developed from the concept that a trial of controlled CSF removal (10ml/hr) for 72 hours might predict shunt responders.The sensitivity of   Figure 2B: CSF infusion study performed via a ventricular access device -patient with probable idiopathic normal pressure hydrocephalus, infusion rate 1.5ml/min.The opening pressure is normal (10mmHg) but CSF infusion produces a plateau around 34mmHg enabling the Rcsf to be calculated (16mmHg/ml/min).This level is just below the 18mmHg/ml/min threshold described by Boon et al. 12 Strong vasogenic waves are also evident at pressures above 25mmHg with an increase in the pulse amplitude.The derived Pressure Volume Index (PVI) was elevated (9.1ml) reflecting poor compensatory reserve.

Neurosurgery Article
the test has been reported as 50-100% with a specificity of 60-80% and a positive predictive value of 80-100%. 10

CSF infusion test
The resistance to CSF absorption by the arachnoid villi can be measured and helps predict shunt responsive patients.
The test is commonly performed in the left lateral position using a constant infusion technique.Lumbar puncture needles are inserted at 2 levels; the use of a solitary needle with a three-way tap is not as reliable.A pressure transducer is connected to one needle and the baseline opening pressure recorded.Normal saline is infused at 1.5ml/min through the second needle whilst the pressure is continuously measured.In most patients the pressure rises steadily and then reaches a plateau.The resistance (Rcsf) to CSF absorption can be calculated using an Ohm's Law analogy: Infusion rate (ml/min) The Rcsf in normal subjects ranges from 6 to 10mmHg/ml/min.It increases in the elderly.In such cases the rate of CSF production probably decreases to prevent hydrocephalus ensuing. 11y using an infusion rate of 1.5ml/min a 30-mmHg increase in CSF pressure provides evidence of the Rcsf exceeding 20mmHg/ml/min.The use of higher infusion rates (eg.3ml/min) imposes limitations in that the pressure needs to rise by 60mmHg to confirm an Rcsf of 20mmHg/ml/min.We recommend aborting the test if CSF pressure exceeds 50mmHg.In this case a minimum value for the Rcsf can still be calculated using the (peak pressure -baseline pressure) as the numerator in the equation.Boon et al. have reported that in patients with probable NPH (mainly idiopathic but also including some secondary cases) a positive response to shunting was likely if Rcsf exceeded 18mmHg/ml/min with a PPV of 92% and a likelihood ratio of 3.5 in their series of 95 patients.However, the sensitivity of the test at this threshold was only 46% although the specificity was high at 87%. 12 Performing the infusion test via a frontal ventricular access device appears to minimise the effect of CSF leakage around lumbar needles and may increase the predictive value of the investigation (Figures 2A and  2B).With sophisticated computer analysis (see www.neurosurg.cam.ac.uk/icmplus) of the pressure waveform further information about the elastance and compliance of the craniospinal axis, including the Pressure Volume Index (PVI), can be derived both in lumbar and ventricular CSF infusion studies.This may assist the decision making process in borderline cases.

Choosing a CSF shunt
Most neurosurgeons advocate a ventriculo-peritoneal shunt (VP shunt) as the preferred system for implantation.In some circumstances (eg.inadequate absorption in a patient with multiple previous abdominal operations) alternative sites are required (eg.ventriculo-pleural, ventriculo-atrial).VP shunt insertion is associated with numerous potential complications.These include: • Peri-operative intracranial bleeding Attention to detail during the placement of a VP shunt is crucial to minimise the risks of shunt insertion.Meticulous sterility, accurate catheter placement and secure connections between shunt components are essential.Consideration needs to be applied to the choice of shunt hardware.The ventricular catheter most widely used is a straight non-flanged device with multiple apertures in proximity to the catheter tip.There is no consensus over the best anatomical site for catheter placement.The distal catheter provides a conduit to drain CSF to the peritoneal cavity.Distal slit valves are unnecessary and may increase the risk of distal obstruction provided a valve is utilised proximally.Antimicrobial impregnated ventricular and peritoneal catheters have been developed in an attempt to reduce shunt infection rates.

Valves
Valve systems with different hydrodynamic properties have been developed to try and minimise complications such as over-drainage with lowpressure postural headaches and subdural fluid collections.The properties of valves have been independently evaluated in vivo. 13Valves are designed to be (1) flow regulated or (2) differential pressure regulated (Figure 3).The Orbis Sigma Valve is the archetypal flow controlled device.CSF flows though a diaphragmatic aperture whose diameter decreases as the flow rate rises above 20ml/hr.This increases the resistance of the valve, regulating flow.A safety mechanism leading to a reduction of resistance at differential pressures of 25-30mmHg is incorporated to avoid acute severe elevations in intracranial pressure.
Most valves are differential pressure regulated.These devices are

Figure 1 :
Figure 1: Overnight ICP monitoring in a normal pressure hydrocephalus patient who responded to subsequent VP shunt insertion.Graphs show ICP; Amplitude; Slow B waves and RAP coefficient.The baseline pressure was normal (8-10mmHg) with many vasogenic waves exceeding 20mmHg.The pulse amplitude of the ICP waveform was elevated, especially during the vasogenic waves.The averaged amplitude of the slow B waves was above 5mmHg and the derived RAP coefficient was above 0.7 most of the time, signifying poor compensatory reserve.

Figure 2A :
Figure 2A: CSF infusion study performed via a ventricular access device -normal result showing ICP, heart rate and ICP pulse amplitude; infusion rate 1.5ml/min.The opening pressure (5mmHg) and amplitude were normal.During infusion the ICP increased to a plateau of 15mmHg, enabling calculation of resistance to CSF outflow (7mmHg/ml/min).The low pulse amplitude and absence of vasogenic waves are characteristic of a normal study.Measurement of the heart rate from the pulse amplitude enables the technical quality of the recording to be assessed.

Figure 3A :
Figure 3A: Flow-pressure curves for a differential pressure-regulating valve.The pressure regulating mechanisms try to maintain the same differential pressure across the valve regardless of the flow rate.In practice most manufacturers market high, medium and lowpressure valves each with different pressure flow characteristics.

Figure 3B :
Figure 3B: Flow-pressure curve for a flow regulated valve.The flow regulator attempts to change its resistance in response to the differential pressure thereby maintaining flow at a constant level.The Orbis Sigma Valve has a variable resistance that increases in the mid-range acting as a flow control mechanism.

Neurosurgery Article Marek Czosnyka PhD (Warsaw) DSc (Warsaw) in Biomedical Engineering is
Reader in Brain Physics and Director of Neurosurgical Physics in the Academic Neurosurgical Unit, University of Cambridge, UK.He is also Associate Professor at Warsaw University of Technology, Faculty of Electronics, Poland.He is interested in hydrocephalus research with an emphasis on CSF dynamics, cerebrovascular factors and mathematical modeling.
Peter Whitfield is Consultant Neurosurgeon at the South West Neurosurgical Centre, Plymouth.He has a PhD in the molecular biology of cerebral ischaemia.Clinical interests include vascular neurosurgery, image guided tumour surgery and microsurgical spinal surgery.