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SHUNT SELECTION MODEL: "Application of the DiaCeph Test with In-Office ICP Measurements and Shunt Comparison Data from the University of Heidelberg."

Topic: For the Care and Treatment of Hydrocephalus

Updated: May 25, 2006

By: Stephen Dolle, B.S.B.A., and Ph.D. (Unofficial)

Summary: Tandem In-Office ICP Assessment and DiaCeph Test; a New Method of ICP Monitoring

    This paper presents a new tandem protocol for 24/7 DiaCeph Test non-invasive monitoring of hydrocephalus in concert with a single shunt tap and in-office ICP manometer measurement. The results are then referenced to shunt manufacturer's specifications and shunt comparison studies published by Dr. Aschoff and colleagues at the University of Heidelberg's hydrocephalus research center. Our initial study published in April had not considered the benefits of corroboration with widely accepted in-office ICP assessment. This important corroboration broadens and verifies this critical pre-surgical clinical information, not routinely available in today's neurosurgical setting, and raises the utility and reliability of each method, propelling the tandem protocol beyond that of mere 48 hour in-hospital monitoring. SEE also our new Codman Medos ( Hakim) programmable shunt calculator to determine the most physiologic setting in these and other makes of programmable shunts.

    Click to print/download our hydrocephalus monitoring form and blank graph to chart your shunt performance and shunt outcomes before and after after surgery. This next download is a sample patient DiaCeph User Graph, which illustrates how DiaCeph enables a thorough diagnostic evaluation in tandem with ICP taps and other tests, in preparation for a well planned shunt revision. This Shunt Selection Model paper further elaborates on these results. A more in depth (and at times lighter) look at CNS shunt operation can be explained through this Aschoff et. al. Shunt Technology Perspectives presentation.

    Field professionals active in the treatment of hydrocephalus would agree there is great need for real-time ICP (intra-cranial pressure) and shunt performance monitoring in the normal routine of the patient. Currently, perhaps the best available testing is 48 hour in-hospital monitoring. However, it is infrequently used due to its high costs, risks of infection, and concerns over actual benefit to the patient. Instead, in-office ICP taps and isotope clearance exams are more often performed to ascertain a single instance of ICP measure and shunt patency, respectively. Still, these widely used exams reveal little in view of the common "intermittence" of shunt malfunction, as well as shunt performance and shunt pressure matching in the normal routine of the patient. The resulting advances and reliability of implantable ICP monitoring has been disappointing, and diagnostic shunts remain years away. If and when they do become available, should patients be revised merely to implant diagnostic shunts when other alternative diagnostic systems remain available?

    An efficacious and practical method of hydrocephalus monitoring employs the tandem application of 24/7 DiaCeph home monitoring and a single patient visit and ICP tap in the neurosurgeon's office. Should the neurosurgeon suspect deterioration of shunt status, rather than rely upon traditional CT/MRI/Isotope clearance exams, he can tap the patient's shunt reservoir and take measurements in the upright and supine postures. He can then compare the ICP tap findings and clinical symptoms to comparable data points on the patient's DiaCeph day graph. In corroborating two or more identical points, he can reliably extrapolate the reliability of the other times/days of DiaCeph monitoring. When possible, the neurosurgeon should also aspirate and re-inject CSF fluid to and from the ventricles - to assess patency, compliance, and the patient's present tolerance to abnormal ICP levels. He should also note changes in headache, nausea, dizziness, and cognitive complaints - which are also key parameters of the DiaCeph Test. How closely the in-office shunt tap readings match the patient's same DiaCeph data points is indicative of DiaCeph's overall accuracy in the care of the patient, and the indicated home monitoring techniques the patient or caregiver has followed.

    In our patient study, we examined a single patient using several months of DiaCeph data - overlaid on a single day graph. We plotted eight (8) data points corresponding to incident and typical shunt performance over the previous 3-month period, and noted patient complaints and the activity being undertaken at each data point. We then tapped the patient's shunt in the neurosurgery office, and measured manometer readings of -10 cm H2O sitting, and +25 cm H2O supine. The patient's complaints were verbally queried while we aspirated and re-injected 5-10 cc of CSF/sterile water, to assess the patient's level of compliance and tolerance to raising and lowering his ICP. This patient's physiologic ICP was observed to be within a 10-15 cm H2O "window" of his initial measurements. Then we compared these measurements with the DiaCeph graph. Not only did the initial ICP measurements match up with his DiaCeph data points, but his physiologic "window" and level of complaints at those data points also matched our neurosurgery office findings.

    This information also serves the shunt assessment and selection process in a second manner. The treating neurosurgeon can carefully match the patient's real time measured ICP and CSF outflow requirements against specifications of available shunts, and further compare these needs and shunt specifications to (bench test) shunt opening pressure and CSF outflow charts available by Aschoff and his colleagues at the University of Heidelberg, or other published works. This protocol enables the highest physiologic pre-surgical matching of patients to a particular shunt system, as well as determining the most physiologic (real time) pressure setting in programmable shunts. See also the New in DiaCeph Patent and the DiaCeph Test Power Point presentation which follows two sample patients: one with an incorrectly set programmable shunt, and a second with shunt obstruction. Note to the neurosurgeon: In order for ICP manometer readings to be most accurate when obtained through the shunt reservoir, some distal point of (peritoneal) catheter on the head should be pinched off with finger pressure.

Shunt Selection Model: History, Discussion of Technology and Methods, Conclusion

    This new tandem assessment method substantially aides the treating neurosurgeon when evaluating the hydrocephalic patient for evidence of shunt malfunction, selecting the most physiologic shunt in the event of a revision, selecting the most physiologic pressure setting in programmable shunts, assessment of new patients prior to first time shunt placement for obstructive or communicating hydrocephalus, NPH, and in post discharge assessment of patients following ETV procedures. The use of the medication, Diamox, as part of an interventional diagnostic assessment ( particularly NPH patients), can also be done in tandem with DiaCeph monitoring. SEE also our test literature page. Similarly, spinal taps and cisternography can be used in tandem with DiaCeph monitoring. DiaCeph data collected from a shunted patient can serve as an invaluable reference point for future management of the patient.

    Our Hydrocephalus and DiaCeph Patent sections provide additional useful information on CNS shunts and managing hydrocephalus, including, an up to date historical perspective, key FDA regulatory factors that have impacted shunt devices, assistive (neuro-cognitive) artificial intelligence technologies, and new techniques in managing chronic neurological complaints. Interested neurosurgeons, researchers, and patients should also view our Neuro-Compensatory Study relating to neuro-hypersensitivities, hippocampus dysfunction, and balance disorders secondary to hydrocephalus. We offer additional insights on alternative therapies using Music & Art Therapy, Drum Circles, and AI Technology (neurocompensatory mechanisms and therapies).

    Historically, first time shunt placement involved implanting of a medium pressure shunt of a type most familiar to the neurosurgeon, with the patient seen in follow-up after one to three months. CT or MRI imaging has been, and remains the primary determining exam, in evaluating the patient for proper shunt function - albeit assessment of ventricular size/change after shunting or ETV. The neurosurgeon will also perform an in-office neurological exam, and in the proper setting, may tap (insert) a needle into the shunt reservoir to measure ICP (intra-cranial pressure) and CSF (cerebral spinal fluid) flow resistance within the shunt. If shunt obstruction is believed, he could also order an isotope clearance test, where a radiotracer is injected into the shunt reservoir and imaging performed for up to an hour. More infrequently, he may order an isotope cisternogram, where an isotope is injected into the spinal fluid space and imaged for 72 hours, or he may admit the patient into the hospital for 48 hour ICP monitoring.

    More often than not, after initial shunt placement, a revision to a shunt with different pressure/flow specifications setting is required. And today, some neurosurgeons will elect to implant a "programmable" shunt from the start to avoid having to re-operate to a shunt with a different opening pressure. In these shunts, the opening pressure, or amount of resistance, can be readily adjusted externally using a calibrating instrument provided by the manufacturer. But these do have their drawbacks, as discussed later in this paper. Also quite common today, is the use of "anti-siphon" shunts, and separately attached siphon reducing devices in conjunction with a selected primary shunt. Alternatively, some neurosurgeons will opt to use one of two newer " auto-regulating" shunts, either the Orbis Sigma shunt or Diamond Valve. Again, each of these technologies comes with their own finer points, and drawbacks. It is up to the treating neurosurgeon, in each case, to determine what will best meet the patient's needs. It has been a "trial and error" process. However, our new tandem protocol can substantially improve shunting.

    Over the last seven years, Codman, Medtronic, and Sophysa have each introduced their own models of programmable shunts, with Codman being the first to receive FDA marketing approval in the U.S. In addition, two newer fixed resistance " anti-siphon devices" have been introduced, the Miethke shunt assistant, and the Vygon/Phoenix Biomedical gravity compensating device. Programmable shunts have aided in reducing the number of surgical revisions due to a mis-match in shunt opening pressure. But, they are yet to become the new standard in shunting for two reasons. First, siphon control devices often need to be subsequently added or removed surgically. Secondly, programmable shunt pressure settings can be "tripped" accidentally by various common magnetic fields. Sophysa has introduced a new model, the Polaris, that cannot be tripped by magnetic fields. Amidst the newer shunt technologies, a multi-center study by Dr. Drake et. al. and his U.S. colleagues published in year 2000 did not report significant improvements in overall outcomes and survival rates compared to previous years. Perhaps, a revisiting of this study might reveal a greater improvement in measured outcomes.

    In light of the versatility that these newer shunts have added to shunting outcomes and pre-surgical shunt selection (re: opening pressure models), postural overdrainage and physiologic ICP in all postures remains an on-going problem. We believe the big advance in shunt technology will come via "programmable siphon control devices" - for fine-tuning upright physiologic ICP and overdrainage. When coupled with a sufficiently flowing auto-regulating shunt, a programmable siphon control device would resolve 95% of shunt related complaints and surgical revision due to pressure/flow characteristics. 

    CSF overdrainage, or siphoning, is more prevalent in taller children and adults, in those with slit ventricle syndrome, and in those who have a lesser degree of shunt dependence. Overdrainage, and subsequently low ICP, occurs when the shunted patient (primarily v-p shunts) sits or stands for any period of time. Headaches, small slit like-ventricles, brain shearing and/or bleeding can result if too much overdrainage occurs. Once a patient is fitted with a shunt, most will continue to have some residual CSF clearance, and coinciding with their degree of shunt dependency. Each patient's supplemental CSF outflow needs via shunting, plus age, weight, and other factors specific to each patient, will determine their unique specifications required of a shunt.

    Overdrainage (too much CSF outflow) vs. underdrainage (too little CSF outflow) has historically been difficult to measure, absent daily routine monitoring of the patient. Underdrainage typically occurs with a valve of too high an opening pressure - but it can also be confused with shunt obstruction. Overdrainage usually occurs due to either of two factors: either the opening pressure of the shunt valve is too low, or the patient is more affected by the increased CSF flow, or "siphoning," that occurs when the patient sits or stands. Overdrainage is rather easy to ascertain, but selecting the correct technology solution is more complex. Once a shunt is felt to produce an acceptable response on CT/MRI and upon review of clinical symptoms, the outcome is considered successful and the patient then followed by a neurologist or family physician. Sometimes, more often than most would acknowledge, a shunted patient will continue to experience complaints resembling shunt malfunction, to which the neurosurgeon cannot differentiate upon exam, or correct with shunt revision or a change in pressure setting. This has become a subject of much debate in the neurosurgical community - determining "when" a satisfactory surgical outcome exists, and "when" to intervene further surgically, therapeutically, or medicinally.

    New research we have undertaken suggests that a significant portion of once believed shunt related complaints may in fact be due to neuro-hypersensitivities and/or dysfunction of the hippocampus, brought on by repeated surgical interventions, and/or secondary neurological disorders and neuropathology, including stress and PTSD ( post traumatic stress disorder). As the "hippocampus" is the brain's critical sensory mechanism, lying just beneath the 3rd ventricle, it must processes all stimuli from our five senses. It is also known as the the originating center of most seizure activity. In some cases, neurosurgeons continue to re-operate to find a remedy, while others refer such a patients for biofeedback therapy. On some occasions, patients are mistakenly sent for biofeedback and psychiatric therapy, when in fact they have a surgically treatable problem. This remains the "quagmire" that neurosurgeons are faced with today. It was this ambiguity in determining acceptable shunting outcomes, and unavailability of "real-time" shunt monitoring, that attracted the interests of Stephen Dolle, the author of this paper.

    Many neurosurgeons have the view that a shunted patient will simply adapt, or compensate, to the characteristics of a particular shunt. But physiologic brain CSF fluid clearance is a complex and continuous mechanism. After placement of a CNS shunt, the patient's residual CSF clearance will continue to compete with the implanted shunt in clearing CSF. It has only been in the last few years that less significance is given to ventricular size on CT/MRI in the post operative evaluation. The true goal of shunting is to achieve an acceptable level of " total CSF clearance" and physiologic ICP, or compensation. So today - more consideration is given to the degree of symptoms and activities made possible with a particular shunt. Neuropsychological testing is useful over the longer term, but not normally used as a determinant of shunting outcome or performance. But it is indicative of shunting outcomes and degree of compensation, and ought to be more widely used. Insurance reimbursement remains a significant barrier in managing hydrocephalus in the U.S., in that neuropsychological testing and key critical diagnostic tests for shunt malfunction frequently face denial of coverage.

    It is widely accepted that the best physiologically matched and functioning shunt will produce the fewest complaints, fewest long term health complications, enable higher level cognitive function and productivity, and benefit the patient through the living of a more normal lifestyle. Still, it remains too much a matter of chance that patients are broadly benefiting from today's more physiologic shunt systems. Why a matter of chance? There is no widely accepted standard for pre-surgical shunt selection, no standard for qualifying physiologic shunt outcomes, no technology for early documenting of shunt malfunction, and no effective technology to determine if shunt malfunction is due to a non-physiologic shunt system, shunt failure, neuro-hypersensitivities, or if some combination of all are occurring.

    The assessment for shunt revision does lend itself to arbitrary judgment by the neurosurgeon, and has historically often led to mis-interpretation. When the DiaCeph Test was designed in 1997, Stephen Dolle did so with the hope that it could become an " in-vivo" assessment standard, as similar efforts were undertaken by the IMPACT Test to assess and standardize concussion. The DiaCeph Test aids the neurosurgeon through a number of specific determinations. It allows the patient and family to capture critical ( often misunderstood) data in real time, and standardize it within the parameters of their own case management program. It collects data indicative of whether the patient requires more, or less CSF outflow, and in various body postures and activities. It can indicate a failed valve, an obstruction, a disconnected catheter, or any compromise in flow to any portion of the shunt system. It is an artificial intelligent (AI) palm computer that will print key patient data and comparative charts when connected to its PC program. No more remembering, assorted notes, or physician speculation!

    This paper describes our protocol for using the DiaCeph Test, with or w/o corroboration with in-office ICP shunt tap manometer measurements, as described above, and in concert with available bench test shunt comparison data from the 1999 paper, Aschoff Shunt Reviews (in MS Word). Aschoff and his colleagues at the University of Heidelberg put 482 shunt valves through a series of elaborate bench tests. Two of those charts (Figures 13 and 14) are utilized in the discussion below. The neurosurgeon can review this information in concert with other exams/ clinical information when making a shunt selection, or changing the pressure setting on a programmable shunt. This protocol echoes published research where the most productive, and least symptomatic patient, is implanted with his or her most physiologic and ideal shunt. Use of the DiaCeph Test in tandem with an in-office ICP/shunt tap manometer evaluation, and Aschoff comparative charts, creates a highly accurate shunt selection model for determining CSF outflow needs - without the high costs and risks of in-hospital ICP monitoring.

    In this discussion, Patient No. 1 is followed to illustrate how DiaCeph data can help in selecting the most physiologic shunt. See also our two examples in the DiaCeph Power Point presentation. Patient No. 1 is currently shunted with an Orbis-Sigma I (OSV-1) shunt valve in a VP system. DiaCeph data revealed a patent (clear) shunt system. This patient also tends to underdrain (too little CSF outflow) during sleep at night while supine (when ICP is at its highest), with some reciprocal overdrainage immediately following REM sleep - corresponding to surges in ICP. During the daytime, this patient tends to overdrain (too much CSF outflow) slightly over the course of the day when sitting and/ or standing with the OSV-I.

    It is well known that sleep and supine posture produce higher ICP levels, so the shunt must have a low enough opening pressure and ICP responsiveness to accommodate ICP spikes during REM sleep. Patient No. 1 is also 6 feet tall, and has about 15-20% (partial) shunt dependence. These two factors render this patient more susceptible to CSF overdrainage when upright than the typical patient. The taller height creates a longer catheter distance between the inlet point of flow on the patient's head, and the emptying point in the abdomen. Also, with partial shunt dependence, there is ongoing CSF clearance that continues to compete with the shunt to clear CSF. Each individual's brain physiology, pain tolerance, and brain compliance further determines how well an individual will adapt to non-physiologic ICP. This adaptation is invariably influenced by other complex factors such as age, mental stature, and secondary health conditions. Another complicating issue, can be the presence of chronic neurological changes in the brain secondary to hydrocephalus, such as inflammation of the hippocampus, that can mask as shunt malfunction or valve mismatch, and be misleading to the neurosurgeon when revising or re-programming a shunt. Hippocampus complaints are best evaluated via PET (positron emission tomography), fMRI (functional MRI) imaging, and to some degree, through neuropsychological testing.

    The desired shunt system here must have a sufficiently high enough opening pressure, or outflow resistance, to match the physical flow/siphon control needs of the patient when sitting and standing. Historical remedies to offset upright overdrainage have included SCD and ASD anti-siphon devices and integral shunts by Medtronic/PS Medical and Integra/Heyer-Schulte respectively. A siphon control device, Siphon Guard, is also offered by Codman separately, and within their programmable shunt. There are also two auto-regulating models of the Orbis-Sigma shunt, the OSV-2 and the Integra Low Flow Valve. Meithke offers a gravitational device, or shunt assistant, and several shunt models. Vygon Neuro (formerly Phoenix Biomedical) offers an auto-regulating shunt, the Diamond Valve, and a new shunt assistant. These shunt assistant devices are secondary devices, and added to a shunt in surgery.

    In Figure 14 below, Aschoff provides a flow curve of the Codman Siphon Guard device, that appears to limit CSF outflow to 20-25 ml/hour (normal CSF production rate), though after allowing an initial surge to 150 ml/hour. The Meithke Shunt Assistant anti-siphon device, not evaluated here, comes in 6 pre-set resistance levels of 10, 15, 20, 25, 30, and 35 cm of H2O, that add the rated cm of H2O to the opening pressure of the patient's primary shunt valve when he/she is upright. The device is activated by change in angle from the supine posture. Aschoff only provides a chart of an earlier Cordis shunt assistant in Figure 16. It is postulated the Meithke device would display a similar chart appearance.

    Figures 13 and Figure 14 below were taken from page 15 of Aschoff et. al.'s study, "482 Valves Tested in Vitro and a Review on 652 Tests Reported in Literature." The full excerpt of this study can be found in this MS Word format file Aschoff Shunt Reviews.

    As is seen in Figure 13, each of the eight (8) shunt valves evaluated had unique flow (ml/hour) patterns in response to increasing pressure (cm H2O), simulating a patient's response. Featured are the Orbis-Sigma I, the Orbis-Sigma II, Holter-Hausner 70 and 100 mmH2O opening pressures, the Phoenix 3 70 mmH2O opening pressure, Denver Low-Flow pressure, Spitz-Holter Medium opening pressure, and the French-Neurone 9 95 mmH2O opening pressure.

Fig. 13   Comparison of pressure-flow-graphs of Orbis-Sigma (type 1) vs. low-flow slit-valves with a little bit sticking silicone.

    Figure 13 chart reveals a higher flow rate for the Orbis-Sigma I above 30 mmH2O over its newer Orbis-Sigma II model, plus a higher flow rate compared to the other valves shown for opening pressures below 15 mmH2O. The Orbis-Sigma II model would be more suitable for patients with low to normal intracranial pressure and outflow needs (lower opening pressure), and more resistance to flow under higher opening pressures (such as when upright). This chart reveals increased outflow resistance (lower flow) during peak flow for the Orbis-Sigma II over the Orbis-Sigma I. The OSV-I was discontinued last year, so we include it only for comparative value. The difference in maximum CSF flow between the OSV-II and OSV-I is measurable on this chart, and suggests that the OSV-II will better control overdrainage. In Patient No. 1, the Orbis-Sigma II would be slightly more effective in limiting CSF outflow (overdrainage) with the patient standing, and offer additional resistance to run-away CSF outflows during REM sleep. It is not known if this additional resistance is sufficient to control this upright overdrainage, and run-away CSF when the patient is laying supine during sleep. The determining shunt issue would be how rapidly it "responds" to the ICP surges of REM sleep.

    Figure 14 below compares Codman's Siphon Guard 3 (anti-siphon device only) to Vygon's Diamond valve and Integra's (discontinued) Orbis-Sigma I valve in a hanging system - to assess control of upright overdrainage. It suggests the Diamond Valve would be beneficial in patients who need anti-siphon control, but having average CSF outflow needs - similar to a low-medium pressure valve. In the case of Patient No. 1, the Diamond valve may resolve the underdrainage problem associated with sleep, but would likely allow overdrainage in the upright posture, and likely not suit this patient. By comparison, the Orbis-Sigma I provides more anti-siphon control, with less overall CSF outflow than the Diamond valve - similar to a medium-high pressure valve. The Siphon Guard appears to limit CSF outflow to 20-25 ml/hour, but it does not reveal how its flow restriction might act under varying postures. The Siphon Guard is available as an integral system with the Codman programmable (Hakim) shunt model, which comes with 18 separate pressure settings, or as a supplemental device.

Fig 14  Pressure-flow-characteristics of Phoenix-Diamond and Codman SiphonGuard 3 compared to the flow of a previously tested Orbis-Sigma 1. The Diamond shows a higher flow, leading a higher risk for over-, but a lower risk for underdrainage compared to Orbis-Sigma 1. The SiphonGuard 3 strictly limits the flow to 20 ml/h.  Note: It is a supplementary valve. The flow near the opening pressure is controlled by the main valve (not shown)!

    In Patient No. 1, neither the Orbis-Sigma I, the Diamond valve, nor Siphon Guard ( supplemental device) appears to be a good physiologic match. The Orbis-Sigma II was not fully evaluated by Aschoff. It's auto-regulating specs as pressure/flows include: Stage I = 30 to 80 mm H2O at 5 to 18 ml/hour, Stage II = 120 to 300 mm H20 at 18 to 30 ml/hour, Stage III = 300+ mm H2O at 30 to 40 ml/hour. It may be a match for this patient. Published studies suggest it to be more responsive to ICP changes than its earlier OSV-II, now discontinued. The Meithke shunt assistant (30 or 35 cm H2O model) could be attached to a Codman or Sophysa programmable valve, and would seem to be a good predictable match. The Codman valve comes with 18 pressure choices, in 10 mm increments from 30 mm H2O to 200 mm H2O. The Sophysa comes in four different models ranging from 10 to 140 mm H2O, 30 to 200 mm H20, 50 to 300 mm H2O, and 80 to 400 mm H2O, with eight pressure selections within each model range. The SM8 model with 30 to 200 mm H20 is their standard model. Sophysa has a new programmable shunt, the Polaris, reported as unaffected by magnetic fields, with the same low and high end pressure ranges, but with only "five" pressure selections to choose from. The SPV is reported to be their widely used standard model. With all other available programmable valves susceptible to unintended reprogramming from exposure to magnetic fields, it is a feature to consider.

    Medtronic's programmable shunt, the Strata valve, comes only with its original SCD siphon control device integrated inside the valve, and has a total of " five" pressure selections, ranging from about 15 mm H2O up to 160 mm H2O. Compared to the Codman and Sophysa valve, which have pressure selections in 10 mm H2O, and 15 to 30 mm H2O increments, respectively, the Strata valve's incremental selections are spaced slightly further apart, 20 to 35 mm H2O on average. It is not known of how significant the physiological limitations of having only five pressure selections (Strata and Polaris), and may well be an occasional individual patient issue. It is inviting to speculate why these manufacturers chose to use the varied pressure selections and ranges. Perhaps it relates to patent and proprietary issues, or merely their effort to target different pressure schemes. A reasonable argument would support Sophysa's use of four (4) separate models/range of pressure selection schemes, provided the neurosurgeon has the necessary ICP/CSF outflows information on each patient - to avoid a mis-match. We note their SM8-140 model offers the lowest opening pressure (10 mm H2O) of all the programmable valves studied, and perhaps more suited for infants.

    The above programmable valve pressure specifications would appear to meet the opening pressure requirements for Patient No. 1 - in the supine posture. But the difficulty comes in the selection of the very critical "siphon control" device ( or auto-regulating shunt valve) that will most physiologically regulate CSF outflow, and ICP, in the upright postures. The upright specifications required for this patient, as described earlier, are critical to controlling his propensity to overdrain. In addition, with a frontally located ventricular catheter (near the top of his head), the valve and accessory placed at this site will have to overcome the additional 2 to 4 cm H2O of resistance created by CSF having to initially flow up vertically before emptying into the abdomen, primarily in upright postures.

    We were unable to locate additional CSF flows specifications on Codman's Siphon Guard device. The Meithke Shunt Assistant has more detailed product literature as a siphon control device, narrowing the pre-surgical guesswork. The Strata valve's integral siphon control device (and separately available SCD), requires the valve body/SCD to be placed at a fairly precise vertical location, or "zero point," on the patient's head relative to the inflow point - the tip of the ventricular catheter. Its placement site then acts as a reference point for the degree of siphon retarding resistance. The Strata's Technical Bulletin PDF file at http://www.medtronic.com/neurosurgery/valves.html illustrates on page 6, Fig. 13, just how critical its vertical placement or "zero point" is on the patient's head - in order to create the desired reference point and upright CSF flow control per its specifications. Fig 13. clearly illustrates how a mere 3 cm vertical misalignment above the zero point will limit upright CSF flow to less than 10 ml/hour, and lead to underdrainage; whereas a 3 cm misalignment below this zero point will allow upright CSF flow rate to reach about 110 ml/hour, leading to overdrainage. Additional resistance is also caused by overlying scalp/scar tissue encapsulation, as well as pressure exerted by lying on it. So, this reference point is extremely critical, and only relates to SCD and ASD anti-siphon device designs.

    In Patient No. 1, it is critical that upright siphon control maintain ICP between -10 and +10 cm H20, by controlling CSF outflow in concert with the programmable valve's opening pressure. An approximate figure of negative hydrostatic pressure (when standing) acting upon a standard 60 to 70 cm peritoneal catheter length is said to be about -50 cm H20. A person at 6 feet in height will have an even longer distance, perhaps 80 cm to 90 cm of catheter, and about -70 cm (negative) hydrostatic pressure. In order to maintain this patient's ICP above -10 cm H20 when standing, the combined valve and siphon control resistance must counteract this -70 cm H20, and match with the patient's own ICP, measured in cm of H20.  The shunt system must also consider the prospects for overdrainage, more significant in patients with a lesser amount of shunt dependence. This same shunt system must feature resistance and valve opening pressures to accommodate the patient in supine posture, where we can estimate the siphon effect and hydrostatic pressure to be negligible. In the supine posture, the shunt system should maintain ICP between +10 cm H20 and +20 cm. Either a programmable shunt with a good siphon control device for upright posture flow control, or the Orbis Sigma II auto-regulating shunt could be used in this patient. 

    Most shunt models provide specifications according to "opening pressure," to which CSF flow is determined, in part upon the amount of pressure (in ICP) in the user's ventricles, the dimensions of the valve and catheter (including a distal slit valve), and per the presence of a siphon control device in the system. Distal slit valves have been employed at the tip of a peritoneal catheter to reduce hydrostatic pressure, but have been mostly discontinued due to problems with debris clogging the outflow path. The critical specification in CNS shunts is "CSF outflow," because when it is correctly controlled under various postures and physical exertion, so will the ICP of the user. There are well published elaborate formulas for determining "CSF outflow," which we will not cover here. Instead, we would advocate the creation of "opening pressure/CSF flow/ICP templates," so neurosurgeons can readily correlate shunt specification data, rated in resistance, to a patient's measured ICP and required CSF outflow figures.

    Patient No. 1 has a frontally located 5 cm ventricular catheter pointing downwards. So, if an SCD device or Strata valve (SCD located distal end) were implanted, it would need to be placed at a vertical site on the head within a 1 to 2 cm window of the "zero point," or same horizontal plane as the tip of the catheter, to achieve the needed upright CSF flow control. In effect, site placement in a Strata or SCD creates a customizable level of upright siphon flow control - but must be strategically located to create the desired level of resistance. It is important to have "pre-surgically ascertained data" of the patient's upright and supine ICP and outflow needs, in order to determine the customizable level of upright resistance. Otherwise, the neurosurgeon is guessing. Programmable siphon control devices would address this issue, and enable external fine-tuning of upright ICP. 

    Medtronic's literature states that a Strata valve placement and misalignment by more than 3 cm of distance from the required zero point will lead to dramatically altered CSF flow rates in the upright posture - and may not permit any of its five pressure settings to achieve a safe and acceptable CSF outflow and patient ICP in both upright and supine postures. In consideration, we contacted Medtronic and asked them to create a special pre-surgical placement protocol to better enable the Strata, Delta, and SCD devices to be efficaciously placed in accordance with their literature. Medtronic indicated they are not interested in providing any such protocol. SEE the END of this paper for UPDATE.

    Siphon control provided by the Meithke shunt assistant and Vygon/Phoenix gravitational device are based upon pre-calibrated fixed resistance measurements in cm of H20. Meithke's only stipulation is that it be placed vertically within a shunt system, limiting its placement to a limited area on the back of the head, or in the chest. The Meithke, Vygon, and Siphon Guard devices all claim variable control of anti-siphon action to patient posture, a physiologic feature if carried out correctly. This variable control is also a feature of the Orbis Sigma and Diamond Valve shunts. The Meithke and Vygon devices must be sutured in during surgery, and the neurosurgeon must be comfortable with doing so.  

    All three programmable shunt valves, the Codman, Medtronic, and Sophysa likely meet the supine and overall needs of Patient No. 1, except for the appropriate siphon control. We have contacted Medtronic and asked them to provide a new pre-surgical site placement protocol to enable more reliable and physiologic placement of the Strata valve and SCD. The Meithke Shunt Assistant has not been widely used in the U.S., and most neurosurgeons remain unwilling to try it. We did earlier ask Codman for additional information on their Siphon Guard, and they are yet to provide us with any further information. We met with Integra Life Sciences and discussed their OSV II and Low Flow valves. Integra reports that their design and manufacturing improvements in the OSV II allow this shunt to better migrate between Stages I, II, and III, particularly in the control of overdrainage in the upright posture. We are awaiting to review new published studies, and new comparative bench tests findings as was previously reported by Aschoff et. al at the University of Heidelberg. The OSV II could well be a correct shunt match for this sample patient. But, we would like to see studies that support this hypothesis. We retain some "apprehension" in recommending the OSV II for this sample patient, mostly with respect to its ability to maintain upright physiologic ICP. At this juncture, our conclusions preliminarily find for using the OSV II as the best possible shunt valve match in this sample patient.

    In closing, we also believe the new tandem application of the DiaCeph Test with a single in-office ICP manometer evaluation (with or w/o shunt comparison charts), is an efficacious and reliable pre-surgical shunt selection protocol, with consideration of patient age and developmental level.

UPDATE ON SURGICAL SITE PLACEMENT WITH MEDTRONIC STRATA AND DELTA VALVES

    We earlier asked Medtronic PS Medical if they would provide a new "pre-surgical placement protocol" for their Strata and Delta shunts to help neurosurgeons better place these shunts for correct "anti-siphon" operation. Medtronic denied our request, and we later learned that the company is misleading neurosurgeons regarding site placement of the Strata and Delta shunts with respect to the user's correct zero point. An incorrect surgical placement can lead to poor outcomes, chronic complaints, and unnecessary reoperations.