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Hydrocephalus: Overdrainage by Ventricular Shunts. A Review and Recommendations Robert H. Pudenz, M.D., and Eldon L. Foltz, M.D. Huntington Medical Research Institute and Division of Neurosurgery, University of California Irvine Medical Center, Orange, California

Pudenz RH, Foltz EL. Hydrocephalus: overdrainage by ventricular shunts. A review and recommendations. Surg Neurol 1991;35:200-12.

Selected literature review of the clinical course of patients with ventricular shunts for hydrocephalus shows that the effects of cerebrospinal fluid overdrainage are subdural hematoma, craniosynostosis, slit ventricle syndrome, and low intracranial pressure syndrome. These occur sequentially at different age groups, but approximate averages of incidence and time of occurrence after first shunt reveal an overall incidence of 10%-12% for at least one of these appearing at 6.5 years after shunting. The basic etiology, diagnosis, and variety of treatment modalities available are reviewed, including the need for shunt closing intracranial pressure control. Included is a hydrocephalus program designed to minimize the need for long-term extracranial shunts and to maximize therapeutic intracranial procedures for hydrocephalus. KEY WORDS: Hydrocephalus; Overdrainage; Ventricular shunts; Slit ventricles; Review; Recommendation

Prior to mid 1950, hydrocephalus was rarely treated. Fatal outcome was usual, often after prolonged nursing care and hospitalizations. In 1956, ventricular shunts dramatically changed the outlook for hydrocephalus. Quality survival and near normal life expectancy were possible. In spite of continued upgrading of shunt techniques, however, ventricular shunting of cerebrospinal fluid (CSF) in hydrocephalic patients is generally accepted as associated with more complications than any other neurosurgical procedure [ 1,2,20,21,23,26,35,44,52]. The major complications are infection, shunt obstruction, and overdrainage [9,15,18,30, Foltz, Meyer, personal communication, 1989]. Overdrainage can predis-

Address reprint requests to: Eldon L. Foltz, M.D., Professor of Neurological Surgery, Division of Neurosurgery, University of California Irvine Medical Center, 101 City Drive South, Orange, California 92668. Received April 23, 1990; accepted August 15, 1990.

© 1991 by Elsevier Science Publishing Co., Inc.

pose the patient to subdural hematoma [54], premature closure of cranial sutures [34], the slit ventricle syndrome (SVS) [4], and low intracranial pressure (ICP) syndrome [12]. If ventricular shunting has been done inappropriately for communicating hydrocephalus, stenosis or occlusion of the sylvian aqueduct may occur [15]. This report is concerned with the incidence, diagnosis, and treatment of overdrainage problems, with a primary focus on the SVS and low ICP syndrome. The overall incidence of ventricular overshunting, however, is surprisingly high, and the age of incidence of several clinical problems included is predictable relative to age of the patients involved. In this report, the variety of methods used to control CSF overdrainage in ventricular shunts is reviewed, including the new zero pressure shunt system. The reports reviewed are difficult to compare, largely because of the different methods used to record ICP, especially as related to a generally accepted reference point and an understanding of the normal range of ICP in supine and upright body positions. Intracranial Hydrodynamics The effects of overdrainage of CSF shunting require a discussion of basic intracranial hydrodynamics. It must be emphasized that all of the recorded ICPs are based on ambient atmospheric pressure. The latter is considered as transmitted to the CSF system via arteries and veins with primary access to the CSF via the pulmonary system. The pressures in the cerebral ventricles, the basal cisterns, and the spinal subarachnoid spaces vary from individual to individual, depending on body size and build, general health, posture, activity, and body position (upright or supine), as well as the presence of Valsalva maneuvers. Intracranial pressures are considerably lower in infants and young children than in adults, in part related to less-than-adult resistance because of the expansile nature of the skull. In an interesting 1980 publication, Welch [57] re0090-3019/91/$3.50

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ported ICP measurements in 28 babies hospitalized for conditions not associated with increased ICP. Pressures measured by lumbar puncture in the lateral decubitus position had mean values of 45 -+ 12 mm H20. Welch stated that it was possible to estimate ICP within a range of 10 mm H20 by employing the aplanation principle, ie, observing the plane of the anterior fontanelle with the raising and lowering of the infant's head. Under normal conditions, ICP in the lateral ventricles and the spinal subarachnoid space in adults is approximately 100 +- 50 mm H20 in the lateral decubitus. The normal variation is considerable. With 45 ° of head elevation, ventricular pressures are subatmospheric, ranging from + 10 to - 70 mm H20. In the sitting position, the height of the CSF column as observed in a fluid manometer is approximately at the level of the cisterna magna, or even at the foramen of Monro level when the head is partially flexed at the neck. Intraventricular pressures in the sitting and standing positions average - 70 to - 100 mm H20. The reference point for ICP in this report is the uppermost level of the CSF system for the body position [57]. Pressure measurements in infants with hydrocephalus were reported in 1975 by Yamada et al [58] in which ventricular, right atrial, and peritoneal pressures were recorded in six infants prior to ventriculoatrial (VA) or ventriculoperitoneal (VP) shunting. In this group, the ventricular pressures varied from 100 to 250 mm H20, atrial pressures from - 10 to 0 mm H20, and peritoneal pressures from 0 to 50 mm H20 when the infants are quiet. During crying and straining (Valsalva maneuvers), ventricular ICP varied from 200 to 500 mm H20 on inspiration and 350 to 900 mm H20 on expiration. Atrial pressures varied from - 200 to 0 mm H20 during inspiration and 300 to 800 mm H20 during expiration. Similarily, peritoneal pressures were 30 to 150 mm H20 on inspiration and 400 to 750 mm H20 during expiration. The observations of Yamada et al emphasize a significant variation of intraventricular, atrial, and peritoneal pressures and stress their importance in evaluating shunt flow as a function of relative and variable pressure gradients from the lateral ventricle to the absorption site of the CSF shunt. Shunting and Cerebrospinal Fluid Hydrodynamics The purpose of extracranial shunting in hydrocephalus is to prevent brain mass destruction by ventriculomegaly. It has been assumed for years that ventriculomegaly is due to increased mean ICP, but recent evidence [5] clearly indicates that pulse ICP is possibly even more important in producing ventricular enlargement and loss ofintracranial compliance [10,12,14]. The interrelation-

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ship of pulse and mean ICP is slowly emerging, and it seems that both factors are important [14]. To date, ventricular shunt design has not addressed the issue of recovery of normal pulse ICP. Design has recently emphasized flow control of CSF through the shunt, hopefully to approximate the need to the individual patient, though that particular individual's need is difficult to identify [12]. In hydrocephalus, the complex deficit in reabsorption of CSF, especially its individual variation, has prevented a true physiologic correction by artificial shunt and valve systems. Gravity-influenced CSF flow in ventricular shunts needs continual emphasis [Foltz, Meyer, personal communication, 1989]. All present shunts respond to gravity and therefore to body position as to rate of CSF flow produced. Because of gravity effect on our relative opening pressure shunt systems, the supine position causes less flow than the upright position, due simply to the length of fluid-filled catheter affected by gravity, which produces a negative pressure or sucking effect on the ventricular cavities. Pormoy [46,47] and Pormoy et al [48] state that the pressure that drives CSF through a shunt system is expressed by the formula: PP = IVP + HP - (DCP + CP), where PP = perfusion pressure of CSF, IVP = intraventricular pressure, HP -- hydrostatic pressure (length of gravity-effected CSF column), DCP = distal cavity pressure, and CP = closing pressure of the differential pressure valve. This formula has considerable merit, but the closing pressure of the differential pressure valve is exceedingly variable, and all valves involved in the testing are in fact relative pressure valves based on an opening pressure. The formula completely ignores the position of the body relative to the function of the shunts, but HP, the hydrostatic pressure, is strongly emphasized. Recently, Sainte-Rose et al [51] have expressed CSF flow through a shunt as: F -- DP/R, where F = flow of CSF through the shunt, DP = difference between input and output pressures of the shunt, and R = resistance of the shunt system. Again, this contribution fails to recognize the difference in shunt flow based on hydrostatic pressures being variable in different body positions. However, Sainte-Rose et al do clearly reemphasize that this valve described is based on the physiological principle that flow through the shunt should not exceed the CSF production rate of 20-30 mL/h. Flow is controlled at these rates in stages I and II, when the patient is supine or erect, and in stage III, when differential pressures rise above 350 mm H20, flow rate increases rapidly. In vitro testing of this valve

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is described in detail. Evaluation of hydrocephalus in patients aged 2 months to 15 years is said to be promising. In regard to this point, differential pressure valves are available from several manufacturers and are designed to operate in low, medium, and high opening pressure ranges. Pressure and flow data on the valves are provided, but precision and valve function cannot meet biological needs. These data do, however, conform to the standards set by the American Society for Testing and Materials (ASTM F647 Standard Practice for Evaluating and Specifying Implantable Shunt Assemblies for Neurosurgical Application). The neurosurgeon does evaluate the opening and closing of such valves prior to implantation, but the pressure and flow characteristics require specialized equipment. Shunt flow is affected by the distal cavity pressures since all present shunts are based on gradient pressures. For example, in VA shunting the pressure in the atrium is approximately 60 mm H 2 0 in the supine position and 0 mm H 2 0 in the upright position [49]. Similarly, during standing the neck veins are partially collapsed and the pressure in the sagittal sinus becomes about - 130 mm H20. When the heart is pumping vigorously or when blood flow into the heart is depressed, the atrial pressure may lower to - 40 to - 60 mm H20. On the other hand, right atrial pressure is known to be very high, eg, in heart failure. Peritoneal pressures are also subject to wide variations in posture, body build, and respiration, and during any type o f Valsalva maneuver. Subdiaphragmatic pressures in the standing position have been recorded from - 170 mm to ÷ 120 mm H 2 0 , with the average pressure being 0 mm H 2 0 . Furthermore, the 0 mm H 2 0 in the subdiaphragmatic region may be accompanied by ÷ 400 mm H 2 0 pressure in the pelvis. The effects of crying and straining on intraperitoneal pressures are significant and highly variable. It is therefore obvious that distal cavity pressures will alter CSF flow through a shunt system numerous times throughout the course of a single day irrespective of type o f shunt, and also irrespective of the opening pressure of that shunt. From the foregoing, it is apparent that current shunt systems cannot be expected to operate in a steady-state mode. Cerebrospinal fluid flow will vary depending on the size, position, and activity of the person. Many factors must therefore be considered in the selection of an appropriate shunt system. There are many reports in the literature on the effects o f VA and VP shunting on CSF hydrodynamics. In 1972 and 1973 Fox et al [16,17] and McCuUough et al [37] recorded intraventricular pressures in patients with suspected normal pressure hydrocephalus before and after shunt surgery. In their 1973 report, preoperative and

Pudenz and Foltz

Figure 1. Universal ICP measurement reference point for usual body position; ICP stated as pressure in millimeters of water at this point for all body positions.

postoperative ventricular pressures o f 18 patients in the supine, semirecumbent, sitting, and standing positions were recorded. The pressures were made via a 22-gauge needle inserted into the dome o f a Mishler or Ommaya reservoir, which in turn was connected to a catheter in the frontal horn of the lateral ventricle. Pressures were recorded using an external pressure transducer coupled to an amplifier that provided an instant readout. All pressures were corrected to a horizontal plane passing through the foramen of Monro. The shunts employed Pudenz catheters for the atrium and Raimondi catheters for the peritoneum. In the preoperative patients, the ICP varied from ÷ 100 to ÷ 150 mm H 2 0 with the patient supine and - 20 to - 80 mm H 2 0 on sitting and standing. Following shunting, the supine ICP in both the VA and VP shunt patients varied from ÷ 70 to - 20 mm H 2 0 , with the majority of pressures at 0 or below. On sitting or standing the average drops in ICP were - 2 5 0 mm H 2 0 in VP shunts. On the basis o f these findings the authors concluded that the greater negativity in VP shunting was more likely to be associated with development of subdural hematoma. There are recent data concerning chronic shunted hydrocephalus patients that reemphasize three major points in hydrocephalus diagnosis and treatment that have contributed to a continued lack o f precision in hydrocephalus treatment by shunting [Foltz and Meyer, personal communication, 1989]. 1. A universally used reference point for ICP measurements does not exist in the extensive literature (Figure 1). 2. An accurate history o f symptoms present when the patient is up and about or when lying down is rarely

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1. Subdural hematoma: occurs within 1 to 2 years of IOO"

50

[

0-

( - I - '-50" I

-I00-150

- 200 • ~I~I~T~

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- 225

initial shunting [16,17,37,54]. 2. Premature closure of cranial sutures, skull deformities: in infants, obvious by 2 to 3 years after initial shunting [30,34]. 3. Stenosis or occlusion of the aqueduct: variable, 3 to 5 years after inappropriate ventricular shunting of communicating hydrocephalus [ 15 ]. 4. SVS: occurs at unpredictable times, 4 to 10 years after initial shunting [4,22,50]. 5. Low ICP syndrome: symptomatic at 5 to 17 years after initial shunting [11,12, Foltz and Meyer, personal communication, 1989].

UPRIGHT

Figure 2. Differences in ICP measuredin supine and upright positions (normal and shunted hydrocephaluspatients).

done, and therefore symptoms of occlusion or overdrainage are confused. 3. ICP measurements are not done routinely in shunt revisions and are rarely, if ever, done in the supine and upright body positions, as is mandatory for complete study of such patients when symptomatic of shunt dysfunction, either overdrainage or obstruction (Figure 2). These clinical studies [9,11,12, Fohz and Meyer, personal communication, 1989] led to reemphasis that normal ICP is different in the supine and upright body positions, but that shunts currently used are not designed to adjust to this fact, though normal brain physiology is based on this variability. These studies on long-term shunted hydrocephalic patients derive the functional, or "normal" ICP during this upright body position by using a new "zero pressure shunt system." The ICP in upright positions of shunted hydrocephalic patients can be predicted with relative accuracy by the use of the zero pressure shunt system. It is based on an absolute closing valve pressure technique and does not respond to. high negative pressures distal to the zero pressure device. The result of this type of shunt is a controlled upright ICP that is commensurate with asymptomatic, maximum brain function in these shunted patients. These "target ICP studies" implicitly recommend use of such shunt systems in the initial hydrocephalus operations, as the currently popular relative pressure shunts rely only on opening pressures, are relative pressure gradient shunts, and do not produce normal upright ICPs. A d v e r s e E f f e c t s o f CSF O v e r d r a i n a g e b y V e n t r i c u l a r CSF S h u n t s The major problems arising from the overdralnage of CSF are the following:

Subdural Hematoma and Hydroma The incidence of postshunt subdural hematoma and hydroma varies considerably in published reports. In the earlier reports published before computed tomography (CT) scans and magnetic resonance imaging (MRI) became routine procedures in the follow-up of shunt patients, the incidence varied from 4.5% to 21% [40,41, 47,54]. Since the advent of scanning, asymptomatic postoperative subdural hematomas and hydromas have been noted frequently. Faulhauer and Schmitz [9] reported on 400 shunted patients with 17 subdural hematomas (4%). Only five of these patients required surgical evacuation, which was combined with occlusion of the shunt to promote relative increase in ICP to "occlude" subdural spaces.

Premature Closure of Cranial Sutures, Skull Deformity, and Microcephaly There is a large body of literature concerning the skull changes that develop in shunted infants. In this report only selected papers are identified. In 1973, Kaufman et al [30] noted the skull changes in 176 infants followed up to 15 years after VA shunting. The changes included thickening and inward growth of bone in both the base and vault of the skull, synostosis of sutures, decrease in the size of the sella turcica, and reduction in the size of foramina with remineralization. With the recurrence of intracranial hypertension following shunt obstruction, many of these changes were reversible, except when complete synostosis was present. More recently, Faulhauer and Schmitz [9] evaluated 400 shunted hydrocephalic patients for signs of excessive CSF drainage. In this group 336 patients were seen regularly with an average observation time of 4 years. The principal skull deformity was dolichocephaly, or sagittal suture synostosis. Microcephaly developed in 33 babies, and 18 of these have synostosis of the sagittal

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suture. The overall incidence of microcephaly was approximately 6%. In 1971 Loop and Foltz [34] were one of the first to point out that craniostenosis and skull diploic lamination followed shunting for hydrocephalus. This could occur within 2 years of shunting.

Stenosis or Occlusion of the Sylvian Aqueduct In 1966, in their original paper on stenosis or occlusion of the aqueduct in 12 patients, Foltz and Shurtleff [15] noted that marked reduction of ventriculomegaly by a shunt may cause collapse of the aqueduct, converting communicating into obstructive hydrocephalus. Although they did not stress this as the sole etiologic factor, the bypassing of flow in the aqueduct appeared to have been contributory. The strong implication of such a study warrants ventricular shunting only for aqueduct stenosis already developed, and not for communicating hydrocephalus.

Slit Ventricle Syndrome Slit ventricle syndrome requires definition [39,55] and separation from the low ICP syndrome, in particular. Slit ventricle syndrome is the reduction of the ventriculomegaly of hydrocephalus to subnormal size as determined by CT scans or MRI. In this syndrome, it is generally accepted that encroachment of the ventricular walls of the ventricles on the ventricular catheter leads to intermittent, or even complete, obstruction of CSF flow with concomitant signs and symptoms of sudden ICP increase. This is not the same syndrome as the low ICP syndrome. The SVS is characterized by: 1. Slitlike configuration of the lateral ventricles as noted with imaging techniques. 2. Intermittent headaches, unrelated to posture and frequently accompanied by nausea, vomiting, drowsiness, irritability and impaired mentation. Incontinence, increases in focal neurological deficits already present, possible seizures, and lethargy [27] suggest rather sudden increased ICP. 3. Slow refilling of the ventricular shunt reservoir at testing by palpation, present during apparent temporary obstruction. The intermittency of symptoms and signs of high ICP may be related to sudden disappearance of slow refilling of the shunt reservoirs, reverting to normal reservoir function by palpation at a time when symptoms suddenly disappear--presumably secondary to clearing of the catheter obstruction.

Incidenceof SVS, ie, with clinical symptoms, as compared with discovery of only slitlike ventricles on scans, is

difficult to determine [28,39]. McLaurin and Olivi [38] state that slitlike ventricles will be noted on CT scans of approximately 5% of successfully shunted patients, and that one third of these will have the signs and symptoms associated with SVS. Time after operation is not cited. In 1982, Kiekens et al [31] reported a 5.3% incidence of SVS in 302 shunted hydrocephalus patients. The interval between the last revision of the shunt and development of SVS was 4.3 years. The authors noted that SVS did not appear in infants. Oi and Matsumoto [43] evaluated the development of SVS in patients and experimental animals. Kaolininduced hydrocephalus in dogs disclosed that slit ventricles showed ependymal and subependymal gliosis, dilated vascular beds, reduction in transependymal absorption of CSF, and decreased compliance. In their 1986 publication they noted a total of 94 cases of SVS in the literature; none of the patients were infants. The peak incidence was between 4 and 6 years of age. They reported the syndrome in eight patients, two of whom were infants. They noted further that slit ventricles were twice as common in premature or mature neonates (85.7%) than in infants 6 or 12 months of age (42.9% and 14.3%, respectively). Oi and Matsumoto [44] further described occlusion of the contralateral foramen of Monro in patients with SVS leading to an isolated dilated ventricle. In pediatric patients, Hubballah and Hoffman [26] confirmed this finding. Associated with the subependymal gliosis of the ventricular wall is an increase in elastance, which accounts for failure of the ventricle to dilate after overdralnage is corrected [4]. Engel et al [7] noted this problem in shunted patients with normal-sized ventricles; others have called this "stiff ventricles" [10]. In 1982 Hirayama [22] noted that slit ventricles developed in approximately one third of his 106 patients with VP shunts. He emphasized that close clinical monitoring of these patients was needed because of the risk of acute intracranial hypertension from obstruction. He considered this as a possible cause of death in five patients, though no confirmatory autopsies were done. Slit ventricles as a goal in shunting hydrocephalus, though advocated by some initially [22, Walker, personal communication], does not have support today. In summary, slit ventricles in hydrocephalus may be associated with: 1. No clinical symptoms, only demonstration by CT or MRI of slit ventricles. 2. Slit ventricle images associated with clinical symptoms of intermittent high ICP associated with temporary ventricular catheter occlusions, ie, the SVS.

Treatment of the SVS has been somewhat varied. Anti-

Overdrainage by Ventricular Shunts

siphon valves (ASVs) have been developed and used principally for prophylaxis and treatment of SVS, but prior to these, a variety of surgical techniques were used. An original technique was reported by Yelin and Ehni [59] in 1969, who surrounded the ventricular catheter with a perforated red rubber tube and were successful in preventing obstruction from coaptation of the ventricular walls. In 1970 Salmon [53] reported success in four patients with collapsed ventricles by implanting a valve operating at a higher pressure. In 12 other children who were originally shunted in the neonatal period, a higher pressure valve was implanted prophylactically. O'Brien [41] has implanted catheters in the frontal horns of both lateral ventricles via a bicoronal approach in managing SVS. The basis for this is not clear. Subtemporal craniectomy for the treatment of SVS was introduced by Epstein et al [8] in 1974. They noted that this procedure makes three important contributions to the management of shunt-dependent children with SVS: (1) venting of increased ICP, (2) enlargement of the ipsilateral ventricle, and (3) ability of the surgeon to evaluate ICP by observation and palpation of the craniectomy area. The effectiveness of subtemporal craniectomy in venting the increased ICP is also dependent on the size of the craniectomy and expansibility of the dura mater [45]. To promote the latter Epstein et al [8] incised the outer dural layer. In adults cruciate incision of the dura mater may be necessary. In 1979 Holness et al [24] stated that shunt revisions were required less frequently after a subtemporal decompression. Subtemporal craniectomy is not invariably accompanied by enlargement of the ipsilateral ventricle. In 1983 Linder et al [33] reported three patients who had an average of 48% reduction in ventricular size as determined by using a computer-digitized technique on CT scans. Despite the reduction in ventricular volume the procedure was effective. The authors speculate that favorable changes in the pressure-volume elasticity function may explain the effectiveness of the procedure. In 1982 Walsh and James [56] advised both subtemporal decompression and higher pressure valves in treating SVS, but the basis of such was not clearly defined. Third ventriculostomy was successfully used by Reddy et al [50] in 1988 on five patients with SVS. Prior treatment with antisiphon devices, addition of shunt valves operating at higher pressures, and subtemporal craniectomy did not provide relief. The mean follow-up time was 78 weeks. The third ventriculostomy was carried out via a subfrontal approach, opening of the chiasmatic and lamina terminalis cisterns, and making a 5-10-mm opening in the lamina terminalis. In the discussion of his paper, Hoffman [23] noted

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that a concurrent shunt probably facilitates the function of the third ventriculostomy by keeping the subarachnoid space patent. In additional discussion Kelly noted that patients with aqueduct stenosis or obstruction of the third or fourth ventricles have gratifying responses to CT stereotactic third ventriculostomy. Cohen [3] described a 15-year operative success of this procedure in 1949. In his paper on transcerebral fistula Foltz [ 13] points out that the success of these operations to direct CSF from ventricle to subarachnoid spaces depends on two major points: (1) successful measurement of CSF absorbing capacity of the space into which the CSF is directed from the ventricle in order to assure absorbing capacity there, and (2) successful closure of the arachnoid over the fistula to prevent dural adhesions that block the fistula or direct the fluid into the subdural space instead of the subarachnoid space. Antisiphon valves incorporated in the shunt system have become the method of choice in management of SVS. The original ASV was reported by Pormoy et al [48] in 1973. They demonstrated that such a device was effective in preventing abnormally low intraventricular pressures in 13 shunted patients. The ASV was designed to prevent the development of subdural hematoma by preventing excessive negative ICP, but six of these patients developed this complication. Explanation of this was not clear. However, they did state that the ASV operated on the principle that a diaphragm valve located just distal and in tandem with the flow control valve would close in the presence of negative hydrostatic ICP. Unfortunately, their ICP recordings were not as complete as might be desired. Location of the device in the system was not related to any specific vertical parameter. Gruber et al [19] stated that they began to use the ASV routinely in 1977 in the management of 41 pediatric hydrocephalus patients. They demonstrated the effectiveness of the device in preventing siphoning. Of particular interest was a significant reduction in postoperative ventricular catheter obstruction. The annual complication rate was four times less frequent after ASV implementation. They implanted the device in 10 primary procedures and 31 revisions in children. Hyde-Rowan et al [27] observed reexpansion of slit ventricles in six hydrocephalic children following highresistance valves incorporated with ASVs in 1982. In primary operations for hydrocephalus, Hoffman [23] in 1982 recognized the value of the ASV in preventing development of slit ventricles but would not use the device in young infants because negative intraventricular pressure is "helpful in restoring the cerebral mantle." The ICP reference point, however, is not mentioned and the degree of negative ICP was not actually recorded or commented on.

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In 1986 McCullough [36] reported use of the ASV in 40 children and young adults with extreme hydrocephalus. All of these patients were considered at risk for subdural hematoma, but nine developed severe neurological dysfunction despite shunt patency, with four of these showing alarming disturbances of consciousness and unusual neurological deficits. After the ASV, ventricular enlargement did occur as shown on CT scan. McCullough commented that these tall children and adults might have an obligatory opening pressure to open the shunt in the upright position, which was too low. His patients were relieved, or so it was implied, by increasing the valve from a low to medium pressure. He likewise stated that atmospheric or ambient pressure must be compensated by free, mobile scalp over the ASV to allow such to happen. In 1987, McLaurin and Olivi [38] succeeded in relieving the symptoms of the SVS in 15 patients by implantation of an ASV and upgrading the valve resistance. Postoperative CT scans showed some enlargement of the ventricles in nine patients. In the other six patients symptoms were relieved despite the lack of change in the ventricular size. In 1988, Jaskolska and MacKinnon [29] reported on 30 infants and children in whom an ASV was incorporated with a high- or medium-pressure valve during a primary shunt operation. In the remaining 20, the ASV was used to treat a suspected SVS. This relieved symptoms associated with overdrainage in the majority of patients. However, expansion of slit ventricles seldom occurred in the secondary procedures. They decided they would continue to use an ASV in combination with a medium-pressure valve during a primary shunt procedure despite their lack of conclusive evidence. The zero pressure shunt system has been recently developed [11,25, Foltz, Meyer, personal communication, 1989] to treat the low ICP syndrome initially, and now the SVS. The system incorporates the available siphon control device (SCD) and emphasizes the need for a universal ICP reference point to which the vertical location of the SCD in the system can be related when the patient is upright. Thus, the SCD is located at the optimum location (gravity related) to keep the upright negative ICP at the clinically indicated level. It is a closing pressure system based on absolute pressures, unlike the usual opening pressure, relative pressure gradient systems in general use. The initial series is under investigation.

Low Intracranial Pressure Syndrome The low ICP syndrome, at times called negative pressure syndrome, has been described in a number of reports on shunting [4,8,11,23,32,40, Foltz and Meyer, personal

Pudenz and Foltz

communication, 1989]. It is characterized by headache, nausea, emesis, lethargy, and even diplopia and paresis of upward gaze with strabismus and vision impairment, usually associated with the upright position, and frequently relieved by lying down if such observations are routinely carried out. Acute effects of CSF overdrainage, however, may produce even more dramatic effects such as tachycardia, loss of consciousness, and other possible brain stem deficits that are presumed by some to be secondary to a rostral shift of the brain stem but may even more logically be secondary to a very rapid lowering of normal ICP. The signs and symptoms of low ICP and overdrainage during shunting do simulate the signs and symptoms of elevated ICP, except the history and examination shows the presence of such when the patient is up and about, not lying down [ 10-12, Foltz, Meyer, personal communication, 1989]. Intraventricular pressure measurements during change from supine to erect positions is required for differentiation. This low ICP syndrome is not recognized easily in infants because of the lack of expressed symptoms due to age, but the infants who show marked depression of the anterior fontanelle and overriding of their cranial bones when in the position of head elevation obviously have lower than normal ICP. Patients with long-standing overdrainage of CSF by shunts may not tolerate procedures designed to restore their intraventricular pressures to homeostatic levels. Faulhauer and Schmitz [9] reported patients in whom elevations of ICP from below normal to 60 or 80 mm H20 (considered normal) caused pressure symptoms, including impairment of consciousness and even the risk of respiratory arrest. They considered these effects due to loss of CSF buffering effect with a decrease of brain compliance. Foltz and Blanks [12] reported similar experiences in 14 patients with the low ICP syndrome. They differentiated this syndrome from high ICP secondary to shunt obstruction by noting that the low ICP symptoms occurred when the patient was upright and active and that relief was obtained by lying down. Considerable relief was obtained by incorporating a high-pressure flow control valve in the shunt system. One third of these patients, however, did not maintain long-term improvement [Foltz, Meyer, personal communication, 1989]. In a report on 44 patients with low ICP syndrome, Foltz [11, personal communication] showed that all patients had very small or slit ventricles by CT scans of the head. Initial symptoms were clearly related to low ICP. However, episodes of shunt occlusion occurred in these patients if not treated promptly to alleviate the strikingly dramatic fall in ICP when they assumed the upright position. In some, relief was not immediately achieved

Overdrainage by Ventricular Shunts

for they could not tolerate rapid return to normal ICP, as related previously by Faulhauer and Schmitz [9], and yet, if not relieved, recurrent shunt obstructions with additional fixed central nervous system defects occurred.

Discussion The entire field of proper long-term ICP control in shunted hydrocephalus requires continued reemphasis on several major issues, clearly evident in our literature reviews and multiple discussions with colleagues. 1. What type of patient should have a ventricular shunt initially? In order to avoid some of the problems apparently inherent in ventricular shunting, only patients with ventricular obstructions at the foramen of Monro, third ventricle, aqueduct, and fourth ventricle should have initial ventricular shunts. This would obviate many of the problems of long-term ventricular shunts in communicating hydrocephalus, a type of hydrocephalus that should have LP shunts based on well-known basic principles in CSF physiology. 2. Since ventricular shunts work on a pressure gradient, how important is the resistance to flow at the CSF dump site of VA versus VP shunts? In this discussion, heavy emphasis has not been placed on the distal resistance to CSF flow. The literature reviewed shows quite conclusively that such pressures are extremely variable and overall probably have little effect on shunt functions over time. It is important to realize, however, that the VP shunts are much more likely to overdrain because of the long length of their tubing and the tendency for more of a "sucking effect" on the ventricular CSF than the shorter tube (shorter distance) of the VA shunts. This is clear in all papers discussing CSF overdrainage in shunted hydrocephalus. It is corrected by the recent zero pressure shunt system, which is equally effective in VA and VP shunts in maintaining the "target ICP." 3. What is "normal" ICP? Most papers do not clearly define a reference point that is valid for any body position or related to usual body positions of everyday life. Comparison of data from different centers is therefore difficult. An ICP reference point should be easily identified externally and reliably oriented to the measure device (Figure 1). From this review, emphasis on body position and normal ICP is warranted since (a) supine body position shows average ICP of 100 mm H 2 0 (range 50-150 mm H20), and (b) upright body position shows an average ICP of - 1 0 0 mm H20, with a range of - 2 0 to - 1 5 0 mm H20. 4. In what body position is ICP being measured? Upright, supine, or lateral decubitus? It is increasingly

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clear that ICP in shunted hydrocephalus must be measured in upright and supine positions if full and relevant data are to be achieved concerning low ICP signs and overdrainage symptoms. To do so accurately, the easily identified reference point (Figure 1) as described is mandatory. 5. Is ICP measured with the subject awake and no anesthesia? All general anesthetics increase ICP due to several factors. Only ICP measurements under local anesthesia with the patient quiet and undisturbed are valid measurements. 6. What is the target ICP for long-term shunted hydrocephalus? Such data by one of us (E.L.F.) are unique and show variability significant enough to conclude that the proper ICP is a level at which that individual patient (long-term shunt) can function satisfactorily when he is up and about. Measurement of ICP with the patient upright is mandatory. Supine, prone, or decubitus body position is not reliable for ICP records in the long-term shunt hydrocephalic patient who has overdrained. With the patient sitting or upright, the published target ICP to achieve recovery from the low ICP syndrome and to a lesser degree from SVS is the following: range, + 40 to - 2 5 0 mm H20; average, - 6 5 mm H~O (N -- 38). This review may have significant data if the overdrainage syndromes identified--subdural hematoma, craniosynostosis, aqueduct stenosis, SVS, and low ICP synd r o m e - a r e lumped together for averaging both incidence in the various series reported and time of occurrence since initial shunt. This interesting fact then becomes significant: 1 0 % - 12 % of ventricular shunt patients will develop at least one of the overdrainage problems identified herein by approximately 6.5 years after the initial shunt. Neurosurgery can do better than this. Hydrocephalus patients can be quite normal if the hydrocephalus is controlled properly over years and years. Avoiding overdrainage of CSF seems an obvious goal in the long-term treatment of hydrocephalus. The CSF overdrainage occurs almost exclusively when hydrocephalus patients are treated with ventricular shunts. Overdrainage has not been reported as a complication in LP shunts, Occurrence of CSF overdrainage can be reduced if the following occur. 1. Ventricular shunts, effective for 35 years in relieving high ICP of hydrocephalus, are used only in cases of obstructive hydrocephalus, such as aqueduct stenosis or fourth ventricle outlet obstruction to CSF bulk flow. Communicating hydrocephalus patients are treated preferably by an LP shunt, even in infants. These shunts seldom overdrain. 2. Ventricular shunts for long-term treatment of hydro-

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HYDROCEPHALUS PATIENT - Ventriculomegaly - Progressing/high I.C.P. Identify CSF flow site of obstruction

j--...

Obstructive Hydrocephalus -Aqueduct/4th Ventricle Obstruction

Communicating Hydrocephalus

LP Shunt

Step 1.

VA/VP Shunt

I

2-3 Weeks

Periodic follow up

Step 2.

*CSF a b s o r p t i o n d e f i c i t (cc/hr) - Ventriculostomy

Step 3,

*CSF a b s o r p t i o n s t i l l p o s s i b l e in 4th v e n t r i c l e and SA space. -

in v e n t r i c l e s

LP maximum infusion test: stable I.C.P. (cc/hr) -

CSF absorption equals or exceeds ventricular deficit.

CSF absorption and deficit do not match.

Intracran@al operation - Torkildsen - 3rd V e n t r i c u l o s t o m y - V-SA F i s t u l a

VA/VP Sh~unt Zero Pressure System ( ? plexectomy)

J

Periodic follow-up

cephalus are used only when intracranial operations cannot be used. A program based on this principle has been started (Figure 3) wherein obstructed hydrocephalus is initially treated by ventricular shunt only long enough to control ICP and stabilize the clinical status (step 1). During this period of several weeks, external CSF drainage techniques establish the CSF absorption deficits cubic centimeters per hour over a 12- to 24-hour period of time, using an averaged normal ICP threshold for drainage (step 2). This number represents the cubic centimeters per hour of CSF that must be absorbed somewhere other

Figure 3. Hydrocephalusprogram.

than the ventricle system proximal to the bulk flow occlusion site. In the hope of matching this deficiency with CSF absorption potentials distal to the obstruction, each patient then has a CSF absorption capacity in the posterior fossa and subarachnoid space calculated individually after the ICP has been normal for 2 to 3 weeks. This is done by doing the CSF infusion study of Katzman, by lumbar puncture (step 3). In this

Overdrainage by Ventricular Shunts

step, infusion rate equal to or above the deficiency of absorption (ventricular) in cubic centimeters per hour must be associated with a stable and normal CSF pressure for I hour. If such a result occurs, the patient may be a candidate for third ventriculostomy, Torkildsen procedure, transcerebral fistula, or ventriculosubarachnoid fistula. This program should identify candidates for intracranial operations with an 8 0 % - 8 5 % success probability, compared with the current success rate of less than 50%. If the CSF infusion shows CSF absorption rate less than the ventricular absorption need (ie, 10 cm3/h vs 15 cm3/h need), then an intracranial operation most likely will not be adequate and extracranial ventricular shunt is almost mandatory. Choroid plexectomy might have a role at this point before the ventricles become too small, but this option needs continued development. 3. In those cases of CSF "proximal" absorption deficit mismatch with CSF "distal" absorption capacity, ventricular shunting (VP or VA) is the procedure of choice, using the zero pressure shunt system to prevent overdrainage. The usual pressure gradient, opening pressure (relative) shunts have not controlled long-term (years) overdrainage problems since the differences in ICP when the patient is upright (lower, negative) as compared with the supine position (higher, positive) have not been considered in the design (Figure 4). Overdrainage therefore occurs because of very low, negative ICP produced by these shunts when the patient is upright. This has been erroneously termed "siphon effect," but occurs mainly due to direct gravity drainage related in degree to the length of the distal tube of the CSF shunt when the patient is upright (Figure 5). Thus, this sucking effect on the ventricular CSF effectively drains all CSF possible and the ICP falls well below normal for upright position because there is no absolute pressure limitation on the flow in such systems, only a relative pressure gradient. In a VP or VA shunt, the resistance of the abdominal cavity (VP) or the superior vena cava (VA) provides only minimal and variable flow reduction. These resistances are very variable and not highly significant, as interpreted presently. The zero pressure shunt system controls low ICP by preventing excessive CSF shunt flow when the patient is upright (Figure 5). Zero pressure device made as a SCD is the critical unit for such control. The system functions on an opening valve basis, and will function only when positive CSF pressure is present above the zero pressure device. Negative pressure below the device will not cause flow.

Surg Neurol 1991;35:200-12

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Therefore, when this system is installed, the zero pressure device is located at operation at a definite distance in centimeters below the vertex in a gravitational axis when the patient is upright. Since the CSF pressure immediately at the device level within the shunt system is barely positive at the input to the device, pressure at the ICP reference point (vertex when upright) will on average be of negative value equal only to the vertical distance from the vertex (ICP reference point) to the device. Pressure within the tubing below the device will of course be negative but will have no effect on the ICP or drainage of CSF. Studies on over 50 patients [Foltz and Meyer, personal communication, 1989] show that an average, predicted upright ICP with this system is accurate to 1.8 cm of water. Averaged studies are necessary in such evaluations since the ICP is very dynamic and changeable. Intermittent Valsalva effects, such as coughing and talking, can cause temporary positive ICP proximal to the zero pressure device with consequent temporary CSF shunt flow. After Valsalva cessation, ICP may therefore temporarily show a greater negative ICP than usual, but will return to expected near normal levels promptly. This zero pressure shunt system has been proved valid as used and studied clinically [Foltz and Meyer, personal communication, 1989]. It is used as an initial shunt and to convert overdraining shunts in patients already shunted (Figure 5). It is effective in correcting and preventing overdrainage. It should be considered for all ventricular shunts, but the usual shunt problems other than overdrainage are of course to be expected.

Summary and Conclusion 1. Overdrainage of CSF in shunted hydrocephalic patients can predispose to the development of subdural hematoma and hydroma, premature closure of the skuU sutures, stenosis or occlusion of the sylvian aqueduct, SVS, and low ICP syndrome. Overall analysis shows that 1 0 % - 1 2 % of long-term ventricle shunt patients will develop at least one of these symptoms at approximately 6.5 years after the initial shunt. 2. The incidence, diagnosis, and treatment of these shunt overdrainage problems are discussed in this report. 3. The control of CSF overdrainage in long-term shunted hydrocephalic patients is reviewed and discussed, including the methods of (a) subtemporal cra-

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A

Pudenz and Foltz

UPRIGHT

UPRIGHT

REFERENCE POINT REFERENCE POINT

HIGHEST CATHETER LEVEL

A VENTRICULAR DRAINAGE LEVEL

__

C

SUPINE

SUPINE

REFERENCE POINT REFERENCE POINT

Y

/P

D

Figure 4. Current shunts for hydrocephalus contain valves that function on a relativepressure gradient after the initial opening ICP is exceeded;excessive gravity drainage therefore when upright," more controlled, direct nonsiphon drainage by openingpressures only when body is in supine positions. (A) Siphon drainage can occuronly with frontal ventricle catheterplacement when upright. (B) Nonsiphon drainage when supine. (C) Nonsiphon direct drainage occurs with standard posterior ventricle catheter when upright and (D) when supine.

niectomy-decompression, (b) third ventriculostomy, and (c) SCDs and the recent zero pressure shunt system. . In view of the clear indications in this review that overdrainage by extracranial shunts occurs, the sug-

gestion is made that intracranial operations for hydrocephalus to use residual biologic CSF absorptive capacity be reassessed and further developed. A brief outline of such a hydrocephalus program already underway is included.

Overdrainage by Ventricular Shunts

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211

UPRIGHT

UPRIGHT

REFERENCE POINT REFERENCE POINT BRAIN LEVEL

~* '

LEVEL

1 ...........

i, el

ZERO PRESSURE DEVICE

DRAINAG~

L~VgC

t a, I

V A SHUNT

O~STANC~~

VPSHUNT DISTANCE

~

VP

A

B

¢r-f

Figure 5. Zero pressure shunt system. This is a zero pressure (positive CSF pressure above, negative CSF pressure below) point in the shunt system that controls the negative ICP reference point based on the vertical distance from the device (subgaleal location) to the reference point. High degree of negative CSF pressure below device will not cause CSF drainage; only positive CSF pressure above device causes CSF flow. Device functions as a closing pressure valve responding only to a positive CSF pressure above it; it is not responsive to a relative pressure gradient when the ICP is measured at zero at the device level. (A) The upright position, zero pressure shunt system, frontal ventricular catheter. B equals maximum negative ICP when upright; surgical placement can be varied for X. (B) The upright position, zero pressure shunt system, posterior ventricular catheter; surgical placement can be varied for X. Supine body position irrelevant since pressure gradient is minimal.

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