US3886948A - Ventricular shunt having a variable pressure valve - Google Patents

Abstract

In the ventricular shunt system disclosed herein, proper cerebral hydraulic conditions for the treatment of hydrocephalus are maintained by a one-way valve incorporating adjustable biasing means for altering the back pressure maintained by the valve.

Description

United States Patent [191 Hakim June 3, 1975 VENTRICULAR SHUNT HAVING A VARIABLE PRESSURE VALVE [75] Inventor: Salomon Hakim, Bogota, Colombia [73] Assignee: Hakim Company Limited, Saint Vincent, British W. Indies [22] Filed: Jan. 30, 1974 [21] Appl. No.: 437,767

Related U.S. Application Data [63] Continuation-impart of Ser. No, 280,451, Aug. 14,

[52] U.S. C1 128/350 V [51] Int.Cl A61m 27/00 [58] Field of Search... 128/350, 351, 350 V, 349 R, 128/349 B [56] References Cited UNITED STATES PATENTS 1,103,967 7/1914 Hughes 128/350 R 2,969,066 1/1961 Holter 128/350 V 3,504,676 4/1970 Lomholt 128/351 3,527,226 9/1970 Hakim 128/350 V 3,654,932 4/1972 Newkirk 128/350 V Primary Examiner-Richard A. Gaudet Assistant Examiner-Henry J. Recla Attorney, Agent, or FirmKenway & .lenney [57] ABSTRACT In the ventricular shunt system disclosed herein, proper cerebral hydraulic conditions for the treatment of hydrocephalus are maintained by a one-way valve incorporating adjustable biasing means for altering the back pressure maintained by the valve.

8 Claims, 10 Drawing Figures SHEET FIG.

l/ll/l/l/ FIG. 2

I VENTRICULAR SHUNT HAVING A VARIABLE PRESSURE VALVE This application is a continuation-in-part of Ser. No. 280,451 Aug. 14, 1972.

BACKGROUND OF THE INVENTION The treatment of hydrocephalus frequently involves the provision of a ventricular shunt for draining excess cerebral spinal fluid (CSF) from the ventricle in the brain. The shunt generally consists of a cerebral catheter inserted through the brain tissue into the ventricle and connected through a one-way valve system to drain into the jugular vein or another reservoir in the body. The shunt provides for removal of excess CSF from the ventricle and consequent reduction in its size. Control over the drainage is provided by the one-way valve, which normally operates at a fixed closing pressure.

Although hydrocephalus is frequently associated with high CSF pressure, there are numerous cases where hydrocephalus is associated with the CSF at normal pressure, see Ojemann, Robert 0., Normal Pressure Hydrocephalus, Clinical Neurosurgery. Vol. 18, pp. 337-370, 1971.

An analysis of the hydromechanics involved in normal pressure hydrocephalus syndrome leads to the conclusion that the effective expansive force from the ventricles is not dependent on the CSF pressure alone. but is the product of the CSF pressure and ventricular area. In other words, in the presence of ventricular enlargement a normal pressure is acting. Thus, in normal pressure hydrocephalus the ventricle remains enlarged, because the area subjected to the pressure of the CSF is larger than normal and hence the total force on the brain tissue, the product of the pressure times the area, is excessive, (See S. Hakim and R. D. Adams, The special clinical problem of symptomatic hydrocephalus with normal cerebrospinal fluid pressure: Observations on cerebrospinal fluid hydrodynamics. J. Neurol. Sci., Vol. 2 pp. 307-327, 1965).

In addition to the forces developed by the CSF pressure, the brain tissue is subjected to a eounterforee developed by the venous pressure within the intraparenchymatous system, within the brain tissue itself. Whereas CSF pressure tends to enlarge the ventricles, the venous pressure tends to reduce their size. These two forces are normally in balance so that the vetricular size does not increase nor decrease, but remains constant through life.

Accordingly, the aim in treating hydrocephalus by shunting procedures is not merely to arrest the condition, but to restore, as much as possible, normal ventricular size. Once hydrocephalus has developed, this restoration is accomplished by reversing the imbalance of the two forces acting on the brain parenchyma. The CSF pressure must be reduced by an amount inversely proportional to the size of the ventricles to offset the increase force developed by virtue of their enlarged area. Then the forces developed within the venous system can cause the compressed brain tissue to spring back" against the lower CSF force, and the venous bed will regain its lost volume and free flow."lhis way brain metabolism becomes normal and the tissue recuperates.

In treating hydrocephalus, a reduced CSF pressure is established by a shunt which includes a one-way valve having an operating pressure equal to the desired CSF pressure. With the shunt in place the CSF pressure remains at a maximum level determined by the implanted valve and drainage of CSF fluid from the ventricles continues long as CSF pressure is not less than the operating pressure of the valve. Since the correction of normal pressure hydrocephalus requires the implantation of a valve having a lower than normal operating pressure, e.g. 30-40 mm. H O, the CSF pressure remains lower than normal as the ventricle decreases in size. On the other hand the venous pressure remains normal; as a consequence the force imbalance is reversed. The venous system force becomes greater than the CSF system force because of the progressively smaller ventricular area and lowered CSF pressure. Accordingly, once the ventricle is again normal size the intraventricular CSF pressure must be brought back to normal levels. Otherwise there is not enough force within the ventricle to keep the brain normally expanded.

If on the other hand lower than normal CSF pressure is maintained, overcorrection of hydrocephalus may cause undesirable pathological consequences, such as swelling" or engorgement of the veins, cerebral edema, slit ventricles" and microcephaly. In other cases complications such a subdural hygromas, hematomas and overlapping of the skull bones are known to occur.

The ability to control the CSF pressure with respect to the ventricular size is important to the proper treatment of hydrocephalus, because proper balance within the brain must eventually be established.

In brief the problem in correcting and maintaining correct normal hydrodynamic balance in the cranial cavity is to maintain the interventricular CSF at a pressure corresponding to the ventricular area. When the ventricle attains normal size, the valve provided for the initial drainage should be replaced with one having a closing pressure equivalent to a normal CSF pressure (l25l50) mm. H O.

To date there has been little recognition of the problem of maintaining the correct balance of CSF pressure, ventricular area and venous pressure within the brain.

In an ideal shunt system the valve should provide for initial drainage at a lower than normal pressure and thereafter operate at an operating pressure that maintains the correct intraeranial balance.

Accordingly, in one aspect this invention provides a ventricular shunt valve having means to adjust and vary the valve operating pressure with respect to ventricular size such that the proper balance of forces within the cranium may be maintained.

In another aspect the invention makes use of the fact that the brain tissue is itself a viscoelastic solid which transmits the force developed at the ventricles outwardly to the dura region. This force may be sensed and utilized to control the operating pressure of the ventriculoatrial shunt valve appropriately to maintain proper drainage condition and balance of forces.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a valve for a ventricular shunt characterized by an operating pressure that is variable with respect to the ventricular size so as to permit proper balancing of the force exerted on the brain by the CSF pressure in the ventricle with the constraining force developed by the venous pressure. This valve may accordingly be implanted to regulate the drainage of CSF from the ventricle at a properly low valve operating pressure, (e.g. 45 mm. H O) when the force of the CSF is excessive, and to provide for an elevated operating pressure as the brain tissue relaxes and the ventricular area decreases.

Applicant has discovered that the force on the brain resulting from the CSF pressure exerted over the area of the ventricle is transmitted through the brain tissue as a viscoelastic solid and may be sensed in the subdural space where the brain lies adjacent to the skull. This invention features a sensor adapted to be inserted into the dural region between the brain and the skull, in association with a ventricular valve having a working pressure that varies inversely with the force applied to the sensor.

In the preferred embodiment of the invention, inverse variation of the operating pressure of the valve is provided by a feedback arrangement, preferably hydraulic, wherein the force applied to the sensor is transmitted to counteract the spring bias which controls the operating pressure, thus to lower the operating pressure as the force increases. The spring which is biased to hold the valve closed, is unloaded in response to the force applied by the brain to the sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of this invention is described below with reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration showing the valve system in its physiological environment;

FIG. 2 is a longitudinal cross-sectional view of the preferred valve mechanism and sensor of this invention;

FIG. 3 is a longitudinal cross-sectional view illustrating an embodiment featuring a resilient slotted tube valve mechanism;

FIG. 4 is a longitudinal cross-sectional view illustrating a valve mechanism featuring a mechanical brain force sensing control element;

FIG. 5 is a plan view showing the implantation of the valve mechanism illustrated in FIG. 4;

FIG. 6 is a longitudinal cross-sectional view, including a partial sagital section, illustrating a resilient slotted tube valve mechanism directly operable by brain forces;

FIG. 7 is a top plan view of the embodiment shown in FIG. 6;

FIG. 8 is a transverse cross-section taken in 8-8 in FIG. 7;

FIG. 9 is a cross-section of the alternative embodiment employing direct sensing of sub-dural force; and

FIG. 0 is a cross-sectional view illustrating an alternative construction employing a separate sensor.

The implantation of the ventricular shunt is illustrated in FIG. 1. Aventricular catheter 50 inserted through aburr hole 52 in theskull 54 and through thebrain tissue 58 into theventricle 56 connects through a one-way drainage valve 60 to adrainage catheter 62 which will normally lead to the right atrium, the peritoneal cavity or some other suitable reservoir. The improved shunt valve of this invention is contained in thevalve 60. The force exerted on the brain is sensed by a fluid filledbladder 24 which hydraulically connects bytube 25 to the valve 69 mechanism described below.

The sensing bladder is inserted also through the burr hole, and preferably through thedural membrane 64 to lie against thearachnoidal membrane 66 over one or more convolutions, e.g. 68. Best response to the forces in the brain require close association of the sensing bladder with the brain tissue (cortex). The subarachnoid space surrounding the convolutions is itself subject to CSF pressure, but this pressure is not transmitted to the sensor because the arachnoid membrane is tethered to the piamater membrane by the arachnoidal strands. The object, it will be noted, is to sense the force exerted by the brain tissue as distinguished from the pressure of the CSF.

As in general with ventricular shunts the entire drainage assembly is ultimately covered and held in place by the scalp (not shown).

The hydraulic servo shunt valve of this invention as embodied in FIG. 2 constructed generally as described in Applicants U.S. Pat. No. 3,288,142, and features ahousing 10, avalve body 12 formed with aconical valve seat 15 which forms the outlet from aninlet channel 14. Aspherical valve member 16, held down by aspring 18, rests within the conical sear and maintains the valve closed until the fluid pressure at this inlet is sufficient to overcome the bias force of the spring.

Thespring 18 is mounted on abase plate 20 which is pivoted to thebody 12 at the end of the spring away from the valve sphere. A preloadingspring 21 extends from aforward shoulder 19 of the body and positionsthe'base plate 20 for normal or maximum valve spring bias. Thebase plate 20 lies in spaced relation to a recess portion of the valve body on which is mounted avalve bladder 22 which is hydraulically connected bytube 25 to thesensing bladder 24. Thevalve bladder 22 is constructed to expand when hydraulically loaded to push thebase plate 20 against the force of the preloadingspring 21 and thereby unload the valve spring by moving it in the direction away from thevalve sphere 16. Thevalve spring 18 is thus unstrained such that the force applied to the sphere is reduced, correspondingly reducing the working pressure of the valve; which is the fluid pressure at the inlet required to open the valve. When implanted the outward force exerted by the brain is applied to thesensing bladder 24 situated in the subdural region between the brain and the skull, and is transmitted hydraulically to thevalve bladder 22. Thus when the force, produced by the CSF pressure applied over the ventricular area, is increased the operating pressure of the valve is reduced, and vice versa. Accordingly, as the hydraulic brain contracts from CSF drainage, the resultant reduction in the brain force brings about an increased operative pressure effective to maintain proper drainage and a balance of forces.

Both the sensingbladder 24 and thevalve bladder 22 are conveniently constructed of silicone rubber discs cemented together at their edges by means of silicone cement which may be of the RTV type or polymerizable by ionizing radiation.Silicone tubing 25 attached to the discs provides for hydraulic connection between the bladders. The bladders and tubing are conveniently filled with a radiopaque oil such as ethyl iodophenyl undecylate which provides appropriate viscous dampening of the valve mechanism, permits the hydraulic servo connection to be radiologically viewed, and is a safe material frequently used in myelography.

It will be understood that the valve mechanism described in FIG. 2 is the downstream end of the check valve pair described in U.S. Patv No. 3.288.142. and is mounted in a flexible length of hollowing tubing 11 which surrounds thehousing 10.

The actual construction is more or less conventional, with stainless steel being preferred for thehousing 10,valve body 12,spring 18, base plate and preloadingspring 21. Thespherical valve member 16 is preferably synthetic sapphire. The biasingspring 18 is spot-welded to thebase plate 20 and thepreloading spring 21 is spot-welded to thetransverse shoulder 19. The pivotal mounting of thebase plate 20 to the downstream end of the valve body consists of apin member 23 spotwelded to the lower side of thebase plate 20, received at its ends withinsleeves 26, conveniently formed of pieces of hypodermic needle tubing, spot-welded to the top side of the downstream end of thevalve body 12. Thetube 25 passes through anopening 27 in thevalve body 12 and also through the tubing 11 to which it may be sealed by silicone cement.

Implantation of the valve of this invention in a ventriculoatrial shunt system follows standard surgical procedures, with the additional procedure of inserting the sensingbladder 24 into the subdural region. Most conveniently this is introduced through the burr hole and then laterally a short distance away to lie between the brain and the skull.

After the system has been implanted, it may be desirable to make hydraulic adjustments to the servo connection to insure that the valve will open and close properly in response to variations in the force at the sensor. This may be accomplished by shimming the sensor with thin pieces of silicone. or by injecting or removing fluid with a hypodermic needle, preferably through a side branch tube (not shown) which may subsequently be sealed.

In the embodiment illustrated in FIG. 3 the valve consists in a resilient hollow closed tube formed with alongitudinal slit 72. The CSF fluid enters thevalve tube 70, and under sufficient pressure causes theslit 72 to open for drainage. The slit is also under the control of an internalspherical bladder 22a, hydraulically connected to thesensing bladder 24 and expansible under hydraulic pressure to urge the slit to open.

In the embodiment illustrated in FIG. 4 and 5, the construction is generally as described with reference to FIG. 2 with the exception that the control mechanism consists of apin 76 mounted to the underside of thebase plate 20, passing through asilicone rubber seal 78. Thepin 76 is adjustably connected, e.g. threaded, to asensing button 80, adapted to be placed in contact with the exterior of the brain in the dural region. The threaded engagement between thepin 76 and thesensing button 80 provides for adjustment to the individual patient.

The mounting of this embodiment is illustrated in FIG. 5 and features two burr holes and 86, the former accommodating thecatheter 50 and the latter accommodating the sensing mechanism. The valve mechanism is conveniently attached to the skull by a mounting pin which passes through thevalve body 12 and terminates in eyes by which the valve can be fastened to the skull, eg by screws 92.

In the embodiment illustrated in FIGS. 6, 7, and 8, the valve mechanism is contained within aresilient chamber 102, e.g. silicone rubber. formed for direct placement in the dural region in contact with the brain. Thecatheter 50 leads into thechamber 102 and termiiates in a resilient closed tube having atransverse slit 108 on a side wall portion. The top and bottom of thetube 100 contact opposite top and bottom walls of thechamber 102 and may incorporatesmall metal discs 106. Adrainage catheter 104 leads from thechamber 102.

In operation, sufficient CSF pressure will cause theslit 108 to open and provide drainage from thecatheter 50 into thechamber 102. Should excessive forces develop, thechamber 102 becomes squeezed and thediscs 106 compress thetube 100 causing theslit 108 to tend to open. Drainage at a lower CSF pressure is thus provided. As the brain contracts the force applied by thediscs 106 is lessened and an increased CSF pressure becomes necessary for drainage. Thus proper drainage conditions and balance of forces may be maintained.

The embodiment of FIG. 9, like that of FIG. 4, is adapted to employ direct mechanical sensing of the sub-dural force. For this purpose, the valve employs a generally cylindrical body member which is of a size to fit within a burr hole through the patients skull, as indicated at 122 in FIG. 9, the lower portion of the valve assembly projecting below the inner surface of the skull, as indicated, for directly contacting the brain. A pair of thin, disk-like plates 124 and 126, one on either side of thebody member 120, are coupled together bycylindrical pins 132 which extend through corresponding axial apertures l34-136 in thevalve body 120. The pins 130-132 are slightly longer than the depth of thebody member 120, so that some movement of theplates 124 and 126, with respect to the body member, is permitted.

Aninlet port 137 communicates with theopening 134 and thus also with the spaces between theplates 124 and 126 and thevalve body 120.Conduit 137 is adapted for connection into theventricular catheter 50. Anoutlet conduit 139, adapted to be connected to thedrainage catheter 62, communicates with acentral bore 140 in thevalve body 120. The upper end of thebore 140 is sealed by acap 142 while the lower end of the bore is shaped to provide aconical valve seat 144. Aspherical valve member 146 is biased againstseat 144 by ahelical spring 148. Ashort pin 147 welded to theplate 126 allows theball valve 146 to be lifted from its seat by a pressure exerted upwardly against theplate 126. An envelope 127 of silastic, i.e. silicone rubber, surrounds theplates 124 and 126, together with thevalve body 120, this envelope being sealed with adhesive around the inlet andoutlet conduits 137 and 139 respectively.

The operation of this embodiment is essentially similar to that of the device illustrated in FIG. 4. An increase in sub-dural force is sensed by theplate 126 which bears against theball valve member 146, reducing the pressure necessary to permit venting of CSF from the ventricles. Accordingly, a servo valve operation is obtained in which the sub-dural force is automatically regulated to a level essentially preselected by the characteristics of thespring 148.

The embodiment illustrated in FIG. 10, is adapted to employ a separate sensor in a manner similar to the embodiment illustrated in FIG. 1, the valve assembly itself being implantable underneath the patients scalp but outside of the skull. In common with the embodiment illustrated in FIG. 9, the FIG. 10 device employs a pair of disk-like members 150 and 152 connected by pins 154-156 extending throughcorresponding apertures 158 160 in avalve body 162. In this version, each of thedisks 150 and 152 is. however, sealed to the valve body by arespective silastic diaphragm 162, 164, thediaphragms 162 and 164 being clamped and sealed to the valve body byrespective caps 166 and 168. As in the previous embodiment, the entire valve assembly is surrounded by asilastic envelope 184 which is sealed to all inlet and outlet conduits.

Aninlet conduit 170 communicates with acentral cavity 172 which is shaped to provide a conical valve seat as indicated at 174. Aball valve member 176 is biased against thevalve seat 174 by aconvoluted spring 178. Thespring 178 carries anextension 179 which can be engaged by ahook 181 carried by the disk-like member 150. The downstream side of the ball valve communicates with the space between thedisks 150 and 152 and also with anoutlet conduit 180. The sensingbladder 24, located as illustrated in FIG. 1, is coupled through aconduit 181 with the space betweendiaphragm 164 and thecorresponding cap 168. The space between thediaphragm 162 and thecap 166 is vented, through anaperture 186 incap 166, to the space between the valve assembly and thesurrounding envelope 184. Thus, this space may be considered to be at atmo spheric pressure in considering the operation of this valve mechanism. In practice, this space may be filled with a suitable hydraulic fluid to provide pressure communication. A radiopaque oil such as ethyl iodophenyl undecylate could be used.

The operation of this valve mechanism is analogous to the operation of the device illustrated in FIG. 1. An increase in the sub-dural force drives hydraulic fluid from thebladder 24, throughconduit 181, into the space betweencap 168 anddiaphragm 164. This displacement will raise the disk-like members 152 together withmember 150, owing to the coupling provided by pins 154156. Lifting of theplate 150 causes the hool-like member 181 to lift thespring extension 179, thereby decreasing the force biasing theball valve 176 againstseat 174. This reduces the back pressure exerted by the valve on the flow of CSF from the patients ventricles to the drain conduit. As will be understood. the loop gain of this servo system will be quite high so that the system will tend to maintain the subdural force at a preselected level. corresponding to the nominal tension exerted by thespring 178. An advantage of this arrangement is that the hydraulic pressure developed by thesensor 24 operates against atmo' spheric pressure rather than the pressure at the downstream side of the valve so that siphon effects are mini mized.

In view of the foregoing. it may be seen that several objects of the present invention are achieved and other advantageous results have been attained.

As various changes could be made in the above constructions without departing from the scope of the invention. it should be understood that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A cerebral-spinal fluid shunt system comprising:

a conduit for venting fluid from a patients cerebralspinal fluid system;

a conduit for draining cerebral-spinal fluid into a selected drainage site within the patients body; means for sensing the force exerted by the brain in the dural region;

a one-way bypass valve connecting said vent conduit and said drain conduit. said bypass valve including biasing means for maintaining a pressure differential between said vent conduit and said drain conduit', and

means interconnected with said biasing means responsive to said sensing means for adjusting the biasing of said bypass valve as a function of the sensed force thereby to maintain said force substantially at the desired level.

2. The improvement defined byclaim 1 wherein the biasing means comprises a spring mounted to a movable base with means for adjustably moving said base.

3. The improvement defined byclaim 1 wherein the means for controlling the biasing means includes hydraulically interconnected sensor bladder and control bladder, the sensor bladder being adapted to contact the brain directly or through its enveloping membranes and the control bladder being operatively associated with the biasing means thereby to vary the biasing force exerted by the biasing means.

4. A cerebral-spinal fluid shunt valve comprising a conduit member having an inlet and an outlet. a valve body within said conduit member having a passageway between said inlet and outlet terminating in a valve seat, a valve member overlying said seat and forming a one-way closure to said passageway, resilient means movably mounted relative to said valve body biasing said valve member against said seat thereby to maintain a preselected back pressure in the ventricle. force responsive means outside said conduit adapted to contact brain in the epior subdural region, and movable means responsive to said force responsive means and operatively interconnected with said resilient biasing means to increase the bias thereof as the force decreases and vice versa thereby to maintain said force at a preselected level.

5. The cerebral-spinal fluid shunt valve defined by claim 4 wherein the force responsive means and movable means comprise hydraulically connected bladders including a sensing bladder adapted to be inserted into the dural region and a control bladder operatively associated with the resilient means thereby to vary the biasing force exerted by the biasing member.

6. A cerebral-spinal fluid shunt valve comprising a conduit member having an inlet and an outlet, a valve within said conduit including resilient means biasing the valve closed. and an hydraulic servo mechanism comprising a sensing bladder adapted to contact the brain in the dural region and a control bladder in said valve operatively associated with said resilient means opposing said biasing, said sensing bladder and control bladder being hydraulically interconnected.

7. The valve defined by claim 6 wherein the hydraulic interconnection comprises a radiopaque fluid.

8. The method of treating hydrocephalus comprising:

providing a conduit for venting cerebral-spinal fluid from a patient's cerebral-spinal fluid system through a one-way valve providing an adjustable back prcssure;

sensing the force exerted by the brain against the skull in the dural region; and

automatically adjusting said valve pressure responsive to the sensed force thereby to vent said cerebral spinal fluid to maintain said sensed force substantially at a desired level.

Claims (8)

1. A cerebral-spinal fluid shunt system comprising: a conduit for venting fluid from a patient''s cerebral-spinal fluid system; a conduit for draining cerebral-spinal fluid into a selected drainage site within the patient''s body; means for sensing the force exerted by the brain in the dural region; a one-way bypass valve connecting said vent conduit and said drain conduit, said bypass valve including biasing means for maintaining a pressure differential between said vent conduit and said drain conduit; and means interconnected with said biasing means responsive to said sensing means for adjusting the biasing of said bypass valve as a function of the sensed force thereby to maintain said force substantially at the desired level.

1. A cerebral-spinal fluid shunt system comprising: a conduit for venting fluid from a patient''s cerebral-spinal fluid system; a conduit for draining cerebral-spinal fluid into a selected drainage site within the patient''s body; means for sensing the force exerted by the brain in the dural region; a one-way bypass valve connecting said vent conduit and said drain conduit, said bypass valve including biasing means for maintaining a pressure differential between said vent conduit and said drain conduit; and means interconnected with said biasing means responsive to said sensing means for adjusting the biasing of said bypass valve as a function of the sensed force thereby to maintain said force substantially at the desired level.

2. The improvement defined by claim 1 wherein the biasing means comprises a spring mounted to a movable base with means for adjustably moving said base.

3. The improvement defined by claim 1 wherein the means for controlling the biasing means includes hydraulically interconnected sensor bladder and control bladder, the sensor bladder being adapted to contact the brain directly or through its enveloping membranes and the control bladder being operatively associated with the biasing means thereby to vary the biasing force exerted by the biasing means.

4. A cerebral-spinal fluid shunt valve comprising a conduit member having an inlet and an outlet, a valve body within said conduit member having a passageway between said inlet and outlet terminating in a valve seat, a valve member overlying said seat and forming a one-way closure to said passageway, resilient means movably mounted relative to said valve body biasing said valve member against said seat thereby to maintain a preselected back pressure in the ventricle, force responsive means outside said conduit adapted to contact brain in the epi- or sub- dural region, and movable means responsive to said force responsive means and operatively interconnected with said resilient biasing means to increase the bias thereof as the force decreases and vice versa thereby to maintain said force at a preselected level.

5. The cerebral-spinal fluid shunt valve defined by claim 4 wherein the force responsive means and movable means comprise hydraulically connected bladders including a sensing bladder adapted to be inserted into the dural region and a control bladder operatively associated with the resilient means thereby to vary the biasing force exerted by the biasing member.

6. A cerebral-spinal fluid shunt valve comprising a conduit member having an inlet and an outlet, a valve within said conduit including resilient means biasing the valve closed, and an hydraulic servo mechanism comprising a sensing bladder adapted to contact the brain in the dural region and a control bladder in said valve operatively associated with said resilient means opposing said biasing, said sensing bladder and control bladder being hydraulically interconnected.

7. The valve defined by claim 6 wherein the hydraulic interconnection comprises a radiopaque fluid.

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