Patent Publication Number: US-2006020239-A1

Title: Cerebral spinal fluid flow sensing device

Description:
This application claims the benefit of U.S. provisional application No. 60/589,350, filed Jul. 20, 2004, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The invention relates to medical devices and, more particularly, devices for draining cerebrospinal fluid (CSF).  
     BACKGROUND  
      Hydrocephalus is an excess accumulation of cerebrospinal fluid (CSF) in the ventricles of the brain. This fluid, which protects, nourishes and cleanses the brain and spinal cord, is manufactured daily in the ventricles. Accumulation occurs when the fluid cannot flow freely throughout the ventricles and the central nervous system due to various forms of blockage. Hydrocephalus can result from genetic conditions or trauma to the brain.  
      There are a number of accepted treatments available for hydrocephalus, most of which involve the surgical implantation of a ventricular shunt. A shunt diverts CSF from the ventricles to the peritoneal cavity or the cardiovascular system. The shunt may include a valve that controls the rate of flow of CSF through a catheter of the shunt. In some cases, the valve may be controllable to adjust the flow rate.  
      Some CSF shunt valves include a telemetric CSF flow or pressure sensor. U.S. Pat. No. 6,533,733 to Ericson et al. describes an implantable device for sensing intracranial pressure (ICP) and CSF flow, and transmitting sensed information to an external device by wireless telemetry. Zumkehr et al. describe an implantable telemetric CSF flow sensor in “Project: Packaging and Industrialization of an Implantable Flow Sensor for Neurological Applications,” CCP Center for Computational Physics, 2003. U.S. Pat. No. 4,519,401 to Ko et al. describes a battery-powered implantable ICP monitor with low power pressure sensing circuitry and wireless telemetry. U.S. Pat. No. 6,113,553 to Chubbuck describes an implantable, inductively powered ICP sensor providing wireless telemetry. U.S. Pat. No. 6,248,080 to Miesel et al. describes an implantable, battery-powered ICP sensor with wireless telemetry.  
      Table 1 below lists documents that disclose implantable telemetric CSF flow or ICP monitors.  
                       TABLE 1                       Patent               Number   Inventors   Title                  6,113,553   Chubbuck   Telemetric intracranial pressure monitoring               system       6,533,733   Ericson   Implantable device for in-vivo intracranial and           et al.   cerebrospinal fluid pressure monitoring       6,248,080   Miesel   Intracranial monitoring and therapy delivery           et al.   control device, system and method       4,519,401   Ko et al.   Pressure telemetry implant       Not   Zumkehr   Project: Packaging and Industrialization of an       applicable   et al.   Implantable Flow Sensor for Neurological               Applications                  
 
      All documents listed in Table I above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary, Detailed Description and Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the techniques of the present invention.  
     SUMMARY OF THE INVENTION  
      In general, the invention is directed to an implantable device for sensing CSF flow through an implantable ventricular shunt. The sensing device is implanted with the CSF shunt, and includes a flow sensor to sense flow rate or shunt blockage. The sensing device transmits the sensed flow rate to an external monitoring device by wireless telemetry.  
      Various embodiments of the present invention provide solutions to one or more problems existing in the prior art with respect to prior art systems for CSF flow control in an implanted shunt. These problems include the insufficiency of ventricular size and ICP level, in some forms of hydrocephalus, as indicators of shunt performance. In particular, ventricular size or ICP level may not be a reliable indicator of the actual CSF flow rate through the shunt. The inability to accurately sense CSF flow rate can undermine the therapeutic efficacy of the shunt in treating hydrocephalus. In addition, without accurate CSF flow data, it may be difficult to identify an optimal valve pressure setting or changes in flow over time.  
      Various embodiments of the present invention are capable of solving at least one of the foregoing problems. When embodied in a system or method for monitoring CSF flow through an implantable ventricular shunt, the invention includes features that facilitate more accurate sensing of CSF flow rate. In one embodiment, the invention provides a system for monitoring CSF flow within a shunt. The system includes an implantable flow sensor configured to sense flow within the shunt and generate a flow signal, and a telemetry unit to transmit the flow signal by wireless telemetry. The sensing device may be integrally formed as part of the shunt, or clamped onto a portion of the shunt, in which case the sensing device may be resusable. An external monitor receives the transmitted flow signal and presents information based on the flow signal. The sensing device may be inductively powered or include its own power supply. In some embodiments, the flow sensor includes an optical sensor, such as a laser Doppler sensor, to sense CSF flow within a transparent or translucent inline tube that couples the CSF flow valve to a ventricular catheter within the shunt.  
      In comparison to known techniques for monitoring and controlling CSF flow in an implanted shunt, various embodiments of the invention may provide one or more advantages. For example, the invention enables a care-giver to better evaluate the performance of the ventricular shunt, and take remedial action if necessary. For example, if the sensed CSF flow rate is too low, too high, or indicates a blockage, the care-giver may adjust a flow control valve or take other action to address the situation. In either case, the invention provides a caregiver with the ability to determine and report shunt performance, and evaluate CSF flow data that may be helpful in identifying an optimal valve pressure setting, flow changes over time, or an obstruction in the system. Hence, an implantable CSF flow sensing device as described herein may aid the care-giver in ensuring proper shunt performance and adjustment. In addition, in some embodiments, the flow sensing device may be reusable upon replacement of all or a portion of an implanted shunt within a patient.  
      The above summary of the present invention is not intended to describe each embodiment or every embodiment of the present invention or each and every feature of the invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.  
      The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram illustrating a CSF flow monitoring system having an implantable ventricular shunt and a CSF flow sensor in conjunction with a patient.  
       FIG. 2  is an enlarged side cross-sectional view of an inline CSF flow sensor.  
       FIG. 3  is a cross-sectional view of a CSF flow sensor taken along line  1 - 1 ′ of  FIG. 2 .  
       FIG. 4  is a cross-sectional view of an alternative inline CSF flow sensor.  
       FIG. 5  is a cross-sectional view of the flow sensor of  FIG. 4  in an open configuration.  
       FIG. 6  is a block diagram illustrating a CSF flow sensor.  
       FIG. 7  is a block diagram illustrating an external monitor for receiving flow information from an implantable CSF flow sensor.  
       FIG. 8  is a flow diagram illustrating a method for monitoring CSF flow. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  is a schematic diagram illustrating a CSF flow monitoring system  10  having an implantable ventricular shunt  12  and a CSF flow sensor  14  in conjunction with a patient  16 . CSF flow sensor  14  transmits CSF flow information to an external CSF flow monitor  18  via wireless telemetry. As shown in  FIG. 1 , shunt  12  is implanted within the cranium of patient  16 , and includes a ventricular catheter  20 , a CSF flow control valve  22  and a drainage catheter  24 . Ventricular catheter  20 , flow control valve  22  and drainage catheter  24  define a CSF flow path within shunt  12 .  
      Ventricular catheter  20  may be bent and angled downward to extend into a brain ventricle. Alternatively, ventricular catheter  20  may be substantially straight, but couple to an angled coupling joint extending from control valve  22 . Ventricular catheter  20  may define a plurality of holes  26  to receive CSF from a brain ventricle. Shunt  12  may be, for example, a ventriculoperitoneal shunt, a ventriculoatrial shunt, or a lumboperitoneal shunt.  
      CSF flow sensor  14  is constructed as an inline component that couples between one end of ventricular catheter  20  and one end of flow control valve  22 . Hence, CSF flowing from ventricular catheter  20  to flow control valve  22  flows through CSF flow sensor  14 . CSF flow sensor  14  includes a sensor to sense flow, and a wireless telemetry interface to transmit information based on the sensed flow rate to external CSF flow monitor  16 . External CSF flow monitor  18  may provide further processing, analysis and presentation of the flow rate information for a care-giver.  
      Using the CSF flow information generated by CSF flow sensor  14 , external monitor  18  may compute instantaneous flow rate, average flow rate, and generate trend data over a period of time. With the aid of this information, a care-giver may determine a more appropriate setting for valve  22  associated with shunt  12 . CSF flow sensor  14  may be inductively powered, i.e., by an external inductive telemetric power source, or may include its own power source, such as a battery.  
      CSF flow sensor  14  and external monitor  18  provide a caregiver with the ability to determine and report shunt performance, and evaluate CSF flow data that may be helpful in identifying an optimal valve pressure setting, flow changes over time, or an obstruction in shunt. CSF flow sensor  14  may be useful with various CSF shunt and valve systems, such as those manufactured by Medtronic, Inc. of Minneapolis, Minn. Examples include the PS Medical® Delta®, Strata®, and CSF-Flow shunt and valve component systems commercially available from Medtronic.  
       FIG. 2  is an enlarged side cross-sectional view of inline CSF flow sensor  14  of  FIG. 1 .  FIG. 3  is an enlarged cross-sectional view of inline CSF flow sensor  14  taken along line  1 - 1 ′ of  FIG. 2 . As shown in  FIG. 2 , CSF flow sensor  14  is constructed for inline coupling between ventricular catheter  20  and CSF flow control valve  22  such that CSF flows from the ventricular catheter, through the CSF flow sensor, and to the flow control valve. To that end, CSF flow sensor  14  includes a housing  28  with an integrally formed tube  30  defining an inner lumen  32  to accommodate CSF flow.  
      Opposite ends of tube  30  receive and are coupled to ventricular catheter  20  and control valve  22 , e.g., by crimping, adhesive bonding, ultrasonic bonding, or the like. In particular, an input of tube  30  is coupled to an output of ventricular catheter  20 , and an output of tube  30  is coupled to an input of control valve  22 . An output of control valve  22  is then coupled to an input of drainage catheter  24 . In some cases, a length of an existing ventricular catheter  20  may be cut to accommodate attachment of tube  30  and CSF flow sensor  14 .  
      Hence, flow control valve  22  has an input in fluid communication with an output of the ventricular catheter  20  and an output in fluid communication with an input of the drainage catheter  24 . However, CSF flow sensor  14  is placed between the input of flow control valve  22  and the output of ventricular catheter  20 . In other embodiments, CSF flow sensor  14  may be placed on an opposite side of control valve  22 , i.e., between the output of the flow control valve and the input of the drainage catheter.  
      By constructing CSF flow sensor  14  as an integrally formed inline sensor, it may be incorporated in a variety of different shunt systems, and serve as a coupling member between ventricular catheter  20  and control valve  22 . In some embodiments, housing  28  and tube  30  may be integrally molded from biocompatible plastic material, such as silicone or polyurethane.  
      In the example of  FIG. 2 , CSF flow sensor  14  includes an optical flow sensor. In other embodiments, CSF flow sensor  14  may incorporate alternative sensing devices such as electromagnetic flow meters, magnetic field flow meters, ultrasonic Doppler sensors, hot wire anemometers, thermal convection velocity sensors, or the like. As shown in  FIG. 2 , CSF flow sensor  14  includes an optical emitter  34 , an optical receiver  36 , and circuitry  38  for driving the optical emitter and receiver, as well as wireless telemetry circuitry. Emitter  34 , receiver  36  and circuitry  38  may be mounted on a small circuit board  40 . Emitter  34  may be a light emitting diode (LED) oriented to transmit light into inner lumen  32  of tube  30 . Receiver  36  may be a photodiode sensitive to wavelengths of light transmitted by emitter  34 , and may be positioned on the same side of tube  30  as emitter  34 .  
      Optical emitter  34  transmits light through a translucent or transparent portion of the shunt tube  30  and into the flow field of the shunt  12 , which may be integrally formed with the tube or retrofitted to the tube as part of the CSF flow sensor  14 . In either case, optical receiver  36  receives reflected light and generates a CSF flow level signal based on the frequency, phase, or level of received light. CSF flow will tend to interrupt, attenuate, or shift the received light, and will provide an indication of CSF flow.  
      To permit transmission of light from emitter  34  into inner lumen  32 , tube  30  may be constructed from a transparent or translucent material. Alternatively, tube  30  may include a transparent or translucent window oriented to permit transmission of light from emitter  34 . Any of a variety of transparent or translucent materials may be used to construct tube  30 . For example, tube  30  may be constructed from any of a variety of biocompatible glass or polymer compositions, such as transparent polyester, PTFE, or PVC. As an alternative, emitter  34  and receiver  36  may be designed to protrude, at least partially, into inner lumen  32  through fluid-sealed holes in tube  30 . In this case, a transparent or translucent tube  30  is not necessary.  
      As an option, a reflector  42  may be mounted or coated onto an inner surface of tube  30 . In this case, light produced by emitter  34  passes through CSF fluid within inner lumen  32  and reflects off of reflector  42  and toward receiver  36 . Receiver  36  senses reflected light that passes through the CSF fluid twice, i.e., once from emitter  34  to reflector  42  and then again from the reflector to the receiver. Alternatively, receiver  36  may sense light reflected from the CSF fluid within inner lumen  32 , rather than a reflector  42 . As a further alternative, emitter  34  and receiver  36  may be mounted on opposing sides of tube  30  such that the receiver senses light that is transmitted from the emitter and through the CSF fluid within inner lumen  32 .  
      In some embodiments, emitter  34  and receiver  36  may be constructed to form a laser Doppler sensor. In particular, emitter  34  may be constructed to illuminate the CSF flow within inner lumen  32  with a coherent beam of monochromatic light. Particles carried by the CSF, such as blood cells, reflect the light to produce back-scattered light that is Doppler-shifted relative to the transmitted light. Receiver  36  senses the back-scattered light, and generates an output signal for circuitry  38 .  
      Circuitry  38  processes the output signal to produce a flow rate signal. For example, circuitry  38  may be configured to mix the output signal with an input signal used to drive emitter  34 , and thereby generate a difference signal. The difference signal is based on a difference between the frequency of the light transmitted by emitter  34  and the Doppler-shifted frequency of the reflected light received by receiver  36 . The magnitude of the difference signal indicates the flow rate of the CSF through the inner lumen  32  of tube  30 , and hence through shunt  12 . In particular, the difference signal is proportional to the rate at which particles carried by the CSF fluid travel through inner lumen  32 .  
      With this information, circuitry  38  generates a CSF flow rate signal. In some embodiments, circuitry  38  may include additional processing circuitry to evaluate the CSF flow rate signal relative to a desired maximum or minimum flow rate and possibly identify blockages. Alternatively, such processing may be performed within external CSF flow monitor  18  to reduce processing overhead and power consumption within CSF flow rate sensor  14 . In some embodiments, external CSF flow monitor  18  may provide advisories in the event the CSF flow rate deviates from a desired range. In addition, external CSF monitor  18  may present recommendations for adjustments to flow control valve  22 .  
      Circuitry  38  further includes telemetry circuitry that drives an inductive coil  44  to transmit a telemetry signal based on the computed flow rate. As will be described, inductive coil  44  also may be used to receive power from an external power source. In this case, circuitry  38  also may include inductive power generation circuitry to convert current induced in inductive coil  44  into operating power for the sensing and telemetry functions performed by CSF flow sensor  14 . Hence, CSF flow sensor  14  may be telemetrically powered. As an alternative, in some embodiments, CSF flow sensor  14  may include a rechargeable or non-rechargeable battery. A rechargeable battery may be recharged using current induced in inductive coil  44  by an external power source.  
       FIG. 4  is a cross-sectional view of an alternative CSF flow sensor  14 B. In the example of  FIGS. 2 and 3 , CSF flow sensor  14  included an integrally formed housing  28  and tube  30  designed for inline coupling between ventricular catheter  20  and control valve  22 . CSF flow sensor  14 B generally corresponds to CSF flow sensor  14  of  FIGS. 2 and 3 . In the example of  FIG. 4 , however, CSF flow sensor  14 B is designed to attached to a section of ventricular catheter  20  or a section of tubing between the ventricular catheter and control valve  22 . In particular, CSF flow sensor  14  B of  FIG. 4  is designed with a “clam-shell” configuration that permits it to be clamped onto an existing section of transparent or translucent tubing, and possibly detached for reuse, if desired.  
       FIG. 5  is a cross-sectional view of flow sensor  14 B of  FIG. 4  in an open configuration. As shown in  FIGS. 4 and 5 , flow sensor  14 B includes a housing  28  with a first half  45  and a second half  47  that are coupled together with a snap-fit post  49  and a hinge  51 . First half  45  contains coil  44  and optional reflector  42 , while second half  47  contains circuit board  40  and circuitry  38 . Flexible electrical cabling may extend between coil  44  and circuit board  40 , e.g., along the hinge point defined by hinge  51 .  
      First and second housing halves  45 ,  47  pivot about hinge  51  to permit the halves to be placed about a tube  30 . Halves  45 ,  47  define an inner cavity  53  sized to accommodate tube  30 . By closing halves  45 ,  47  about hinge  51  and engaging snap-fit post  49  with socket  55 , flow sensor  14 B can be clamped around tube  30 . Cavity  53  is sized so that emitter  34  and receiver  36  (not shown in  FIGS. 4 and 5 ) are placed sufficiently close to tube  30  to sense flow within inner lumen  32 .  
      If shunt  12  is replaced, either partially or in its entirety, flow sensor  14 B can be detached and then reapplied to the replacement shunt using the snap-fit arrangement. For example, snap-fit post  49  and socket  55  may be sized so that the post remains engaged once it is urged into the socket, but can be disengaged from the socket upon application of force to permit reuse. Although a clam-shell arrangement is depicted in  FIGS. 4 and 5 , other arrangements such as screw-tightened or friction fit C-clamps may be used, provided they do not cause an obstruction in tube  30 . In each case, flow sensor  14 B can be quickly added to an existing shunt  12  with transparent or translucent tubing, and reused with another shunt, as needed.  
       FIG. 6  is a block diagram illustrating a CSF flow sensor  14 . As shown in  FIG. 6 , flow sensor  14  may include processing circuitry  46 , sensing element  48 , memory  50 , telemetry interface  52  and inductive power interface  54 . Sensing element  48  may include an emitter and receiver, as described with reference to  FIGS. 2 and 3 , for optical flow sensing. Processing circuitry  46  may form part of circuitry  38  shown in  FIGS. 2 and 3 . Processing circuitry  46  controls the operation of the various components of sensing element  48  and provides the processing resources to handle processing of output signals from the sensing element. Processing circuitry  46  also controls telemetry interface  52  to transmit signals to external CSF flow monitor  18 .  
      Processing circuitry  46  may include one or more microprocessors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other equivalent processing circuitry. Memory  50  may include random access memory (RAM), read-only memory (ROM), electronically-erasable programmable ROM (EEPROM), flash memory, or the like, or a combination thereof. Memory  50  may store program instructions that are executed by processing circuitry  46  to perform some of the functions described herein. For example, memory  50  may store instructions for processing circuitry  46  to execute in support of control of telemetry interface  52  and control of, and processing of information obtained from, CSF flow sensor  14 . Memory  50  may include separate memories for storage of instructions and archived CSF flow information.  
      Telemetry interface  52  includes an inductive coil for wireless communication as well as inductive generation of operating power for flow sensor  14 . Telemetry interface  52  also may include appropriate modulation, filtering and amplification circuitry for transmission of signals under control of processor  46 . Inductive power interface  54  converts alternating current (ac) induced in the inductive coil into operating power for processing circuitry  46 , sensing element  48 , memory  50 , and telemetry interface  52 . For example, inductive power interface  54  may include an ac/dc conversion circuit, such as a rectifier, that converts the ac current induced in the coil into dc operating power.  
      Inductive power interface  54  also may include a capacitor or other storage device to store a dc potential as a source of operating power. The capacitor may store energy temporarily to power flow sensor  14 , e.g., only during the time that inductive coil actually receives energy external CSF flow monitor  18 . Alternatively, a battery may be provided to power flow sensor  14  over an extended period of time. In some embodiments, inductive power interface  54  may generally correspond to similar circuitry described in U.S. Pat. No. 6,731,976 to Penn et al., the entire content of which is incorporated herein by reference.  
      Processing circuitry  46  filters, amplifies, and processes the ICP measurement signal, as necessary. Telemetry interface  52  then generates telemetry signals for wireless transmission to external monitor  18 . Telemetry interface  52  includes appropriate amplifier, filtering and modulation circuitry to convert the ICP measurement signal into a telemetry signal.  
       FIG. 7  is a functional block diagram illustrating an external monitor  18  for receiving flow information from an implantable CSF flow sensor  14  and powering the implantable CSF flow sensor. External CSF flow monitor  18  provides an indication of CSF flow rate measured by implantable CSF flow sensor  14 , e.g., on a display device or printout. In addition, external monitor  18  may be configured to apply additional processing to CSF flow information received from CSF flow sensor  14  to generate average, maximum, minimum and trend data. In some embodiments, external monitor  18  may generate recommendations for adjustments to control valve  22  to achieve an optimum flow rate. Also, external monitor  18  may invoke advisory levels at which a CSF flow measurement may trigger an alarm or other indicator for the attention of a care-giver. For example, external monitor  18  may alert a care-giver to the presence of a blockage or an undesirably high or low flow rate.  
      As shown in  FIG. 7 , external monitor  18  may include a processor  56 , blockage indicator  58 , memory  60 , user input device  62 , display  64 , telemetry interface  66  and inductive power interface  68 . Processor  56  controls the operation of the various components of external monitor  18 . For example, processor  56  controls inductive power interface  68  and telemetry interface  66 , and handles processing and storage of information obtained from implantable CSF sensor  14 . Processor  56  may include one or more microprocessors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other equivalent logic circuitry.  
      Processor  56  also may accept input from user input device  62 , e.g., to select different formats, or time or amplitude scales, for presentation of CSF flow information on display  64 . Display  64  may include any of a variety of different displays, such as a liquid crystal display (LCD), plasma display, or cathode ray tube (CRT) display. In addition, processor  56  may archive CSF flow information within memory  60  for retrieval or transmission to other devices, such as remote monitors distributed within a network.  
      Memory  60  may include any magnetic, electronic, or optical media, such as random access memory (RAM), read-only memory (ROM), electronically-erasable programmable ROM (EEPROM), flash memory, or the like, or a combination thereof. Memory  60  may store program instructions that, when executed by processor  56 , cause the processor to perform the functions ascribed to it herein. For example, memory  60  may store instructions for processor  56  to execute in support of control of wireless telemetry interface  66  and control of, and processing of information obtained from implantable CSF sensor  14 . Memory  60  may include separate memories for storage of instructions and archived CSF flow or ICP information.  
      Telemetry interface  66  may include a wireless radio frequency (RF) receiver and suitable demodulation, amplification and filtering circuitry to permit reception of information transmitted by implanted CSF flow sensor  14 . In some embodiments, CSF flow sensor  14  may be equipped for bidirectional communication, and may be responsive to commands transmitted via telemetry interface  66 . In each case, telemetry interface  66  includes an antenna, which may take the form of an inductive coil placed adjacent the patient&#39;s head to ensure reliable telemetry. In particular, the inductive coil may be embedded in a wand-like instrument.  
      Inductive power interface  66  applies current to an inductive coil to support inductive transfer of electromagnetic energy to implanted CSF flow sensor  14 . Typically, the same inductive coil associated with external monitor  18  will be used for both telemetry and inductive power transfer, as is the case with the inductive coil within CSF flow sensor  14 . Telemetry interface  66  and inductive power interface  68  enable CSF flow sensor  14  to be operated passively. In other words, all of the power for operation of CSF flow sensor  14  is provided by external monitor  18 . When inductive power interface  68  is activated, CSF flow sensor  14  senses CSF flow rate within shunt  12  and transmits flow rate information to external monitor  18  by wireless telemetry.  
       FIG. 8  is a flow diagram illustrating a method for monitoring CSF flow. The method may be implemented by implanted CSF flow sensor  14  and external monitor  18 . As shown in  FIG. 8 , external monitor  18  inductively powers CSF flow sensor  14  ( 70 ). In response, CSF flow sensor  14  optically senses CSF flow ( 72 ) within shunt  12 , and transmits a CSF flow signal ( 74 ) to external monitor  18  by wireless telemetry. External monitor  18  displays CSF flow information based on the CSF flow signal ( 76 ) for evaluation by a care-giver.  
      In addition, external monitor  18  may be configured to apply further processing and analysis to the CSF flow information. For example, external monitor  18  may compare the CSF flow rate to a minimum flow threshold ( 78 ). If the CSF flow rate is less than the minimum flow threshold, external monitor  18  generates a blockage advisory ( 80 ), which may be presented to the care-giver visually or audibly. For example, the blockage advisory may be presented visually via display  64  or a dedicated light, and audibly in the form of an audible tone or speech. In each case, the care-giver may take appropriate intervention steps to address the detected blockage.  
      If the CSF flow is not less than the minimum flow threshold ( 78 ), but does not fall within a desired operational range ( 82 ), external monitor  18  may generate a valve adjustment advisory ( 84 ) for the care-giver. In some embodiments, the valve adjustment advisory may simply indicate that a valve adjustment is necessary. In other embodiments, the valve adjustment advisory may specify the magnitude of the valve adjustment necessary to restore CSF flow to the desired range. In this manner, external monitor  18  may be helpful in determining an optimum valve setting.  
      In general, CSF flow sensor  14  provides a caregiver with the ability to determine and report shunt performance, and evaluate CSF flow data that may be helpful in identifying an optimal valve pressure setting, flow changes over time, or an obstruction in shunt  12 . CSF flow sensor may take a variety of forms, as described herein, but preferably is constructed as a sensor that is either placed within or adjacent to the fluid flow path through shunt  12 .  
      Again, CSF flow sensor  14  may incorporate an optical sensor, such as laser Doppler sensor, as described with reference to  FIGS. 2-5 . However, other types of sensors may be suitable, such as electromagnetic flow meters, pressure-based flow sensors, magnetic field flow sensors, ultrasonic Doppler flow sensors, thermal flow sensors, and other types of optical sensors. As one example, although a laser Doppler sensor has been described, an alternative type of optical sensor may simply rely on a level of reflected or transmitted light passing through the CSF flow. In this case, blood cells in the CSF serve to interrupt or attenuate transmitted light, such as infrared light. The resulting level of received light, attenuated by the CSF, indicates the rate of CSF flow.  
      The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, therefore, that other expedients known to those skilled in the art or disclosed herein may be employed without departing from the invention or the scope of the claims.  
      In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts a nail and a screw are equivalent structures.  
      Various embodiments of the invention have been described. Various modifications may be made without departing from the scope of the claims. These and other embodiments are within the scope of the following claims.