Neurosurgery notes/Procedures/Cranial procedures/CSF procedures/Shunts

Shunts

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Parent item
CSF procedures
Sub-item
Types of shunt
Shunt siphoning
Shunts companies
Shunt complications
Shunt tap
Shunt externalization

Theory

  • Shunts aim to divert CSF to an alternative drainage site
  • Maintenance of physiological drainage is not possible
  • Current strategies employ valves to maintain EITHER:
    • Constant pressure
    • Constant flow

Poiseuille’s law

  • Volume Flowrate, F =
(P₁−P₂)/R=π(Pressuredifference)(radius)⁴/8(viscosity)(length)(P₁-P₂)/R=π(Pressure difference)(radius)⁴/8(viscosity)(length)
  • Resistance to Flow, R =
8nL/πr⁴8nL/πr⁴
  • Imagine a flowrate of 100cm³/s:
    • Double length: 50cm³/s
    • Double viscosity: 50cm³/s
    • Double pressure: 200cm³/s
    • Double radius: 1600cm³/s

Shunt hydrodynamics

  • The opposing factor to resistance in determining flow is pressure; specifically the pressure difference or "driving pressure"
  • Q =
ΔP/RΔP/R
  • What constitutes the driving pressure in the shunted CSF circulation?
  • ΔP = intraventricular pressure (IVP) + hydrostatic pressure (HP) - opening pressure of valve (OPV) - intra-abdominal pressure (IAP)
  • IVP =
HP−OPV−IAPHP - OPV - IAP
  • Hydrostatic pressure =
densityoffluid×gravitationalconstant×verticalheightdifferencedensity of fluid × gravitational constant × vertical height difference
  • The OPV is usually fixed; thus HP plays a large role in determining IVP, ΔP, and thus flow
  • Flow = pressure difference / resistance (Q=ΔP/R)
  • Shunts offer a low-resistance alternative pathway for CSF
  • "In the case of laminar flow, the volume of flow rate is given by the pressure difference divided by the viscous resistance. Resistance depends linearly on viscosity and length, but the fourth power dependence on the radius is dramatically different."

Shunt valve types

Differential pressure valves
  • Open when pressure difference across valve exceeds a predetermined threshold
    • When open, valve has a very low resistance to flow
    • When pressure differential falls below threshold, valve closes
  • Attempt to maintain a constant intraventricular pressure (IVP)
  • Types
    • Fixed
      • Available in various predetermined pressure settings (e.g. low, medium, high)
      • Changing fixed pressure does not prevent siphoning
      • Various technologies
        • Slit valve (Holter; Codman Univalve)
        • Diaphragm valve (Codman HAKIM, Integra, Medtronic)
        • Ball valve (proGav 2.0 or Gav 2.0, Mblue
    • Programmable
      • Programmable (Codman MEDOS, Medtronic STRATA, Sophy POLARIS)
Differential Pressure Valves 1 Pressure
Reality Pressure
Flow-regulated valves
  • Aim
    • To increase resistance when the pressure differential increases, thus maintaining a constant flow
  • Mechanism
    • Allows aperture to become smaller as pressure increases, increasing resistance and maintaining flow
    • Has a "safety valve" mechanism by which if pressure becomes very high, resistance drops and flow increases
  • Pro
    • Thought to reduce over-drainage complications
  • Cons
    • More prone to failure (small orifice)
    • Can cause scalp collections in children
      • Because more back pressure from flow regulated valve
Flow-Regulated Valves 1 Pressure
FLOW-REGULATED VALVES Contoured synthetic ruby flow control pin that fits inside a movable ruby ring As the pressure increases , the ruby ring is deflected downwards, the ruby ring is tapered the flow aperture decreases which increases resistance and reduces flow. _ -If Zwo LOW Resistance Resistance at High pressure (Gafety pressure release) High Resistance rbis Si If the pressure is further increased the ruby ring is further deflected down until resistance is lowered to allow rapid increase in flow rate.
  • Example:
    • Orbis-Sigma valve (OSV)
        • 2 types
          • The Integra OSV II Valve System suits patients needing drainage close to CSF secretion rate (18-30 ml/hr).
          • For lower drainage rates (8-17 ml/hr), the Integra Flow Regulating Valve Low Flow is recommended.
        • To differentiate the two
          • The Flow Regulating Valve Low Flow features a black radiopaque flow direction arrow for visual and X-ray differentiation from OSV II Valve.
        • Mechanism
          • The variable flow restrictor comprises
            • Silicone elastomer diaphragm,
            • Synthetic ruby seat
            • Notched pin.
          • Diaphragm reacts to differential pressure variations, adjusting clearance between seat and pin to regulate flow rates.
          • Three stages of operation for OSV II Valve System:
            • Stage I: Low Differential Pressure, CSF flow rates up to 18 ml/hr.
            • Stage II: Flow Regulation, restricts flow between 18 and 30 ml/hr.
            • Stage III: Pressure Relief Mode, rapid flow rate to normalize intraventricular pressure.
        • Unsuitable for
          • Untreated choroid plexus tumors or
            • As these system produce excessive CSF that the flow rate would not be able to divert all the CSF
          • Drainage of extraventricular fluid collections.
            • As overdrainage might occurs
            • Rather use very low differential pressure valves
        Integra' OSV Il' Low Pro Valve @ Valve mechanism 1 Antechamber Integra' OSV IIS Valve I Antechamber @ Valve mechanism
        Integra' OSV IIS Low Pro Valve Integrae OSV 11@Valve (j) Antechamber (2) Valve mechanism Ant hamber @ Valve mechanism
        Integra@ OSV Low Pro Valve Integra' OSV IIS Valve
         
Gravitational valves
  • Aim
    • A strategy to reduce siphoning
  • Has to be inserted parallel to the floor.
  • Mechanism
    • Use a weighted-ball system to increase resistance to CSF flow in the upright position
        • GRAVITATIONAL VALVES They attempt to prohibit or reduce siphoning by increasing opening pressure with the assistance of gravity. Inlet valve = ball spring valve and does not change resistance with position Outlet valve has a synthetic ruby ball that sits in a conical seat and there are three stainless steel balls that sit on top Horizontal-vertical Valve Cordis horizontal vertic I valve of it which wiegh it down in upright position and fall away in recumbent position.
  • Some brands come in adjustable versions, including the ability to set opening pressures for both supine and upright positions
    • MIETHKE BLUE
  • Examples:
    • Aesculap MIETHKE valves (GAV, proGAV, etc)
    • Cordis Horizontal-Vertical valve
GRAVITATIONAL VALVES Ball-in-Cone Valve Gravitational Unit
GRAVITATIONAL VALVES
Summary
Valve
Operating Principle
Example
Fixed differential pressure valve
Valve opens and flow occurs if the inlet pressure exceeds the outlet pressure by a certain amount (usually ranges described as low, medium, or high pressure). However, CSF flow occurs even when the measured mean ventricular ICP is lower than the differential pressure of the valve because the systolic peak of the pulsatile ICP may exceed the threshold for opening. Similar CSF egress may be due to coughing and Valsalva maneuvers, as well as intrinsic dural vasomotor changes. Four broad categories: slit valves, miter valves, diaphragm valves, and ball-in-cone valves. These devices all attempt to achieve the same goal of keeping IVP from climbing too high or falling too low. The valve’s opening pressure is not necessarily the same as its closing pressure because of the length of the hysteresis. However, the length and diameter of distal catheter can have just as large an effect as the valve on the effective opening pressure.
- Codman Hakim fixed pressure
- Medtronic Delta fixed pressure
- Chhabra
Programmable (adjustable) differential pressure valve
Are externally adjustable differential pressure valves. They act in the same fashion as nondisputable differential pressure valves, except ability to alter the opening pressure with an external device, thereby obviating the need for surgical shunt revision. This increases convenience and marginally decreases risk, but it is not clear whether this benefit outweighs the increased cost of using these valves in all patients. Most valves are adjusted with an external magnet, and some are disposed to inadvertent reprogramming in the presence of strong magnetic fields
- Codman Hakim Programmable
- Medtronic Strata
- Sophy Programmable Pressure valve
Flow regulated valve
Designed to increase hydrodynamic resistance as the pressure gradient increases in an attempt to keep the flow rate constant. It is in fact the differential pressure that controls the resistance (i.e. pressure-controlled). A contoured synthetic ruby flow control pin that fits inside a movable synthetic ruby ring. At low pressures the valve behaves as a differential pressure valve until flow rates reach about 20 mL/h. As the pressure increases, the ruby ring is deflected downward, the flow aperture decreases, which increases resistance and reduces flow. This will tend to maintain flow at a constant level over a range of physiologic pressures (8-35 cm H₂O). If the pressure increases further, beyond a predetermined threshold, typically around 35 cm H₂O, the ruby ring is deflected further downward beyond the pin, thereby increasing the aperture for CSF flow and lowering resistance to allow "venting" of CSF for mitigating any pathologic rise in CSF pressure. These devices produce pressure-flow curves with a sigmoid shape
Integra Orbis-Sigma II
Antisiphon device
Usually placed distal (in series) with a differential/programmable valve. Generally act to prevent CSF overdrainage in situations when ICP is less than atmospheric pressure (e.g. membrane of Delta chamber sucked in to close the pathway) or if CSF flow rate is excessive
- Medtronic Delta Chamber (found in Strata and Delta valves),
- Miethke Shunt-Assist
- Codman SiphonGuard
Gravitational valves
Attempt to prohibit or reduce siphoning by increasing opening pressure with the assistance of gravity when a patient sits or stands. All fixed gravitational valves are differential pressure valves but operate at two different opening pressures that is dependent on whether the patient is in the horizontal or vertical position. It is extremely important to ensure that gravity-actuated valves are secure and in the proper vertical position
- Integra horizontal-vertical valve
- Miethke paedi/proGAV