- General
- All shunts are pressure-passive
- Adjustability—reasonable aim helping to minimize revision rate
- Majority of valves have unphysiologically low resistance (when open)
- Few offer reasonable prevention against posture-related overdrainage
- Adjustable valves distort MRI image
- Most of adjustable valves (except ProGAV and Polaris) may be accidentally re-adjusted by external magnetic field
- Prices vary: from 300£ to 1600£
- Findings of Cambridge shunt evaluation laboratory
- Clinical
- 3 common pressure-flow performance curves
- Autoregulation valve - OrbisSigma
- Stabilizes flow, not a pressure
- Prevents overdrainage
- Contraindicated in patients suffering from intermittent (vasogenic) ICP waves
- Mechanism
- Performance
- Adjustable valves
- Some programmable valves are sensitive to external magnetic field (> 10mT). New models like Polaris, ProGAV and ProSa, Certas are not sensitive.
- Set up
- Valve structure
- Strata valve: “Ball-on-Spring” adjustable valve+Delta siphon controlling device
- Performance
- Shunt technology
- Features
- Closing mechanism
- Ball on spring
- Silicone membrane (membrane valves)
- Mitre valves
- Programmable valves
- Preventing overdrainage
- Risk
- Most valves have non-physiologically low resistance (from 1.5 to 3mmHg/ml/min)
- The use of long catheter can increase this resistance by 100-200% → risk of overdrainage
- Codman Hakim valve with siphon guard
- Mechanism
- Two channels
- Middle-low resistance
- Spiral outer - high resistance (around 40 mmHg/(ml/min))
- Switching may be unreliable
- Structure
- Performance
- Images
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