Types of shunts

Programmable

Some shunts have valves that can be adjusted to open at varying pressures, by using a special device placed on the skin. This way, the neurosurgeon can alter the way the shunt works without having to replace it. This is particularly helpful in children, as the drainage pattern changes as the child grows, and also for people with NPH. Currently the most commonly used programmable shunts are adjusted with a magnetic device. This can mean that occasionally some types can be accidentally reset by magnetic fields coming close to the valve area, or by powerful magnets such as those in MRI scanners. Some types of programmable valves, such as ProGAV, need physical pressure to be applied to ‘unlock’ the mechanism before the magnet resets it, so it can’t be changed unintentionally by magnetic fields of 3 Tesla or less.

Others, such as Codman Certas, can only be changed by magnetic fields in a certain direction, meaning MRI should not change the setting.

Children with other programmable shunts should be supervised when playing with magnetic toys etc. or around children using Vagal Nerve Stimulators (VNS) to control seizures.

Gravitational

Shunts drain at different rates depending on whether we are standing or lying, owing to the effects of gravity. Shunts tend to drain more when we are upright, owing to a ‘siphon’ effect, and the greater the height between the head and the bottom end of the shunt, the more drainage tends to occur. To counteract this effect, some shunts (such as GAV, and PaediGAV) operate with two settings, one for standing, and another for lying down, to help reduce over drainage. Over drainage only happens with shunts, not Endoscopic Third Ventriculostomy (ETV).

Antimicrobial

Bactiseal shunts

Bactiseal shunts are impregnated with two antibiotics to kill bacteria that are introduced at the time of the shunt surgery. The antibiotics are embedded throughout the shunt’s material, rather than being coated onto the inside of the shunt. This means the antibiotics aren’t washed off the shunt by the flow of the CSF, so continues to kill bacteria for several weeks after the shunt is inserted. 

The BASICS trial was led by a neurosurgery team in Liverpool (Professor Conor Mallucci and Professor Michael Jenkinson). The trial showed that antibiotic shunts (such as Bactiseal) reduced the risk of infection from 6% to 2% in all ages of patients and in all types of hydrocephalus. The Society of British Neurological Surgeons (SBNS) recommends that antibiotic shunts are used as standard in the UK for all new shunts.

Shunt accessories 

Antisiphon devices

Some shunts have devices included or added, to reduce or prevent over drainage owing to gravity when standing up (siphoning).

Although not strictly a shunt, Ommaya reservoirs are often used alongside shunts or ETVs. They are formed of silicon, with one end in the ventricle. The other is a closed end placed just under the surface of the scalp. No CSF flows through the device, but it provides direct access to the ventricle, and can be useful for measuring intracranial pressure, or for doctors to draw off CSF in the case of suspected shunt or ETV blockage, to relieve the pressure.