Exposure to powerful magnetic fields may alter the settings of programmable ventriculoperitoneal shunt valves or even cause permanent damage to these devices. Transcranial magnetic stimulation (TMS) and magnetic resonance imaging both generate a high-intensity, focal magnetic field. To the best of our knowledge, there is no literature data on the compatibility of TMS with neurosurgical implants. The aim of the present in vitro study was to investigate the effects of TMS on four types of pressure-programmable valves (the Strata 2 from Medtronic, the Polaris from Sophysa, the ProGAV from Miethke, and a cylindrical valve from Codman–Hakim).
We used a Magpro X100 stimulator (Medtronic) for monophasic or biphasic TMS via a circular or a figure-of-eight coil. Each valve setting was tested before and after exposure to TMS. Experiment 1: The effect of the coil–valve distance (10, 5, 2.5, and 1 cm) was assessed. Experiment 2: We mimicked in situ stimulation with a human mannequin by placing the valve in a retroauricular position, the TMS circular coil on the apex, and figure-of-eight coil centered over the primary motor area site. Temperature changes were monitored throughout the experiments. Experience 3: TMS-induced valve movements were assessed by using an in-house accelerometric setup.
Our results primarily demonstrated that the Strata 2 and Codman–Hakim valves' settings were perturbed by TMS. There was no heating effect for any of the valves. However, TMS induced movements of the Strata 2, Polaris, and ProGAV valves. Experiment 1: The unsetting frequencies observed for the Strata 2 and the Codman–Hakim valve showed an influence of the distance, the coil model, and the magnetic field characteristics, whereas the Polaris and ProGAV's settings remained unchanged. Experiment 2: Unsetting occurred for Strata 2 valve with the circular coil only, whereas the Polaris, ProGAV, and Codman–Hakim valves' settings remained stable. Experiment 3: The Strata 2, Polaris, and ProGAV valves showed high-amplitude oscillations during TMS under all stimulation conditions, whereas the Codman–Hakim valve did not move.
Our in vitro experiments showed that TMS can interfere with programmable shunt valves by inducing unsetting or movement. This finding suggests that great care must be taken if applying TMS in hydrocephalic, shunted patients.
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We are indebted to Mr Pascal Sohier for his expertise and technical assistance during X-ray experiments with the Codman–Hakim valve. We wish to thank Dr David Fraser (Biotech Communication, Damery, France) for his helpful advice on the English language in this paper.
Disclosure of competing interests
The work reported in the present manuscript did not have any specific funding source. None of the authors has a conflict of interest.
The adjustable valves are programmed or reset with a special programming tool, which is a strong external magnet. They are therefore at risk for inadvertent resetting by exposure to external magnetic fields. In daily life patients are exposed to various devices that generate magnetic fields. The question whether such devices can change the settings of adjustable hydrocephalus valves has been addressed before. Previous studies have revealed that external magnetic fields can change the settings in some models of programmable valves.
Starting with low-intensity magnetic fields (appliances producing electromagnetic fields but more often using magnetic holders, latches, or toys) and finishing with exposure to extremely strong magnetic fields (as in MR imaging), the risk of an interaction between external fields and the magnets used in all types of adjustable valves causes different kinds of safety concerns. Research has shown that most metallic implants are safe for patients undergoing MR imaging procedures if they are nonferromagnetic and if the magnetic attraction or torque is acceptably low. However, MR imaging in patients with a shunt implant continues to be a concern as relatively little published information exists regarding safety. The main risk for a patient with an implanted shunt valve who is undergoing MR imaging is related to the resetting and, less likely, possible heating and dislodgment of the implant during the imaging procedure. Moreover, shunt valves can generate considerable artifacts, thus invalidating the imaging procedure itself. As MR imaging proves to be an important diagnostic tool in the follow-up of hydrocephalic patients, the compatibility characteristics of different shunt valves might direct the neurosurgeon's choice of a particular model. Problems with interaction TMS and programmable shunts has not been addressed before, and the present study fills this niche.
Lavinio A, Harding S, Van Der Boogaard F, Czosnyka M, Smielewski P, Richards HK, Pickard JD, Czosnyka ZH.Magnetic field interactions in adjustable hydrocephalus shunts. J Neurosurg Pediatr. 2008 Sep;2(3):222-8.
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Lefranc, M., Yeung Lam Ko, J., Peltier, J. et al. Effect of transcranial magnetic stimulation on four types of pressure-programmable valves. Acta Neurochir 152, 689–697 (2010). https://doi.org/10.1007/s00701-009-0564-2
- Cerebrospinal fluid shunt (CSF)
- Transcranial magnetic stimulation (TMS)
- Programmable valve