Telemetric measurements of ICP becomes gain rising importance in modern neurosurgery. The NEUROVENT®-P-tel probe has been proven to be safe and effective in telemetric ICP monitoring and is used as reference in acute therapy of elevated ICP as well as for the diagnosis of neurological diseases such as pseudotumor cerebri and idiopathic normal pressure hydrocephalus (iNPH) [1,2,4, 5, 8,10,11].
The sensor reservoir® (Miethke, Aesculap, Germany) is available to determine pressure conditions continuously within a shunt system and thus to assess over- or under-drainage during shunt therapy. Radiation exposure of CT scan or long and expensive MRI for the diagnosis of over- or under-drainage symptoms can thus be avoided.
Since the approval of the sensor reservoir® for clinical use in 2015, no direct prospective comparison with another established system for measuring ICP has been available.
Management of telemetric devices
All devices were implanted for clinical reasons according to their licensed purpose, following the routine procedures. During the time of parallel implantation absolute ICP values gained by NEUROVENT®-P-tel probes and VP shunts with sensor reservoir® could be compared directly.
The advantage of both telemetric systems is the possibility of continued monitoring after transfer to the neurosurgical ward, rehabilitation, or domesticity, as well as during different body positions of the patient.
To the best of our knowledge there are three publications about the sensor reservoir® [7, 8, 23].
Antes and colleagues  who co-developed the sensor reservoir® implanted the sensor reservoir® in patients who had already received conventional VPS and were suspected to have suboptimal valve settings. The sensor reservoir® was thus used to try to detect shunt-associated complications such as over- and underdrainage in time and to treat them with appropriate changes in valve settings.
Ertl et al  used the sensor reservoir® to analyze the change in ICP values in relation to the change in the patient's body position. Our study confirms his findings in standing and lying position.
The most recent study is a technical note by Norager et al. , who describes technical advantages and disadvantages and present one illustrative case for each device.
A comparison of measured ICP values in the cerebral CSF compartment and the brain parenchyma has already been described in the literature [13-17]. Brean, Eide et al.  analyzed the ICP dynamics of wired intraventricular and parenchymatous ICP probes in comparison. In some patients with primary SAH, a parenchymatous sensor was implanted alongside the EVD in the same hemisphere. The ICP values were then measured simultaneously using both devices. In contrast, a one-sided implantation of both measuring cells of telemetric devices proved to be impracticable in the present study due to the design of the corresponding reading devices. For this reason, both hemispheres were used for implantation in our patient population even though imbalanced intracranial pathologies could have tampered the results. However, the positive correlation of ICP curves for both devices indicates comparable measurements while differences of absolute ICP values could be attributed to implantation site, calibration, and other technical issues. This difference in measurement is to be expected and can be justified on the one hand by the different measurement situations of the two measurement sensors. The sensor reservoir® measures the CSF pressure changes that occur on a corrugated, biocompatible membrane, which changes are passed on to the pressure sensor via a chamber filled with air or special gas . In this case, the measurement location is the reservoir that is implanted in the calotte. The NEUROVENT®-P-tel probe is a piezo- resistive pressure sensor that is located on the tip of a 3 cm long intra-parenchymatous catheter. The pressure transducer contains several electrical resistors that are doped on a flexible membrane. This membrane is in direct contact with the pulsating brain tissue. An increase in the ICP leads to an expansion of the membrane. These changes in resistance are registered by a pressure transducer and converted into ICP values [2; 18]. In this case, the measurement location is approximately 3 cm deep. The different absolute ICP values can presumably be attributed to a hydrostatic pressure difference .
Due to the elastic properties of the shunt catheter, it is assumed that the transmission of pulsating ICP components is damped . In addition, the ICP measured via the sensor reservoir depends in part on the valve setting.
A technical error could also arise. The technical error rate of the sensor reservoir is 8% , and that of the intraparenchymal ICP probes is 3-16% [1-2; 8; 19-20]. Another factor that could explain the differences in the absolute values is the zero-point drift of both measurement methods. A zero drift of the NEUROVENT®-P-tel of + -2.5 mmHg has already been described in the literature [2; 5; 20].
ICP measurement via sensor reservoir® versus NEUROVENT®-P-tel probe
In this study, the absolute ICP values of the sensor reservoir® did not match the absolute ICP values of the NEUROVENT®-P-tel probe. However, the difference of the mean ICP values was ± 4 mmHg (1.3-13.6 mmHg). This difference was to be expected and can be explained by the different measurement locations of both sensors, elastic properties of the shunt, by technical errors or by zero-point drift (see section above).
However, the tendency of the ICP dynamics of both systems is largely synchronous in the present study despite the difference between the absolute ICP values. The correlation coefficient was significant in nine cases (81.8 %).
The study also shows that ICP values change accordingly to the patient’s position. In the case of the programmable differential valves without gravitational unit measured via sensor reservoir® and NEUROVENT®-P-tel probe in the same patient, the pressure gradient between lying and standing position was significantly greater than in patients with additionally implanted fixed or adjustable gravitational unit. The regulation of the CSF outflow rate during standing position and thus the avoidance of over-drainage complications with the existing gravitational valve can be recorded with ICP continuously monitoring via the sensor reservoir®. This information can help during diagnosis and therapy of over-drainage. The telemetrically acquired ICP data can also be used to determine the indication for implantation of an additional gravitational unit. The SVASONA study showed the efficacy of gravitational units through avoidance of over-drainage complications for patients with idiopathic normal pressure hydrocephalus [21, 22]. Using the adjustable proSA valve (Miethke, Germanmy) as a gravitational unit, the shunt system can be adjusted even more precisely to the individual needs of a patient.
A disadvantage of the sensor reservoir is that the large RFID antenna of the reading device must be placed over the sensor reservoir in order to measure and store the ICP values. A permanent fixation of the heavy antenna on the head of patient is not possible with the current version of the device. A long-term ICP monitoring for 24-48 hours to determine the Lundberg A and B waves is therefore not possible .
Because the sensor reservoir has a height of 7.7 mm, there is also a cosmetic disadvantage after implantation because a swelling remains visible. A wound dehiscence above the sensor reservoir was not observed in our hospital but seems possible in elderly patients or thinned skin.
In the study by Ertl et al  the sensor reservoir was implanted in two patients with normal pressure hydrocephalus. A measurement with a frequency of 1 Hz was performed. As in our study, a similar change in ICP values was measured depending on changes in the patients' body position. It can thus be concluded that the sensor reservoir® provides traceable real-time values. In our opinion this telemetric technique can be used for the diagnosis and therapy of over- or under-drainage in shunt patients.
Freimann et al  implanted a NEUROVENT®-P-tel probe in addition to the programmable shunt valve in four patients with hydrocephalus. In these patients telemetric ICP measurements were helpful in valve adjustment and enabled regular evaluation of the position-dependent ICP values as a therapeutic target. However, the NEUROVENT®-P-tel probe can be implanted for only 3 months. The sensor reservoir®, on the other hand, enables permanent ICP measurement. In the work of Antes  the sensor reservoir® was implanted in 25 patients. Complications such as over- or underdrainage could also be detected and quantified with the sensor reservoir®. The valves could be individually adjusted according to ICP measurements. This study was unique in describing the use of the sensor reservoir® was to diagnose shunt complications. An additional adjustable gravitational unit (pro-SA) was implanted, which was connected distally of the fixed gravitational unit. During the control examination after 4 months ICP measurements showed a reduction in to -7 mmHg and clinical improvement with reduced headaches was observed.
As a limitation of the study is the partially incomplete data collection should be mentioned. Parallel measurements of the ICP via the sensor reservoir® and NEUROVENT®-P-tel probe for approx. 5 minutes 3 times a day was a great challenge despite the 24-hour service in our clinic. Sometimes only one ICP value was obtained via both telemetric devices.
In addition, the small number of cases is a significantly limiting factor.