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 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 [8] 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 [7] 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. [23], 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. [16] 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.
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. In the sensor reservoir®, the measurement of CSF pressure changes is performed on a corrugated, biocompatible membrane. The pressure gradient is transmitted to the pressure transducer via a chamber filled with air or gas [9]. The measurement is performed in the reservoir itself.
The NEUROVENT®-P-tel probe is a piezo resistive pressure sensor located at the tip of a 3 cm long intraparenchymatous catheter. The pressure transducer contains several electrical resistors doped onto a flexible diaphragm. This membrane is in direct contact with the pulsating brain tissue. An increase in ICP leads to a stretching of the membrane. These resistance changes are registered by a pressure transducer and converted into ICP values [2, 18]. In this case, measurements are performed in the brain parenchyma at a depth of approx. 3 cm.
Furthermore, the different ICP absolute values are due to a hydrostatic pressure difference [16].
Due to the elastic properties of the shunt catheter it is assumed that the transmission of pulsating ICP components is attenuated [7]. In addition, the measured ICP via the sensor reservoir® depends partly on the valve setting.
Technical errors might be another reason for diverging measurements. Antes at al, registered a technical error rate of the sensor reservoir® of 8% [8]. Several authors found error rates between 3 and 16% for the parenchymatous ICP probes [1, 2, 8, 19, 20].
Another factor that could explain the differences in absolute ICP values is the zero drift of both measuring methods. A zero-point drift of the NEUROVENT®-P-tel probe of ± 2.5 mmHg has already been described in the literature [2, 5, 20]. 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, 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 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 CT scan or MRI in the event of under- or over-drainage symptoms could be avoided in the future.
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 [8].
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 [7] 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 reliable 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 [10] 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 [8] 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.