Systems for deep brain stimulation: review of technical features

Since its introduction more than 25 years ago (Benabid et al. 1987), deep brain stimulation (DBS) for movement disorders has become an efficient and widely used tool in the therapeutic armamentarium of various neurological conditions. Parkinson disease (PD), tremor, dystonia, epilepsy, and obsessive–compulsive disorder (OCD) are indications for treatment with deep brain stimulation. Table 1 summarizes approved indications for each system.

A typical DBS system consists of an electrode that is placed into the targeted cerebral structure, an implantable pulse generator (IPG), and an extension that connects the electrode to the IPG. The IPG itself consists of a case that houses a battery and electronic circuitry, which generates the electric signal going to the brain.

To achieve the desired therapeutic effect and to avoid undesired co-stimulation, the electric signal and the anatomical structures have to be aligned. This can be achieved by a well-shaped electric stimulation field, which can be adjusted using the DBS system.

In cases of perfectly implanted electrodes, the importance of the highly sophisticated technical features may be limited, but in cases where stimulation induces side effects, and those with little therapeutic window (defined as the difference between the amplitude of the electric signal at which side effects occur and the amplitude of the electric signal at which a significant therapeutic effect is observed), the availability of these features should be highly valued.

This review compares the technical features of DBS systems, deliberately limited to those approved for the European market. Information was primarily obtained from practical experience, promotional material, websites of the manufacturers, instructions for use, and competent support from representatives of the manufacturers. Altogether, DBS systems available from Boston Scientific (B), Medtronic (M) and St. Jude (S)¹ are reviewed.

The electrical source is either a voltage source or a current source. Today the use of current sources is increasingly prevalent, but voltage sources represent the classical approach.

According to Ohm’s law, the current (C) is directly proportional to the voltage (V) and inversely proportional to the resistance (R). Depending on the electrical source, either the voltage or the current can be adjusted. The resistance depends on the electrical characteristics of the tissue, the electrode–tissue interface of the electrode itself, and in part on the stimulation. The resistance may vary over time, and it is not possible to anticipate the exact impedance behavior (Benabid et al. 1996; Hartmann et al. 2015).

With a voltage source, the desired voltage level can be adjusted. The resistance of the device and the anatomical structure results in a certain current value. This current value determines the therapeutic effect. The resistance can be subject to fluctuations due to subject- or device-related reasons. According to Ohm’s law, fluctuations in resistance result in fluctuations in the current. These current fluctuations can be avoided using a current source, as the current level can be adjusted directly (Bronstein et al. 2015).

All devices reviewed in this study can be operated with a current source. The devices marketed by B are equipped with multiple sources, with a dedicated current source for each contact on the electrode. The devices marketed by M also have the option to be operated using a voltage source. The technical features regarding current and voltage mode are summarized in Table 2.

The systems can perform either unilateral or bilateral stimulation, depending on the number of connectors provided on the header of the IPG. To stimulate both hemispheres one can use two IPGs that offer the option of unilateral stimulation, or one IPG that offers the option of bilateral stimulation. A single IPG for bilateral stimulation was first implanted in 1998 (Vesper et al. 2002). Figure 1 shows a schematic illustration of an IPG and electrode combination for unilateral and bilateral stimulation. Table 2 summarizes this feature for each system provided by B, M and S.

A targeted and precise configuration of the electric stimulation field allows a localized neuronal response. The configuration, therefore, has a high impact on the efficacy of the therapy. Furthermore, the location of the electrode contacts in the brain target is essential for a potentially good therapy outcome.

The electric stimulation field depends on the distribution of the current in the targeted brain region, the pulse width, and the stimulation rate. The current distribution can be altered according to the electrode design, the polarity and proportion of current coming from each contact, and the amplitude. Thus, the technical features can be adapted to the requirements of the anatomical structure of the patient.

DBS systems are designed as either rechargeable or non-rechargeable. Non-rechargeable systems store energy for the complete lifetime of the battery. The longevity of the systems depends on parameters used for the electric signal, such as stimulation amplitude, stimulation rate, pulse width, the number of active contacts, and the battery capacity. Furthermore, the energy consumption of the device itself, such as the electronic circuit or impedances of the electrode, affects battery longevity. Rechargeable systems have to be recharged on a regular basis. Rechargeable and non-rechargeable systems currently marketed are summarized in Table 2.

The stimulation field can be set and adjusted using a programmer (Volkmann et al. 2006). The desired parameters for the stimulation field are transmitted from the programmer to the IPG by radio frequency coupling. B, M and S provide their own programmers developed for physicians and patients. The Infinity system provided by S uses Apple digital devices for its programmers, which have a Bluetooth link to communicate with the IPG.

The rising diversity of DBS systems also focuses attention on the interconnectivity of different DBS systems. While geometrical aspects of the connectors are very similar, the design is specific and does not allow a simple connection.

DBS systems consist of an electrode, an IPG and an extension that connects the electrode to the IPG. Each manufacturer provides these components for its own DBS systems. The combination of components from different manufacturers can be achieved using adaptors. These adaptors connect the extension to the IPG and thereby allow the use of different electrode-IPG-combinations. Electrodes from M can potentially be connected to any IPG from B, M and S. However, no adaptors are provided to connect the electrodes from B and S to IPGs from M (Table 5). Additionally, adaptors are currently also offered for electrodes used with the classical IPGs provided by M (Itrel, Soletra, Kinetra). These IPGs are not on the market anymore, but patients may have them implanted. In case of a replacement, IPGs from different manufacturers can be connected with an adaptor to the implanted electrodes.

This review focuses on technical features that are primarily relevant for electrical stimulation. The systems may differ with regard to electrical source, possible stimulation regions, and the stimulation field. When DBS stimulation was first introduced, voltage-only sources were used. In recent years, current sources have been used more often. All DBS systems currently on the market provide current sources. The number of electrode contacts has increased from four contacts to up to eight contacts. In the latest developments in electrode design, one single contact is segmented into three contacts. This allows the current distribution to be adjusted and directed more specifically. The possible settings for the stimulation rate, the pulse width, and the stimulation amplitude have not changed significantly over time, and most of the DBS systems may be operated in a uni-, bi- and multipolar configuration. Some manufacturers also provide the option to adjust specific settings with the concept of interleaving stimulation, which can reflect specific patient requirements. Nevertheless, the setting options are limited with regard to the possible stimulation rates and polarity configurations of the electrode contacts. In the past, only non-rechargeable systems were marketed; today, all manufacturers provide rechargeable and non-rechargeable systems. The diversity of DBS systems also raises the question on interconnectivity between components of different manufacturers. The combination of technical features of an impulse generator from one manufacturer and an electrode from another manufacturer may be a desirable requirement in clinical practice. Consequently, a uniform standard would be required to simplify efforts for interconnecting components from different manufacturers.

The DBS systems appear quite similar, but when it comes to technical features and their combination systems differ significantly from each other. To achieve the best therapeutic effect, technical features, its combination possibilities, and the limitations of the systems have to be considered.