The system

The neuromodulation system consists of:

• 1.
  

  a multicontact stimulating lead

  
  
• 2.
  

  a combination implantable pulse control system and self-contained power supply (IPG)

  
  
• 3.
  

  an extension cable that connects (1) to (2) (see Chapter 9 ).

At this time, there are two Food and Drug Adminstration (FDA) approved leads for deep brain stimulation (DBS): Medtronic models 3387 and 3389, with other manufacturers running or getting ready to run clinical trials. For spinal cord stimulation (SCS), there is a much wider choice of approved electrodes and implantable pulse generators (IPGs) from multiple manufacturers. The Appendix contains a list of all approved systems with images.

In all cases, the lead needs to be secured. For DBS, this occurs where it exits the skull, and for SCS this occurs with a friction suture cover. For DBS, the excess length of lead wire is coiled beneath the scalp and connected to an extension wire , which is thicker and more durable than the lead. For SCS, the lead may or may not use an extension wire. Conductors in the Medtronic DBS and SCS extensions are made from silver core MP35N. Each conductor is coiled and set in an individual cylindrical opening which reduces the chance of shorting.

The DBS circuit-Paradigm for all fault testing

The DBS extension is passed through a subcutaneous tract that traverses the retrosigmoid sinus region and neck to an ipsilateral subcutaneous pocket in the subclavicular area of the upper chest. There the extension is connected to the IPG. For a dual stimulation device, each lead is connected to a single extension wire via a ‘Y’-adapter (‘Y’ in shape only – all contacts are still individual). A silastic cover (boot) is placed over the lead–extension connection and two suture ties are placed on each end creating a water-tight seal for the connection ( Fig. 15.1 ). The connector screws are made from titanium and the connector blocks from stainless steel. The extension insulation is silicone rubber and polyurethane while the connector block is sealed in silicone rubber and siloxane-coated silicone rubber. The maximum resistance of the complete extension wire is 7 Ω.

Example of the connection between the lead and the extension. The lead slides into a connector that has 1 to 4 screws for securing the lead in the connector. A silastic cover is placed over the connection to keep fluid from interfering with the electrical contacts. Finally ties are placed on either end of the silastic cover to make the connection watertight.

In order for current to flow through an electrical circuit, the circuit needs to be configured in a closed loop. The electrical circuit that contains the DBS system is depicted in Figure 15.2 . The power source provides a constant voltage pulse of potential V that, when activated, sends a current around the circuit. The current (I) is determined by the potential (V) and the impedance (Z) that the potential needs to overcome.

A graphical representation of the DBS and SCS systems. For the DBS system the IPG is usually placed in the chest, but can also be placed elsewhere if the patient desires or for other medical reasons. For the SCS system the IPG is usually placed in the upper buttock or abdomen.

For neuromodulation, impedance includes both the circuit resistance and the effects of capacitance and inductance at the biomechanical interface of the electrode and tissue. Therefore, the total circuit impedance is composed of three elements:

• 1.
  

  the connections between the system components

  
  
• 2.
  

  the impedances of the conductors (wires) used in both the extension cable (<7 Ω ) and the lead (<100 Ω )

  
  
• 3.
  

  the brain–body–electrode interface, which contributes the largest impedance.

It is also important to note that the impedance of tissue varies with the stimulation frequency. Thus, comparing impedances over time is only useful if the same test frequency and pulsewidth were used. In general, as the frequency increases, the measured impedance of the biologic material decreases . For example, for an intact DBS system, normal impedance values for a test pulse of 210 μs at 30 Hz, when referenced to the IPG case (i.e. monopolar configuration), should range between 600 and 2000 ohms with a current between 9 and 25 μA using 2.0 Volts. This is true for electrodes located within the subthalamic nucleus (STN), globus pallidus pars interna (GPi), and ventral intermedius (VIM) when using the Medtronic model 8840 programmer in the electrode impedance test mode, not during therapy measurement testing. Future systems may use different test parameters and will yield different normal impedance values. Also, normal impedance values in other brain regions may differ from those observed in these three areas.

The literature describes very little anatomical change at the electrode–brain interface as a consequence of chronic DBS . Therefore, one may conclude that a major change in the measured electrical properties of impedance and current over time will most likely occur at the other two primary circuit impedance points (i.e. the conductors and connection points). It should be noted that within 3 months of implantation there may be large changes in impedance, likely due to surgical healing. Three types of electrical failure modes have been identified in implanted neuromodulation systems:

• 1.
  

  foreign body accumulation at the connection points

  
  
• 2.
  

  an ‘open’ circuit (i.e. a break in the circuit path)

  
  
• 3.
  

  a ‘short’, which is a new unexpected and unwanted circuit pathway between what should be independent circuit elements.

An internal failure of the IPG is exceedingly rare, but possible. However, locating a problem in the IPG is more complex and is arrived at through a process of elimination, when all other testing, to be described below, fails to localize the failure.

Under the ‘open’ circuit condition, current is unable to flow due to a break in the pathway ( Fig. 15.3 ). If the circuit is completely ‘open’, the measured current will be zero and the impedance will be ‘infinite’. If the circuit contains an intermittent ‘open’ and ‘closed’ condition, current will flows some of the time (transient mode failure), during the time in which current flows through the circuit may appear normal. Intermittent ‘open’ circuits are very difficult to troubleshoot, and may only be found during the actual ‘open’ period. An intermittent ‘open’ circuit could be seen when a break in the conductor leaves the two ends in close proximity. When the ends are in contact, the circuit will function normally, however, if the extension or the lead are moved (e.g. while turning the head), the ends separate and the ‘open’ circuit condition occurs.

A schematic representation of current flow in an electrical circuit. (A) shows a normal circuit. (B) shows an open circuit where no current can flow into the load. (C) shows a short circuit where current will be shunted away from the load via the short.

In patients with quickly reacting symptoms such as tremor, which varies quickly in relation to the state of stimulation, the ability to diagnose an intermittent ‘open’ circuit is easier than in patients whose symptoms change more slowly. If the intermittent condition is very brief, no abnormality may be detected. Patients with brief intermittent ‘open’ circuits may derive benefit from stimulation, but the results will be suboptimal. Therefore, if a patient presents with an unexplained reduction in therapeutic efficacy, but the system appears to be functioning properly, one must consider a transient mode failure.

In a ‘short’ circuit situation, current is shunted away from the electrode contacts in the brain. This is because the new circuit pathway, created by the ‘short’, is of lower impedance and ‘draws’ current away from the lead tip. For the internalized stimulation system, there may be multiple short-circuit types. The first type involves a break (‘open’-circuit) in the extension or lead insulation. The wires on the IPG side of the break may touch each other causing the current to flow only in the electrical circuit and not in the body. Under this condition, one will measure very low impedance and very high current during the therapeutic test (see below). The terms ‘high’ and ‘low’ are used because the normal values depend on the therapeutic settings being employed, the target tissue, and the device model. Thus, it is critical to look at the therapeutic parameters at each visit so that a reference exists for each particular patient.

A second type of ‘short’ occurs if the insulation between the conductors in the extension breaks down and the conductors begin to short due to contact with biological tissue. Since there is no ‘open’ circuit, some of the current flows back to the IPG via the shorted wire, while the rest flows to the conductors in the brain. As a consequence, some ‘inactive’ contacts may transmit current, stimulating unintended areas of the brain. Short circuits can cause excessive current flow because, when the impedance trends toward zero, the current will exceed the maximum desired, rapidly draining the power source. One dangerous problem with ‘short’ circuits, and the high current that results, is that this high current may break down the insulation, causing additional unwanted current paths. Also, higher current can generate heat at the site of the short which will, in turn, heat adjacent tissue, generating potential burns. In Figure 15.4 burns can be seen on the extensions wires removed from this patient.

Example of a burn inside the lead insulation.

A third type of ‘short’ circuit condition can arise when fluid enters the connection between the extension and the lead or the extension and the IPG. The fluid can act as a conductor, shunting the current away from the stimulating electrode surfaces to other unintended contacts. In monopolar configurations, the shunted current may activate an alternate conductor, again sending current to an inactive electrode, stimulating an area of brain inadvertently.

Monopolar and bipolar stimulation behave, to some extent, differently in failure modes due to the differing return pathways for the current. During monopolar stimulation, the return is the casing of the IPG. If the lead and extension wire insulations are intact (i.e. the break is inside an intact insulation), the insulation will create a very high resistance thus still allowing the current to flow to the lead and not from the break point to the IPG. However, if the insulation has a break, current will escape from the opening back to the IPG and the patient may feel a ‘shock’ at the break point. For the ‘short’ circuit situation in a monopolar configuration, the current will be split between the two shorting leads if only one lead is active. No changes will be seen if both of the leads are active, assuming the insulation is not broken. If the short is between the connector and the IPG, the patient may feel intense pain at the IPG site. A note of caution: under normal circumstances thin patients may feel a sensation at the location where the IPG is implanted during monopolar stimulation, which may be mistaken for a short circuit. However, interrogation of the system will reveal normal impedance. If the insulation is broken, a shock may be felt at the break point. If the impedance at the break point is lower than that of the electrode contacts in the brain, current will pass from the break point to the case, taking the path of least resistance.

During bipolar stimulation, multiple types of ‘open’ circuit situations can occur. If the insulation is intact, no current will flow in the circuit. If, however, the insulation surrounding one of the conductors is broken, current will flow along two pathways. The first pathway is from the insulation break to the reference electrode. The second is the intended pathway between the active and reference electrodes. ¹

1 At first one may think that, if there is a break in the wire and conductor, then no current will get to the target. Yet, if there is fluid in the conductor between the breaks, then a current pathway may exist for energy to get the target.

‘Short’ circuits also present in multiple ways depending upon the state of the insulation and the state of the conductors at the location of the ‘short’. If the insulation and the conductors are fully intact, minimal current will get to the brain because the impedance at the short is very low. If the insulation is intact and multiple electrodes are being employed for therapy, two conditions could arise. First, if the short is between the active contact and another (an electrode not used for the patient’s particular therapy program), or the reference and another, the current will be split between the normal circuit and the new path, stimulating an unintended region of the brain. Second, if the short is between two active contacts or between two reference contacts, no difference will be seen. If the insulation is broken, the current has multiple pathways it can travel and the current to the electrodes will most likely be reduced due to the low power supply resistance.

Note that in contrast to constant voltage devices, the internal resistance of constant-current stimulators must be very high to ensure that the power supply and not the load controls the current delivered. In every day life, load typically controls current. For example, when we turn on a brighter light at home, the lamp draws more current from the power supply. In a patient, the electronics need to operate in reverse, so that when there is a short, the power supply will automatically ‘limit’ the current to a safe level. In the home, there are fuses and circuit breakers for protection.