Motor Threshold and Motor Cortex Stimulation

Motor cortex stimulation (MCS) is a form of brain stimulation for patients with medically refractory neuropathic pain. The strength of stimulation can be individually adjusted by changing the voltage of stimulation. Too high voltage will produce side effects (e.g. seizures) while too low voltage will not provide pain control. The aim of this study is to analyze the optimal stimulation parameters in patients already implanted with a motor cortex stimulation who have received good pain relief. The actual voltage may vary widely between patients (because of the individual variations in tissue resistance) but the investigators feel that there may be an "ideal" voltage if it is measured as a percentage of motor threshold (PMT).

If motor threshold is the voltage that can evoke a muscle contraction then a PMT = 80% would be a voltage that was eighty percent of that value. Although the actual voltage may be different between patients, the effective PMT may be similar since it represents a more physiologic measure of stimulation.

Systematic analysis of the findings of this study might help the individual participant and future patients by reducing voltage to the lowest effective setting and reducing the chance of seizures.

Motor cortex stimulation is used in the treatment of neuropathic pain since 1991 but still no guidelines for programming exist and programming is therefore mainly bases on trial and error. This is mostly due to many variables influencing the choice of stimulation parameters and significant individual differences in susceptibility to stimulation. Routinely the motor threshold is determined during programming to identify the superior limit of voltage at which twitching is induced. No stimulation above the motor threshold should be performed as this is known to cause seizures. The voltage of simulation that will be effective for an individual is unknown at the beginning of the treatment.

The investigators try to find the lowest effective voltage because that will reduce the risk of stimulation-induced seizures and prolong the life of the pacemaker.

Inclusion criteria

• Adult patient more than 18 years of age
• Chronic neuropathic pain effectively treated with motor cortex stimulation
• Stable medication during the trial
• Willing and able to comply with the study protocol and to return per the follow-up visit schedule and able to provide informed consent.

Exclusion criteria

• Evidence of an active disruptive psychiatric disorder or other known condition significant enough to impact the perception of pain, compliance to intervention and/or ability to evaluate treatment outcome as determined by the investigator
• Technical malfunction of the MCS device
• History of seizures
• Unable to provide informed consent