Comparison of Constant Current to Constant Voltage Stimulation in Subthalamic Deep Brain Stimulation for Parkinson's Disease

Deep brain stimulation (DBS) is an established therapy for neurological and psychiatric disorders, being one of the most important therapies in functional neurosurgery to date. Class I evidence shows that subthalamic nucleus stimulation (STN) for Parkinson's disease (PD) is more effective than the best medical therapy (1,2). Globus pallidus internus stimulation for primary dystonia has an improvement rate of up to 70%(4) and is considered the main surgical treatment for this pathology. Symptom improvement in essential tremor is well documented. Pain, epilepsy, psychiatric disorders such as Tourette syndrome, obsessive - compulsive disorder and depression are promising applications of DBS. Obesity, memory improvement, aggressiveness, drug addiction, minimal conscious state and hypertension are areas of continuing investigation as potential applications of DBS.

The clinical effects of DBS result from the delivery of electrical charge to brain tissue. The way this electrical charge is delivered depends on the electronics of implantable pulse generators (IPGs). IPGs apply pulses of electrical stimulation in which voltage or current increases from zero to a maximum value for a period of time (order of tens of microseconds) and then returns to zero.

There are two types IPGs , differing on whether voltage or electrical current is controlled (3).

1. Constant - voltage IPGs: in this type of IPG there is control over the maximum voltage associated with each pulse. The maximum current will vary depending on the impedance. A specific voltage is programmed. The amount of electrical current delivered with a constant voltage depends on the impedance of the tissue and the electrodes. Thus, the voltage will not indicate how much electrical current is given; the resistance will determine this parameter.
2. Constant - current IPGs: provide a specific electrical current and will automatically adjust the voltage depending on the impedance. The strength of stimulation will remain the same regardless of changes in the impedance. The difference in impedances on each electrode and the intersubject variability in impedances will not alter the electrical current by modifying the voltage.

Until fairly recently, only constant voltage IPG's have been available for use. A newer generation of IPG's have been available for more than a year that can provide either constant voltage or constant current according to physician and patient preference. There is no evidence yet to support the preferential use of one modality over the other.

IMPORTANCE OF THE STUDY Following DBS electrode implantation, the impedances within the same patient can vary widely. Surgical implantation changes tissue impedance, being usually higher immediately after surgery. In this case the patient will require higher electrical currents. After the initial tissue reaction subsides, the reduction of impedance will generate an increase of the charge density. For this reason it is commonplace to wait at least 2 weeks after electrode implantation before programming the IPG.

Furthermore, different contacts on the same DBS lead can have widely varying impedances. Effects or side effects associated with stimulation of one electrode contact often cannot be applied to a different contact on the same DBS lead in the same patient because the impedances may differ.

The range of impedances that can be measured will depend on the choice of voltage. In some patients, the voltage used therapeutically may not be enough to allow an accurate measure of the impedances, causing confusion regarding the structural and electrical integrity of the IPG.

For all the above mentioned factors, the clinical results in patients with constant - voltage IPGs will depend on the experience of the operator to deal with the different aspects related to tissue and electrode impedances to achieve the desired current. The electronic properties of constant - current IPGs allows the system to make voltage adjustments in order to maintain the desired current and the subsequent clinical effect. This has been the theoretical background for the development of constant current IPG's; whether or not this theory is true in practice is the subject of this study.