General risks, as in any operation, include bleeding at the operative site, infection related to the implantation and the rare death.
Side effects depend on the site chosen for stimulation. The reported complications associated with:
a) pallidal stimulation is minimal. These include visual phenomenon with certain stimulation parameters. There is a trend for decreased verbal fluency though this is much less than with ablative procedures.
b)subthalamic nucleus as the target, the risk is of abnormal movements called chorea.
The drawbacks of caring for a chronically implanted device are significant and include: infection, migration, breakage, battery replacement. These are expensive devices and require periodic programming. This translates into large investment of time, personnel and, of course money.
The main thing going for DBS is that it is a reversible procedure. If in the future a revolutionary treatment is found the implant would not prevent the patient from trying out such a treatment. DBS works as long as it is switched on, though there may be a time lag in onset and disappearance depending on the individual and site of stimulus. As Parkinson's disease is progressive in nature, another feature of DBS is that the stimulation parameters can be programmed differently over time to keep the disease under control. Side effects are similarly reversal. If a particular side effect becomes prominent, it can simply be reduced or stopped depending on the stimulation parameters.
The effectiveness of the treatment is dependent on several factors. Obviously the implant must be very precisely placed to be effective and at the same time economical with regards to battery consumption. The trend is to move towards imaging methods which are not invasive. Ventriculography, where dye is injected into the fluid space within the brain, is practiced by very few. Instead MRI and CT scan are used together to complement each others advantages using image fusion techniques. Others like physiological feedback from the electrode tip such as electrical activity recording from a group of cells or even a single cell, impedance measurements, response to stimulation by electrical pulses, all add to the efficacy of the treatment.
Standardised rating scales are used for assessing treatments for PD. These include Unified PD Rating Scale (UPDRS) and Hohn and Yahr staging scale. STN stimulation is potentially an excellent target. Its stimulation is highly effective for bradykinesia, rigidity, tremors as well as posture instability and freezing.