Deep Brain Stimulation (DBS) has transformed the management of several movement disorders, including Parkinson’s disease, essential tremor, and dystonia. The therapy involves implanting electrodes into specific brain regions, which connect to a pulse generator—a device similar to a pacemaker—placed under the skin, typically in the chest. While DBS offers substantial relief from motor symptoms, having a permanently implanted neurodevice introduces chronic concerns. This discussion focuses on the persistent, delayed, or cumulative side effects that emerge months or years after the initial stabilization period.
Hardware and Component Longevity
Living with an implanted medical device requires long-term maintenance and accounts for potential mechanical failure. The most frequent long-term hardware issue is the finite lifespan of the battery, housed within the Implantable Pulse Generator (IPG). Non-rechargeable systems typically last three to five years, varying based on the required stimulation settings. Once depleted, the IPG must be replaced through a minor surgical procedure.
Rechargeable systems offer a longer service life, often lasting up to fifteen years, but require consistent daily charging by the patient. Multiple replacement surgeries over a patient’s lifetime increase the risk of late-onset complications, such as infection or skin erosion, years after the initial procedure. Infections at the IPG site are a concern, especially following repeated device replacements.
Beyond the battery, the leads are subject to wear over time. Lead migration, where the electrode shifts position, is a recognized complication, occurring in roughly 12% to 18.5% of implanted electrodes in long-term studies. This shift can diminish therapeutic benefit and require re-programming or, rarely, revision surgery to reposition the lead. Lead fractures, a break in the wire connecting the electrode to the IPG, are less common but reported in up to 15.2% of implanted leads, often necessitating surgical replacement.
Enduring Motor and Speech Changes
The continuous electrical current delivered by the DBS system can lead to specific physical side effects or interact with the underlying disease progression. A common stimulation-related issue is stimulation-induced dyskinesia (SID), which presents as involuntary, writhing movements. This occurs when the stimulation is set too high, over-activating the motor pathways. SID is manageable through careful re-programming of the device settings to balance symptom relief and side effects.
Changes in speech function, known as dysarthria, are a persistent long-term consequence, particularly with stimulation of the subthalamic nucleus (STN). The stimulation can inadvertently affect adjacent neural circuits involved in voice and articulation, leading to slurred speech or reduced vocal volume. Speech intelligibility may deteriorate in a majority of patients within a few years post-DBS, even as core motor symptoms improve. This decline is often directly related to the parameters used, since higher voltage settings increase the likelihood of speech disturbance.
Long-term stability of gait and balance is a challenge, especially for patients with Parkinson’s disease. While DBS initially improves core symptoms like tremor and rigidity, benefits for axial symptoms—such as postural instability and freezing of gait (FOG)—may not be maintained. These issues may worsen over time because they reflect aspects of the disease less responsive to stimulation or progress independently of the therapy. Overly aggressive stimulation settings may also exacerbate FOG or balance problems.
Therapeutic tolerance is sometimes observed, requiring the device to use increasingly higher voltage to maintain symptom control over many years. This need for higher amplitude can be due to subtle changes in the electrode-tissue interface or the progression of the underlying condition. Increasing the stimulation amplitude to chase therapeutic effect simultaneously increases the risk of inducing stimulation-related side effects, including dysarthria.
Neuropsychiatric and Behavioral Adjustments
Precise targeting of brain structures can lead to complex and delayed changes in mood, cognition, and emotional regulation. A specific long-term cognitive concern is a subtle decline in verbal fluency—the ability to quickly generate words. This decline is a recognized consequence of STN stimulation, with some studies reporting up to a 30% reduction in verbal fluency scores. While generally mild, this change represents a specific cognitive adjustment associated with the treatment.
Changes in mood are another area of adjustment; some patients experience improved mood post-surgery due to better motor control, while others may develop persistent apathy or depression. Apathy, characterized by a lack of motivation or interest, has been reported as an adverse effect of STN-DBS. This can be linked to the stimulation itself or the rapid reduction of dopaminergic medication following the procedure. Mood symptoms require monitoring and adjustment of stimulation parameters.
The relationship between DBS and Impulse Control Disorders (ICDs) is complex. Before surgery, ICDs such as pathological gambling, hypersexuality, or compulsive shopping are often linked to high doses of dopaminergic medication. Since DBS allows for a significant reduction in medication, many patients see an improvement or remission of these compulsive behaviors post-surgery. However, in a minority of cases, new ICDs (de novo ICDs) can emerge after DBS, or existing ones may worsen.
These behavioral changes result from the stimulation inadvertently affecting the limbic (emotional) parts of the target nucleus. Some patients also experience shifts in personality, including changes in executive function and social behavior. While these alterations are difficult to distinguish from the effects of the progressive disease, they underscore the need for comprehensive long-term psychological and psychiatric follow-up.