Thalamic Deep Brain Stimulation for Secondary Dystonia in Children and Young Adults

Dystonia is a movement disorder seen in both children and adults that is characterized by "sustained or intermittent muscle contractions causing abnormal, often repetitive, movements, postures, or both." Secondary dystonia has evolved to refer to dystonia resulting from damage to the nervous system or degenerative disease processes. While primary dystonia is generally thought to arise from genetic causes, secondary dystonias have a variety of causes including perinatal injuries (cerebral palsy), central nervous system infections, traumatic brain injuries, and many different metabolic, neurodegenerative, and mitochondrial conditions. Secondary dystonia is far more common in pediatric populations than primary dystonia, and far more recalcitrant to standard pharmacologic and surgical treatments including Deep Brain Stimulation. Given that most treatments for dystonia are developed for primary dystonia and then applied to secondary dystonia, it is not surprising that this effectiveness gap exists. Thus, there exists a large unmet need to develop new therapeutics, treatment strategies, and outcome measures for pediatric secondary dystonia.

Deep Brain Stimulation (DBS) is one such therapeutic intervention that has potential to improve secondary dystonia. DBS is a surgical treatment for several different movement disorders that evolved from functional stereotactic neurosurgery techniques initially used to lesion specific deep brain structures. While Essential Tremor and Idiopathic Parkinson's Disease have predictable and consistent response rates to DBS in carefully selected patients, response rates of dystonia have been much more inconsistent. One predictor of success has been the presence of DYT-1 mutation, the most common known genetic cause of primary dystonia. Success rates in DYT-1 dystonia are consistently high with reductions in dystonia typically greater than 80%. However, the results in secondary dystonia have been much more modest and inconsistent. A recent meta-analysis found that on average, dystonia symptoms as measured by common rating scales improve 23% following DBS for dystonic cerebral palsy (the most common cause of secondary dystonia), however there are frequent cases of non-responders. Additionally, there have been very few examination, radiological or laboratory predictors of good response to DBS, except for genetic confirmation of DYT-119. However, across both primary and secondary dystonia, younger age at the time of surgery (less than 21 years old) and shorter duration of symptoms (less than 15 years) have been shown to be the most likely predictive factors for a good postoperative outcome. This has led many to suggest that DBS should be offered earlier in the course of intractable dystonia, prior to the development of permanent complications such as orthopedic contractures. Thus, we are setting an upper age limit of 25 to account for the concern that earlier implantation leads to improved outcomes. The lower age limit of 7 reflects the fact that the current humanitarian exemption for DBS for dystonia currently goes down to age 7. Thus, there exists a need to both improve patient selection as well as application of DBS for secondary dystonia in children.

Inclusion criteria

• Ability to give informed consent or assent for the study
• Dystonia symptoms that are sufficiently severe, in spite of best medical therapy, to warrant surgical implantation of deep brain stimulators according to standard clinical criteria
• Age 7-25
• Stable doses of anti-dystonia medications (such as levodopa, baclofen, or diazepam) for at least 30 days prior to baseline assessment
• If patient receives botulinum toxin injections, patient should be on a stable injection regimen
• Intact thalamic anatomy as determined by standard clinical MRI

Exclusion criteria

• Pregnancy or breast feeding
• Major comorbidity increasing the risk of surgery (severe hypertension, severe diabetes, or need for chronic anticoagulation other than aspirin)
• Inability to comply with study follow-up visits
• Any prior intracranial surgery
• Uncontrolled epilepsy
• Immunocompromised
• Has an active infection
• Requires diathermy, electroconvulsive therapy (ECT) or transcranial magnetic stimulation (TMS) to treat a chronic condition
• Has an existing implanted neurostimulator or cardiac pacemaker.
• Dystonia caused by known genetic mutation in any DYT genes