Patient Selection Criteria for Deep Brain Stimulation in Movement Disorders

Neurological Evaluation

The preoperative evaluation for DBS normally begins with assessment by a neurologist with specific expertise in the management of movement disorders. This evaluation should be focused on establishing the correct diagnosis and ensuring that all reasonable medical therapies have been tried. Approved indications for DBS include essential tremor (ET), Parkinson disease (PD), and primary dystonia. Other forms of tremor (i.e., midbrain, cerebellar, and orthostatic), tics, and choreas (Huntington chorea and neuroacanthocytosis) have also been targeted with DBS but are considered off-label uses of the approved device. Box 106.1 summarizes movement disorders for which DBS is currently indicated.

Once the correct diagnosis is established, the neurologist should review the medical management history to make sure that all reasonable pharmacologic therapies have been properly attempted. This can be a very controversial point because opinions vary greatly as to what constitutes the “minimal” medication therapy to be attempted before proceeding with DBS. When a patient with PD is being considered for DBS, examination both “off” and “on” medications should be performed because determination of benefit from levodopa has well-documented diagnostic implications. In addition to neurological and motor deficits, functional disability should be documented using an accepted clinical rating scale that is specific to the patient’s diagnosis. Videotaped examinations are also of great value for clinical documentation and monitoring of long-term outcome.

The value of the initial neurological visit does not end with the selection process. It also establishes a clinical baseline for postoperative comparison and provides an opportunity to educate patients and caregivers on the risks of surgery and expected outcomes. It is far more challenging to program a patient’s DBS devices and manage his or her medications postoperatively if the neurologist is unfamiliar with the patient. Unrealistic expectations on the part of patients or family members are common reasons for “DBS failures,” so it is wise to moderate expectations through consultation early in the process. Finally, DBS management represents a significant commitment of time, so it is important that the treating neurologist assess the patient’s and family’s motivation to undertake the challenge.

Neurosurgical Evaluation

If the neurologist determines that the diagnosis is correct and that reasonable medical strategies have not provided acceptable benefit, the patient is referred to a functional neurosurgeon for further evaluation. During this meeting, the surgeon confirms that the patient is an appropriate surgical candidate and discusses the remainder of the preoperative process; the surgical options, including the available targets, devices, and surgical methodologies (e.g., frame versus frameless); and the risks and realistic results of DBS.

A fundamental task for the neurosurgeon is to define whether the risk-benefit ratio of the surgical procedure is acceptable to the patient. The most fearsome risk associated with the DBS procedure is the occurrence of intracerebral hemorrhage, which may cause a variety of neurological deficits, including death in the most severe cases. The risk for hemorrhage may vary from center to center, based on the experience of the surgeon, but it is never nil. In addition, there are device-related risks of system failure and infection, which can occur in up to 25% of patients even in the best centers. ^, Patients are also reminded that battery replacement for certain models of implanted pulse generator, requiring minor surgery, will be necessary after 2 to 5 years, depending on the electrical energy usage associated with their therapeutic stimulation. Newer devices that have recently entered the market are expected to last even longer (up to 15 years for some rechargeable models).

In addition, potential contraindications to surgery should be explored. Patients with poorly controlled hypertension, diabetes, coronary artery disease, cardiac pacemakers, liver or kidney failure, seizure disorders, or coagulopathies may be poor candidates, although the risk-benefit ratio of DBS surgery should be assessed for each case. If the patient is a good surgical candidate and agrees, a tentative date for surgery is scheduled and the remainder of the preoperative evaluation is completed. If there is disagreement about the best surgical target or treatment modality, the patient’s case should be reviewed at a multidisciplinary conference to arrive at a consensus.

Neurocognitive and Psychiatric Evaluation

A neuropsychologist, psychiatrist, and social workers are essential members of a strong DBS program because proper cognitive assessment, psychiatric screening, and social considerations are fundamental steps in patient selection. Although fewer data are available on patients with premorbid psychiatric conditions undergoing DBS of the globus pallidus interna (GPi) or ventral intermediate (VIM) nucleus of the thalamus, the available evidence suggests that behavioral and cognitive side effects may occur, warranting preoperative neuropsychological testing for all patients considering DBS.

The prevalence of dementia is high in patients with PD, and even early preoperative dementia is a risk factor for permanent cognitive decline after DBS. ^, Because no one neuropsychological assessment tool has proven to be predictive of postoperative cognitive decline, several test batteries may be used to screen surgical candidates, with the goal of excluding those with dementia or significant deficits in executive function. No consensus guidelines or “gold standards” are available, and detailed comparative data have not been published. In designing a neuropsychological battery for DBS candidates, it is important to cover the major neurocognitive domains for both patient selection and follow-up; at the same time, the battery should not be exhaustive because patients with PD fatigue easily, leading to inaccurate results. Moreover, many PD patients with severe medication-related “on-off” fluctuations can sometimes experience significant cognitive disturbance during their “off” period yet perform relatively normally during their “on” period. ^{, ,} Unfortunately, neuropsychological testing is not universally performed before DBS, potentially leading to negative postsurgical outcomes.

A full psychiatric evaluation is essential for patients who pass cognitive screening but have signs of untreated depression or psychosis, including assessment for dopaminergic dysregulation syndrome, medication-induced hypomania/mania, and suicide risk.

The need for systematic neuropsychological and neuropsychiatric evaluation is less established for other DBS indications, including ET and dystonia, because dementia is not typically a feature of these conditions. On the other hand, because ET is a disorder of the aging brain, if there are concerns about a patient’s cognitive function, a neurocognitive evaluation may certainly be warranted. Dystonia patients who are DBS candidates are typically young and less prone to suffer cognitive abnormalities after surgery but may have neuropsychiatric issues surrounding their disabilities and may therefore benefit from psychiatric and social work support.

Neuroimaging

Potential surgical candidates should undergo preoperative imaging, preferably magnetic resonance imaging (MRI), in order to rule out structural lesions or anatomic distortions that either may interfere with proper targeting (e.g., areas of encephalomalacia) or represent an increased risk for hemorrhage (e.g., abnormally enlarged lateral ventricles, severe brain atrophy). In addition, when the diagnosis of idiopathic PD is questioned, MRI can show abnormalities typical of multiple system atrophy (MSA) or progressive supranuclear palsy (PSP), parkinsonian syndromes that are not currently considered appropriate indications for DBS surgery.

Nuclear imaging studies (single-photon emission computed tomography, positron emission tomography) can occasionally be helpful in differentiating atypical parkinsonism from idiopathic PD. Although performed in the past to predict pallidotomy outcomes, the use of metabolic imaging modalities for screening patients undergoing DBS therapy is currently not part of routine DBS screening but may be used as needed to establish or rule out a diagnosis of idiopathic PD.

Medical Clearance

Medical clearance is the last screening step that is recommended before DBS surgery and normally includes standard preoperative blood tests and electrocardiography when indicated. DBS candidates should also be instructed to discontinue vitamin E, aspirin, and other medications that interfere with normal coagulation at least 2 weeks before surgery to reduce the risk for hemorrhage. In patients with mechanical heart valves, the surgeon may employ enoxaparin during this 2-week period and then discontinue these injections the night before surgery, thereby minimizing the risk of thrombotic events during the 2-week washout period.

Parkinson Disease