Surgical Management of Tremor

This chapter includes an accompanying lecture presentation that has been prepared by the authors: .

Key Concepts

Tremor is probably the most common indication for surgery in patients with essential tremor (ET), symptoms of Parkinson disease, (PD) or any other etiology.
Deep brain stimulation (DBS) is currently the most commonly used technique in the surgical treatment of tremor.
There is a renewed interest in lesional procedures, especially MR-guided focused ultrasound, which requires neither an incision nor chronically implanted hardware.
In Parkinsonian tremor, the subthalamic nucleus is the target of choice for DBS because of its additional effects on other PD symptoms.
The ventrolateral thalamus including the VIM nucleus has been the traditional target for DBS and lesioning in ET, but an increasing number of patients are being implanted in the posterior subthalamic area (PSA), including the caudal zona incerta (cZI).
The PSA can be visualized on MRI, and increasing evidence suggests that it may be a more effective target in DBS for tremor.
Asleep DBS surgery for movement disorders is becoming more common, but only one paper has been published regarding asleep DBS for ET.
Stereotactic functional neurosurgery for tremor is currently evolving quickly as a result of the discovery/development of new brain targets, new lesioning techniques, improved structural MRI, advances in connectivity-based targeting, directional DBS, and asleep surgery.

Tremor is the most common movement disorder. Whether the rest tremor of Parkinson disease (PD), a kinetic-action tremor as in essential tremor (ET), or a dystonic tremor, this symptom greatly affects patients’ well-being and quality of life. The impact of tremor on an individual’s activities of daily living, mood, and social life should not be underestimated: tremor is visible, difficult to hide, socially embarrassing, and, as such, a highly stigmatizing symptom.

Tremor of any type is often resistant to medications, although beta blockers, anticholinergics, or anticonvulsants such as primidone, topiramate, or gabapentine can be effective in some patients; however, they are often sedating in the doses required to control tremor. Regular use of benzodiazepines or alcohol as a treatment is not an option. Similarly, for PD, it is known that tremor is a symptom that may not respond as well to dopaminergic medications as do other PD symptoms such as akinesia and rigidity. Hence it is not a coincidence that surgery for movement disorders has historically tried to target mainly the symptom of tremor, regardless of its etiology. It is also not a coincidence that results of surgery for tremor and graphic motion pictures of tremor patients before and after surgery (whether thalamotomy or DBS) have had a great impact on the popularization of these procedures. After all, the worldwide success of thalamotomy introduced by Hassler et al. in 1954 was a result of the very dramatic, instantaneous, and robust effect of this procedure on the symptom of tremor.

This chapter starts with a very basic classification of tremor, an ultra-brief summary concerning its circuitry and pathophysiology, and a short history of stereotactic neurosurgery for this symptom. For details concerning these aspects, the reader is referred to Chapter 81, Chapter 84 . Following this introduction, we will detail stereotactic procedures for the three most common tremors (Parkinsonian, essential, and dystonic) including radiofrequency thalamotomy and deep brain stimulation (DBS) of various brain targets. Finally, we will briefly discuss emerging imaging techniques, such as connectivity imaging and tractography, and emerging ablative surgical techniques for tremor, such as Gamma Knife (GK) radiosurgery, MR-guided focused ultrasound (MRgFUS), and MR-guided laser interstitial thermal therapy (MRgLITT).

Definition and Classification of Tremor

Tremor is defined as an involuntary rhythmic oscillatory movement of a body part, usually the upper limbs. It is caused by rhythmic fluctuations of the muscle tonus between the flexors and extensors. There are two main types of tremor: resting tremor, which is present when the muscle is at rest and disappears as soon as the muscle becomes activated, as in PD; and kinetic (action) tremor, also called intention tremor, which is absent or minimal when the muscle is relaxed, but starts or worsens when the muscle becomes or remains activated. These are the most common tremor forms that have been and are subject to surgical therapy. Other less common tremors that can be subject to surgery are dystonic tremor (a coarse, irregular tremor accompanying a particular body posture), rubral tremor (a combination of rest, postural, and action tremor), and tremor of multiple sclerosis (a cerebellar intention-action tremor with high amplitude, proximal muscle engagement, and dysmetria).

Pathophysiology and Circuitries of Tremor

Oscillations in the cerebello-thalamo-cortical pathway occur in virtually all tremor disorders, regardless of their etiology. In PD, tremor-related neuronal activity has been demonstrated to occur both in the basal ganglia and their receiving areas of the thalamus, the ventral oral nuclei, and the cerebello-thalamo-cortical circuit passing through the thalamic ventral intermediate (VIM) nucleus. ^, In essential tremor, thalamic neurons show firing patterns at tremor frequency, predominantly in the VIM nucleus, which is the cerebellar receiving zone. ^, These two pathways of tremor, the basal ganglia-thalamic in PD and the cerebello-thalamic in ET, traverse the posterior subthalamic area and converge in the motor ventrolateral (VL) thalamus, which provides the anatomic-physiologic rationale for targeting the VL thalamus and the subthalamic area for surgical intervention.

History of Stereotactic Surgery for Tremor

Though human stereotactic neurosurgery was initially conceived for psychiatric disorders, it has been most successful in surgical treatment of tremor, with the procedures aimed at interrupting the oscillating pallidothalamic and/or dentatothalamic pathways at various loci. Stereotaxis for Parkinsonian tremor and rigidity included pallidotomy and pallidoansotomy, the Hassler VL thalamotomy, the Spiegel campotomy in the field (campus) of Forel, the Andy subthalamotomy, and various target areas mainly in the VL thalamic complex, extending from the VIM nucleus posteriorly to the nucleus ventro-oralis anterior of the thalamus (Voa) anteriorly.

With the introduction of microelectrode techniques, the VIM area of the thalamus, just posterior to the nucleus ventro-oralis posterior of the thalamus (Vop), was shown to contain cells that fired in a tremor-synchronous manner. The VIM nucleus thus became, and remains, the stereotactic target par excellence for the treatment of all tremors, regardless of etiology. ^, After the introduction of L-dopa therapy in the late 1960s and early 1970s, there was a dramatic decline in stereotactic surgery for Parkinson disease, including surgery for tremor. It was not until the late 1980s that a renaissance of stereotactic surgery for movement disorders took place, pioneered by Laitinen’s rediscovery of the Leksell posteroventral pallidotomy for PD. It was well known by then that L-dopa therapy was not a lifelong treatment for PD and that chronic administration caused motor fluctuations and dyskinesias, which became the main indications for pallidotomy.

Deep Brain Stimulation

Since the 1960s, various workers occasionally tried chronic electrical stimulation through electrodes permanently implanted in thalamic and subthalamic targets for treatment of tremor. The rationale was that this method was more lenient than ablative lesions in poor-risk patients or in those who needed bilateral surgery because bilateral thalamotomy harbored unacceptable complications, especially when performed simultaneously.

The 1987 publication by Benabid et al. heralded the new era of DBS. Initially, Benabid used DBS contralaterally to a previous thalamotomy in patients with bilateral Parkinsonian or essential tremor. Eventually, DBS became the primary procedure and the method of choice in surgery for tremor, enabling a safe and bilateral simultaneous surgery with few and mostly reversible side effects compared with ablative surgery. The extension of DBS to the STN and globus pallidus pars interna (GPi) in the 1990s ^, established DBS as the leading procedure worldwide for the treatment of movement disorders in general, and tremor in particular.

Contemporary Surgery for Tremor

Since the turn of the millennium, surgery for tremor has mainly consisted of DBS, not ablation. However, ablative surgery for tremor is witnessing a modest renaissance as a result of the development of two new ablative techniques: MRgFUS and MRgLITT. The primary targets for DBS for tremor are the VIM nucleus and various targets into the subthalamic area/ZI. In the following sections, we describe these targets and their imaging as well as the surgical techniques used for DBS. In addition, we discuss some basic principles concerning radiofrequency (RF) lesioning techniques, especially RF thalamotomy for tremor. Finally, we provide a brief discussion of emerging tools for performing ablative treatment for tremor, such as GK radiosurgical thalamotomy, MRgFUS thalamotomy, and, most recently, MRgLITT thalamotomy.

Targets

Subthalamic Nucleus

The subthalamic nucleus (STN) is the target of choice for DBS in PD, whether or not tremor is the dominant symptom. It is unclear where in the STN the best tremor effect is achieved. The effect of STN-DBS in relation to the location of the electrode was evaluated in 260 patients in a total of 13 different studies. Interestingly, most of these studies found that the best effect on tremor is not achieved in the STN itself, but in the rostral ZI (rZI) above the STN. Sometimes, the best effect on tremor is achieved when the electrode exits the STN posteromedially and enters the caudal ZI (cZI).

Ventral Intermediate Nucleus

Some differences exist among different groups regarding the anterior commissure–posterior commissure (AC-PC) coordinates of the VIM nucleus, a structure that still cannot be visualized on conventional structural MRI scans. Many groups aiming at the VIM area will also explore the underlying posterior subthalamic area (PSA) during surgery. Several studies performed on electrode locations in the VIM nucleus have demonstrated that the best effect on tremor is often not achieved with stimulation in the VIM nucleus itself, but from contacts located below the thalamus in the PSA.

Posterior Subthalamic Area

The PSA is the area situated inferior to the motor ventral thalamus, lateral to the red nucleus and posteromedial to the STN. The primary components of the PSA include the sparse nuclear structure called the ZI, the fiber tracts of the prelemniscal radiations, the lemniscus, the fasciculus-cerebello-thalamicus, and the Forel fields (which consist of the ansa and fasciculus lenticularis). When treating tremor, different groups have chosen slightly different target points within the PSA. ^,

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