The distributed somatotopy of tremor: a window into the motor system

Connecting tremors - a circuits perspective

PURPOSE OF REVIEW

Tremor is one of the most prevalent movement disorders in clinical practice. Here, we review new insights in the pathophysiology of tremor. We focus on the three most common tremor disorders: essential tremor (ET), dystonic tremor syndrome (DTS), and Parkinson's disease (PD) tremor.

RECENT FINDINGS

Converging evidence suggests that ET, DTS, and PD tremor are all associated with (partly) overlapping cerebral networks involving the basal ganglia and cerebello-thalamo-cortical circuit. Recent studies have assessed the role of these networks in tremor by measuring tremor-related activity and connectivity with electrophysiology and neuroimaging, and by perturbing network components using invasive and noninvasive brain stimulation. The cerebellum plays a more dominant and causal role in action tremors than in rest tremor, as exemplified by recent findings in ET, DTS, and re-emergent tremor in PD. Furthermore, the role of the cerebellum in DTS is related to clinical differences between patients, for example, whether or not the tremor occurs in a dystonic limb, and whether the tremor is jerky or sinusoidal.

SUMMARY

Insight into the pathophysiological mechanisms of tremor may provide a more direct window into mechanism-based treatment options than either the etiology or the clinical phenotype of a tremor syndrome.

STN Versus GPi Deep Brain Stimulation for Action and Rest Tremor in Parkinson's Disease

Objective

To investigate the effects of subthalamic nucleus (STN) and globus pallidus internus (GPi), deep brain stimulation (DBS) on individual action tremor/postural tremor (AT) and rest tremor (RT) in Parkinson’s disease (PD). Randomized DBS studies have reported marked benefit in tremor with both GPi and STN and DBS, however, there is a paucity of information available on AT vs RT when separated by the surgical target.

Methods

We retrospectively reviewed the 1-year clinical outcome of PD patients treated with STN and GPi DBS at the University of Florida. We specifically selected patients with moderate to severe AT. Eighty-eight patients (57 STN and 31 GPi) were evaluated at 6 and 12 months for changes in AT and RT in the OFF-medication/ON stimulation state. A comparison of “response” was performed and defined as greater than or equal to a 2-point decrease in tremor score.

Results

STN and GPi DBS both improved AT at 6- and 12-months post-implantation (p < 0.001 and p < 0.001). The STN DBS group experienced a greater improvement in AT at 6 months compared to the GPi group (p = 0.005) but not at the 12 months follow-up (p = 0.301). Both STN and GPi DBS also improved RT at 6- and 12-months post-implantation (p < 0.001 and p < 0.001). There was no difference in RT scores between the two groups at 6 months (p = 0.23) or 12 months (p = 0.74). The STN group had a larger proportion of patients who achieved a “response” in AT at 6 months (p < 0.01), however, this finding was not present at 12 months (p = 0.23). A sub-analysis revealed that in RT, the STN group had a larger percentage of “responders” when followed through 12 months (p < 0.01).

Conclusion

Both STN and GPi DBS reduced PD associated AT and RT at 12 months follow-up. There was no advantage of either brain target in the management of RT or AT. One nuance of the study was that STN DBS was more effective in suppressing AT in the early postoperative period, however, this effect diminished over time. Clinicians should be aware that it may take longer to achieve a similar tremor outcome when utilizing the GPi target.

Effect of deep brain simulation on arm, leg, and chin tremor in Parkinson disease

OBJECTIVE

The efficacy of deep brain stimulation (DBS) of the subthalamic nucleus (STN) in the treatment of Parkinson disease (PD)-related tremor has been well established. However, the relative impact on arm, leg, and chin tremor has been less clearly elucidated. The authors evaluated the distribution of tremors in a PD cohort undergoing STN DBS and sought to evaluate the differential impact of DBS as a function of tremor location.

METHODS

A retrospective study of patients with PD with tremor who underwent DBS surgery between 2012 and 2016 was performed to evaluate the impact of STN stimulation on overall and regional tremor scores.

RESULTS

Across 66 patients the authors found an average of 78% overall reduction in tremor after 6 months. In this cohort, the authors found that tremor reduction was somewhat better for arm than for leg tremors, especially in instances of higher preoperative tremor (84% vs 71% reduction, respectively, for initial tremor scores ≥ 2). No significant difference in response was found between patients with medication-responsive versus medication-nonresponsive tremors.

CONCLUSIONS

The authors found that although DBS improved tremor in all regions, the improvement was not uniform between chin, arm, and leg-even within the same patient. The reasons behind these differing responses are speculative but suggest that STN DBS may more reliably reduce arm tremors than leg tremors.

Tremor in motor neuron disease may be central rather than peripheral in origin

BACKGROUND AND PURPOSE

Motor neuron disease (MND) refers to a spectrum of degenerative diseases affecting motor neurons. Recent clinical and post-mortem observations have revealed considerable variability in the phenotype. Rhythmic involuntary oscillations of the hands during action, resembling tremor, can occur in MND, but their pathophysiology has not yet been investigated.

METHODS

A total of 120 consecutive patients with MND were screened for tremor. Twelve patients with action tremor and no other movement disorders were found. Ten took part in the study. Tremor was recorded bilaterally using surface electromyography (EMG) and triaxial accelerometer, with and without a variable weight load. Power spectra of rectified EMG and accelerometric signal were calculated. To investigate a possible cerebellar involvement, eyeblink classic conditioning was performed in five patients.

RESULTS

Action tremor was present in about 10% of our population. All patients showed distal postural tremor of low amplitude and constant frequency, bilateral with a small degree of asymmetry. Two also showed simple kinetic tremor. A peak at the EMG and accelerometric recordings ranging from 4 to 12 Hz was found in all patients. Loading did not change peak frequency in either the electromyographic or accelerometric power spectra. Compared with healthy volunteers, patients had a smaller number of conditioned responses during eyeblink classic conditioning.

CONCLUSIONS

Our data suggest that patients with MND can present with action tremor of a central origin, possibly due to a cerebellar dysfunction. This evidence supports the novel idea of MND as a multisystem neurodegenerative disease and that action tremor can be part of this condition.

Thalamomuscular coherence in essential tremor: hen or egg in the emergence of tremor?

Thalamomuscular coherence in essential tremor (ET) has consistently been detected in numerous neurophysiological studies. Thereby, spatial properties of coherence indicate a differentiated, somatotopic organization; so far, however, little attention has been paid to temporal aspects of this interdependency. Further insight into the relationship between tremor onset and the onset of coherence could pave the way to more efficient deep brain stimulation (DBS) algorithms for tremor. We studied 10 severely affected ET patients (six females, four males) during surgery for DBS-electrode implantation and simultaneously recorded local field potentials (LFPs) and surface electromyographic signals (EMGs) from the extensor and flexor muscles of the contralateral forearm during its elevation. The temporal relationship between the onset of significant wavelet cross spectrum (WCS) and tremor onset was determined. Moreover, we examined the influence of electrode location within one recording depth on this latency and the coincidence of coherence and tremor for depths with strong overall coherence ("tremor clusters") and those without. Data analysis revealed tremor onset occurring 220 ± 460 ms before the start of significant LFP-EMG coherence. Furthermore, we could detect an anterolateral gradient of WCS onset within one recording depth. Finally, the coincidence of tremor and coherence was significantly higher in tremor clusters. We conclude that tremor onset precedes the beginning of coherence. Besides, within one recording depth there is a spread of the tremor signal. This reflects the importance of somatosensory feedback for ET and questions the suitability of thalamomuscular coherence as a biomarker for "closed-loop" DBS systems to prevent tremor emergence.