Dystonic Tremor Disappearance after Internal Capsule Stroke

Dystonic tremor is de ﬁ ned by the occurrence of tremor and dystonia in the same body part. 1 Dystonia secondary to stroke is a well-known phenomenon, 2 however the disappearance of dystonia or dystonic tremor after stroke has not been reported. 3 We present a patient with dystonic tremor, which was abolished on the right side after contralateral stroke in the posterior limb of the internal capsule.

Dystonic tremor is defined by the occurrence of tremor and dystonia in the same body part. 1 Dystonia secondary to stroke is a well-known phenomenon, 2 however the disappearance of dystonia or dystonic tremor after stroke has not been reported. 3 We present a patient with dystonic tremor, which was abolished on the right side after contralateral stroke in the posterior limb of the internal capsule.

Case Report
A 64-year-old woman was referred to us as a potential candidate for stereotactic surgery for tremor. She experienced shaking of the arms that had progressively worsened over the past six years, without sufficient effect of primidone, propranolol and gabapentin. The patient is right-handed. During examination, we saw a bilateral position-dependent and jerky action tremor, most severe when the arms were pronated (Video 1, part 1). There was dystonic posturing of the right hand, with hyperextension of the fingers, wrist flexion, ulnar deviation of the hand, as well as abnormal finger tapping. At rest, there was an intermittent tremor of the right index finger. A very mild, intermittent horizontal head tremor combined with subtle right-sided rotation of the head was observed, suggestive of cervical dystonia. There was no bradykinesia, rigidity or ataxia. Polymyography showed tremulous activity with a frequency between 4.3 and 5.6 Hz (Fig. 1A). Brain magnetic resonance imaging (MRI) showed no lesions in the basal ganglia and cerebellum (Fig. 1B). Hence, she was diagnosed with idiopathic dystonic tremor syndrome. 1 Six weeks later upon waking up in the morning, she noticed a minor weakness of the right leg, but also disappearance of tremor in the right arm. Computer tomography (CT) of the brain did not show any abnormalities in the acute setting, and she was started on antiplatelet therapy under suspicion of a stroke. MRI two weeks later showed a T2-hyperintense lesion with diffusion restriction on diffusionweighted imaging, in the posterior limb of the internal capsule, suggestive of a stroke in the vascular territory of the anterior choroidal artery (Fig. 1B, C).
During a second visit to our outpatient clinic (Video 1, part 2) the tremor on the right arm had disappeared. Hyperreflexia and a Babinski reflex were noted on the right bodyside. A subtle dysmetria during finger-chase and heel-to-shin tests was observed. Dystonic posturing at the right wrist remained unchanged, however dystonic posturing around the right thumb has improved. There was no more need for stereotactic surgery, Video 1. In part 1 a bilateral position-dependent and jerky action tremor is noted, most severe when the arms are pronated. There is dystonic posturing of the right hand. In part 2 (after stroke) the tremor on the right arm has disappeared. Hyperreflexia and a Babinski reflex, and subtle dysmetria during finger-chase and heel-to-shin tests are noted on the right bodyside, indicating involvement of the pyramidal tracts. Dystonic posturing of the right arm is unchanged. Video content can be viewed at https://onlinelibrary.wiley.com/ doi/10.1002/mdc3.13790 the patient was able to perform all daily activities with her right hand to her satisfaction.

Discussion
This is the first report of disappearance of dystonic tremor after stroke. Using an atlas-based reconstruction, we suggest that in this case a lesion in either the thalamocortical radiations exiting the thalamic ventralis intermediate (Vim) nucleus and ventral oralis posterior (VOp) nucleus, or the parieto-pontine projections, might be responsible for attenuation of dystonic tremor (Fig. 2). This is in line with previous literature suggesting that any lesion within the cortico-cerebello-thalamo-cortical circuit can disrupt tremor. 3 Whether previously reported cases with essential tremor also had subtle signs of dystonia, and would at present be classified as dystonic tremor, is unknown, as for most of these cases video recordings are unavailable.
In 1953, Cooper already described the therapeutic effect of anterior choroidal artery ligation on Parkinsonian tremor. 4 In a previous report by Dupuis and colleagues, a similar lesion as in our case is described in the posterior limb of the internal capsule in case 2, alleviating essential tremor, probably by involvement of thalamocortical projections. 5 The Vim relays cerebellar input and the VOp relays pallidal input to the cortex. As can be seen in Figure 2A-B, the lesion is in close proximity to both thalamic nuclei. A recent functional MRI study suggests that dystonic tremor involves both pallido-thalamo-cortical and cerebello-thalamo-cortical circuits. 6 In contrast, in essential tremor abnormal activity in the cerebello-thalamo-cortical circuit, but not the basal ganglia, is observed. 7 Furthermore, the optimal target for DBS in dystonic tremor appears to be at the border of the Vim, the conventional target for essential tremor, and the VOp. 8 The lesion in our case might be ideally placed at the latero-superior surface of the Vim and VOp nuclei, thereby interfering with abnormal activity in both circuits. An alternative explanation is that the lesion interrupted cortico-pontine projections (Fig. 2C-D), which relay cortical activity to the cerebellum via the pons. Interestingly, right-sided dysmetria as observed in our case might also be explained by involvement of these projections, as this will also interfere with activity in the cortico-cerebello-thalamocortical circuit.
In conclusion, we report disappearance of dystonic tremor after a stroke in the contralateral posterior limb of the internal capsule. We hypothesize involvement of the thalamocortical radiation or cortico-pontine tracts. In either case, the common factor is disruption of the cortico-cerebello-thalamo-cortical circuit, as has been described for other tremor disorders.  Coronal slices indicating involvement of mainly parieto-pontine tracts using the same method as for Figure 2C. Please see the supplementary materials for further details on used methods and references. DWI and FLAIR-based masks are available from the corresponding author, upon reasonable request.