mdc312030-sup-0001-VideoS1.m4vvideo/x-m4v71755KVideo. Sequence 1 (8.5 years old): Sitting position is autonomous. Standing position and gait are possible with assistance, but impaired by ataxia and spasticity. Sequence 2 (11 years old): Sitting position, standing position, and gait are not autonomous. Dystonia and spasticity have worsened. Cerebellar signs with dysmetria and abnormal ocular movements are elicited. Sequence 3 (13 years old): Marked spasticity and skeletal deformities with bilateral fixed flexor knee, trunk ataxia, extensor trunk and neck spasms, severe worsening of the cerebellar signs involving the upper limbs, and intention tremor. One can see worsening of the abnormal ocular movements, observed at each blinking reflex. Sequence 4 (15 years old): Rapid and severe worsening of dystonia resulting in dystonic storm necessitating admission in the ICU for sedation. The child exhibits intense axial dystonic spasms, hyperthermia, and sweating. As consequence of severe dystonia, the child developed rhabdomyolisis and also had an aspiration pneumonia. Sequence 5 (15 years old): One month after DBS surgery: the child recovered from status dystonicus; she is able to sit with aid; she does not have dystonic spasms; she is fed orally; and upper-limb cerebellar signs improved, as did oculogyric crisis. Postural and intention tremor decreased, thereby allowing the patient to hold a pen and draw a circle. Sequence 6 (15.5 years old): Three months after DBS surgery: there is no relapse of severe dystonic spasms. The child is now able to shortly sit unaided. Grasping and dysmetria improved, compared with the pre-DBS condition. Ocular pursuit and saccades are satisfactory, and she has no episodic oculogyric crisis.

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