PMC6133908_01
Neuroimaging and surface electromyography. A - MRI scan revealed an intracranial dermoid cyst causing severe midbrain and pons dislocation. B - Surface EMG was evaluated in six different conditions: at rest or with arms outstretched at shoulder level (top traces), during mass loading or contralateral movements (at the middle), during ballistic movements or making a finger tapping by synchronizing contralateral movements with a 3 Hz metronome (bottom traces). Note the different frequency between postural and rest jerks, the increase of tremor amplitude with mass loading, as well as jerks' synchronization during voluntary contralateral movements. Both tremor inhibition and co-activation sign (black arrow) appeared while asking the patient to make a ballistic movement (R = right; L = left; EDC = extensor digitorum communis; FCR = flexor carpi radialis).
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Magnetic Resonance Imaging (MRI), performed before our examination because of the occurrence of dizziness and postural instability, revealed a bulky intracranial dermoid cyst at the right pontocerebellar angle with severe brainstem and midbrain dislocation (see Fig. 1-A). Surface electromyography showed an alternating pattern, with a different frequency between postural and rest jerks (Fig. 1-B); this paradoxical variation has not been described before, although it may be related to a combined involvement of cerebello-thalamic and nigrostriatal pathways as occurs in rubral tremor. Co-activation sign was also present: together with a transient arrest during ballistic movements, a tonic discharge of antagonistic muscles approximately 300 ms before tremor onset was observed (Fig. 1-B).