REPETITIVE AXIAL MYOCLONUS IN A BOY WITH THE MCT8 GENE MUTATION

Objectives: Some patients with thyroid hormone transporter MCT8 defects were reported to have paroxysmal movements1), however the relationship between defects in MCT8 and the movements is not clear. We reported that focal dystonic movements in a boy with MCT8 gene mutation were associated with a contralateral putamen lesion2). He also exhibited involuntary, irregular, rhythmic axial jerks during drowsy states. The movements arose spontaneously, but not in response to any stimulus. The objective of this report is to investigate the axial jerks electrophysiologically in order to reveal how defects in MCT8 affect the movements.

Methods: EEG, nerve conduction velocity (NCV) studies, somatosensory evoked potentials (SEP), brainstem auditory evoked potentials (BAEP) and a surface EMG were performed.

Results: NCV and BAEP studies gave normal results. Repeated EEG showed moderate slow background activity without any paroxysmal discharges. An EEG recording during the movement revealed no abnormalities and gave no evidence for epileptic discharges associated with the jerky movements. Back-averaging of the EEG disclosed no time-locked cortical potentials in the one second preceding and following the spontaneous jerks. A SEP study showed no giant cortical SEPs. EMG analysis showed brief (less than 50 ms) jerks in the bilateral paraspinal muscles.

Conclusion: Electrophysiological studies revealed the boy showed axial myoclonus originating from either the brain stem or spinal cord. Although axial myoclonus was not reported in other patients with the MCT8 gene mutation, further studies are needed to investigate the relationship between axial myoclonus and defects in MCT8. 1) Brockmann K et al J Neurol. 2005 Jun;252(6):663–6. 2) Kakinuma H et al J Pediatr. 2005 Oct;147(4):552–4.