Involuntary Jaw Movements During Eating

A 65-year-old woman with multiple system atrophy with predominant cerebellar ataxia (MSA-C) presented with difficulty eating for several months. Her abnormal jaw movements during eating resulted in impaired bolus formation and spillage of food (Video 1, part 1). The surface electromyography (sEMG) of the right masseter and left anterior digastric muscles during eating was recorded simultaneously (Video 1, part 2). Water intake remained unaffected (Video 1, part 3). The abnormal jaw movements were not observed at rest or while chewing solid objects, whereas head titubation was observed (Video 1, part 4). She had aphonia due to the progression of MSA; therefore, tremor during speech could not be confirmed. She reported no newly initiated medications. We diagnosed the condition as task-specific masticatory tremor, because the alternating discharge pattern appeared only during eating. Oromandibular dystonia was unlikely because sEMG revealed phasic rhythmic bursts, rather than sustained contractions. Geniospasm was excluded because of onset age and clinical features. There was no involuntary movement of the palate, and the observed 4–5 Hz frequency differed from the typical 1–3 Hz rhythm of palatal tremor extending to the jaw, making this diagnosis unlikely. Task-specific masticatory tremor is a rare disorder characterized by involuntary rhythmic oscillations of the jaw during oral intake (eating or drinking).1, 2 In our patient, the tremor occurred exclusively during eating. Most reported cases lack underlying neurodegenerative diseases,1 and no previous reports have described task-specific masticatory tremor associated with MSA. This tremor often responds poorly to pharmacological treatment,1 as observed in our case. Simple physical measures such as manually supporting the jaw improved food transfer (Video 1, part 5). With this method, eating a spoonful of gruel took 15 s rather than 40 s. This finding suggests that external stabilization of the mandible may compensate for impaired coordination of agonist–antagonist muscle activation during feeding. (1) Research project: A. Conception, B. Organization, C. Execution; (2) Statistical analysis: A. Design, B. Execution, C. Review and critique; (3) Manuscript preparation: A. Writing of the first draft, B. Review and critique. T.A.: 1A, 1C, 3A. K.K.: 1A. T.S.: 3B. We would like to thank the patient for her cooperation and participation in this study. Ethical Compliance Statement: This study was approved by our local ethics committee. Patient consent was obtained. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. We obtained patient consent using forms created by our hospital. We received a signed consent form from this patient and her family authorizing the offline and/or online distribution of the videotaped material. Funding Sources and Conflict of Interest: We submit a completed version of the ICMJE conflict of interest disclosure form. Author disclosures are available in the Supporting Information. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions. Data S1. Coi_disclosure. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.