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Letter to the editor
Jaw Opening Myoclonus in Subacute Sclerosing Panencephalitis: A New Phenotypic Observation
Divyani Garg1orcid, Vanshika Kakkar2orcid, Suvasini Sharma2corresp_iconorcid
Journal of Movement Disorders 2024;17(1):106-108.
Published online: September 14, 2023

1Department of Neurology, All India Institute of Medical Sciences, New Delhi, India

2Department of Pediatrics (Neurology Division), Lady Hardinge Medical College and associated Kalawati Saran Children’s Hospital, New Delhi, India

Corresponding author: Suvasini Sharma, DM Department of Pediatrics (Neurology Division), Lady Hardinge Medical College and associated Kalawati Saran Children’s Hospital, Shaheed Bhagat Singh Marg, New Delhi 110001, India / Tel: +91-011-23365792 / E-mail:
• Received: August 7, 2023   • Revised: September 5, 2023   • Accepted: September 14, 2023

Copyright © 2024 The Korean Movement Disorder Society

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Dear Editor,
Subacute sclerosing panencephalitis (SSPE) is a neurodegenerative disorder due to persistent infection of the brain with a mutant measles virus, with clinical signs emerging in late childhood and adolescence. The phenotypic sine qua non is the presence of a slow axial myoclonus. Movement disorders are seen in up to half of all patients with SSPE [1]. We have recently reported jaw myoclonus for the first time in SSPE [2] and have subsequently observed it in other patients.
A cross-sectional study was conducted between November 1, 2020, and August 31, 2022. During this period, 56 patients with SSPE were identified at Kalawati Saran Children’s Hospital, New Delhi, India. SSPE diagnosis was based on a modified form of Dyken’s criteria.
The median age (range) at presentation was 6 years (2.5 months–19 years), and the median age at onset was 5.9 (1.5–17.6) years. The median disease duration was 0.3 (4–36) months. The median Jabbour stage was 2 (2–5). Of 56 patients, seven (12.5%), including 5 males, demonstrated jaw opening myoclonus (JOM) (Table 1, Supplementary Videos 1-6 in the online-only Data Supplement). The age range was 3–18 years. The median duration of symptoms was 4 (1.5–24) months. Five patients were in Jabbour stage 3. All patients also demonstrated myoclonus in other body regions. JOM was synchronous with myoclonus in other regions in five patients and asynchronous in two. In one patient, JOM followed myoclonus in other body regions. Ocular myoclonus was observed in two patients (Cases 2 and 3) and was synchronous with JOM in both. Associated dystonia was seen in four patients. In two patients, JOM was seen despite normal magnetic resonance imaging (MRI) and a lack of periodic electroencephalography (EEG) discharges. Apart from the median Jabbour stage, which was higher in patients with JOM, we did not find any significant differences in terms of EEG or MRI compared to the patients without JOM.
The typical SSPE myoclonus is periodic, axial, and stereotyped, with a slow relaxation phase. Jaw myoclonus is a new phenotypic observation that may assist in disease recognition, especially when MRI and EEG do not contribute to the evidence of SSPE. Jaw myoclonus may be a feature of advanced disease in SSPE. The origin of myoclonus in SSPE has been debated but may correlate with the disease stage, with cortical or cortico-subcortical origin in earlier stages and brainstem origin in advanced stages, suggesting a cephalocaudal spread of pathology [3]. In our series, we considered the patients’ abnormal jaw movement to be myoclonus rather than dystonia, despite its slow character. The slowness of the relaxation phase was consistent with the typical presentation of SSPE myoclonus.
Involuntary movement involving the jaw has been reported in other conditions: jaw myoclonus induced by drugs such as cefepime and gabapentin [4], jaw closure myoclonus in sleep-related faciomandibular myoclonus [5], and jaw closure dystonia in anti-Ri-related encephalitis [6].
However, in our experience, jaw-opening myoclonus seems to be exclusive to patients with SSPE as opposed to any other disease involving the jaw. As slow myoclonus in SSPE serves as a diagnostic hallmark, jaw-opening myoclonus may have additional diagnostic potential. We do not know at what point in the disease course jaw myoclonus first appears. It would be useful to track the disease longitudinally and assess whether jaw myoclonus provides any prognostic value.
The online-only Data Supplement is available with this article at

Video 1.

Case 1. An 18-year-old male with recurrent jaw opening, limb and axial myoclonus, tremors and retrocollis.

Video 2.

Case 2. An 18-year-old male with recurrent jaw opening and limb myoclonus, right upper limb dystonia and forced deviation of the contralateral eye.

Video 3.

Case 3. A 6-year-old male with recurrent jaw-opening myoclonus accompanied by ocular myoclonus with gaze direction to the left.

Video 4.

Case 4. A 9-year-old male with jaw-opening myoclonus synchronized with right upper limb myoclonus.

Video 5.

Case 5. A 3-year-old female with recurrent jaw opening following limb myoclonus and left-sided dystonia.

Video 6.

Case 6. A 12-year-old male with jaw-opening myoclonus synchronized with left upper and lower limb myoclonus.

Ethics Statement

This study was approved by the Lady Hardinge Medical College Institutional Ethics Committee (Reference no. F.LHMC/IEC/2022/03/31). Informed consent was obtained from the participant’s parents and assent from the child, whenever applicable. 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.

Conflicts of Interest

The authors have no financial conflicts of interest.

Funding Statement


Author contributions

Conceptualization: Divyani Garg, Suvasini Sharma. Data curation: all authors. Formal analysis: Divyani Garg. Investigation: Suvasini Sharma. Methodology: Divyani Garg, Suvasini Sharma. Project administration: Suvasini Sharma. Resources: Suvasini Sharma. Software: Divyani Garg. Supervision: Suvasini Sharma. Validation: Vanshika Kakkar, Suvasini Sharma. Visualization: Divyani Garg, Suvasini Sharma. Writing—original draft: Divyani Garg. Writing—review & editing: Vanshika Kakkar, Suvasini Sharma.

We thank the patients and their families for their co-operation.
Table 1.
Detailed characteristics of SSPE patients with jaw myoclonus
Case Age (yr)/sex Duration of symptoms Type of jaw myoclonus Associated features MRI features EEG features Stage
1 18/M 2 years Jaw opening with lateral deviation of lips and protrusion of tongue asynchronous with limb myoclonus Limb myoclonus, neck myoclonus, tremors, seizures, cognitive impairment, dystonia (severe retrocollis) Asymmetrical frontoparietal white matter signal change, basal ganglia signal change (T2/FLAIR) Pseudo-periodic, short-interval, generalized discharges 3
2 18/M 4 months Jaw opening with lateral deviation synchronous with limb myoclonus Asymmetrical limb and truncal myoclonus, seizures, cognitive impairment, forced conjugate lateral eye deviation, ocular myoclonus*, right upper limb dystonia Small, discrete frontal white matter hyperintensity (T2/FLAIR) Pseudo-periodic, short-interval, generalized discharges 3
3 6/M 6 months Jaw opening with sustained open phase synchronous with limb/axial myoclonus Limb myoclonus, cognitive impairment, ocular myoclonus* Generalized cerebral atrophy Pseudo-periodic, long-interval, generalized discharges 3
4 9/M 4 months Jaw opening synchronous with right upper limb myoclonus Frequent falls, seizures, axial and limb myoclonus Normal Pseudo-periodic, long-interval, generalized discharges 2
5 6/F (published case) [6] 2.5 months Jaw opening synchronous with limb and axial myoclonus Limb myoclonus, left upper and lower limb dystonia, ataxia, aggressive behavior Normal Generalized theta slowing 3
6 3/F 3 months Jaw opening asynchronous with limb myoclonus Limb myoclonus, left upper and lower limb dystonia, cognitive impairment Bilateral hippocampal signal change (T2/FLAIR) Pseudoperiodic long interval generalized discharges 3
7 12/M 1.5 months Jaw opening synchronous with left upper and lower limb myoclonus Frequent falls, limb myoclonus, increased aggression Normal Generalized theta slowing 2

* we believe this case was a slow myoclonus involving extraocular muscles, as it was observed in other body parts in SSPE;

stages correspond to the Jabbour staging system.

SSPE, subacute sclerosing panencephalitis; MRI, magnetic resonance imaging; EEG, electroencephalography; M, male; F, female; FLAIR, fluid attenuated inversion recovery.

  • 1. Garg D, Kakkar V, Kumar A, Kapoor D, Abbey P, Pemde H, et al. Spectrum of movement disorders among children with subacute sclerosing panencephalitis: a cross-sectional study. J Child Neurol 2022;37:491–496.ArticlePubMedPDF
  • 2. Garg D, Kumar A, Sharma S. Periodic jaw-opening myoclonus in subacute sclerosing panencephalitis. J Mov Disord 2023;16:210–212.ArticlePubMedPMCPDF
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  • 4. Hui CH, Leung JK, Chang RS, Shea YF. Reversible dysphagia due to gabapentin-induced jaw myoclonus. Chin Med J (Engl) 2019;132:1485–1486.ArticlePubMedPMC
  • 5. Zhang S, Aung T, Lv Z, Zhao X, Wang P, Shi T, et al. Epilepsy imitator: tongue biting caused by sleep-related facio-mandibular myoclonus. Seizure 2020;81:186–191.ArticlePubMed
  • 6. Pittock SJ, Parisi JE, McKeon A, Roemer SF, Lucchinetti CF, Tan KM, et al. Paraneoplastic jaw dystonia and laryngospasm with antineuronal nuclear autoantibody type 2 (anti-Ri). Arch Neurol 2010;67:1109–1115.ArticlePubMed

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