Beyond Slow Axial Jerks: Periodic Jaw-Opening Myoclonus and Dystonia as the Initial Manifestation of Subacute Sclerosing Panencephalitis

Subhajit Roy; Raghavendra Kenchaiah; Jigil Joy; Jalumuri Satya Soma Anjush; Ajay Asranna

doi:10.26815/acn.2025.01235

Subacute sclerosing panencephalitis (SSPE) is a rare, progressive disorder caused by measles virus infection of the central nervous system, classically presenting with behavioral changes, cognitive regression, and periodic slow axial myoclonus time-locked to high-amplitude generalized epileptiform discharges on electroencephalography (EEG) [1-3]. Recently, measles has re-emerged as a significant global public health concern, with the World Health Organization noting a resurgence of cases and large outbreaks in multiple regions due to gaps in vaccination coverage and declining herd immunity, underscoring the continued risk of severe complications such as SSPE [4]. Although movement disorders beyond the prototypical axial myoclonus—including dystonia, chorea, tremor, and parkinsonism—have been recognized, they are usually described as adjunctive features or as occurring later in the disease course [5,6]. Among these, craniofacial or jaw-involving myoclonus is rare and often underrecognized [7-9]. We report a 10-year-old girl in whom periodic jaw-opening dystonia–myoclonus was the initial and predominant motor manifestation, preceding the classical axial jerks and progressive encephalopathy. This case underscores a focal craniofacial phenotype that may herald SSPE and reinforces the need to consider SSPE in the differential diagnosis of new-onset periodic craniofacial myoclonus in children, providing a bedside clue that should prompt timely electrophysiological and serological evaluation.

A 10-year-old girl—the second child of nonconsanguineous parents, with a normal developmental history—had been functioning normally until 8 months before presentation, when she developed involuntary periodic jaw-opening movements with slight rotation of the head to the left. These movements developed gradually and were initially infrequent and subtle. Over the subsequent 3 months, the jaw-opening dystonia–myoclonus increased in amplitude and frequency, eventually occurring approximately once per minute with further intensification.

Concurrently, the patient exhibited insidious cognitive decline (difficulty concentrating, inattention, and mild behavioral changes) and a decline in scholastic performance. She had no history of measles or exanthematous fever; however, she was unvaccinated.

At initial presentation, neurological examination revealed an alert child with difficulty sustaining attention. Cranial nerves were intact. She exhibited intermittent, periodic jaw-opening dystonic–myoclonic contractions, often associated with a slight leftward head turn (initial part of Supplementary Video 1). No focal weakness, ataxia, or other focal neurological deficits were observed; reflexes and plantar responses were normal.

Given the clinical scenario of periodic myoclonus with cognitive decline and behavioral deterioration, SSPE was suspected, and the patient underwent evaluation including EEG, brain magnetic resonance imaging (MRI), and cerebrospinal fluid (CSF) analysis. EEG (Fig. 1) revealed high-amplitude periodic generalized discharges, consistent with classical Radermecker complexes, synchronized with the clinical events of periodic jaw-opening dystonia–myoclonus. Brain MRI demonstrated hyperintensities in the bilateral subcortical frontal and parietal white matter and periventricular regions (Fig. 2). There was no enhancement or obvious cortical atrophy. CSF assay revealed elevated measles immunoglobulin G (IgG) antibody titers (titer 625 IU/mL), fulfilling the diagnostic criteria (modified Dyken criteria).

Approximately 4 months into the disease course, the patient developed slow periodic axial jerks involving the trunk and neck (later part of Supplementary Video 1). Her Full-Scale Intelligence Quotient (FSIQ) was <70, and her modified Rankin Scale (mRS) score was 4. Thus, her presentation comprised periodic jaw-opening dystonia–myoclonus that evolved into combined periodic axial jerks, along with cognitive regression and behavioral disturbances. Notably, the periodic slow jaw-opening myoclonus served as an early, craniofacial-dominant manifestation, allowing targeted evaluation and establishment of the diagnosis before the emergence of slow axial myoclonus.

The patient was initiated on isoprinosine and valproate. Antiepileptic therapy was subsequently tailored with the addition of clonazepam followed by carbamazepine, aimed at suppressing myoclonus. Over 4 months of follow-up, her clinical status remained unchanged (FSIQ <70, mRS score 4), with continued need for assistance due to myoclonus.

The classical motor hallmark of SSPE is slow, periodic, generalized myoclonus involving axial muscles, occurring synchronously with generalized, high-amplitude periodic EEG discharges [1-3]. Craniofacial or jaw-opening myoclonus, however, has been described only in isolated reports and typically as a later or concurrent feature rather than the initial presentation [7,9]. Pandey et al. [10] described two cases in which jaw-opening myoclonus was the presenting feature, with concurrent right limb myoclonus in one and truncal myoclonus in the other.

In our patient, the earliest symptom was periodic jaw-opening dystonia–myoclonus associated with subtle head deviation, appearing months before the emergence of the more typical slow axial periodic jerks. This sequential evolution—from a craniofacial-dominant phenotype to mixed axial involvement—is rarely documented. The persistence of both patterns over time further distinguishes this case from prior reports and highlights the phenotypic heterogeneity of SSPE.

Recognition of this rhythmic craniofacial myoclonus pattern is crucial, as it may be misinterpreted in the early phase as focal motor seizures, oromandibular dystonia, or a psychogenic movement disorder, delaying appropriate diagnosis.

The prominence of jaw-opening dystonic–myoclonic activity suggests preferential early involvement of corticobulbar and basal ganglia circuits that regulate orofacial motor output. Oga et al. [6] demonstrated that periodic dystonic–myoclonic movements in SSPE likely arise from abnormal sensorimotor integration within the cortical-subcortical motor network. Ser et al. [7] further observed that myoclonus subtypes may evolve over the disease course, correlating with progressive propagation of measles-induced pathology from cortical to subcortical regions. A similar mechanism was proposed by Garg et al. [9], who suggested that jaw myoclonus may reflect disease spread from cortical/subcortical regions toward the brainstem.

In this context, early jaw-dominant periodic myoclonus may reflect selective vulnerability of brainstem or orofacial motor networks to the viral insult, preceding the more diffuse cortical-subcortical involvement that later manifests as generalized slow axial myoclonus. This evolution underscores a pathophysiological continuum between focal craniofacial and generalized myoclonus in SSPE. However, further studies are required to elucidate the underlying mechanisms.

SSPE-related periodic myoclonus is typically highly stereotyped, rhythmic, and periodic, occurring at regular intervals (usually every 4 to 10 seconds), and most commonly involving axial or orofacial musculature. Crucially, each motor event displays precise time-locking to high-amplitude generalized periodic EEG complexes (Radermecker complexes), without ictal evolution in frequency, morphology, or spatial distribution; this feature helps distinguish them from focal epileptic seizures [9,11]. In contrast, epileptic myoclonic or focal seizures typically exhibit variable inter-event timing, evolving ictal EEG patterns, and inconsistent electroclinical coupling, and they are more likely to respond to antiseizure medications [9,11]. SSPE-related periodic myoclonus, including jaw-opening spells, often shows a poor or transient response to conventional antiseizure therapy. Recognition of this phenomenon is therefore essential to avoid misdiagnosis, inappropriate treatment escalation, and delays in establishing the correct etiological diagnosis.

The diagnostic turning point in this case was clinical suspicion supported by EEG correlation, which revealed periodic, high-amplitude generalized discharges synchronized with the clinical events, consistent with Radermecker complexes and helping to differentiate SSPE from mimics. Subsequent detection of elevated CSF anti-measles IgG (625 IU/mL) confirmed the diagnosis. Had the craniofacial periodicity been overlooked, diagnosis might have been delayed, as behavioral regression and dystonia alone can mimic a range of metabolic, autoimmune, or degenerative disorders.

Despite advances in the understanding of its pathogenesis, effective therapy for SSPE remains limited. Immunomodulatory and antiviral agents such as isoprinosine, interferon-α, and ribavirin have shown inconsistent benefits [1,3]. Our patient was initiated on isoprinosine alongside appropriately titrated antiepileptic medication, with clinical stabilization over a 4-month follow-up period.

Early recognition and diagnosis of SSPE are critical because, once symptomatic, the disease usually follows a relentlessly progressive neurodegenerative course. SSPE typically manifests years after measles infection, and by the time classic clinical features become apparent, extensive cortical degeneration and irreversible neuronal loss may have already occurred. Timely diagnosis—based on clinical suspicion supported by characteristic periodic EEG discharges and CSF anti-measles antibody testing—enables initiation of disease-modifying therapies such as isoprinosine and interferon-α at an earlier stage. These therapies have been associated in some reports with prolonged survival and disease stabilization, compared with the otherwise fatal natural history of SSPE [12-14]. Early detection also facilitates appropriate counseling, supportive care planning, and avoidance of unnecessary interventions, potentially delaying progression and preserving neurological function for a longer period. Consequently, incorporating early diagnostic strategies into clinical practice is essential to improve outcomes and mitigate cortical degeneration before irreversible neurological injury ensues [12].

Thus, this case underscores the clinical heterogeneity of SSPE and expands its motor spectrum by demonstrating periodic jaw-opening dystonia–myoclonus as an initial and persistent manifestation preceding classical slow axial myoclonus. Recognition of this rhythmic craniofacial movement pattern—particularly when accompanied by cognitive or behavioral regression—should alert clinicians to the possibility of SSPE and prompt EEG and CSF evaluation. Early identification remains essential for timely diagnosis, therapeutic intervention, and prognostication.

The authors confirm that they have read the journal’s position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. Institutional Review Board approval was obtained for this case report (Approval No. NIMH/DO/IEC [BS & NS DIV]/2023 National Institute of Mental Health and Neurosciences, Bengaluru).

Written informed consent was obtained from the patient’s father for publication of this case report, including the accompanying video material. The caregiver was informed that the video contains identifiable facial features which could be disseminated and agreed to its publication without masking, as this was considered scientifically important. The consent form explicitly stated that photographs/videos may be used for medical record documentation, research, teaching, and presentation in medical forums (including conferences and medical websites), as well as publication in journals, and therefore may be publicly displayed.