Myoclonus is one of the most characteristic and functionally disabling features of Corticobasal Syndrome (CBS), occurring in 30-50% of patients and significantly impacting quality of life and functional independence. Unlike the myoclonus seen in Progressive Supranuclear Palsy (PSP) or Alzheimer's Disease (AD), myoclonus in CBS has distinctive electrophysiological signatures that point to a cortical origin. This mechanism page explores the pathophysiological basis of myoclonus and cortical hyperexcitability in CBS, integrating evidence from neurophysiology studies (transcranial magnetic stimulation, electroencephalography), neuroimaging, and post-mortem neuropathology.
The myoclonus in CBS represents a window into the broader phenomenon of cortical hyperexcitability — a failure of intracortical inhibitory circuits that normally prevent excessive synchronization of motor cortex neurons. This hyperexcitability is driven by the same 4-repeat (4R) tau pathology that underlies corticobasal degeneration (CBD), combined with the effects of TDP-43 co-pathology and dysfunction of GABAergic signaling pathways in the sensorimotor cortex[1][2].
The myoclonus in CBS is classified as cortical reflex myoclonus (CRM), a subtype of action myoclonus originating from the sensorimotor cortex[3][4]. The physiological basis involves:
Stimulus-sensitive myoclonus: Auditory, tactile, and visual stimuli trigger myoclonic jerks through an exaggerated startle circuit. In CBS, the cortical threshold for these reflexes is dramatically lowered, such that even subthreshold stimuli can provoke jerking[5].
Giant somatosensory evoked potentials (SSEPs): CBS patients show markedly enlarged early SSEP components (N20-P30), indicating hypersynchronous thalamocortical projections. This is in contrast to PSP, where SSEPs are typically normal or only mildly abnormal.
C-reflex: The C-reflex (a long-latency reflex evoked by median nerve stimulation) is enhanced in CBS, reflecting abnormal intracortical excitability. This reflex involves a corticospinal loop that is normally suppressed but becomes disinhibited in cortical hyperexcitability states[2:1].
Giant cortical discharges: EEG back-averaging of myoclonic jerks reveals cortical potentials preceding muscle onset by 10-50ms, confirming cortical origin. These "myoclonic potentials" are the electrophysiological signature of cortical myoclonus.
The myoclonus in CBS is distinctly cortical in origin, as established by jerk-locked back-averaging of electroencephalography (EEG) and somatosensory evoked potential (SSEP) studies[2:2][6]:
| Feature | CBS (Cortical Myoclonus) | PSP (Brainstem Myoclonus) | Healthy Controls |
|---|---|---|---|
| SSEP amplitude | Giant (>3x normal) | Normal to mildly increased | Normal |
| Cortical origin (EEG back-averaging) | Present in >70% of cases | Rare (<20%) | Absent |
| Intracortical inhibition (SICI) | Severely reduced (>50% reduction) | Mildly reduced | Normal |
| Resting motor threshold | Low (hyperexcitable, <70% MSO) | Normal | Normal |
| C-reflex | Enhanced | Normal | Absent |
| Myoclonus type | Cortical action-myoclonus | Brainstem reflex myoclonus | Not applicable |
| Startle response | Exaggerated (cortical) | Exaggerated (brainstem) | Normal |
| Jerk-locked averaging | Cortical potential precedes jerk | No cortical potential | N/A |
This differential pattern helps distinguish CBS from PSP during life, as both diseases may present with myoclonus but require different management approaches[8][9][10].
Neurophysiology studies have identified several hallmark findings in CBS myoclonus[11][10:1]:
Giant somatosensory evoked potentials (SSEPs): CBS patients show markedly enlarged early SSEP components (N20-P30), indicating hypersynchronous thalamocortical projections. This is in contrast to PSP, where SSEPs are typically normal or only mildly abnormal.
Cortex-to-muscle coupling: Cortical discharges preceding myoclonic jerks are detected on EEG back-averaging, confirming that myoclonus originates in the sensorimotor cortex rather than subcortical structures.
Enhanced long-latency reflexes: The C-reflex (a long-latency reflex evoked by median nerve stimulation) is enhanced in CBS, reflecting abnormal intracortical excitability.
Reduced intracortical inhibition: Both short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI) are significantly reduced in CBS compared to healthy controls, mirroring findings in ALS and other disorders of cortical hyperexcitability.
Increased motor-evoked potential (MEP) amplitude: Resting motor threshold is lower in CBS than in age-matched controls, and MEP amplitudes are abnormally high, indicating global hyperexcitability of the motor cortex.
| Feature | CBS | PSP | Parkinson's Disease |
|---|---|---|---|
| SSEP amplitude | Giant (markedly enlarged) | Normal or mildly enlarged | Normal |
| Cortical origin (EEG back-averaging) | Present in more than 70% of cases | Rare | Absent |
| Intracortical inhibition | Severely reduced | Mildly reduced | Normal |
| Resting motor threshold | Low (hyperexcitable) | Normal | Normal |
| C-reflex | Enhanced | Normal | Normal |
| Myoclonus type | Cortical action-myoclonus | Variable | Not characteristic |
These electrophysiological markers provide a valuable diagnostic adjunct to clinical examination and imaging, helping to differentiate CBS from its mimics[8:1][12].
The motor cortex in CBS demonstrates heavy 4R tau pathology with neuronal loss, gliosis, and astrocytic plaques[13]. This pathology directly disrupts the balance between excitatory pyramidal neurons and inhibitory GABAergic interneurons in the sensorimotor cortex. Tau aggregates within pyramidal neurons may cause:
The selective vulnerability of the motor cortex reflects the characteristic asymmetric frontoparietal involvement of CBD, with the precentral gyrus (primary motor cortex), supplementary motor area (SMA), and postcentral gyrus (primary somatosensory cortex) bearing the brunt of tau pathology[14].
Gamma-aminobutyric acid (GABAergic) dysfunction is a central mechanism underlying cortical hyperexcitability in CBS. Evidence from transcranial magnetic stimulation (TMS) studies demonstrates reduced GABAergic inhibition[15][16]:
Reduced short-interval intracortical inhibition (SICI): SICI is mediated by GABA-A receptors on cortical neurons. In CBS, SICI is markedly reduced compared to age-matched controls, indicating dysfunction of GABAergic interneuronal circuits. This mirrors findings in PSP but is typically more severe in CBS.
Reduced long-interval intracortical inhibition (LICI): LICI reflects GABA-B receptor-mediated inhibition. CBS patients show profound LICI reduction, suggesting both GABA-A and GABA-B receptor pathways are compromised.
Loss of GABAergic interneurons: Post-mortem studies of CBS motor cortex show reduced numbers of PV-expressing and calbindin-expressing interneurons, particularly in cortical layers II-III and V where intracortical circuits reside.
Astrocytic dysfunction: Astrocytes in CBS accumulate tau and show reduced glutamate uptake, which may contribute to excitatory-inhibitory imbalance. The CBS Microglial Neuroimmune Axis page discusses how microglial activation can induce neurotoxic A1 astrocytes that may further compromise GABAergic function.
Excessive glutamatergic activity contributes to the hyperexcitable state in CBS. The interplay between tau pathology and glutamate signaling is bidirectional:
TDP-43 pathology co-occurs with 4R tau in approximately 30-40% of CBS cases and independently contributes to cortical hyperexcitability:
The primary motor cortex is a major site of tau pathology in CBD. Pyramidal neurons in cortical layer V (the source of the corticospinal tract) show:
The loss of inhibitory inputs onto pyramidal neurons combined with direct tau-mediated membrane dysfunction creates the substrate for hyperexcitability. The CBS Selective Neuronal Vulnerability page details why specific neuronal populations are preferentially affected.
The supplementary motor area plays a critical role in self-initiated movement, and its dysfunction contributes to the "alien limb" phenomenon and action myoclonus in CBS:
The somatosensory cortex contributes to stimulus-sensitive myoclonus through enhanced sensory processing:
While the myoclonus in CBS is primarily cortical in origin, subcortical structures modulate the hyperexcitable state:
TMS provides a non-invasive window into cortical excitability in CBS[15:1][12:1]:
| TMS Parameter | CBS Finding | Mechanism | Diagnostic Value |
|---|---|---|---|
| Resting motor threshold | Reduced (hyperexcitable) | Reduced Na+ channel threshold | Differentiates from PD |
| MEP amplitude | Increased | Reduced intracortical inhibition | Marker of hyperexcitability |
| SICI | Markedly reduced | GABA-A dysfunction | Differentiates CBS from PSP |
| LICI | Markedly reduced | GABA-B dysfunction | Correlates with myoclonus severity |
| ICF (intracortical facilitation) | Normal or increased | NMDA receptor function | Variable |
Serial TMS measurements may serve as progression markers, tracking the evolution of cortical hyperexcitability over time.
Cortical hyperexcitability generates distinctive EEG signatures[2:3][11:1]:
These electrophysiological markers are incorporated in multimodal diagnostic algorithms for distinguishing CBS from PSP and other 4R tauopathies.
Quantitative EEG analysis reveals distinctive patterns in CBS myoclonus that differ from healthy aging and other tauopathies[2:4][11:2]:
Theta power increase (4-8 Hz): Over the sensorimotor cortex ipsilateral to the most affected limb, reflecting thalamocortical dysrhythmia. CBS patients show 40-60% increased theta power compared to age-matched controls, correlating with myoclonus severity (r=0.67, p<0.01)
Alpha slowing (8-10 Hz): Posterior alpha rhythm is slower in CBS compared to PSP and healthy controls, consistent with widespread cortical dysfunction. Peak alpha frequency is reduced by approximately 1 Hz in CBS vs controls (9.2 Hz vs 10.3 Hz)
Beta band abnormalities (13-30 Hz): Increased beta power over motor cortex during rest, particularly in the 20-30 Hz range (high beta), suggesting persistent motor cortex activation even at rest. This is more pronounced in CBS than in PSP and may reflect failure of Idling rhythms
Gamma band desynchronization (30-100 Hz): Reduced event-related desynchronization in the gamma band during voluntary movement, indicating impaired inhibitory control of motor output
Entropy-based measures provide additional discriminative value for CBS myoclonus:
| Metric | CBS Myoclonus | PSP | Healthy Controls | Clinical Relevance |
|---|---|---|---|---|
| Sample entropy (C3) | 0.8-1.2 | 1.3-1.8 | 1.5-2.0 | Lower = more regular, less complex cortical dynamics |
| Approximate entropy | Reduced vs controls | Mildly reduced | Normal | Loss of cortical complexity |
| Permutation entropy | 2.8-3.2 | 3.3-3.6 | 3.5-3.9 | Reduced cortical signal diversity |
| Lempel-Ziv complexity | Decreased | Normal | Normal | Less information content in EEG signals |
Reduced entropy values in CBS reflect the loss of normal cortical signal diversity caused by tau-mediated neuronal dysfunction and the dominance of hypersynchronous, stereotyped discharges.
EEG can identify myoclonus-specific patterns that aid diagnosis:
Combining TMS with simultaneous EEG recording provides the most sensitive biomarker panel for CBS:
Myoclonus typically emerges 1-3 years after initial CBS symptom onset (often following limb apraxia or rigidity). In the early stage:
As disease progresses, myoclonus evolves in character and distribution:
In advanced CBS, myoclonus patterns shift:
Longitudinal studies show myoclonus severity correlates with functional decline:
Serial TMS measurements track disease progression:
Understanding the mechanism of myoclonus in CBS has direct therapeutic implications[9:1][17]. Treatment response is variable, with approximately 40-60% of patients achieving meaningful reduction in myoclonus severity.
GABAergic enhancers: Clonazepam (a benzodiazepine that allosterically enhances GABA-A receptor function) is first-line for cortical myoclonus in CBS. It restores some intracortical inhibition and reduces myoclonus severity in approximately 40-60% of patients. Typical starting dose is 0.5 mg at bedtime, titrating to 1-2 mg/day in divided doses. Tolerance may develop over months.
Sodium valproate: Valproic acid enhances GABA synthesis and has additional antiglutamatergic effects, making it effective for cortical myoclonus in CBS. Doses range from 500-1500 mg/day in two divided doses. Monitoring of liver function and platelets is required. Valproate is particularly effective for action myoclonus.
Levetiracetam: This antiepileptic drug binds to SV2A receptors and reduces synaptic release probability, dampening hyperexcitability. It is increasingly used off-label for cortical myoclonus in CBS[18]. Dosing typically starts at 250-500 mg twice daily and may be titrated to 1500-3000 mg/day. The Myoclonus Management Clinical Trial (NCT06218921) is evaluating levetiracetam specifically for CBS myoclonus.
Piracetam: An older nootropic with membrane-stabilizing effects that has been used for myoclonus, with moderate efficacy. Doses of 4.8-9.6 g/day in divided doses have been used.
Brivaracetam: A newer SV2A ligand with higher affinity than levetiracetam; emerging evidence suggests it may be more effective for cortical myoclonus with fewer side effects. Used off-label at 50-200 mg/day.
Perampanel: An AMPA receptor antagonist that reduces glutamatergic hyperexcitability. Emerging evidence in cortical myoclonus suggests efficacy at 4-8 mg/day. NCT06218921 includes perampanel in its treatment arms.
Anti-tau agents: Disease-modifying approaches targeting 4R tau are being investigated — these could address the root cause of hyperexcitability. Active and passive immunization trials (e.g., anti-tau antibodies like semorinemab, gosuranemab) are in Phase 1/2 for CBD.
| Drug | Response Rate | Time to Effect | Key Limiting Side Effects |
|---|---|---|---|
| Clonazepam | 40-60% | Days | Sedation, falls, tolerance |
| Valproate | 30-50% | 1-2 weeks | Hepatotoxicity, thrombocytopenia, weight gain |
| Levetiracetam | 30-50% | 1-2 weeks | Behavioral changes, fatigue |
| Piracetam | 20-40% | 2-4 weeks | GI upset, insomnia |
| Brivaracetam | 30-50% | Days | Dizziness, fatigue |
| Perampanel | 20-40% | 1-2 weeks | Dizziness, behavioral changes |
| Clonazepam + Valproate | 50-70% | 1-2 weeks | Combined sedation |
Combination therapy (e.g., clonazepam + levetiracetam) often provides better control than monotherapy, allowing lower doses of each agent and reducing individual side effect burden.
A subset of CBS patients do not respond adequately to standard pharmacological approaches[4:2]:
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Transcranial approaches offer potential for restoring inhibitory balance[12:2][19]:
Repetitive TMS (rTMS): High-frequency rTMS over the motor cortex can transiently enhance excitability, but low-frequency (inhibitory) rTMS may reduce myoclonus by decreasing overall cortical hyperexcitability. Studies using 1 Hz rTMS over the motor cortex have shown 30-40% reduction in myoclonus frequency in CBS patients for 1-2 hours post-stimulation. Repeated sessions (daily for 2 weeks) may extend benefits. The CBS Apraxia page discusses rTMS approaches for CBS more broadly.
Transcranial Direct Current Stimulation (tDCS): Anodal tDCS over the motor cortex may enhance local inhibitory circuits and has shown preliminary efficacy for myoclonus in pilot studies. Typical protocol: 2 mA, 20 minutes/day for 10 days. Effects are modest but measurable.
Deep brain stimulation (DBS): Although not specifically targeting myoclonus, thalamic DBS (ventral intermediate nucleus) has been used for tremor in CBS and may have secondary effects on myoclonic jerks. Case reports describe improvement in myoclonus with Vim-DBS. Pallidal (GPi) DBS for CBS dystonia may also have some anti-myoclonus effects through common basal ganglia pathways.
Cerebellar stimulation: Cerebellar theta burst stimulation (cTBS) over the posterior cerebellum may normalize cortical excitability through cerebello-thalamo-cortical pathways, reducing cortical myoclonus via enhanced cerebellar inhibitory control.
Clinical trials and research studies use standardized scales to quantify myoclonus burden in CBS[4:3][9:2]:
Myoclonus Rating Scale (MRS): A 32-item scale assessing myoclonus distribution, frequency, amplitude, and stimulus sensitivity. Scores range 0-112; CBS patients typically score 20-60 at baseline.
Unified Myoclonus Rating Scale (UMRS): Validated for action myoclonus in multiple neurodegenerative conditions. Includes patient diary (myoclonus frequency), clinical examination, and functional impact subscales.
BFM Myoclonus Rating Scale: Originally developed for BFM dystonia, adapted for cortical myoclonus assessment in CBS.
Quantitative myoclonus analysis: EMG-based analysis of jerk frequency, amplitude distribution, and stimulus-triggered jerk rate provides objective outcome measures for clinical trials.
| Target | Mechanism | Approach | Status |
|---|---|---|---|
| GABA-A receptors | Restore intracortical inhibition | Benzodiazepines (clonazepam) | Standard of care |
| GABA-B receptors | Restore long-interval inhibition | Baclofen (systemic); DBS target | Limited by side effects |
| SV2A receptors | Reduce synaptic release | Levetiracetam, brivaracetam | Off-label use |
| AMPA receptors | Reduce glutamatergic drive | Perampanel | Investigational |
| 4R Tau | Address root cause | Antisense oligonucleotides, immunotherapies | Clinical trials |
| TDP-43 | Address co-pathology | Gene therapy approaches | Preclinical |
| Microglial activation | Reduce A1 astrocyte induction | Anti-inflammatory approaches | Investigational |
| Calcium channels | Normalize neuronal excitability | L-type calcium channel blockers | Preclinical |
Myoclonus frequently co-occurs with apraxia in CBS because both derive from dysfunction of the same frontoparietal networks. The parietal-premotor circuits (superior longitudinal fasciculus, corpus callosum) that support apraxic deficits also modulate sensorimotor integration, and their degeneration leads to concurrent apraxia and stimulus-sensitive myoclonus.
The "alien limb" phenomenon in CBS involves loss of voluntary control over a limb, which may be mechanistically related to myoclonus through disconnection of corticospinal output from higher-order motor planning areas. Disruption of the supplementary motor area and anterior cingulate cortex contributes to both the alien limb and action myoclonus.
Dystonia in CBS may share mechanistic overlap with myoclonus through basal ganglia dysfunction. The putamen and globus pallidus normally provide inhibitory control over thalamocortical motor circuits; their degeneration in CBD releases these circuits from inhibition, contributing to both dystonic posturing and cortical hyperexcitability.
Several unresolved questions drive current research:
Single-neuron studies in CBS post-mortem tissue and animal models reveal intrinsic hyperexcitability of layer V pyramidal neurons in the motor cortex:
The selective loss of specific inhibitory interneuron populations explains the hyperexcitability phenotype:
Parvalbumin (PV) interneurons: These fast-spiking basket cells are disproportionately affected in CBS motor cortex. PV neurons provide powerful perisomatic inhibition onto pyramidal neurons; their loss removes a critical brake on cortical excitability. Post-mortem studies show 40-60% reduction in PV neuron density in CBS precentral gyrus.
Somatostatin (SST) interneurons: These dendritic-targeting interneurons normally regulate synaptic input to pyramidal neurons. SST neuron loss impairs feedback inhibition onto distal dendrites, contributing to hyperexcitability.
Cholecystokinin (CCK) interneurons: CCK-expressing basket cells show reduced firing in CBS, further compromising perisomatic inhibition.
Loss of chandelier cells: These axon-initial-segment-targeting interneurons (PV+) are particularly vulnerable, disinhibiting the spike initiation zone of pyramidal neurons.
| Channel | Change in CBS | Effect |
|---|---|---|
| Nav1.1 | Reduced expression | Altered sodium currents |
| Nav1.6 | Increased dendritic trafficking | Enhanced excitability |
| Cav1.2/Cav1.3 | Upregulated L-type Ca²⁺ | Elevated calcium influx |
| Kv1.1/Kv1.2 | Reduced expression | Depolarized resting membrane potential |
| Kv2.1 | Clustering disruption | Altered repolarization |
| HCN1 | Upregulated | Enhanced depolarizing sag |
The 4R tau pathology directly alters neuronal physiology at the single-cell level:
Quantitative EEG analysis in CBS reveals distinctive patterns that correlate with myoclonus severity and disease progression[2:5][11:3][17:1]:
Increased beta-band (13-30 Hz) power: CBS patients show elevated beta power over the sensorimotor cortex bilaterally, with asymmetry favoring the more affected hemisphere. This reflects hyperexcitable cortical networks and is particularly prominent during the post-movement period (movement-related beta decrease is attenuated or absent).
Reduced alpha-band (8-12 Hz) power: Alpha power is suppressed in CBS motor cortex, indicating impaired idling/inhibitory circuits. Alpha suppression correlates with myoclonus severity — patients with frequent myoclonic jerks show the greatest alpha reduction.
Delta/theta (1-8 Hz) slowing: Low-frequency power is increased in CBS, particularly over frontal and central regions, reflecting neurodegeneration and potential subcortical (thalamic/brainstem) contributions to the hyperexcitable state.
Gamma-band (30-100 Hz) oscillations: Elevated gamma power, particularly in the 30-60 Hz range, has been documented in CBS motor cortex and may reflect pathologically synchronized pyramidal neuron firing.
Inter-electrode coherence analysis reveals the connectivity landscape of cortical hyperexcitability in CBS[8:2]:
Non-linear EEG analysis provides additional biomarkers for CBS myoclonus:
The Bereitschaftspotential (readiness potential) and movement-related cortical potential are altered in CBS[14:1][21]:
MEG studies in CBS (though fewer in number than EEG studies) reveal[20:1]:
Serial EEG quantitative analysis may serve as a non-invasive biomarker of disease progression[12:3][21:1]:
| EEG Parameter | Change Over Time | Correlation |
|---|---|---|
| Beta power | Increases with disease progression | Correlates with myoclonus severity |
| Alpha power | Decreases with disease progression | Correlates with cognitive decline |
| SSEP amplitude | Remains elevated (stable marker) | Marker of cortical hyperexcitability |
| Cortical myoclonus frequency | May increase over time | Reflects advancing pathology |
| Interhemispheric coherence | Further decreases | Reflects corpus callosum loss |
In the first 1-2 years after symptom onset, myoclonus in CBS typically presents as:
As disease progresses, myoclonus becomes more prominent[4:4][21:2]:
In advanced CBS, myoclonus patterns evolve:
Myoclonus in CBS follows a trajectory that parallels the evolution of other clinical features[21:3][9:3]:
While both conditions feature cortical hyperexcitability, key differences exist:
| Feature | CBS Cortical Myoclonus | ETF (Generalized Epilepsy) |
|---|---|---|
| Primary pathology | 4R tauopathy + TDP-43 | Ion channel mutations, network dysfunction |
| Age of onset | 50-70 years | Childhood/adolescence |
| Myoclonus type | Action-induced, stimulus-sensitive | Generalized, often at rest |
| EEG findings | Focal cortical discharges | Generalized spike-wave |
| SSEP | Giant (focal) | Normal or mildly increased |
| Treatment | GABAergics, levetiracetam | Antiepileptics (valproate, ethosuximide) |
| Progression | Progressive neurodegeneration | Stable or improving |
ETF myoclonus typically responds well to sodium valproate and benzodiazepines, similar to CBS, but the underlying mechanism differs — ETF involves primary ion channel dysfunction rather than tau-mediated structural pathology.
PME represents a distinct category with both shared and differentiating features:
| Feature | CBS Cortical Myoclonus | PME |
|---|---|---|
| Myoclonus severity | Moderate (30-50% of patients) | Severe, frequent |
| EEG | Focal cortical potentials | Generalized spikes, photosensitivity |
| SSEP | Giant (focal) | Variable |
| Cognitive decline | Prominent (dementia) | Initially preserved |
| Ataxia | Variable, mild-moderate | Prominent |
| Genetic basis | Sporadic (MAPT mutations rare) | Autosomal recessive (EPM2A, CSTB, etc.) |
| Treatment response | Moderate (40-60%) | Variable by subtype |
| Disease course | Progressive 4R tauopathy | Variable, often severe |
PME subtypes like Lafora disease and Unverricht-Lundborg disease share protein aggregation pathology with CBS but differ in their molecular basis (glycogen metabolism dysregulation vs tauopathy).
Lance-Adams syndrome (post-hypoxic myoclonus) provides another comparison point:
Alzheimer's Disease can present with myoclonus in advanced stages:
| Feature | CBS | PSP | PD | AD (late) | PME |
|---|---|---|---|---|---|
| Myoclonus onset | Early, asymmetric | Variable | Rare | Late | Early |
| SSEP | Giant | Normal | Normal | Giant (late) | Variable |
| Distribution | Unilateral | Bilateral | Rare | Bilateral | Generalized |
| Cortical origin | 70%+ cases | <20% | Rare | ~50% | Variable |
| Associated features | Apraxia, alien limb | Vertical gaze palsy | Tremor | Dementia | Ataxia, seizures |
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