Causes of Myoclonic Jerks
Myoclonic jerks arise from diverse etiologies spanning physiological, epileptic, toxic-metabolic, infectious, neurodegenerative, and drug-induced causes, requiring systematic evaluation to distinguish benign from serious underlying conditions. 1
Classification Framework
Myoclonic jerks are organized into four major clinical categories that guide diagnostic approach 1:
- Physiological myoclonus: Includes benign sleep jerks, hiccups, and exercise-induced myoclonus in healthy individuals 1
- Essential myoclonus: Isolated myoclonus without other neurological abnormalities 1
- Epileptic myoclonus: Seizure-related jerks occurring in various epilepsy syndromes 1
- Symptomatic myoclonus: Secondary to identifiable underlying disorders, representing the most common category 1
Epileptic Causes
Juvenile Myoclonic Epilepsy (JME)
JME is characterized by the triad of myoclonic jerks on awakening (100% of patients), generalized tonic-clonic seizures (>90%), and typical absences (one-third of patients). 2
- Onset is age-related with circadian distribution, frequently precipitated by sleep deprivation, fatigue, and alcohol intake 2
- EEG shows generalized pattern of spikes and/or polyspikes and waves 2
- Intelligence remains normal 2
- This syndrome is commonly under-diagnosed despite being well-defined clinically 2
Other Epileptic Syndromes
- Myoclonic seizures occur in idiopathic generalized epilepsies and severe epileptic encephalopathies 3
- Status myoclonicus can present in generalized epilepsy syndromes, though extremely uncommon in elderly patients 4
- Myoclonic seizures are characterized by sudden, brief, involuntary jerks lasting only a fraction of a second 5
Symptomatic (Secondary) Causes
Post-Hypoxic Myoclonus
Status myoclonus starting within 48 hours after cardiac arrest is associated with poor neurological outcomes, though some patients can recover. 5, 6
- Post-anoxic myoclonus may evolve into Lance-Adams syndrome with chronic action myoclonus in patients with good neurological recovery 6
- EEG recording helps identify awareness and reactivity in post-arrest patients 5
Toxic-Metabolic Disorders
Immediately review the medication list to identify drug-induced myoclonus, particularly from opioids, as this may be reversible. 7
- Opioid-induced myoclonus results from accumulation of toxic metabolites 7
- Hypocalcemia may induce or worsen myoclonic movements 8
- Hypomagnesemia can trigger myoclonic jerks 8
- Glucose metabolism disorders are recognized causes 8
Infectious Causes
- Viral encephalitis, particularly measles virus, causes myoclonic jerks with characteristic EEG showing periodic complexes with 1:1 relationship to jerks 8, 5
- Nipah virus infection leads to myoclonus along with dystonia, areflexia, and hypotonia 5
- JC virus causing Progressive Multifocal Leukoencephalopathy in immunocompromised patients presents with myoclonic movements 5
Neurodegenerative Disorders
- Storage diseases cause symptomatic myoclonus 1
- Various neurodegenerative conditions present with myoclonic features 1
- Creutzfeldt-Jakob disease (CJD) is characterized by rapidly progressive dementia followed by myoclonic jerks with bisynchronous periodic sharp wave discharges on EEG that have 1:1 relationship with jerks 8
Structural/Metabolic Conditions
- Calcium-phosphate metabolism disorders including hypoparathyroidism, pseudoparathyroidism, and primary familial brain calcification 8
- Hyperthyroidism 8
- Kernicterus 8
- Cerebrovascular disease 8
- Brain trauma 8
Benign Conditions in Infancy
Benign Myoclonus of Early Infancy (BMEI)
BMEI is a nonepileptic disorder characterized by myoclonic jerks of the head and/or upper limbs occurring in clusters, mimicking infantile spasms but with preserved consciousness. 8
- Onset occurs in the first year of life, mainly between 4-7 months 8
- Attacks usually cease by age 2 years 8
- Ictal EEG, neurological status, and development must be normal to confirm diagnosis 8
- Attacks are frequently triggered by excitement, frustration, postural changes, or sensory stimuli 8
Febrile Myoclonus
- Massive myoclonic jerks during febrile illness are usually benign with no neurological sequelae 9
- Often misinterpreted as febrile seizures, leading to unnecessary diagnostic workup 9
- Recognition prevents unwarranted hospitalizations and procedures like lumbar puncture and EEG 9
Anatomical Origins
The motor cortex is the most common source of myoclonus, but origins also include 1:
Critical Diagnostic Distinctions
Distinguish myoclonic seizures from syncope-related movements: epileptic myoclonus begins at or shortly after loss of consciousness, while syncope-related movements occur after loss of consciousness and fall, are typically fewer and asymmetrical. 5
- Distinguish myoclonic seizures from tonic seizures (sustained muscle contractions) 5
- EEG monitoring is essential in patients with clinical seizure manifestations to detect epileptic activity 6
Red Flags for Secondary Causes
When evaluating myoclonic jerks, the following suggest secondary causes or alternative diagnoses 8: